Patent Publication Number: US-2020291414-A1

Title: Novel plants and methods for producing muconic acid

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The application claims priority to U.S. Provisional Patent Application Ser. No. 62/808,188, filed Feb. 20, 2019, which is herein incorporated by reference in its entirety. 
    
    
     STATEMENT OF GOVERNMENTAL SUPPORT 
     This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the United States Department of Energy. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to producing muconic acid. 
     BACKGROUND OF THE INVENTION 
     Muconic acid (MA) is a platform chemical that serves as a precursor for the synthesis of products such as adipic acid, terephthalic acid, and caprolactam which are widely used in the nylon and thermoplastic polymer industries. Current processes for the manufacturing of MA or its derivatives mainly rely on non-renewable petroleum-based chemicals. Such processes are not sustainable and eco-friendly since they require a high energy input and yield large quantities of toxic by-products (Xie et al., 2014). 
     As an alternative, the biological production of MA using engineered microorganisms and inexpensive carbohydrate feedstocks has received increasing attention over the past 20 years (Xie et al., 2014). Most of the established biological routes consist in the production catechol and its subsequent conversion into MA by ring-cleaving catechol 1,2-dioxygenase (Vaillancourt et al., 2006). All these routes exploit the intrinsic shikimate pathway for the biosynthesis of catechol precursors such as protocatechuate, anthranilate, salicylic acid (SA), and 2,3-dihydroxybenzoic acid (Kruyer and Peralta-Yahya, 2017). Recently, MA biosynthetic pathways have been implemented in various microbial strains capable of growing in the presence of aromatics derived from lignocellulosic biomass. These include engineered strains of  Escherichia coli  (Sonoki et al., 2014, Wu et al., 2017),  Amycolatopsis  sp. (Barton et al., 2017),  Pseudomonas  sp. (Vardon et al., 2015, Johnson et al., 2016, Johnson et al., 2017, Sonoki et al., 2017), and  Sphingobium  sp. (Sonoki et al., 2017). 
     In addition to microbial synthesis, the metabolic engineering of photosynthetic organisms like plants also provides a sustainable approach for the production of valuable metabolites and materials (Börnke and Broer, 2010, Farré et al., 2014). These chemicals, when produced in engineered bioenergy and oilseed crops, represent value-added renewable co-products on top of the lignocellulose and seed oil used to generate energy (Snell et al., 2015). Because plants are autotrophs able to capture solar energy, they represent an attractive chassis for implementing de novo metabolic pathways for cost-effective production of important chemicals (Yuan and Grotewold, 2015). 
     SUMMARY OF THE INVENTION 
     The present invention provides a genetically modified plant or plant cell comprising a nucleic acid encoding one or more heterologous enzymes operatively linked a promoter, wherein one or more heterologous enzymes synthesizes muconic acid (MA) from a salicylic acid (SA). The genetically modified host cell can comprise one of the enzymatic pathways necessary for producing a muconic acid described herein. 
     In some embodiments, the genetically modified plant or plant cell comprises nucleic acid encoding one or more heterologous enzymes wherein the plant cell or transgenic plant is capable of producing a muconic acid. The genetically modified plant cell or transgenic plant can comprise one of the enzymatic pathways for producing a muconic acid described herein, such as starting with DAHP, shikimate or chorismate as a precursor. 
     The present invention provides for a method for producing a muconic acid comprising: (a) optionally genetically modifying a plant cell or transgenic plant to produce a genetically modified plant cell of the present invention, (b) growing or culturing the genetically modified plant cell or transgenic plant to produce a muconic acid, (c) optionally pretreating the plant cell or transgenic plant, and (d) optionally converting the muconic acid into an adipic acid, terephthalic acid, and/or caprolactam. 
     In some embodiments, the heterologous enzymes are salicylate hydroxylase and/or catechol 1,2-dioxygenase. In some embodiments, the salicylate hydroxylase (NahG) is a bacterial or  Pseudomonas  salicylate hydroxylase (NahG). In some embodiments, the salicylate hydroxylase (NahG) comprises an amino acid sequence that is at least 70%, 80%, 90%, 95%, or 99% identical to the amino acid sequence of  Pseudomonas  salicylate hydroxylase (NahG), and comprises the enzymatic activity of salicylate hydroxylase (NahG). In some embodiments, the salicylate hydroxylase comprises one or both of the following ADP binding site amino acid sequences: GXGXXG (SEQ ID NO:5) and/or HGRXXLXGD (SEQ ID NO:6), wherein X is any naturally occurring amino acid. In some embodiments, the salicylate hydroxylase comprises one or more of the conserved amino acid sequences described in You et al.,  Biochem.  30:1635-1641 (1991). 
     The amino acid sequence of  Pseudomonas putida  salicylate hydroxylase (NahG) comprises: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 1) 
                   
               
               
                         10         20         30         40         50 
                   
               
               
                 MKNNKLGLRI GIVGGGISGV ALALELCRYS HIQVQLFEAA PAFGEVGAGV 
               
               
                   
               
               
                         60         70         80         90        100 
               
               
                 SFGPNAVRAI VGLGLGEAYL QVADRTSEPW EDVWFEWRRG SDASYLGATI 
               
               
                   
               
               
                        110        120        130        140        150 
               
               
                 APGVGQSSVH RADFIDALVT HLPEGIAQFG KRATQVEQQG GEVQVLFTDG 
               
               
                   
               
               
                        160        170        180        190        200 
               
               
                 TEYRCDLLIG ADGIKSALRS HVLEGQGLAP QVPRFSGTCA YRGMVDSLHL 
               
               
                   
               
               
                        210        220        230        240        250 
               
               
                 REAYRAHGID EHLVDVPQMY LGLDGHILTF PVRNGGIINV VAFISDRSEP 
               
               
                   
               
               
                        260        270        280        290        300 
               
               
                 KPTWPADAPW VREASQREML DAFAGWGDAA RALLECIPAP TLWALHDLAE 
               
               
                   
               
               
                        310        320        330        340        350 
               
               
                 LPGYVHGRVV LIGDAAHAML PHQGAGAGQG LEDAYFLARL LGDTQADAGN 
               
               
                   
               
               
                        360        370        380        390        400 
               
               
                 LAELLEAYDD LRRPRACRVQ QTSWETGELY ELRDPVVGAN EQLLGENLAT 
               
               
                   
               
               
                        410        420        430 
               
               
                 RFDWLWNHDL DTDLAEARAR LGWEHGGGGA LRQG 
               
            
           
         
       
     
     In some embodiments, the catechol 1,2-dioxygenase (CatA) is a bacterial or  Pseudomonas  catechol 1,2-dioxygenase (CatA). In some embodiments, the catechol 1,2-dioxygenase (CatA) comprises an amino acid sequence that is at least 70%, 80%, 90%, 95%, or 99% identical to the amino acid sequence of  Pseudomonas  catechol 1,2-dioxygenase (CatA), and comprising the enzymatic activity of catechol 1,2-dioxygenase (CatA). In some embodiments, the catechol 1,2-dioxygenase comprises one or more of the following amino acid residues acting as Fe ligands, or associated with thereof: Y at position 163 of SEQ ID NO:2, Y at position 197 of SEQ ID NO:2, and/or RPAHXH (SEQ ID NO:7), wherein X is any naturally occurring amino acid. 
     The amino acid sequence of  Pseudomonas putida  catechol 1,2-dioxygenase (CatA) comprises: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 2) 
                   
               
               
                         10         20         30         40         50 
                   
               
               
                 MTVKISHTAD IQAFFNRVAG LDHAEGNPRF KQIILRVLQD TARLIEDLEI 
               
               
                   
               
               
                         60         70         80         90        100 
               
               
                 TEDEFWHAVD YLNRLGGRNE AGLLAAGLGI EHFLDLLQDA KDAEAGLGGG 
               
               
                   
               
               
                        110        120        130        140        150 
               
               
                 TPRTIEGPLY VAGAPLAQGE ARMDDGTDPG VVMFLQGQVF DADGKPLAGA 
               
               
                   
               
               
                        160        170        180        190        200 
               
               
                 TVDLWHANTQ GT Y SYFDSTQ SEFNLRRRII TDAEGRYRAR SIVPSG Y GCD 
               
               
                   
               
               
                        210        220        230        240        250 
               
               
                 PQGPTQECLD LLGRHGQ RPA   H V H FFISAPG HRHLTTQINF AGDKYLWDDF 
               
               
                   
               
               
                        260        270        280        290        300 
               
               
                 AYATRDGLIG ELRFVEDAAA ARDRGVQGER FAELSFDFRL QGAKSPDAEA 
               
               
                   
               
               
                        310 
               
               
                 RSHRPRALQE G 
               
            
           
         
       
     
     In some embodiments, the genetically modified plant or plant cell endogenously produces salicylic acid (SA). In some embodiments, the genetically modified plant or plant cell further comprises one or more enzymes that in the pathway that converts PEP and/or E4P into SA, such that the genetically modified plant or plant cell produces SA. In some embodiments, the one or more enzymes that in the pathway that converts PEP and/or E4P into SA are heterologous to the genetically modified plant or plant cell. 
     In some embodiments, the genetically modified plant or plant cell further comprises bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9) and/or feedback-resistant DAHP synthase (L175Q) (AroG*). In some embodiments, the bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9) is bacterial or  Yersinia enterocolitica  bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9). In some embodiments, the feedback-resistant DAHP synthase (L175Q) (AroG*) is bacterial or  E. coli  DAHP synthase (AroG) that has a L175Q mutation which causes the AroG to be feedback resistant. 
     In some embodiments, the bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9) comprises an amino acid sequence that is at least 70%, 80%, 90%, 95%, or 99% identical to the amino acid sequence of  Yersinia enterocolitica  bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9), and comprises the enzymatic activity of bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9). In some embodiments, the bifunctional ISOCHA synthase/ISOCHA pyruvate lyase comprises the amino acid sequence from position 173 to position 424 of SEQ ID NO:3 which comprises a chorismate binding domain. 
     The amino acid sequence of  Yersinia enterocolitica  bifunctional ISOCHA synthase/ISOCHA pyruvate lyase (Irp9) comprises: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 3) 
                   
