Gene Expression Profiling in Orchid Mycorrhizae to Decipher the Molecular Mechanisms of Plant–Fungus Interactions

In nature, orchid seed germination and early development both require an interaction between the plant and some symbiotic fungi capable of forming orchid mycorrhizae (OM). In recent years, the use of -omics approaches has been instrumental to start elucidating the molecular mechanisms of this important symbiosis. Transcriptomics is one of the most powerful tools to understand the interplay of genes and their assignment to associated functions. Transcriptomics has been used to study diverse phases of the orchid life cycle and their interaction with OM fungi. For whole-transcriptome analysis in orchids, RNA-sequencing (RNA-seq) has been established as a standard method, and several papers have used transcriptomic approaches to investigate plant responses to mycorrhizal colonization, nutrient transfer between the fungal symbiont and its host, and fungal genes related to the establishment of the symbiotic association. In addition, laser microdissection (LM) technology, which allows the fast isolation of specific cells from microscope slide sections, has been used to understand the fine-tuned regulation of orchid genes in symbiotic tissues. Here, some recent studies on gene expression profiling in orchid mycorrhiza, also in combination with metabolomics, are illustrated and discussed to highlight intriguing findings on OM symbiosis.