Gene expression in mycorrhizal orchid protocorms suggests a friendly plant–fungus relationship

Abstract

Main conclusionOrchid mycorrhiza has been often interpreted as an antagonistic relationship. Our data on mycorrhizal protocorms do not support this view as plant defence genes were not induced, whereas some nodulinlike genes were significantly up-regulated.Orchids fully depend on symbiotic interactions with specific soil fungi for seed germination and early development. Germinated seeds give rise to a protocorm, a heterotrophic organ that acquires nutrients, including organic carbon, from the mycorrhizal partner. It has long been debated if this interaction is mutualistic or antagonistic. To investigate the molecular bases of the orchid response to mycorrhizal invasion, we developed a symbiotic in vitro system between Serapias vomeracea, a Mediterranean green meadow orchid, and the rhizoctonia-like fungus Tulasnella calospora. 454 pyrosequencing was used to generate an inventory of plant and fungal genes expressed in mycorrhizal protocorms, and plant genes could be reliably identified with a customized bioinformatic pipeline. A small panel of plant genes was selected and expression was assessed by real-time quantitative PCR in mycorrhizal and non-mycorrhizal protocorm tissues. Among these genes were some markers of mutualistic (e.g. nodulins) as well as antagonistic (e.g. pathogenesis-related and wound/stress-induced) genes. None of the pathogenesis or wound/stress-related genes were significantly up-regulated in mycorrhizal tissues, suggesting that fungal colonization does not trigger strong plant defence responses. In addition, the highest expression fold change in mycorrhizal tissues was found for a nodulin-like gene similar to the plastocyanin domaincontaining ENOD55. Another nodulin-like gene significantly more expressed in the symbiotic tissues of mycorrhizal protocorms was similar to a sugar transporter of the SWEET family. Two genes coding for mannose-binding lectins were significantly up-regulated in the presence of the mycorrhizal fungus, but their role in the symbiosis is unclear.

Plant and fungal gene expression in mycorrhizal protocorms of the orchidSerapias vomeraceacolonized byTulasnella calospora

“…25 An interesting function was discovered in the plant pathogen Fusarium verticillioides, where a fungalysin was found to truncate 2 maize chitinases, that are part of the plant defense response. 21 As a chitinase was found to be expressed in S. vomeracea mycorrhizal protocorms, 9 it would be intriguing to understand whether any of the fungalysins identified in the T. calospora genome may have a similar role in orchid mycorrhiza.…”

“…We used this model system to generate a normalized cDNA library by 454 GS-FLX Titanium pyrosequencing, which represents an inventory of plant and fungal genes expressed in mycorrhizal protocorms. 9 Gene expression in orchid mycorrhiza has received very little attention, 10,11 and only recently Zhao et al 12 investigated the transcriptome of Dendrobium officinale colonised by Sebacina. Despite the different experimental approaches and model systems, a common finding in the transcriptomes of D. officinale and S. vomeracea mycorrhizal protocorms was the small proportion of plant transcripts related to stress response or response to biotic and abiotic stimuli.…”

“…9 One-step RT-PCR identified the corresponding transcripts in RNA extracted from all protocorm cell types (Fig. 1).…”

“…Despite the different experimental approaches and model systems, a common finding in the transcriptomes of D. officinale and S. vomeracea mycorrhizal protocorms was the small proportion of plant transcripts related to stress response or response to biotic and abiotic stimuli. 9,12 We previously assessed, in mycorrhizal and non-mycorrhizal protocorm tissues of S. vomeracea, expression of some plant genes that may provide insights into the orchid response to the mycorrhizal fungus. 9 Despite the small number of genes tested, none of the pathogenesis or wound/stress related genes were significantly upregulated in mycorrhizal tissues, suggesting that fungal colonization did not trigger strong plant defense responses.…”

“…9 Here, we have identified the complete genomic sequence of these transcripts by Blastn search against the T. calospora genome (MycoCosm JGI portal, http://genome.jgi.doe.gov/Tulca1/Tulca1. home.html).…”