               
               
                         10         20         30         40         50 
                   
               
               
                 MKISEFLHLA LPEEQWLPTI SGVLRQFAEE ECYVYERPPC WYLGKGCQAR 
               
               
                   
               
               
                         60         70         80         90        100 
               
               
                 LHINADGTQA TFIDDAGEQK WAVDSIADCA RRFMAHPQVK GRRVYGQVGF 
               
               
                   
               
               
                        110        120        130        140        150 
               
               
                 NFAAHARGIA FNAGEWPLLT LTVPREELIF EKGNVTVYAD SADGCRRLCE 
               
               
                   
               
               
                        160        170        180        190        200 
               
               
                 WVKEASTTTQ NAPLAVDTAL NGEAYKQQVA RAVAEIRRGE YVKVIVSRAI 
               
               
                   
               
               
                        210        220        230        240        250 
               
               
                 PLPSRIDMPA TLLYGRQANT PVRSFMFRQE GREALGFSPE LVMSVTGNKV 
               
               
                   
               
               
                        260        270        280        290        300 
               
               
                 VTEPLAGTRD RMGNPEHNKA KEAELLHDSK EVLEHILSVK EAIAELEAVC 
               
               
                   
               
               
                        310        320        330        340        350 
               
               
                 LPGSVVVEDL MSVRQRGSVQ HLGSGVSGQL AENKDAWDAF TVLFPSITAS 
               
               
                   
               
               
                        360        370        380        390        400 
               
               
                 GIPKNAALNA IMQIEKTPRE LYSGAILLLD DTRFDAALVL RSVFQDSQRC 
               
               
                   
               
               
                        410        420        430 
               
               
                 WIQAGAGIIA QSTPERELTE TREKLASIAP YLMV 
               
            
           
         
       
     
     In some embodiments, the feedback-resistant DAHP synthase (L175Q) (AroG*) comprises an amino acid sequence that is at least 70%, 80%, 90%, 95%, or 99% identical to the amino acid sequence of  E. coli  DAHP synthase (AroG) and a glutamine at the position corresponding to position 175 of the  E. coli  DAHP synthase, and comprises the enzymatic activity of feedback-resistant DAHP synthase (L175Q) (AroG*). In some embodiments, the DAHP synthase (AroG)comprises one or more of the following amino acid residues acting as metal-binding sites: C at position 61 of SEQ ID NO:4, H at position 268 of SEQ ID NO:4, E at position 302 of SEQ ID NO:4, and/or D at position 326 of SEQ ID NO:4, and/or any conserved amino acid residues disclosed in Wu et al.,  J. Biol. Chem.  278(30):27525-27531 (2003). 
     The amino acid sequence of  E. coli  DAHP synthase (AroG) comprises: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 4) 
                   
               
               
                         10         20         30         40         50 
                   
               
               
                 MNYQNDDLRI KEIKELLPPV ALLEKFPATE NAANTVAHAR KAIHKILKGN 
               
               
                   
               
               
                         60         70         80         90        100 
               
               
                 DDRLLVVIGP  C SIHDPVAAK EYATRLLALR EELKDELEIV MRVYFEKPRT 
               
               
                   
               
               
                        110        120        130        140        150 
               
               
                 TVGWKGLIND PHMDNSFQIN DGLRIARKLL LDINDSGLPA AGEFLDMITP 
               
               
                   
               
               
                        160        170        180        190        200 
               
               
                 QYLADLMSWG AIGARTTESQ VHRELASGLS CPVGFKNGTD GTIKVAIDAI 
               
               
                   
               
               
                        210        220        230        240        250 
               
               
                 NAAGAPHCFL SVTKWGHSAI VNTSGNGDCH IILRGGKEPN YSAKHVAEVK 
               
               
                   
               
               
                        260        270        280        290        300 
               
               
                 EGLNKAGLPA QVMIDFS H AN SSKQFKKQMD VCADVCQQIA GGEKAIIGVM 
               
               
                   
               
               
                        310        320        330        340        350 
               
               
                 V E SHLVEGNQ SLESGEPLAY GKSIT D ACIG WEDTDALLRQ LANAVKARRG 
               
            
           
         
       
