Orchidaceae is one of the largest families of flowering plants with over 700 genera and 25,000 species (Dearnaley 2007; Sathiyadash et al. 2012). Orchids are found in a wide range of habitats and may grow autotrophically or heterotrophically (Sathiyadash et al. 2012; Tan et al. 2014; Fochi et al. 2017). Orchids are economically very important and their sales represent around 8% of the world floriculture trade. The economically most important genera are Cymbidium, Dendrobium, and Phalaenopsis (Dearnaley 2007, 2016; Chugh et al. 2009; Emsa-art et al. 2018). Some orchids, such as Gastrodia (Griesbach 2002; Dearnaley 2007), Dendrobium officinale and D. nobile are used as natural medicines (Li et al. 2009). Furthermore, the economically most important orchid products are the flavours derived from some species of the genus Vanilla, which are grown at a large scale and used in food and drinks (Dearnaley 2007; Gonzalez-Chavez et al. 2018).
Most orchids rely on mycorrhizal fungi for survival, as they are essential for seed germination and early plant growth (Sathiyadash et al. 2012; Fochi et al. 2017). Different fungal symbiotic mycorrhizae have been recorded from orchids. Orchid associated non-mycorrhizal endophytic fungi have been investigated via healthy plant organs including leaves, roots and stems (Ma et al. 2015a), whereas mycorrhizal fungi are generally isolated from root tissues (Tan et al. 2014; Ma et al. 2015a, b). Non-mycorrhizal endophytic fungi represent over 110 genera, including Sordariomycetes (Neonectria, Trichoderma, Nigrospora, Pestalotiopsis) and Dothideomycetes (Cercospora, Lasiodiplodia, Phyllosticta) (Ma et al. 2015a, b). Dark septate endophytes isolated from Dendrobium and Leptodontidium sp. enhanced seedling development of Dendrobium nobile by forming peloton-like structures in the cortical cells of the orchid (Hou and Guo 2009; Ma et al. 2015a, b). Fusarium species promoted seed germination of Cypripedium and Platanthera orchids (Ma et al. 2015a, b). The endophyte Umbelopsis nana isolated from Cymbidium spp., promoted growth of Cymbidium hybridum (Ma et al. 2015a, b).
Many epiphytic and terrestrial orchids produce minute seeds with minimal nutrient reserves, and lack nutrients for seed germination and development in the early growth stage (Cameron et al. 2006, Sathiyadash et al. 2012; Tan et al. 2014). After germination, orchid seeds produce a protocorm (a preseedling stage/ early stage of the plant) that lacks chlorophyll (Leake 2004; Sathiyadash et al. 2012; Fochi et al. 2017). Protocorms grow incomplete dependence on fungal symbionts for nutrients and organic carbon supply (Cameron et al. 2006; Dearnaley 2007). Orchid seedlings develop photosynthetic leaves later and then mature roots are colonized by mycorrhizal fungi (Cameron et al. 2006; Smith and Read 2008; Fochi et al. 2017). The protocorm and mature roots cells are colonized by intracellular fungal coils (pelotons) (Dearnaley 2007; Dearnaley et al. 2016; Fochi et al. 2017).
Orchid mycorrhizal associations are useful in the floriculture trade, as they stimulate seed germination and propagate orchids (Tan et al. 2014). A brief methodology for the inoculation of mycorrhizal fungus (Tulasnella sp.) to orchids according to Nontachaiyapoom et al. (2010) and Tan et al. (2014). A Tulasnella sp. isolated from roots of Dendrobium nobile facilitates significantly higher seed germination of D. officinale than that of the control (without inoculation of Tulasnella sp.) (Tan et al. 2014). In addition, Tulasnella sp. promotes seed development up to stage 5, while the control without the fungus developed only to stage 2 (Tan et al. 2014). However, fungi isolated from orchid plant roots do not always exhibit functional symbiotic associations with the orchid plant (Dearnaley 2007). Microscopic observations of orchid root sections might be useful to confirm the presence of intracellular fungal mycelium (Nontachaiyapoom et al. 2010; Emsa-art et al. 2018). Furthermore, it is important to evaluate seedling growth of mycorrhizal inoculated orchids under natural conditions (Tan et al. 2014).
Orchid mycorrhizal fungi are also important for controlling disease in floriculture trade (Yoder et al. 2000; Emsa-art et al. 2018). Inoculation of orchid mycorrhizal fungi may enhance plant immunity against pathogenic diseases (Wu et al. 2011; Emsa-art et al. 2018). For example, soft rot disease is one of the most devastating diseases caused by Dickeya spp., which kills orchids or causes spots/scars on leaves and flowers (Liau et al. 2003; Emsa-art et al. 2018). One recent study showed that soft rot development in mycorrhizal fungi inoculated orchids was significantly reduced compared to that of non-mycorrhizal fungi inoculated orchids in greenhouse conditions. Phalaenopsis is a popular potted plant species that was used for the study by Emsa-art et al. (2018). A brief overview of the methodology of inoculation of mycorrhizal fungi (Tulasnella deliquescens) to Phalaenopsis to control pathogenic Dickeya.
Several commercial products containing mycorrhizal inoculants exist. These inoculants are available for sale in liquid and powder form for easy and effective usage. Most of these products are organic fertilizers inoculated with mycorrhizae spores and with vitamins, minerals, and nutrients to help bolster the fertility and biological activity of the soil. Mycorrhizal fungi are useful in orchid conservation (Tan et al. 2014). Orchids are a highly diverse plant family and many species may face extinction threats (Reiter et al. 2016) because of habitat loss and overexploitation of attractive species (Dearnaley 2007). With this decline of orchid diversity, it is now an urgent requirement to encourage research on the reintroduction of endangered species to natural habitats (Reiter et al. 2016).