# Biosynthetic Pathways of Hormones in Plants
## 11 Strigolactones  
Strigolactones (SLs), which are carotenoid-derived terpenoids, were initially characterized as germination stimulants for root parasitic plant withcheved (_Striga_ spp.). SLs are a class of carotenoid-derived terpenoids that are involved in diverse developmental processes, such as seed germination, shoot branching, leaf senescence, and root development, as well as responses to various environmental conditions (e.g., light stress and high-temperature stress) [179]. They are also known as rhizosphere chemical signals that can regulate interactions between arbuscular mycorrhizal fungi and root parasitic plants. These groups of phytohormones also affect photosynthesis, synthesis, and action of other phytohormones, as well as regulating the levels of different metabolic compounds [180; 181]. According to their chemical structures, SLs can be classified into two groups: canonical and non-canonical SLs. Canonical SLs contain a tricyclic lactone structure composed of three rings (ABC-rings) connected to a butenolide group (D-rings) through an enol-ether bridge, which is critical for the performance of their biological activities in plants. In contrast, non-canonical SLs do not have typical ABC-rings, though they contain both an enol-ether bridge and D-ring moieties [182]. Strigolactone biosynthesis mainly occurs in roots. The intermediates in the SL synthetic pathway have been also found in the stem. The biosynthesis of this group of phytohormones is tightly regulated by environmental conditions, such as starvation of phosphate or drought stress [183]. The early steps of SL biosynthesis are common for each type and may occur via the MVA or MEP pathways that lead to IPP synthesis, which is sequentially condensed to create DMAPP, geranyl pyrophosphate (GPP), farnesyl pyrophosphate (FPP), and GGPP (Figure 7) [116]. Thus, the first part of the SL biosynthetic pathway startsin chloroplasts. The condensation of two GGPP molecules by phytoene synthase generates phytoene (Figure 12), which is the first uncolored carotenoid [184]. Next, phytoene is transformed via a sequence of reactions of desaturation and _cis_/_trans_-isomerization into all-_trans_-lycopene. Cyclization reactions include conversion of all-_trans_-lycopene into all-_trans_-\(\beta\)-carotene and all-_trans_-\(\alpha\)-carotene [185; 186]. Dwarf27 (D27) isomerizes all-_trans_-\(\beta\)-carotene to 9-_cis_-\(\beta\)-carotene, followed by cleavage induced by carotenoid removal of enzyme dioxygenase 7 (CCD7) from 9-_cis_-\(\beta\)-apo-10\({}^{\prime}\)-caroten, which undergoes cleavage and rearrangement reactions to create carlactone (CL), which is the central intermediate in SL biosynthesis (Figure 12). This reaction is catalyzed by the CCD8 enzyme [187]. CCD7 is encoded by gene _MAX3_ and its orthologs genes: _RMS5_ and _D17_/_HTD1_. Gene _MAX4_ and its orthologs _RMS1_, _D10_, and Defective in Anther Dehiscence 1 (_DAD1_) encode CCD8. Both enzymes act in a progressive manner [188]. Carlactone (CL) contains only A- and D-rings that have an enol-ether bridge and exhibits SL-like properties. For example, CL inhibits shoot branching in SL biosynthetic mutants (i.e., rice, and _A. thaliana_) and promotes seed germination of _Striga hermothica_. CL has been identified as an endogenous precursor in synthetic routes of both canonical and non-canonical SLs [186; 189; 190]. Furthermore, a new compound, which we identified as 3-hydroxyCL, is formed in vitro using 9-_cis_-3-hydroxy-\(\beta\)-apo-10\({}^{\prime}\)-carotenal in a reaction catalyzed by enzymes D27, CCD7, and CCD8. This compound was identified in plants such as rice, _A. thaliana_, and _N. benthamiana_[191]. The SL biosynthesis activates the biochemical conversions of CL through the participation of cytochrome P450 mono-oxygenases and other enzymes, which are responsible for the biological and structural diversity of these phytohormones. Subsequently, CL is trans  
Figure 12: The biosynthetic pathways of carlactone.