     
     In some embodiments, the genetically modified plant or plant cell endogenously produces chorismate (CHA). 
     In some embodiments, the genetically modified plant or plant cell comprising a nucleic acid encoding the following heterologous enzymes: NahG, CatA, optionally Irp9, and optionally AroG*, each operatively linked to a promoter capable of expressing each enzyme in the genetically modified plant or plant cell. 
     In some embodiments, each enzyme is expressed in, or expressed and transport to, a plastid in the genetically modified plant or plant cell. 
     In some embodiments, the promoter is tissue-specific. 
     The present invention provides for a plant capable of producing a diversity of metabolic pathways for the production of muconic acid (MA). There are five main pathways starting directly from or depending on the aromatic amino acid biosynthesis pathway. The latter pathway consumes about 30% of photosynthetically fixed carbon meaning that it offers great potentials for large production at low cost of target products, such as MA. 
     The first pathway starts from 3 dihydroshikimate that get converted into protochatechuate by a plastid localized 3-hydroshikimate dehydratase (3-DHSDH; Eudes et al, 2015), then into catechol by a protocatechuate decarboxylase (PDC), and finally into MA by a catechol dioxygenase (CDO). 
     The second pathway starts from chorismate to produce also protochatechuate (Eudes et al., 2016) by the co-expression two plastid targeted enzymes: chorismate pyruvate-lyase (such as,  E. coli  ubiC) and p-hydroxybenzoate 3-monooxygenase (such as,  Pseudomonas aeruginosa  pobA). Then, protochatechuate can be converted into MA as described herein. 
     The third pathway uses chorismate to produce the 2,3 dihydroxybenzoate by targeting to plastids a cluster of three proteins: entC (isochorimate synthase), entB (isochorismatase) and entA (2,3 dihydroxybenzoate synthase). Then MA is produced from the decarboxylation of 2,3 dihydroxybenzoate by benzoate decarboxylase (BDC) to produce catechol followed by ring opening with a catechol dioxygenase (CDO). 
     The fourth pathway starts from anthranilate that is converted in plastid into catechol by the expression of a multicomponent aromatic ring-dihydroxylating enzyme complex (antABC). Then the catechol is converted into MA by a catechol dioxygenase (CDO). 
     The fifth pathway uses salicylic acid that is mainly derived from chorismate. Salicylic acid gets converted into catechol by salicylate 1-monoxygenase (SMO) followed by ring opening with a catechol dioxygenase (CDO) to produce MA. 
     Flux through several of these pathways can be boosted to enhance yield of the final product MA (booster pathways). In some embodiments, carbon entry through the aromatic amino acid biosynthesis pathway can be enhanced by the expression of an insensitive phospho-2-dehydro-3-deoxyheptonate aldolase (AroG; first enzyme of the pathway); salicylic production derived from chorismate can be increased by the expression of bifunctional salicylate synthase (irp9) or both enzymes; isochorismate synthase (ICS) and isochorismate pyruvate lyase (IPL); anthranilate production could be enhanced by the expression of tryptophan insensitive anthranilate synthase (trp5) to increase flux from chorismate to anthranilate. 
     The present invention provides for transgenic plants transformed with each or stacks of these artificial MA pathways with or without booster pathways, will produce diverse quantities of muconic acid in their tissues. The bioproduction of muconic acid is of interest because of its potential use as a platform chemical for the production of other valuable bioplastics, such as nylon-6,6, polyurethane, and polyethylene terephthalate (PET). 
     Bioproduction of muconic acid using fermenting microbes is being developed, but bioproduction in plants has never before been described nor demonstrated. The present invention which uses plants as factories is more eco-friendly and sustainable. Herein is described the expression of bacterial CDO in plastids which results in plants producing muconic acid, a metabolite that in nature is not found in plants. Two CDOs are screened: CDO from  Pseudomonas putita  is more efficient than CDO from  Acinetobacter radioresistens.  Since most pathways go through the ring opening of catechol, several CDO enzymes (same enzyme differently codon optimized, or from different species) can be stacked and co-expressed. In some embodiments, the nucleotide sequence encoding each heterologous enzyme is codon optimized specifically for the plant or plant cell. 
     In some embodiments, the promoter is a CER1, CER2, CER3, CER4, CER5, CER6, CER10, WSD1, Mah1, WBC11, KCS1, KCS2, FATB, LACS1, LACS2, CYP864A, CYP86A7, CYP86A5, KCS10, or KCS5 promoter. In some embodiments, the tissue-specific promoter are as described herein. In embodiments, the fiber-specific promoter is an NST, NST1, NST2, NST3, or LAC17 promoter. In some embodiments, the vessel-specific promoter is a VND1, VND2, VND3, VND4, VND5, VND6, VND7, VNI2, REF4, or RFR1 promoter. In some embodiments, the secondary cell wall-specific promoter is an IRX1, IRX3, IRX5, IRX8, IRX9, IRX14, IRX7, IRX10, GAUT13, GAUT14, or CESA4 promoter. Suitable tissue-specific secondary wall promoters, and other transcription factors, promoters, regulatory systems, and the like, suitable for this present invention are taught in U.S. Patent Application Pub. Nos. 2014/0298539, 2015/0051376, 2016/0017355, and 2016/0251672. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings. 
         FIG. 1 . The various pathways for producing muconic acid. 
         FIG. 2A . Strategy used for the production of muconic acid in plants. De novo biosynthetic pathway for muconic acid synthesis. Bacterial enzymes AroG*, Irp9, NahG, and CatA are expressed in  Arabidopsis  for the production of muconic acid from salicylic acid derived from the shikimate pathway. Abbreviations are: CAT, catechol; CHA, chorismate; DAHP, 3-deoxy-D-arabino-heptulosonate; DHQ, dehydroquinate; DHS, dehydroshikimate;  E4P , erythrose 4-phosphate; EPSP, 5-enolpyruvoylshikimate 3-phosphate; F6P, fructose 6-phosphate; G3P, glyceraldehyde 3-phosphate; ISOCHA, isochorismate; MA, muconic acid; PEP, phosphoenolpyruvate; PYR, pyruvate; S3P, shikimate 3-phosphate; SA, salicylic acid; SHIK, shikimate. The enzymes are as follows: 1, enolase; AroG*, feedback-resistant DAHP synthase(L175Q); 2, DHQ synthase; 3, DHQ dehydratase; 4, shikimate dehydrogenase; 5, shikimate kinase; 6, EPSP synthase; 7, chorismate synthase; Irp9, bifunctional ISOCHA synthase/ISOCHA pyruvate lyase; NahG, salicylate hydroxylase; CatA, catechol 1,2-dioxygenase. 
         FIG. 2B . Strategy used for the production of muconic acid in plants. Binary vectors used in Example 1. Boxes labelled “schl” denote plastid transit peptides. pA6 and pTKan denote the two binary vector backbones conferring hygromycin (hyg R ) and kanamycin (kan R ) resistance in plants. Abbreviations are: schl1, plastid transit peptide from  Arabidopsis  ferredoxin2 (At1g60950); schl2, plastid transit peptide from pea ( Pisum sativum ) ribulose-1,5-bisphosphate carboxylase small subunit (GenBank: AAG45569.1); schl3, plastid transit peptide sunflower ( Helianthus annuus ) ribulose-1,5-bisphosphate carboxylase small subunit (UniProtKB/Swiss-Prot: P08705.1). pAtIRX5, pAtCCoAOMT, pAtIRX8, and pAtC4H designate the promoters of  Arabidopsis  cellulose synthase 4 (At5G44030), caffeoyl coenzyme A O-methyltransferase 1 (At4G34050), galacturonosyltransferase 12 (At5G54690), and cinnamate 4-hydroxylase(At2G30490) genes, respectively. 
         FIG. 3A . MA and SA production in plants expressing bacterial NahG and CatA. Detection by qRT-PCR of mRNA expression levels in stems from 5-week-old wild type and pTkan-pIRX8-schl3-catA-pC4H-schl3-nahG (nahG-catA) transgenic lines. ACT2is used as an internal control. Expression levels of nahG and catA are normalized to 1 in line nahG-catA-4.10 and are calculated relative to these values in the other lines. Error bars represent the SD from technical duplicates. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the wild type using the unpaired Student&#39;s t-test (*P&lt;0.005). 
         FIG. 3B . MA and SA production in plants expressing bacterial NahG and CatA. MA content in mature senesced dried stems of wild type and nahG-catA transgenic lines. Error bars represent the SE from four biological replicates (n=4). (C) SA content in stems of 5-week-old wild type and nahG-catA transgenic lines. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the wild type using the unpaired Student&#39;s t-test (*P&lt;0.005). 
         FIG. 3C . MA and SA production in plants expressing bacterial NahG and CatA. SA content in stems of 5-week-old wild type and nahG-catA transgenic lines. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the wild type using the unpaired Student&#39;s t-test (*P&lt;0.005). 
         FIG. 4A . Overproduction of SA in plants expressing Irp9 and feedback-resistant DAHP synthase (AroG*). Detection by qRT-PCR of mRNA expression levels in stems from 5-week-old wild type and pA6-pIRX5::schl1-irp9 (irp9) transgenic lines. ACT2is used as an internal control. Irp9 expression level is normalized to 1 in line irp9-6.1 and is calculated relative to this value in the other lines. Error bars represent the SD from technical duplicates. 
         FIG. 4B . Overproduction of SA in plants expressing Irp9 and feedback-resistant DAHP synthase (AroG*). SA content in stems of 5-week-old wild type and irp9 transgenic lines. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the wild type using the unpaired Student&#39;s t-test (*P&lt;0.005). 
         FIG. 4C . Overproduction of SA in plants expressing Irp9 and feedback-resistant DAHP synthase (AroG*). Detection by qRT-PCR of mRNA expression levels in 15-cm stems from wild type and pA6-pIRX5::schl1-irp9-pCCoAOMT::schl2-aroG(irp9-aroG) transgenic lines. ACT2 is used as an internal control. Irp9 and aroG expression levels are normalized to 1 in line irp9-aroG-6.9 and are calculated relative to these values in the other lines. Error bars represent the SD from technical duplicates. 
         FIG. 4D . Overproduction of SA in plants expressing Irp9 and feedback-resistant DAHP synthase (AroG*). SA content in 15-cm stems of wild type and irp9-aroG transgenic lines. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the wild type using the unpaired Student&#39;s t-test (*P&lt;0.001). 
         FIG. 5A . MA production in plants expressing bacterial NahG, CatA, Irp9 and AroG*. Detection by qRT-PCR of mRNA expression levels in stems from 5-week-old nahG-catA-1.2 and nahG-catA-1.2×irp9 transgenic lines. ACT2 is used as an internal control. Irp9 expression level is normalized to 1 in line nahG-catA-1.2×irp9 7.4 and is calculated relative to this value in the other lines. Error bars represent the SD from technical duplicates. 
         FIG. 5B . MA production in plants expressing bacterial NahG, CatA, Irp9 and AroG*. MA content in mature senesced dried stems of nahG-catA-1.2 and nahG-catA-1.2×Irp9 transgenic lines. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the nahG-catA-1.2 line using the unpaired Student&#39;s t-test (*P&lt;0.05). 
         FIG. 5C . MA production in plants expressing bacterial NahG, CatA, Irp9 and AroG*. Detection by qRT-PCR of mRNA expression levels in stems from 5-week-old nahG-catA-1.2 and nahG-catA-1.2×Irp9-aroG transgenic lines. ACT2 is used as an internal control. Irp9 and aroG expression levels are normalized to 1 in line nahG-catA-1.2×Irp9-aroG 4.9 and were calculated relative to these values in the other lines. Error bars represent the SD from technical duplicates. 
         FIG. 5D . MA production in plants expressing bacterial NahG, CatA, Irp9 and AroG*. MA content in mature senesced dried stems of nahG-catA-1.2 and nahG-catA-1.2×irp9-aroG transgenic lines. Error bars represent the SE from four biological replicates (n=4). Asterisks indicate significant differences from the nahG-catA-1.2 line using the unpaired Student&#39;s t-test (*P&lt;0.001). 
         FIG. 6A . Representative LC-TOF MS chromatogram of muconic acid extracted from  Arabidopsis  biomass. A sample from wild-type control plants. 
         FIG. 6B . Representative LC-TOF MS chromatogram of muconic acid extracted from  Arabidopsis  biomass. A sample from nahG-catA transgenic plants. 
         FIG. 6C . Representative LC-TOF MS chromatogram of muconic acid extracted from  Arabidopsis  biomass. cis,cis- and cis,trans-muconic acid standards. 
         FIG. 7A . Salicylic acid (SA) and catechol (CAT) contents in stems of five-week-old wild type and muconic acid (MA) producing lines. Error bars represent the SE from four biological replicates (n=4). 
         FIG. 7B . Salicylic acid (SA) and catechol (CAT) contents in stems of mature senesced wild type and muconic acid (MA) producing lines. Error bars represent the SE from four biological replicates (n=4). 
         FIG. 8 . Release of muconic acid from  Arabidopsis  biomass (line nahG-catA 1.2×irp9-aroG 2.2) upon treatments with dilute acid (1.2% w/w H2SO4) or dilute alkaline (0.62% w/w or 1.2% w/w NaOH) at different temperatures and using 10% (w/w) biomass loadings. Error bars represent the SD from three technical replicates. 
         FIG. 9 . Possible metabolic routes for the production of muconic acid (MA) in plants. Biosynthetic enzymes used in this work are indicated in purple. Bacterial enzymes belonging to alternative MA biosynthetic routes and whose functional expression in plants has been demonstrated are indicated in red. These are: antABC, anthranilate dioxygenase; AroY/KpdB, protocatechuate decarboxylase; HCHL, hydroxycinnamoyl-CoA hydratase-lyase; PobA, 4-hydroxybenzoate hydroxylase; QsuB, 3-dehydroshikimate dehydratase; UbiC, chorismate pyruvate lyase (Eudes et al., 2012; 2015; 2016, Shih et al., 2016; Wu et al., 2017). Expression of feedback-resistant anthranilate synthase alpha subunit (OASAD1) redirects carbon flux through anthranilate (ANT) (Ishihara et al., 2006; Last and Fink, 1988). Abbreviations are: CINN, cinnamate; COUM, p-coumarate; COUM-CoA, p-coumaroyl-CoA; HBA, 4-hydroxybenzoate; PCA, protocatechuate; PHE, phenylalanine. Refer to  FIGS. 2A and 2B  for other abbreviations. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings. 
     The terms “optional” or “optionally” as used herein mean that the subsequently described feature or structure may or may not be present, or that the subsequently described event or circumstance may or may not occur, and that the description includes instances where a particular feature or structure is present and instances where the feature or structure is absent, or instances where the event or circumstance occurs and instances where it does not. 
     The term “about” refers to a value including 10% more than the stated value and 10% less than the stated value. 
     Useful nucleotide sequences for use in expressing the heterologous enzymes are described: 
     attB1-schl1-irp9-attB2: Codon-optimized nucleotide sequence encoding Irp9 from  Yersinia enterocolitica  (in red, GenBank accession number CAB46570.1) preceded with a plastid targeting signal (in green, schl1) and flanked with Gateway attB1 (5′-end) and attB2 (3′-end) recombination sites (in black): 
     
       
         
           
               
            
               
                 (SEQ ID NO: 8) 
               
               
                 ACAAGTTTGTACAAAAAAGCAGGCTTCATGGCATCAACAGCATTATCATC 
               
               
                   
               
               
                 CGCTATCGTGGGAACATCATTCATTAGGAGGAGTCCAGCACCAATCAGTC 
               
               
                   
               
               
                 TTAGATCATTACCTTCTGCAAACACACAATCACTTTTCGGTTTCAAATCT 
               
               
                   
               
               
