Proteomic and Transcriptomic Analyses Indicate Metabolic Changes and Reduced Defense Responses in Mycorrhizal Roots of Oeceoclades maculata (Orchidaceae) Collected in Nature

Abstract

Orchids form endomycorrhizal associations with fungi mainly belonging to basidiomycetes. The molecular events taking place in orchid mycorrhiza are poorly understood, although the cellular changes necessary to accommodate the fungus and to control nutrient exchanges imply a modulation of gene expression. Here, we used proteomics and transcriptomics to identify changes in the steady-state levels of proteins and transcripts in the roots of the green terrestrial orchid Oeceoclades maculata. When mycorrhizal and non-mycorrhizal roots from the same individuals were compared, 94 proteins showed differential accumulation using the label-free protein quantitation approach, 86 using isobaric tagging and 60 using 2D-differential electrophoresis. After de novo assembly of transcriptomic data, 11,179 plant transcripts were found to be differentially expressed, and 2175 were successfully annotated. The annotated plant transcripts allowed the identification of up- and down-regulated metabolic pathways. Overall, proteomics and transcriptomics revealed, in mycorrhizal roots, increased levels of transcription factors and nutrient transporters, as well as ethylene-related proteins. The expression pattern of proteins and transcripts involved in plant defense responses suggested that plant defense was reduced in O. maculata mycorrhizal roots sampled in nature. These results expand our current knowledge towards a better understanding of the orchid mycorrhizal symbiosis in adult plants under natural conditions.

A Transcriptomic Approach Provides Insights on the Mycorrhizal Symbiosis of the Mediterranean Orchid Limodorum abortivum in Nature

“…The list of the most upregulated plant genes in mycorrhizal L. abortivum roots included genes belonging to different functional categories previously identified in AM roots and in mycorrhizal protocorms of the terrestrial orchid S. vomeracea colonized by T. calospora [ 15 , 21 ] as well as genes reported to be upregulated in mycorrhizal roots of O. maculata colonized by P. candolleana [ 28 ]. Among these upregulated genes were genes encoding some membrane transporters, such as two bidirectional SWEET sugar transporters (TRINITY_DN27909_c0_g1_i5 and TRINITY_DN26416_c3_g4_i1), a peptide (TRINITY_DN28361_c1_g4_i3) and a copper transporter (TRINITY_DN63579_c0_g1_i1), a gene coding for an acidic endochitinase (TRINITY_DN28711_c0_g3_i3) and a gene coding for a thaumatin-like protein (TRINITY_DN26462_c1_g2_i2).…”

“…A transcriptomic approach has been used to investigate changes in gene expression in adult OM roots of the epiphytic photosynthetic orchid Cymbidium hybridum , inoculated in vitro with the mycorrhizal fungus Epulorhiza repens [ 16 ]. To our knowledge, Oeceoclades maculata is the only orchid species where gene and protein expression in mycorrhizal and non-mycorrhizal roots was compared under natural conditions [ 28 ]. This tropical terrestrial orchid specifically associates with the wood decomposer Psathyrella candolleana [ 28 ], a fungus found as symbiont of mycoheterotrophic orchids [ 29 ].…”

“…To our knowledge, Oeceoclades maculata is the only orchid species where gene and protein expression in mycorrhizal and non-mycorrhizal roots was compared under natural conditions [ 28 ]. This tropical terrestrial orchid specifically associates with the wood decomposer Psathyrella candolleana [ 28 ], a fungus found as symbiont of mycoheterotrophic orchids [ 29 ]. Suetsugu et al [ 30 ] also used RNAseq to investigate gene expression in mycorrhizal roots of adult orchids in nature, but their comparison of albino and green variants of Epipactis helleborine aimed to uncover the molecular mechanisms related to mycoheterotrophy, rather than to OM interactions.…”

In Vitro Symbiotic Germination: A Revitalized Heuristic Approach for Orchid Species Conservation

“…Symbiosis starts at the early stage of germination. What makes OMF different from other mycorrhiza is that the hyphae form a peloton, which is a hyphal coil inside the host cell where nutrient exchange occurs [ 46 , 47 ]. Hyphae enter from the basal part of the protocorm, and colonization normally only occurs there.…”

“…Although in vitro studies do not always represent realistic environmental conditions, these act as a microenvironment replica and may function as a tool for extensive studies [ 47 , 49 ]. The most convenient way to conduct in vitro study is by simply inoculating seeds and a suitable fungus on a solid agar medium ( Figure 1 a–c).…”