Transcriptome Reveals Roles of Lignin-Modifying Enzymes and Abscisic Acid in the Symbiosis of Mycena and Gastrodia elata

Abstract

Gastrodia elata is a well-known medicinal and heterotrophic orchid. Its germination, limited by the impermeability of seed coat lignin and inhibition by abscisic acid (ABA), is triggered by symbiosis with fungi such as Mycena spp. However, the molecular mechanisms of lignin degradation by Mycena and ABA biosynthesis and signaling in G. elata remain unclear. In order to gain insights into these two processes, this study analyzed the transcriptomes of these organisms during their dynamic symbiosis. Among the 25 lignin-modifying enzyme genes in Mycena, two ligninolytic class II peroxidases and two laccases were significantly upregulated, most likely enabling Mycena hyphae to break through the lignin seed coats of G. elata. Genes related to reduced virulence and loss of pathogenicity in Mycena accounted for more than half of annotated genes, presumably contributing to symbiosis. After coculture, upregulated genes outnumbered downregulated genes in G. elata seeds, suggesting slightly increased biological activity, while Mycena hyphae had fewer upregulated than downregulated genes, indicating decreased biological activity. ABA biosynthesis in G. elata was reduced by the downregulated expression of 9-cis-epoxycarotenoid dioxygenase (NCED-2), and ABA signaling was blocked by the downregulated expression of a receptor protein (PYL12-like). This is the first report to describe the role of NCED-2 and PYL12-like in breaking G. elata seed dormancy by reducing the synthesis and blocking the signaling of the germination inhibitor ABA. This study provides a theoretical basis for screening germination fungi to identify effective symbionts and for reducing ABA inhibition of G. elata seed germination.

The role of symbiotic fungi in the life cycle of Gastrodia elata Blume (Orchidaceae): a comprehensive review

“…Once symbiosis is established, G. elata begins to grow with increased biological activity, while the fungi are restricted ( Fan et al., 1999a ). Ren et al. (2021) showed this through comparative transcriptome analysis of seed symbiotic Mycena hyphae and pure cultured hyphae, and furthermore, they sequenced and analyzed the genome of Mycena and found that 5024 genes were annotated in the pathogen-host interactions database, among which more than half were linked to reduced virulence and loss of pathogenicity.…”

“…Manganese peroxidase is a fungal lignin-modifying enzyme, and when Mycena breaks through the seed coats of G. elata , manganese peroxidase and laccase are responsible for the degradation of lignin ( Manavalan et al., 2015 ). Ren et al. (2021) conducted transcriptome analysis and found that the upregulation of manganese-dependent peroxidase short genes was conducive to the invasion of G. elata seeds by hyphae.…”

“…Gene contraction in G. elata is a characteristic facilitating adaptation to the mycoheterotrophic lifestyle. G. elata seed germination is hampered not only by insufficient nutritional reserves and an impermeable seed coat but also by the presence of seed germination inhibitors such as phenolics and abscisic acid ( Van Waes and Debergh, 1986 ; Ren et al., 2021 ). The downregulation of 9-cis-epoxycarotenoid dioxygenase (NCED-2) expression in G. elata reduces abscisic acid production, thereby alleviating abscisic acid’s suppression of seed germination.…”

“…The downregulation of 9-cis-epoxycarotenoid dioxygenase (NCED-2) expression in G. elata reduces abscisic acid production, thereby alleviating abscisic acid’s suppression of seed germination. Additionally, a significantly downregulated receptor protein (PYL12-like) can hinder abscisic acid signaling and thereby break seed dormancy ( Ren et al., 2021 ).…”

Cell structure of epiphytic orchids’ root cortex and exodermis shape fungal colonization

“…Moreover, on a model of Gastrodia elata (Epidendroideae) and Mycena sp. signi cant up-regulation of fungal lignin-degrading enzyme genes during OM formation is shown (Ren et al 2021).…”