                 GGAACCGCTAGAGGTGGAAGGGTTACAGCTATGGCAACCTATAAGGTTAT 
               
               
                   
               
               
                 GAAAATCAGTGAATTCTTGCATTTGGCACTTCCAGAAGAGCAATGGTTGC 
               
               
                   
               
               
                 CTACTATCTCCGGTGTTTTAAGACAGTTCGCTGAAGAGGAATGTTATGTG 
               
               
                   
               
               
                 TACGAAAGACCACCTTGTTGGTACTTAGGTAAAGGATGCCAAGCTAGACT 
               
               
                   
               
               
                 TCACATCAACGCAGATGGTACCCAAGCTACTTTTATAGATGACGCAGGAG 
               
               
                   
               
               
                 AACAGAAATGGGCTGTTGATTCTATTGCAGACTGCGCTAGAAGGTTCATG 
               
               
                   
               
               
                 GCACATCCACAAGTTAAGGGTAGAAGGGTTTATGGTCAAGTGGGTTTTAA 
               
               
                   
               
               
                 TTTCGCTGCACACGCTAGAGGTATAGCTTTTAATGCTGGAGAATGGCCAT 
               
               
                   
               
               
                 TGTTAACTCTTACAGTTCCTAGAGAGGAATTGATTTTCGAGAAGGGTAAT 
               
               
                   
               
               
                 GTGACTGTTTACGCAGATTCTGCTGACGGATGTAGAAGGTTGTGCGAGTG 
               
               
                   
               
               
                 GGTTAAGGAAGCTTCAACTACAACCCAAAATGCACCTCTTGCTGTTGATA 
               
               
                   
               
               
                 CAGCATTGAACGGTGAAGCTTATAAGCAACAGGTTGCTAGAGCAGTGGCT 
               
               
                   
               
               
                 GAGATCAGAAGGGGAGAATACGTGAAGGTTATCGTGTCTAGAGCTATACC 
               
               
                   
               
               
                 ATTGCCTTCAAGGATAGATATGCCAGCAACCCTTTTGTATGGTAGACAAG 
               
               
                   
               
               
                 CTAATACTCCTGTTAGGTCTTTTATGTTCAGACAGGAGGGTAGGGAAGCA 
               
               
                   
               
               
                 TTAGGATTCTCTCCAGAGCTTGTTATGTCAGTGACTGGAAACAAAGTTGT 
               
               
                   
               
               
                 GACAGAACCATTAGCTGGTACCAGAGATAGGATGGGAAATCCTGAGCATA 
               
               
                   
               
               
                 ACAAAGCAAAGGAGGCTGAATTACTTCATGACAGTAAAGAGGTTTTAGAA 
               
               
                   
               
               
                 CACATACTTTCCGTGAAGGAAGCAATTGCTGAGTTGGAAGCTGTTTGTTT 
               
               
                   
               
               
                 ACCTGGTTCAGTTGTGGTTGAAGATTTGATGAGTGTTAGACAAAGGGGTT 
               
               
                   
               
               
                 CCGTGCAGCACTTGGGTTCTGGAGTTTCAGGACAATTAGCTGAAAACAAG 
               
               
                   
               
               
                 GATGCATGGGACGCTTTTACTGTTCTTTTCCCAAGTATTACAGCATCCGG 
               
               
                   
               
               
                 TATCCCTAAAAATGCTGCACTTAACGCTATAATGCAAATTGAGAAGACTC 
               
               
                   
               
               
                 CAAGAGAATTGTACAGTGGAGCTATATTGCTTCTTGATGACACAAGATTC 
               
               
                   
               
               
                 GATGCTGCATTGGTTTTAAGGAGTGTGTTCCAAGACTCCCAGAGATGCTG 
               
               
                   
               
               
                 GATCCAAGCAGGTGCTGGAATTATCGCTCAGTCAACACCTGAGAGAGAGT 
               
               
                   
               
               
                 TGACTGAGACTAGGGAGAAATTAGCATCAATCGCACCTTACCTTATGGTT 
               
               
                   
               
               
                 TGAGACCCAGCTTTCTTGTACAAAGTGGTC 
               
            
           
         
       
     
     attB4r-tG7-pCCoAOMT-attB3r: Sequence containing the tg7 terminator from  A. tumefaciens  followed by the CCoAOMT1 promoter from  A. thaliana  (in black) containing an AvrII restriction site (in red) in its 3′-end and flanked with the Gateway attB4R (5′-end) and attB3R (3′-end) recombination sites (in blue): 
     
       
         
           
               
            
               
                 (SEQ ID NO: 9) 
               
               
                 GACAACTTTTCTATACAAAGTTGACTGACTAACTAGGATGAGCTAAGCTA 
               
               
                   
               
               
                 GCTATATCATCAATTTATGTATTACACATAATATCGCACTCAGTCTTTCA 
               
               
                   
               
               
                 TCTACGGCAATGTACCAGCTGATATAATCAGTTATTGAAATATTTCTGAA 
               
               
                   
               
               
                 TTTAAACTTGCATCAATAAATTTATGTTTTTGCTTGGACTATAATACCTG 
               
               
                   
               
               
                 ACTTGTTATTTTATCAATAAATATTTAAACTATATTTCTTTCAAGATGGG 
               
               
                   
               
               
                 AATTAACATCTACAAATTGCCTTTTCTTATCGACCATGTACCCCGGGTAC 
               
               
                   
               
               
                 CAAGCTTCTCGAGAGCAGTGGATGAGGGAAGAGAGGATTAAGAGGCGTAG 
               
               
                   
               
               
                 AGATTACATGTGATGAATGATACTATCTTTTCTTACAAACACATTTTCGT 
               
               
                   
               
               
                 GTAATTAAAATTTAATTTGGTTCCAAAGATTTTAATCAAAAGAAGTTTGG 
               
               
                   
               
               
                 TAAATTGAAACAGGCAGACATAATTTATTGTAAAGAGTTTTTATTTATTT 
               
               
                   
               
               
                 ATTCATGACGTTGCTTGATGGTGCTTTACCAATTTTCTTCTCCTACGTTA 
               
               
                   
               
               
                 GATTTTTTTCACTTTTTTTTTTGGTGTTTGTAATAAATGTGAAAAATGGA 
               
               
                   
               
               
                 CCGTTTAAAAACTTAAAGACGTTTGATTACTATATAAAGTAATTGTTTAT 
               
               
                   
               
               
                 AATAGAAAGTTAATTGAGACGTGAAATGGTATAATATTATTGTGTAACAG 
               
               
                   
               
               
                 TTGTGTACACGTAGCTCTCATGCAGTTTTAGTGGACCCATATGGCTTGAC 
               
               
                   
               
               
                 TTGTATTCTGTTTTTGGGCTATTAAAGTCCAAAACAGAGACCCCTCTCAA 
               
               
                   
               
               
                 GCCCTTCCTATTAATCCATCTAGCTAATAGAAACTATAAACGTGTCCTCT 
               
               
                   
               
               
                 CTCTCAATTAAATAAGCTAGAAACATACTCAACCATTCGCATTACGCACT 
               
               
                   
               
               
                 TCATAGCGGTAGGTTTAGATTTGTCTAAAATACTTAAAAAAATTTTTGTC 
               
               
                   
               
               
                 TAAGTTGTTGTCCGTTACAAAGTTTTTTTCTTTGTGACAACTTGACAACA 
               
               
                   
               
               
                 TTGACAAATAGAAAAATAAATTTCGATGAAACCTATGAAATGGGCTATGG 
               
               
                   
               
               
                 CCCAACTAAAAAGAGTGGGAAATTAAAGATGGGATGGTTCAAGTGTATAC 
               
               
                   
               
               
                 TTCGAACTTCCGACATTAGGGTCAAAGGATTTTTAAAAGGCAACCATTTG 
               
               
                   
               
               
                 TTCCACTTTCTCGAACAAAAACGAGCCATTTATTAATATATAGTACGGCT 
               
               
                   
               
               
                 GAATTGGTTTTGTTCGTCATTGTGTAAACACAAAGTCATTCGAATTATGT 
               
               
                   
               
               
                 TAGGGTCCGTTGATAATATAGACGGCCCATCCCACGCACATATTAAGTGT 
               
               
                   
               
               
                 TCAACTCCATAGAATATCATATGGGACACTGTTTTTAATTTATAATCACC 
               
               
                   
               
               
                 ATTTAAAATGTTTAAATGTTTATGCAAATTGGATGGCTTCTTCACACAAC 
               
               
                   
               
               
                 ATTTATTTATTGGCCTTTCATTCCATCAAAGTAAAATAGCTTTTCAAATA 
               
               
                   
               
               
                 CATTATACTCTATACTCCTATACATGTAAATAACCATATGCATATATATT 
               
               
                   
               
               
                 TTTTTCAAATATAGGTCAACGCCATTTAATATAATTTTAAAAAAATTTGT 
               
               
                   
               
               
                 TCGGAAAATATCACATTTCTTTCACTAGACAAGCCTTGTTACCACACAAT 
               
               
                   
               
               
                 GTATCAATATGATCTAAAGGGCAAACGAAAGATCCTGACATGAAACGTTT 
               
               
                   
               
               
                 AATTCTCATTTTCTCCAAATTTTATTTTTTATGTGAAGTAGATAAATTAG 
               
               
                   
               
               
                 TATATATATATATATACCAAACTAGTGTGTTATGTTATGGCAAATGTTAT 
               
               
                   
               
               
                 ATCAATTCGAAGGTTCCGCTATTGCAATATTCATTAATTTTTTCATACCA 
               
               
                   
               
               
                 ATACTATTTTTCTTTCTCTTTTATTTTGTTTTTTAATAAATAAAAGAAAT 
               
               
                   
               
               
                 TAAGGATGATTAGTAAGGAAGTCGCCTACCAAGAGATTCACCTACCACGG 
               
               
                   
               
               
                 TACACTTCAACACCGAAGCAGAGTTGTTGAATCCACTTTTTATTCCCTTC 
               
               
                   
               
               
                 TCTAATCTCTACTCACCAAGTCTCCACTTTTTTTTCTCTTTATTATATAC 
               
               
                   
