Handbook of Vanilla Science and Technology

The Orchidaceae is the largest family of flowering plants, with over 24,000 known species (Dressler 1990; Fay and Chase 2009). Orchids are found on every continent except Antarctica. Orchids have minute, dust-like seeds with no stored nutrients to support germination. In nature, orchid seeds can germinate only if they establish a relationship with a fungus from which they can obtain energy (Rasmussen 1995).

Orchids in temperate areas are terrestrial, whereas tropical and subtropical orchids, which include about 75% of species, can be either terrestrial or epiphytic. In adult stages, terrestrial and epiphytic orchids differ in the extent of their relationships with mycorrhizal fungi: most terrestrial orchids are obligately mycorrhizal, whereas many epiphytic orchids appear to be facultatively mycorrhizal (Rasmussen 1995). Vanilla is an unusual orchid in being both terrestrial and epiphytic: most plants have roots in the soil and roots attached to tree bark (Figure 16.1).

Fig. 16.1 Roots of Vanilla planifolia in forests in Puerto Rico. (A) Epiphytic roots attached to tree bark; (B) extensive system of roots removed from soil.

Orchid mycorrhizae are distinct from those of other plants, in two ways: first, the fungi appear to receive little or nothing from the orchids. Credible evidence that the fungus can receive sugars from the plant has only been published in the last few years (Cameron et al. 2006); it appears that orchids usually parasitize their fungi, in contrast to the mutualistic relationships typical of mycorrhizae (Smith and Read 2002). The second distinctive feature of orchid mycorrhizae is their choice of partners: most orchids associate with fungi in the form-genus Rhizoctonia (Basidiomycota), though many orchids have switched to other groups of fungi (Rasmussen 1995).

16.1.2 The fungal players: Rhizoctonia and related taxa

The Rhizoctonia fungi isolated from orchids rarely produce sexual spores in pure culture, so they have traditionally been identified based on the morphology of their hyphae (Figure 16.2). These characters are convergent, causing unrelated fungi to be grouped together. Molecular systematics studies have shown that the fungi formerly known as Rhizoctonia comprise several sexual genera, of which three have been found in Vanilla: Thanatephorus, Ceratobasidium, and Tulasnella (Porras-Alfaro and Bayman 2007). Rhizoctonia sensu strictu is an asexual genus whose sexual stage belongs in Thanatephorus. The phylogenetic position of these fungi is unclear; they were formerly placed in two orders (Thanatephorus and Ceratobasidium in Ceratobasidiales and Tulasnella in Tulasnellales) but recent studies suggest they may all be related to the chanterelles (Hibbett 2006).

Fig. 16.2 Mycorrhizae and mycorrhizal fungi of Vanilla planifolia. (A) Transverse section of root showing root cortex cells with extensive colonization by fungal pelotons (irregular inclusions in cells above vascular cylinder); (B)Asinglepelotonina rootcortexcell; (C)and (D) Hyphaeof Ceratobasidium and Thanatephorus showing characteristic binucleate and multinucleate cells, respectively, after staining with Safranin O (Sneh et al. 1991); (E) Roots from soil, air,and tree bark (from top); (F) Soil rootwith velamen and partofthe cortex removed; areas with mycorrhizae are indicated by brown color (Porras and Bayman 2003).

Fungi in the Rhizoctonia complex are best known as plant pathogens. In particular, Thanatephorus cucumeris (more widely known by its asexual name, Rhizoctonia solani) includes groups that cause significant losses of rice, wheat, potato, beans, and other crops (Vilgalys and Cubeta 1994). Some R. solani strains can attack a wide range of crops; it is not known why orchids appear to be able to parasitize them in turn. Relationships between plant pathogens and orchid mycorrhizal fungi are not clear; in one of the few studies involving Vanilla, a single strain of R. solani was both a pathogen and a mycorrhizal symbiont (Alconero 1969). This subject is discussed in more detail in Section 16.5.1.

Although tropical orchids comprise the vast majority of species, knowledge of orchid mycorrhizal relationships comes mostly from temperate orchids. Despite the importance of Vanilla from economic, cultural, and historical perspectives, surprisingly little is known about its mycorrhizal fungi. Mycorrhizae of Vanilla were first described over a century ago (Decordenoy 1904), but only a handful of studies have been published in the last 50 years.

16.1.3 Phylogenetic diversity and specificity of mycorrhizal fungi of Vanilla