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

Rafael B S Valadares,Renato M Oliveira,Marcio R Lambais,Adriano R Lucheta,Silvia Perotto,Eder C Santos

doi:10.3390/jof6030148

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.

Highlights

Orchidaceae is the largest flowering plant family and accounts for approximately 10% of the global plant diversity, with more than 28,000 described species [1]
Sequence polymorphism was mostly restricted to a single nucleotide substitution, and all phylotypes clustered in the same clade when a phylogenetic NJ tree was constructed with some reference sequences (Figure S1)
Mycorrhizal colonization was accompanied by significant local changes in the host plant metabolism, especially concerning carbon and nitrogen metabolism

Summary

Introduction

Orchidaceae is the largest flowering plant family and accounts for approximately 10% of the global plant diversity, with more than 28,000 described species [1]. Orchid seeds develop into protocorms [3], heterotrophic structures that receive organic carbon and other nutrients from the mycorrhizal symbiont. This fungus-dependent trophic strategy is known as mycoheterotrophy [4,5]. Nutrient and carbon transfer in orchid mycorrhiza (OM) seems to favor the plant over the fungus, especially during the mycoheterotrophic stages For this reason, the OM relationship has been described by some authors as an example of reverse parasitism [3,6], even though the mycorrhizal association has been found to be mutualistic in the adult green-leaved terrestrial orchid

Results

Discussion

Conclusion