               
               
                 ATTTAAATTATTTAATATACGCCAACTACATACATATCCAGTGTAATTTC 
               
               
                   
               
               
                 TCGTTACGTCACACCCCTTTCGTAATCGTCTAATTTCAGAAAAATATCCA 
               
               
                   
               
               
                 GAGGTTTAAATACATATTCCCATCATTAAATCTAGACATAAACACATCAT 
               
               
                   
               
               
                 ACTCACAAAATTTGGCAGCAAACAGTTACTACAGACCCATAAATGAAAAA 
               
               
                   
               
               
                 ACGTATTCACTTGTTTTCAATTTTCACATAACCACTTCCCTGAGTTTGGT 
               
               
                   
               
               
                 CTCAATTTGATTGCCCCGCCGAGGCATTACTACGCCAAGTGCGATTAAGG 
               
               
                   
               
               
                 TCCCATACAGTGTAACGGGACCCACTATAAGACAGCGACCGACCAATTGC 
               
               
                   
               
               
                 GTGTTAGGAGAGTTTCACCAACCCCGGACCGGTTTTTACCGGATATAACA 
               
               
                   
               
               
                 GAACCGGTACGAACCGGTCTCATTATCTTCCATCTTCTTTATATAGACCT 
               
               
                   
               
               
                 CATGCCATGTGTGTGACTCACCAAGAAAAACACAATCGTTTAATCTCACC 
               
               
                   
               
               
                 CAAGAAGACAAAAACACAGAGAGAGAAAGAGAGAGAAACAACTTTGTATA 
               
               
                   
               
               
                 ATAAAGTTGTC 
               
            
           
         
       
     
     attB3-schl2-aroG-attB2: Codon-optimized nucleotide sequence encoding feedback-insensitive AroG (L175Q) from  E. coli  (in red, NCBI Reference Sequence: WP_032246946.1) preceded with a plastid targeting signal (in green, schl2) and flanked with Gateway attB3 (5′-end) and attB2 (3′-end) recombination sites (in black): 
     
       
         
           
               
            
               
                 ACAACTTTGTATAATAAAGTTGGCCATGGCTTCTATGATATCCTCTTCAG 
               
               
                   
               
               
                 CTGTGACTACAGTCAGCCGTGCTTCTACGGTGCAATCGGCCGCGGTGGCT 
               
               
                   
               
               
                 CCATTCGGCGGCCTCAAATCCATGACTGGATTCCCAGTTAAGAAGGTCAA 
               
               
                   
               
               
                 CACTGACATTACTTCCATTACAAGCAATGGTGGAAGAGTAAAGTGCATGC 
               
               
                   
               
               
                 AGATGAATTACCAGAACGATGACTTAAGAATCAAAGAAATTAAAGAGTTG 
               
               
                   
               
               
                 TTACCACCTGTGGCTCTTTTGGAAAAATTCCCAGCAACTGAGAATGCTGC 
               
               
                   
               
               
                 AAACACAGTTGCTCATGCAAGAAAGGCTATTCACAAAATCTTGAAGGGTA 
               
               
                   
               
               
                 ATGATGACAGGTTACTTGTTGTGATCGGACCATGCTCAATACATGATCCT 
               
               
                   
               
               
                 CTTGCTGCAAAGGAATACGCTACTAGATTGCTTGCATTGAGGGAAGAGTT 
               
               
                   
               
               
                 AAAGGATGAACTTGAGATTGTTATGAGAGTGTACTTCGAGAAACCAAGGA 
               
               
                   
               
               
                 CCACTGTTGGTTGGAAGGGACTTATCAATGATCCTCACATGGACAACTCC 
               
               
                   
               
               
                 TTCCAAATTAATGATGGTTTGAGAATCGCTAGGAAACTTTTGCTTGATAT 
               
               
                   
               
               
                 TAACGACTCAGGTTTGCCAGCTGCAGGAGAATTTTTAGATATGATCACAC 
               
               
                   
               
               
                 CTCAGTACTTAGCTGACCTTATGTGATGGGGTGCTATAGGAGCAAGAACA 
               
               
                   
               
               
                 ACCGAAAGTCAAGTTCATAGGGAGCAGGCTTCCGGTTTGTCTTGTCCAGT 
               
               
                   
               
               
                 GGGATTCAAAAATGGTACTGATGGAACAATTAAGGTTGCTATAGACGCAA 
               
               
                   
               
               
                 TTAACGCTGCAGGTGCTCCTCATTGTTTTCTTTCTGTTACAAAATGGGGA 
               
               
                   
               
               
                 CACTCAGCAATCGTGAATACCAGTGGTAACGGAGATTGCCATATTATCTT 
               
               
                   
               
               
                 GAGAGGTGGAAAAGAACCAAATTATTCAGCTAAGCACGTTGCAGAAGTGA 
               
               
                   
               
               
                 AAGAGGGTTTGAACAAGGCTGGATTACCTGCACAAGTTATGATCGATTTC 
               
               
                   
               
               
                 TCTCATGCTAACTCCTCTAAGCAATTCAAGAAACAGATGGATGTTTGTGC 
               
               
                   
               
               
                 TGACGTGTGCCAACAGATCGCTGGTGGAGAAAAGGCTATTATTGGTGTTA 
               
               
                   
               
               
                 TGGTGGAAAGTCACTTAGTTGAGGGAAATCAATCATTAGAAAGTGGAGAG 
               
               
                   
               
               
                 CCTCTTGCTTACGGAAAATCTATTACCGATGCATGCATCGGTTGGGAAGA 
               
               
                   
               
               
                 TACTGACGCTCTTTTGAGACAGTTGGCTAACGCAGTTAAGGCAAGAAGGG 
               
               
                   
               
               
                 GTTGAGACCCAGCTTTCTTGTACAAAGTGGTC 
               
            
           
         
       
     
     attB1-schl3-catA-attB4: Codon-optimized nucleotide sequence encoding CatA from  Pseudomonas  (in red, NCBI Reference Sequence: WP_010954549.1) preceded with a plastid targeting signal (in green, schl3) and flanked with Gateway attB1 (5′-end) and attB4 (3′-end) recombination sites (in black): 
     
       
         
           
               
            
               
                 (SEQ ID NO: 10) 
               
               
                 ACAAGTTTGTACAAAAAAGCAGGCTTCATGGCGAGTATCTCCAGCAGCGT 
               
               
                   
               
               
                 TGCGACAGTGAGCAGAACAGCACCGGCACAAGCGAATATGGTTGCACCGT 
               
               
                   
               
               
                 TTACGGGATTGAAAAGTAATGCTGCGTTTCCAACCACTAAAAAGGCCAAT 
               
               
                   
               
               
                 GATTTCTCGACACTGCCGTCCAACGGTGGCCGTGTTCAGTGTATGCAAGT 
               
               
                   
               
               
                 GTGGCCGGCTTATGGAAACAAAAAGTTTGAAACTCTGTCTTACCTTCCGC 
               
               
                   
               
               
                 CTTTGTCAACAATGGCGCCTACGGTCATGATGGCATCTTCAGCAACCGCC 
               
               
                   
               
               
                 GTGGCTCCATTCCAGGGTCTGAAAAGCACTGCTAGTCTTCCTGTGGCTCG 
               
               
                   
               
               
                 CCGTTCTTCCCGCTCGCTTGGTAATGTTTCTAATGGAGGTCCTATCAGAT 
               
               
                   
               
               
                 GCATGCAGATGACCGTGAAAATCTCCCACACTGCTGACATACAGGCATTC 
               
               
                   
               
               
                 TTTAACAGAGTTGCTGGATTAGACCACGCTGAAGGTAACCCAAGGTTCAA 
               
               
                   
               
               
                 GCAAATTATCTTGAGAGTTCTTCAGGATACTGCTAGGTTAATCGAAGATC 
               
               
                   
               
               
                 TTGAGATAACAGAAGACGAGTTCTGGCATGCTGTGGATTATCTTAATAGA 
               
               
                   
               
               
                 TTGGGTGGAAGGAACGAAGCAGGTTTGTTAGCTGCAGGACTTGGTATAGA 
               
               
                   
               
               
                 GCACTTTTTGGATCTTTTGCAAGATGCTAAAGACGCTGAAGCAGGTTTGG 
               
               
                   
               
               
                 GTGGTGGTACTCCAAGAACTATTGAGGGACCTTTGTATGTTGCTGGTGCA 
               
               
                   
               
               
                 CCATTAGCTCAAGGAGAAGCAAGGATGGATGACGGAACAGATCCTGGTGT 
               
               
                   
               
               
                 TGTGATGTTTTTACAGGGTCAGGTTTTCGATGCTGACGGAAAGCCACTTG 
               
               
                   
               
               
                 CTGGTGCAACCGTGGATTTGTGGCATGCTAACACACAAGGTACTTATTCT 
               
               
                   
               
               
                 TACTTCGATTCTACCCAGTCAGAATTCAATTTGAGAAGAAGAATTATTAC 
               
               
                   
               
               
                 TGATGCTGAGGGAAGGTATAGAGCAAGGAGTATCGTTCCTTCCGGATACG 
               
               
                   
               
               
                 GTTGTGATCCACAAGGACCTACTCAGGAATGCTTAGACTTACTTGGAAGA 
               
               
                   
               
               
                 CACGGTCAAAGGCCAGCTCATGTTCACTTTTTCATTAGTGCACCTGGTCA 
               
               
                   
               
               
                 TAGACACTTAACTACACAGATCAATTTCGCTGGAGATAAATATCTTTGGG 
               
               
                   
               
               
                 ATGACTTCGCTTACGCAACTAGAGATGGTTTGATTGGTGAATTAAGGTTT 
               
               
                   
               
               
                 GTTGAGGATGCTGCAGCTGCAAGAGACAGGGGAGTGCAAGGTGAAAGATT 
               
               
                   
               
               
                 CGCTGAGTTATCATTTGATTTCAGACTTCAGGGTGCAAAGTCTCCAGACG 
               
               
                   
