# New Insights into the Symbiotic Relationship between Orchids and Fungi
## 4 Molecular Studies on Orchid Mycorrhizal Symbiosis by Next-Generation Sequencing and Proteomics
hybridum_ co-cultured with _Eplurolhiza repens_ ML01 (mycorrhizal) and _Umbelopsis nana_ ZH3A-3 (non-mycorrhizal) [57]. The transcriptome indicated that the mycorrhizal and non-mycorrhizal fungi or the combination of both fungi induced the general defense responses in the symbiotic orchid roots. The differentially expressed defense genes are involved in cell wall modification (probable xyloglucan endo-transglucosidase, beta-D-xylosidase and cellulose synthase), detoxification of reactive oxygen species (glutathione peroxidase, probable glutathione S-transferase and germin-like protein), plant hormone signaling and synthesis (ethylene receptor homolog 2, ethylene-responsive element binding factor 4, lipoxygenase, ABA and gibberell biosynthesis) and secondary metabolism (Table 1). As found in other orchid mycorrhizal systems, two inorganic phosphate transporter genes were induced in the symbiotic roots of _C. hybridum_. In addition, several other nutrient transport genes including major facilitator family protein and oligopeptide transporter 1-like genes and root morphogenesis genes were also up-regulated. Although the non-mycorrhizal fungal strain, ZH3A-3, induced common stress responses with _E. repens_ ML01, the mycorrhizal symbiosis specifically activates genes related to signaling (LysM domain receptor-like kinase 3-like),  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Orchid** & **Fungi Strain** & **Platform** & **Differentially Expressed Genes/proteins** & **Ref. ** \\ \hline \multirow{6}{*}{_Bletilla striata_ (terrestrial) & _Tulsnella sp._ HR1-1 & \ \begin{tabular}{c} Illumina HiSeq 1500 \\ (transcriptomics) \\ \end{tabular}  &
\begin{tabular}{c} **Identification of a set of common** \\ _symbiotic genes (CSG) in the orchid:_ \\ _BScCaMK_ (highly conserved with \\ _LJCCAMK_), _BSAM_ genes Highly similar to \\ the AM marker genes in rice), 17 putative \\ nitrogen and sugar transporter genes \\ (amino acid transporter genes, bidirectional \\ sugar transporter SWEET family genes), \\ oligopeptide transporter genes, hormone \\ transporter genes. \\ \hline \multirow{6}{*}{_Oneidium_ (epiphytic)} & \multirow{6}{*}{_Certabbsidium sp._ (proteomics)} & \multirow{6}{*}{_Identification of 88 differentially_ (expressed proteins in mycorrhizal \\  & & **process:** mainly involved in molecular \\ signaling, cell rescue and defense, energy \\ and secondary metabolism. \\ \hline \multirow{6}{*}{_Oneidium_ (epiphytic)} & \multirow{6}{*}{_Thantateporus sp._ (proteomics)} & \multirow{6}{*}{2D-MALDI-TOF MS  
defense (chitinase 4-like, mannose specific lectin), transport (equilibrative nucleoside transporter, ATPase 7, plasma membrane-type-like, peptide transporter PTR3-A-like), auxin response and protein metabolism and processing. Furthermore, the three fungi treatments also induced genes encoding transcription factors including AP2 domain, ERF, WRKY, MYB, bHLH and zinc-finger families which are generally involved pathogen defense responses and root developmental process. In addition to identification of the orchid transcripts, the putative fungal genes related to remodeling of fungal cell wall, degradation of plant cell wall and nutrient transport were identified in the symbiotic roots. The authors previously reported that ZH3A-3 and another orchid mycorrhizal fungi isolate, CL01, induce a similar mineral nutrient uptake in _C. hybridum_ root [67]. Here, they showed that the effect of ZH3A-3 on the growth of _C. hybridum_ is comparable to that of ML01. Therefore, the ZH3A-3 isolate was designated as a mycorrhizal-associated fungus [57]. The fully achlorophyllous variants are sometimes found in the partially myco-heterotrophic orchids which gain C from both photosynthesis and their mycorrhizal fungi. Therefore, it may help to understand the myco-heterotrophy through investigating and comparing the chlorophyllous and achlorophyllous partially myco-heterotrophic orchids [62]. [PERSON] et al.