New Insights into the Symbiotic Relationship between Orchids and Fungi

Abstract

Mycorrhizas play an important role in plant growth and development. In mycorrhizal symbioses, fungi supply soil mineral nutrients, such as nitrogen and phosphorus, to their host plants in exchange for carbon resources. Plants gain as much as 80% of mineral nutrient requirements from mycorrhizal fungi, which form associations with the roots of over 90% of all plant species. Orchid seeds lack endosperms and contain very limited storage reserves. Therefore, the symbiosis with mycorrhizal fungi that form endomycorrhizas is essential for orchid seed germination and protocorm development under natural conditions. The rapid advancement of next-generation sequencing contributes to identifying the orchid and fungal genes involved in the orchid mycorrhizal symbiosis and unraveling the molecular mechanisms regulating the symbiosis. We aim to update and summarize the current understanding of the mechanisms on orchid-fungus symbiosis, and the main focus will be on the nutrient exchange between orchids and their fungal partners. Appl. Sci. 2019, 9, 585 2 of 14 sheath, which functions as a plant-fungus interface, surrounding the epidermal and outer cortical cells. The interface is used for the bidirectional nutrient transfer between a plant and its fungal partner [7]. (2) Endomycorrhiza: the fungi possess specific types of hyphae such as arbuscules and coils to penetrate and grow inside of the root cells of host plants [3]. Endomycorrhizas can be further classified as ectendomycorrhizas, ericoid, arbutoid, monotropoid, orchid and arbuscular mycorrhizas based on the specificity of plant families and the type of internal hyphae [3,8]. Ectomycorrhizas are also alternatively represented as arbuscular mycorrhizas because the class is the most prevalent type of mycorrhizae, as shown in approximately 71% of all vascular plant species [1,9].The Orchidaceae is the largest family of flowering plants including over 700 genera and about 30,000-35,000 species and still hundreds of new species displaying variable floral features, lifestyles, habitat distributions and trophic patterns are being discovered and developed every year [10,11]. Mycorrhizas in orchid species are highly important throughout their whole life including germination and further development. In this article, we provide a brief review of the roles of mycorrhizas in orchid growth and development and the current understanding of the mechanisms in nutrient exchange between orchids and their fungal partners.

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

“…The molecular mechanisms modulating OM are starting to come to light, especially during seed germination and protocorm development (see [ 9 ]). Suppression subtractive hybridization (SSH) was used to identify genes differentially expressed in symbiotically germinated Dendrobium officinale seeds, as compared to non-germinated seeds [ 10 ], and revealed up-regulation of general defense responses, cell wall modification, reactive oxygen species detoxification, secondary metabolites, and changes in hormone balance.…”

Co-Cultures of Mycorrhizal Fungi Do Not Increase Germination and Seedling Development in the Epiphytic Orchid Dendrobium nobile

“…The majority of orchids relies on insects for successful pollination and subsequent seed development (Tremblay et al, 2005), while mycorrhizal fungi are needed for seed germination, seedling recruitment, and growth to adult plants (Swarts and Dixon, 2009;Mujica et al, 2018;Rasmussen and Rasmussen, 2018). Because most orchid seeds are very small (microspermy) and lack sufficient resources, orchid seeds critically depend on mycorrhizal fungi for essential carbon, water, and mineral nutrients (Meng et al, 2019b;Yeh et al, 2019). Before they appear aboveground, orchids are completely reliant on fungi, a condition called initial mycoheterotrophy (Merckx et al, 2013;Jacquemyn and Merckx, 2019).…”

The Dark Side of Orchid Symbiosis: Can Tulasnella calospora Decompose Host Tissues?

“…These previous papers were mainly focused on the symbiotic regulation of plant and fungal genes involved in N uptake and metabolism, but also led to the identification of genes specifically expressed in symbiotic protocorms (some of them only in pelotons containing cells) that can be considered symbiotic markers [25][26][27]. Transcriptomic analyses have been performed also on other orchid symbiotic systems (see [37] for a review), unveiling the orchid and fungal genes involved in symbiosis and providing novel useful information for a comprehensive understanding of OM biology. However, due to the complexity and the diversity of these interactions, several mechanisms involved in the developing and functioning of these associations remain to be elucidated.…”