               
               
                 CTGAAGCAAGATCACATAGACCAAGAGCTTTGCAAGAGGGATGACACCCA 
               
               
                   
               
               
                 ACTTTTCTATACAAAGTTGTCT 
               
            
           
         
       
     
     attB3-nahG-attB2: Codon-optimized nucleotide sequence encoding NahG from  Pseudomonas  (in red, NCBI Reference Sequence: WP_011475386.1) flanked with Gateway attB3 (5′-end) and attB2 (3′-end) recombination sites (in black): 
     
       
         
           
               
            
               
                 (SEQ ID NO: 11) 
               
               
                 ACAACTTTGTATAATAAAGTTGGCATGAAAAATAATAAGTTGGGTTTGAG 
               
               
                   
               
               
                 AATCGGTATCGTGGGTGGTGGAATATCAGGAGTGGCATTGGCATTGGAAT 
               
               
                   
               
               
                 TGTGCAGGTATAGTCATATCCAAGTTCAGTTATTTGAGGCTGCACCAGCT 
               
               
                   
               
               
                 TTCGGAGAAGTTGGTGCAGGAGTGTCCTTTGGTCCTAATGCAGTTAGAGC 
               
               
                   
               
               
                 TATAGTGGGTTTAGGACTTGGAGAGGCATATCTTCAAGTTGCTGACAGGA 
               
               
                   
               
               
                 CCTCTGAACCTTGGGAGGATGTGTGGTTCGAATGGAGAAGGGGAAGTGAT 
               
               
                   
               
               
                 GCTTCCTACTTGGGTGCAACTATTGCTCCAGGAGTTGGACAGTCTTCAGT 
               
               
                   
               
               
                 GCATAGAGCAGACTTTATCGATGCTTTAGTTACTCACCTTCCTGAAGGAA 
               
               
                   
               
               
                 TAGCTCAATTCGGTAAAAGAGCAACACAGGTGGAGCAACAGGGTGGAGAA 
               
               
                   
               
               
                 GTTCAAGTGTTGTTTACTGATGGTACAGAATATAGATGTGACTTGTTAAT 
               
               
                   
               
               
                 AGGAGCTGATGGTATTAAGAGTGCACTTAGGTCCCATGTTTTGGAAGGAC 
               
               
                   
               
               
                 AAGGTTTAGCTCCACAGGTGCCTAGATTCTCTGGAACTTGCGCTTATAGG 
               
               
                   
               
               
                 GGTATGGTTGATTCATTGCATTTGAGAGAGGCATACAGGGCTCATGGTAT 
               
               
                   
               
               
                 CGACGAACACTTGGTTGATGTGCCACAAATGTACCTTGGATTGGATGGTC 
               
               
                   
               
               
                 ACATTTTAACATTTCCTGTTAGAAATGGTGGAATTATCAACGTTGTGGCT 
               
               
                   
               
               
                 TTCATCTCTGACAGGTCAGAACCTAAACCTACCTGGCCAGCTGATGCACC 
               
               
                   
               
               
                 TTGGGTTAGAGAGGCTTCTCAAAGGGAAATGTTGGACGCTTTTGCAGGAT 
               
               
                   
               
               
                 GGGGAGATGCTGCTAGAGCTTTGTTGGAATGTATACCAGCACCTACTTTA 
               
               
                   
               
               
                 TGGGCTCTTCATGACTTGGCAGAATTACCAGGATATGTTCACGGTAGAGT 
               
               
                   
               
               
                 TGTGTTGATTGGAGATGCTGCACATGCTATGTTACCTCACCAAGGAGCTG 
               
               
                   
               
               
                 GTGCAGGACAGGGTCTTGAAGATGCATACTTCTTGGCTAGATTACTTGGA 
               
               
                   
               
               
                 GACACACAAGCTGATGCAGGTAATCTTGCTGAATTGTTGGAGGCTTATGA 
               
               
                   
               
               
                 TGACTTGAGAAGGCCAAGAGCTTGCAGGGTTCAACAGACCTCATGGGAAA 
               
               
                   
               
               
                 CTGGAGAGCTTTACGAATTGAGAGATCCTGTTGTGGGAGCTAATGAGCAA 
               
               
                   
               
               
                 CTTTTGGGTGAAAACTTAGCAACAAGATTTGATTGGCTTTGGAACCATGA 
               
               
                   
               
               
                 TTTGGACACTGATTTGGCTGAGGCAAGAGCTAGGTTGGGATGGGAACACG 
               
               
                   
               
               
                 GTGGAGGTGGTGCTTTGAGACAGGGTTGAGACCCAGCTTTCTTGTACAAA 
               
               
                   
               
               
                 GTGGTCTGA 
               
            
           
         
       
     
     EXAMPLE 1 
     Production of Muconic Acid in Plants 
     Muconic acid (MA) is a dicarboxylic acid used for the production of industrially relevant chemicals such as adipic acid, terephthalic acid, and caprolactam. Because the synthesis of these polymer precursors generates toxic intermediates by utilizing petroleum-derived chemicals and corrosive catalysts, the development of alternative strategies for the bio-based production of MA has garnered significant interest. Plants produce organic carbon skeletons by harvesting carbon dioxide and energy from the sun, and therefore represent advantageous hosts for engineered metabolic pathways towards the manufacturing of chemicals. In this work, we engineered  Arabidopsis  to demonstrate that plants can serve as green factories for the bio-manufacturing of MA. In particular, dual expression of plastid-targeted bacterial salicylate hydroxylase (NahG) and catechol 1,2-dioxygenase (CatA) resulted in the conversion of the endogenous salicylic acid (SA) pool into MA via catechol. Sequential increase of SA derived from the shikimate pathway was achieved by expressing plastid-targeted versions of bacterial salicylate synthase (Irp9) and feedback-resistant 3-deoxy-D-arabino-heptulosonate synthase (AroG). Introducing this SA over-producing strategy into engineered plants that co-express NahG and CatA resulted in a 50-fold increase in MA titers. Considering that MA is easily recovered from senesced plant biomass after harvest, the phytoproduction of MA is envisioned as a beneficial option to add value to bioenergy crops. 
     In plants, the shikimate pathway is confined to plastids and provides the precursors for the synthesis of aromatic amino acids and derived metabolites, vitamins K 1  and B 9 , and SA (Maeda and Dudareva, 2012).  Arabidopsis  is used as a model system to investigate a novel bio-based approach for the phytoproduction of MA. In particular, the SA pool derived from chorismate via the shikimate pathway is converted to catechol and MA by dual expression and plastid-targeting of bacterial salicylate hydroxylase (NahG) and catechol 1,2-dioxygenase (CatA) ( FIG. 2A ). Additional supply of SA to the MA pathway is achieved via the expression of bacterial salicylate synthase Irp9 and feedback-resistant 3-deoxy-D-arabino-heptulosonate (DAHP) synthase (AroG*), which results in a ˜50-fold increase of MA content. Importantly, MA is recovered from the biomass of senesced mature plants which highlights its stability and suitability for storage when produced in target crops. Therefore, such engineered crops could represent high-potential feedstocks for existing MA microbial production platforms towards sustainable development of bio-based MA. 
     2. Material and Methods 
     2.1. Plant Material and Growth Conditions 
       Arabidopsis thaliana  (ecotype Columbia, Col-0) seeds are germinated directly on soil. Growing conditions are 150 μmol/m 2 /s, 22° C., 60% humidity and 10 h of light per 24-h day cycle. Selection of T2 and identification of T3 homozygous transgenic plants is made on Murashige and Skoog vitamin medium (PhytoTechnology Laboratories, Shawnee Mission, Kans.), supplemented with 1% sucrose, 1.5% agar, 50 μg/mL kanamycin and/or 25 μg/mL hygromycin. 
     2.2. Construction of Plasmids and Plant Transformation 
     To generate the pA6-pIRX5::schl1-irp9 construct, a gene sequence encoding Irp9 from  Yersinia enterocolitica  (GenBank accession number CAB46570.1) containing the encoding sequence of the plastid transit peptide (schl1) from the  Arabidopsis  ferredoxin2 (At1g60950) (Xue et al., 2013), and flanked with the Gateway attB1 (5′-end) and attB2 (3′-end) recombination sites is synthesized for expression in  Arabidopsis  (attB1-schl1-irp9-attB2, Supplementary Data S1) (GenScript, Piscatway, N.J.). This sequence is cloned into the Gateway pDONR221-P1P2 entry vector by BP recombination (Life Technologies, Foster City, Calif.). An entry clone is LR recombined with the pA6-pIRX5::GWR1R2 vector (Vega-Sanchez et al., 2015) to generate the pA6-pIRX5::schl1-irp9 construct. 
     To generate the pA6-pIRX5::schl1-irp9-pCCoAOMT::schl2-aroG construct ( FIG. 2B ), the attB1-schl1-irp9-attB2 sequence is amplified by PCR to replace the Gateway attB2 recombination site (3′-end) by an attB4 recombination site, and cloned into the Gateway pDONR221-P1P4 entry vector by BP recombination (Life Technologies, Foster City, Calif.) to produce a pDONR221-L1-schl1-irp9-L4 construct. A chimeric DNA construct is synthesized (GenScript, Piscatway, N.J.): it is flanked by the gateway sequences attB4r (5′-end) and attB3r (3′-end), and contains the tG7 terminator and a 2.2-Kb sequence corresponding to the  Arabidopsis  CCoAOMT1 (At4g34050) promoter (pCCoAOMT). This attB4r-tG7-pCCoAOMT-attB3r construct (Supplementary Data S1) is then subcloned into the Gateway pDONR221-P4rP3r entry vector by BP recombination (Life Technologies, Foster City, Calif.) to produce pDONR221-L4R-tG7-pCCoAOMT-L3R. A gene sequence encoding feedback-insensitive AroG (L175Q) from  E. coli  (NCBI Reference Sequence: WP_032246946.1) containing the encoding sequence of the transit peptide (schl2) of the pea ( Pisum sativum ) ribulose-1,5-bisphosphate carboxylase small subunit (GenBank: AAG45569.1) (Tzin et al., 2012), and flanked with the Gateway attB3 (5′-end) and attB2 (3′-end) recombination sites is synthesized for expression in  Arabidopsis  (attB3-schn-aroG-attB2, Supplementary Data S1) (GenScript, Piscatway, N.J.). This sequence is cloned into the Gateway pDONR221-P3P2 entry vector by BP recombination (Life Technologies, Foster City, Calif.) to produce the pDONR221-P3-schn-aroG-P2 construct. A multi-site LR recombination (Life technologies, Foster City, Calif., USA) using the pDONR221-L1-schl1-irp9-L4, pDONR221-L4R-tG7-pCCoAOMT-L3R, and pDONR221-L3-schn-aroG-L2 entry vectors and the pA6-pIRX5::GWR1R2 destination vector is performed to generate the pA6-p1RX5::schl1-irp9-pCCoAOMT::schl2-aroG construct. 
     To generate the pTkan-pIRX8-schl3-catA-pC4H-schl3-nahG construct ( FIG. 2B ), a gene sequence encoding CatA from  Pseudomonas putida  (NCBI Reference Sequence: WP_010954549.1) containing the encoding sequence of the transit peptide (schl3) of the sunflower ( Helianthus annuus ) ribulose-1,5-bisphosphate carboxylase small subunit (UniProtKB/Swiss-Prot: P08705.1) (Lebrun et al., 1992, Eudes et al., 2015), and flanked with the Gateway attB1 (5′-end) and attB4 (3′-end) recombination sites is synthesized for expression in  Arabidopsis  (attB1-schl3-catA-attB4, Supplementary Data 51) (GenScript, Piscatway, N.J.). This sequence is cloned into the Gateway pDONR221-P1P4 entry vector by BP recombination (Life Technologies, Foster City, Calif.) to produce the pDONR221-L1-schl3-catA-L4 construct. A gene sequence encoding NahG from  Pseudomonas putida  (NCBI Reference Sequence: WP_011475386.1) and flanked with the Gateway attB3 (5′-end) and attB2 (3′-end) recombination sites is synthesized for expression in  Arabidopsis  (attB3-nahG-attB2, Supplementary Data 51) (GenScript, Piscatway, N.J.). This sequence is cloned into the Gateway pDONR221-P3P2 entry vector by BP recombination (Life Technologies, Foster City, Calif.) to produce the pDONR221-L3-nahG-L2 construct. A multi-site LR recombination (Life Technologies, Foster City, Calif.) using the pDONR221-L1-schl3-catA-L4, pDONR221-L4R-tg7-pC4H::schl3-L3R (Eudes et al., 2015), and pDONR221-L3-nahG-L2 entry vectors and the pTKan-pIRX8::GWR1R2 (Yang et al., 2013) destination vector is performed to generate the pTkan-pIRX8-schl3-catA-pC4H-schl3-nahG construct. The constructs are introduced into wild-type  Arabidopsis  plants (ecotype Col0) via  Agrobacterium tumefaciens -mediated transformation (Bechtold and Pelletier, 1998). 
     2.3. RNA Extraction and qRT-PCR Analysis 
     Total RNA is extracted from stems of 5-week-old wild type and T3 homozygous transgenic lines (pools of three plants per line, ˜100 mg) using the Plant RNeasy extraction kit (Qiagen, Valencia, Calif.), and treated with DNase (Qiagen, Valencia, Calif.) to remove genomic DNA contamination. First-strand cDNAs are synthesized from 2 μg of total RNA using the SuperScript III First-Strand Synthesis SuperMix (Thermo Fisher Scientific, Waltham, Mass.) followed by qPCR analysis using SsoAdvanced Universal SYBR Green Supermix (Bio-Rad, Hercules, Calif.) on a CFX96 Real-Time PCR Detection System (Bio-Rad, Hercules, Calif.) following the manufacturer&#39;s instruction. Oligonucleotide primers (Table 1) are tested in annealing temperature gradients and 58° C. was chosen as the annealing temperature. Melting curve analyses are performed after each run to ensure single amplicons are produced. The data are analyzed using the 2{circumflex over ( )}(−ΔΔCt) method (Livak and Schmittgen, 2011). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Oligonucleotides used in Example 1. 
               
