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

Abstract

The study of orchid mycorrhizal interactions is particularly complex because of the peculiar life cycle of these plants and their diverse trophic strategies. Here, transcriptomics has been applied to investigate gene expression in the mycorrhizal roots of Limodorum abortivum, a terrestrial mixotrophic orchid that associates with ectomycorrhizal fungi in the genus Russula. Our results provide new insights into the mechanisms underlying plant-fungus interactions in adult orchids in nature and in particular into the plant responses to the mycorrhizal symbiont(s) in the roots of mixotrophic orchids. Our results indicate that amino acids may represent the main nitrogen source in mycorrhizal roots of L. abortivum, as already suggested for orchid protocorms and other orchid species. The upregulation, in mycorrhizal L. abortivum roots, of some symbiotic molecular marker genes identified in mycorrhizal roots from other orchids as well as in arbuscular mycorrhiza, may mirror a common core of plant genes involved in endomycorrhizal symbioses. Further efforts will be required to understand whether the specificities of orchid mycorrhiza depend on fine-tuned regulation of these common components, or whether specific additional genes are involved.

Keywords: CAZymes; nitrogen metabolism; orchid mycorrhiza; transcriptomics.

Figure 1

Percentage of fungal reads assigned to different genera in the roots of Limodorum abortivum. (a) Percentage of fungal reads in mycorrhizal (LM) root samples. (b) Percentage of fungal reads in non-mycorrhizal (LS) root samples. The error bar represents the 95% confidence interval of abundance estimate based on three replicates per condition.

Figure 2

PCA analysis of the complete plant transcriptome in the three biological replicates of mycorrhizal (LM) and non-mycorrhizal (LS) root samples of Limodorum abortivum.

Figure 3

Enrichment in GO terms and KEGG pathways in the plant transcriptome of Limodorum abortivum mycorrhizal roots: (a) log₂ enrichment of the 10 most significant terms for each GO category. MF = molecular function, CC = cellular component, BP = biological process; (b) enrichment of KEGG pathways in the plant differentially expressed (DE) genes.

Figure 4

CAZyme families enrichment in Limodorum abortivum genes differentially expressed in mycorrhizal roots. GH: glycoside hydrolases; GT: glycosyl transferases; CE: carbohydrate esterases; AA: auxiliary activities. Significant values according to Fisher’s test (p < 0.05) are indicated with an asterisk (*).