            
           
           
               
               
            
               
                 Primer 
                   
               
               
                 name 
                 Sequence (5′-3′) 
               
               
                   
               
               
                 Irp9-F 
                 TGTAGAAGGTTGTGCGAGTG (SEQ ID NO: 12) 
               
               
                   
               
               
                 Irp9-R 
                 CTTCACGTATTCTCCCCTTCTG (SEQ ID NO: 13) 
               
               
                   
               
               
                 CatA-F 
                 TTGGGATGACTTCGCTTACG (SEQ ID NO: 14) 
               
               
                   
               
               
                 CatA-R 
                 GGAGACTTTGCACCCTGAAG (SEQ ID NO: 15) 
               
               
                   
               
               
                 nahG-F 
                 CAAAGGGAAATGTTGGACGC (SEQ ID NO: 16) 
               
               
                   
               
               
                 nahG-R 
                 GCATCTCCAATCAACACAACTC (SEQ ID NO: 17) 
               
               
                   
               
               
                 aroG-F 
                 GATGTTTGTGCTGACGTGTG (SEQ ID NO: 18) 
               
               
                   
               
               
                 aroG-R 
                 TTCCCTCAACTAAGTGACTTTCC (SEQ ID NO: 19) 
               
               
                   
               
               
                 ACT2-F 
                 GGTAACATTGTGCTCAGTGGTGG (SEQ ID NO: 20) 
               
               
                   
               
               
                 ACT2-R 
                 AACGACCTTAATCTTCATGCTGC (SEQ ID NO: 21) 
               
               
                   
               
            
           
         
       
     
     2.4. Metabolite Extraction 
     SA is extracted from developing stems using 80% (v/v) methanol-water at 70° C. as previously described (Eudes et al., 2015). MA is extracted from stems of mature senesced plants ball-milled with a mixer mill MM 400 (Retsch, Newtown, Pa.). Ball-milled stem material (50 mg) is mixed with 1 mL of 80% (v/v) methanol-water and mixed (1400 rpm) for 15 min at 70° C. This extraction step is repeated twice. Extracts are pooled and cleared by centrifugation (5 min, 20,000×g), mixed with 1.5 mL of analytical grade water and filtered using Amicon Ultra centrifugal filters (3000 Da MW cutoff regenerated cellulose membrane; EMD Millipore, Billerica, Mass.) prior to LC-MS analysis. 
     Alternatively, MA is released from stems of mature senesced plants (line nahG-catA-1.2×irp9-aroG 2.2) using dilute alkaline and dilute acid treatments. For dilute alkaline treatments, 10 mg of ball-milled biomass is soaked with 90 μL of either 1.2% or 0.62% (w/v) NaOH and heated at 100° C. or 130° C. for 30 min in an autoclave, respectively. For dilute acid treatment, biomass (10 mg) is soaked with 90 μL of 1.2% (w/v) H 2 SO 4  and heated at 120° C. for 30 min in an autoclave. After cooling down and centrifugation, an aliquot of the hydrolysates is mixed with 4 volumes of 80% (v/v) methanol-water and filtered using Amicon Ultra centrifugal filters prior to LC-MS analysis. 
     2.5. LC-MS Metabolite Analysis 
     SA and catechol are analyzed using liquid chromatography (LC), electrospray ionization (ESI), and time-of-flight (TOF) mass spectrometry (MS) as previously described (Haushalter et al., 2017). LC-ESI-TOF-MS analysis of muconic acid is carried out with a similar method except that the LC gradient elution is conducted as follows: linearly increased from 5% solvent B (0.1% formic acid in methanol) to 60.9% B in 4.3 min, increased from 60.9% B to 97.1% B in 1.3 min, decreased from 97.1% B to 5% B in 0.4 min, and held at 5% B for 2 min. The flow rate is held at 0.42 mL/min for 5.6 min, increased from 0.42 mL/min to 0.65 mL/min in 0.4 min, and held at 0.65 mL/min for 2 min. The total LC run time is 8 min. All metabolites are quantified via calibration curves of standard compounds (Sigma-Aldrich, St Louis, Mo.) for which the R 2  coefficients were ≥0.99. Cis,trans-MA is prepared from cis,cis-MA as previously described (Matthiesen et al., 2016). 
     3. Results 
     3.1. Muconic Acid (MA) Production in Plants Expressing nahG and catA 
     The plastidial SA pool derived from the shikimate pathway is used as precursor for the biosynthesis of MA in  Arabidopsis  stems. To this end, plastid-targeted versions of the salicylate hydroxylase NahG and catechol 1,2-dioxygenase CatA from  Pseudomonas putida  are co-expressed for the sequential conversion of SA into catechol and MA. Although NahG has been shown previously to be functional in plants (Friedrich et al., 1995), the use of CatA for the synthesis of MA in plants has never been described. Since mature senesced  Arabidopsis  plants mainly consist of stem biomass, two  Arabidopsis  promoters (pIRX8 and pC4H) which are both strongly active in stem tissues that develop secondary cell walls for synchronized expression of nahG and catA ( FIG. 2B ) are selected. Specifically, both promoters are known to be active in xylem vessels and interfascicular fibers: pIRX8 is the promoter of a glycosyltransferase family 8 (GT8) involved in the synthesis of secondary cell wall xylan (Persson et al., 2007), and pC4H is the promoter of the cytochrome P450 cinnamate 4-hydroxylase involved in the general phenylpropanoid pathway and lignin biosynthesis (Bell-Lelong et al., 1997). Five independent lines are selected, and expression of nahG and catA was confirmed by RT-qPCR using mRNA extracted from stems of 5-week-old homozygous plants at the T3 generation ( FIG. 3A ). For these five lines, the content of MA extracted from stem biomass of senesced plants vary between 8.3 and 13.8 μg/g DW ( FIG. 3B ), which validates the dual nahG-catA expression strategy for the production of MA in plants. Although CatA converts catechol into cis,cis-MA, a mixture of cis,cis-MA and cis,trans-MA in the plant extracts are detected ( FIGS. 6A to 6C ), presumably due to the partial conversion of cis,cis-MA acid during the extraction procedure performed at 70° C. Therefore, MA titers reported in this Example 1 are the sum of cis,cis-MA acid and cis,trans-MA. Moreover, SA content measured in 5-week-old stems from the transgenic lines that co-express nahGand catA is reduced 5- to 10-fold compared to wild-type plants ( FIG. 3C ), suggesting that SA can be limiting for MA synthesis in transgenics. 
     3.2. Enhancement of SA Content in Stems by Expressing Bacterial Salicylate Synthase (Irp9) and Feedback-Insensitive DAHP Synthase (AroG*) 
     In order to increase the content of SA in  Arabidopsis  stems, a plastid-targeted version of the salicylate synthase Irp9 from  Yersinia enterocolitica  is expressed using the promoter of the  Arabidopsis  secondary cell wall cellulose synthase gene IRX5 (CESA4) which is specifically active in stem vascular tissues (Eudes et al., 2012). Expression of irp9 has previously been shown to be effective to increase SA content without negative growth consequences in poplar (Xue et al., 2013). Seven independent lines are selected and expression of irp9 is confirmed by RT-qPCR using mRNA extracted from stems of 5-week-old homozygous plants at the T3 generation ( FIG. 4A ). For six of these seven lines, the content of SA extracted from 5-week-old stems is increased significantly 1.8- to 4.4-fold compared to wild-type plants ( FIG. 4B ), which validates the important role of Irp9 to increase SA content in  Arabidopsis  stems. None of these transgenic lines show any visible growth defects. Next, in order to further increase the carbon flux through the SA biosynthesis pathway in stem, a new set of transgenic lines is generated for co-expression of Irp9 with a mutant feedback-insensitive DAHP synthase (AroG*) from  E. coli.  Expression of plastid-targeted AroG* in  Arabidopsis  is previously shown to increase the content of metabolites derived from the shikimate pathway such as aromatic amino acids and hydroxycinnamates (Tzin et al., 2012). The promoter pCCoAOMT of the caffeoyl-CoA O-methyltransferase gene involved in the monolignol pathway and lignin biosynthesis in  Arabidopsis  is chosen to express aroG* in stem vascular tissues that produce secondary cell walls (Do et al., 2007). Expression of irp9 and aroG* is verified in six independent lines using mRNA extracted from 15-cm stems of homozygous plants at the T3 generation ( FIG. 4C ). For these six lines, the content of SA extracted from 15-cm stems is increased significantly 11- to 65-fold compared to wild-type plants ( FIG. 4D ). The two lines showing the highest SA increase (irp9-aroG-1.3 and irp9-aroG-3.11) have an obvious dwarf phenotype compared to the other lines and wild-type plants, a phenomenon previously observed in  Arabidopsis  mutants that overproduce SA (Sha, 2003). Therefore, co-expression of Irp9 and AroG* genes under secondary cell wall promoters represents an efficient strategy for over-producing SA in  Arabidopsis  stems. 
     3.3. MA Production in Plants Expressing NahG, CatA, Irp9, and AroG* 
     To assess the effect of SA over-accumulation on MA production, the line nahG-catA-1.2 is transformed with the constructs used for expression of irp9 and for co-expression of irp9 and aroG* ( FIG. 2B ). First, several transgenic lines are selected that show expression of irp9 in the nahG-catA-1.2 background ( FIG. 5A ). Muconic acid content in stems of senesced nahG-catA-1.2×irp9 lines is increased 2- to 24-fold compared to the parental line ( FIG. 5B ). Second, several transgenic lines that show co-expression of irp9 and aroG* in the nahG-catA-1.2 genetic background are selected ( FIG. 5C ). In these lines, muconic acid content range between 483 and 637 μg/g DW, which represents a 39- to 51-fold increase compared to the values measured in the nahG-catA-1.2 parental line ( FIG. 5D ). These results demonstrate the positive effect of increasing the carbon flux through the SA route to enhance the production of MA in the proposed biosynthetic pathway. Furthermore, measurement of SA and catechol in two MA-producing lines reveals contents far below those of MA, suggesting that these two intermediates could limit MA biosynthesis ( FIGS. 7A and 7B ). 
     4. Discussion and Conclusions 
     This Example 1 demonstrates that bacterial catechol 1,2-dioxygenase (CatA) is functional in  Arabidopsis  plastids and thus can be exploited for the production of MA in plants. A biosynthetic route for catechol has been elegantly demonstrated in white campion flowers: it originates from phenylalanine and uses cinnamic acid, benzoic acid, and SA as intermediates (Akhtar and Pichersky, 2013). In this pathway, the biosynthetic genes involved in the steps for sequential conversion of benzoic acid into SA and catechol remain to be identified. Therefore, a plastid-targeted bacterial salicylate hydroxylase (NahG) is used in this work for the conversion of chorismate-derived SA into catechol. Gratifyingly, co-expression of NahG and CatA in plastids results in the production of MA, and MA titers can be further enhanced by increasing the carbon flux through SA biosynthetic pathway. Although the synthesis of SA from chorismate involves known plastidial isochorismate synthases, the plant enzyme(s) involved in the conversion of isochorismate into SA remain to be identified (Widhalm and Dudareva, 2015). Similarly, the identity and sub-cellular localization(s) of the enzymes that contribute to SA biosynthesis from cinnamic acid—an alternative SA pathway described in several plant species—have not all been elucidated (Dempsey et al., 2011). Therefore, a characterized bacterial bi-functional SA synthase (isochorismate synthase/isochorismate pyruvate lyase, Irp9) that has been previously validated in planta to enhance SA synthesis in  Arabidopsis  is targeted to plastids. Moreover, expression of plastid-targeted bacterial feedback-insensitive AroG enhanced SA production when co-expressed with Irp9, which confirms previous observations in  Arabidopsis  about the positive effect of AroG expression on the accumulation of metabolites derived from the shikimate pathway (Tzin et al., 2012). Considering low SA titers measured in transgenic  Arabidopsis  lines that produce MA ( FIGS. 7A and 7B ), additional engineering to enhance carbon flux through SA in these lines could improve MA titers. Whether the overexpression of other enzymes from the shikimate pathway ( FIG. 2A , steps 2-7) would further increase the SA pool remains to be investigated. Similarly, the enolase responsible for the synthesis of phosphoenolpyruvate ( FIG. 2A , step 1) could be targeted to increase SA content via the shikimate pathway since its overexpression was shown to drive carbon flux towards aromatic amino acid biosynthesis in tomato (Zhang et al., 2015). Specifically, the lack of correlation observed between aroG expression levels and SA ( FIG. 4C-D ) or MA ( FIG. 5C-D ) titers in our engineered plants suggests that one of the two aroG substrates could become limiting ( FIG. 2A ). Ultimately, since this MA biosynthetic route is confined to plastids, a trapping of SA inside plastids should be considered to avoid leak of this precursor, which could be achieved by downregulating known SA plastid exporters (Serrano et al., 2013). 
     More generally, certain crops engineered for reduced biomass recalcitrance and enhanced digestibility overproduce SA (Gallego-Giraldo et al., 2011), making them ideal genetic backgrounds for the production of both fermentable sugars and value-added MA Likewise, bioenergy  Populus  species (e.g., Salicaceae family) known to accumulate extremely high amounts of endogenous SA and SA-derived metabolites (up to 10% leaf dry weight) would represent adequate plant chassis for MA bioproduction (Lindroth and Hwang, 1996, Morse et al., 2007). In addition, it is anticipated that bioenergy crops engineered for MA accumulation can serve as compatible feedstock for MA-producing microbial strains able to grow on lignin-enriched streams derived from lignocellulosic biomass (Vardon et al., 2015, Rodriguez et al., 2017). Because such streams are generated with high solids loadings (&gt;10% w/v), their enrichment with MA could be achieved using biomass containing MA. As an illustration, biomass containing 5% MA DW could potentially generate streams with 5 g/L MA (at 10% w/v biomass loading), a value similar to the best titers accomplished using engineered microbes and glucose as carbon source (Johnson et al., 2016). In this scenario, the MA titer reported for  Arabidopsis  (0.64 mg/g DW) would need to be improved by less than two orders of magnitude. More research will be needed to determine the optimal biomass pretreatment conditions for the release of MA. Preliminary study indicates that optimal dilute alkaline biomass pretreatments used to generate lignin-rich fractions for downstream biological upgrading (Karp et al., 2014) can efficiently release MA from biomass of the engineered  Arabidopsis  plants ( FIG. 8 ). On the other hand, lower amount of MA is recovered when biomass was treated with dilute acid, possibly due to the cyclization of MA into muconolactone under these conditions (Carraher et al., 2017). 
     As complementary approaches to the strategy presented in this work, the synthesis of catechol towards MA production could be achieved from alternate precursors such as anthranilate, protocatechuate, or 4-hydroxybenzoate as previously achieved in microorganisms (Kruyer and Peralta-Yahya, 2017). For this purpose, preliminary work conducted in tobacco validated that both anthranilate 1,2-dioxygenase and protocatechuate decarboxylase can be functionally expressed in plastids for the synthesis of catechol from anthranilate and protocatechuate, respectively (Shih et al., 2016a). In addition, previous engineering strategies in  Arabidopsis  have demonstrated the overproduction of anthranilate, protocatechuate, and 4-hydroxybenzoate from chorismate (Eudes et al., 2016, Ishihara et al., 2006, Last and Fink, 1988). Since protocatechuate and 4-hydroxybenzoate synthesis can also be accomplished from the precursors 3-dehydroshikimate and p-coumaroyl-CoA, respectively (Eudes et al., 2012, Eudes et al., 2016, Wu et al., 2017), production of high MA titers in plants could be envisioned by stacking branched biosynthetic routes that use diverse intermediates and products of the shikimate pathway ( FIG. 9 ), and assisted by the use of synthetic promoters to optimize and synchronize the expression of multiple biosynthetic genes (Shih et al., 2016b). 
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     While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.