Figure 1: Fungal culture grown from pelotons [Photo credit: Melissa McCormick, Eranga Wettewa]

Figure 2: Adult Platanthera clavellata in bloom [Photo credit: Melissa McCormick, Eranga Wettewa]

In the "1KFG: Deep Sequencing of Ecologically-relevant Dikarya" project (CSP1974), we are conducting genomic sequencing of key lineages of plant-interacting fungi and saprophytic fungi that are of significant ecological importance for comprehending terrestrial ecosystems. Furthermore, comparative genomic analysis of saprotrophic, mycorrhizal, and pathogenic fungi provides novel insights into specific and conserved adaptations associated with each fungal lifestyle.

Ceratobasidium spp.

Ceratobasidium includes orchid mycorrhizal symbionts, plant pathogens and biocontrol agents of soilborne plant pathogens (Mosquera-Espinosa et al., 2013). These species are reported as among the major mycorrhizal fungi of epiphytic orchids (Otero et al., 2003, Graham & Dearnaley, 2012), that support orchid seedlings through to adulthood. Studies have shown that they help to promote the growth of orchid seedlings by increasing the surface area of the roots or total vegetative parts, increasing the fresh mass, plant height, number of leaves, and root length of the seedlings (Suresh et al., 2024).

Ceratobasidium isolates are found to display functional diversity among isolates within a single OTU (Operational Taxonomic Unit), including differing percentage germination (Freestone et al. 2021), suggesting considerable functional diversity is present.

In Ceratobasidium species, the number of nuclei per cell can vary depending on the specific species and the stage of the life cycle. Generally, Ceratobasidium species are known to have binucleate cells during their vegetative phase. However, some species, like Ceratobasidium flavescens, can have cells with a range of nuclei, from 2 to 14 nuclei per cell (Currah et al., 1997a, Hossain et al., 2013). Uni-nucleate strains have also been reported but rarely found in orchids (Hietala et al., 2001).

Ceratobasidium species may also display pathogenic behavior. Studies have measured aerial hyphal growth as an indication of the degree of pathogen-like behavior of the Ceratobasidium isolate. Aggressive aerial hyphal growth is often observed in unstable orchid-fungus relationships when protocorms are parasitized by the fungus (Zettler 1997; Hajong et al. 2013). Aerial hyphal growth was observed as moderate or aggressive.

In addition to symbiotic relationships with Orchids, Ceratobasidium species are found to form tripartite relationships. Studies have confirmed such a relationship between Rhizanthella gardneri (a unique subterranean orchid species entirely grown under soil, even during flowering), Ceratobasidium sp. (a fungus associated with R. gardneri), and Melaleuca scalene (an autotrophic shrub) through exploration of isotopically labeled tracers (Bougoure et al., 2010).

Figure 1 is from a fungal culture grown from pelotons isolated from adult roots of Platanthera clavellata (Figure 2). Isolated pelotons were grown on Basal Salt (BS) medium at 25 °C for 1–4 weeks. Observations, measurements and photographic images of microscopic fungal structures were recorded using a Hirox Digital microscope.

Understanding the specific mycorrhizal relationships between fungi like Ceratobasidium and orchids is crucial for the successful cultivation and conservation of orchid species, as these relationships play a significant role in the orchids' life cycle and growth.

Researchers who wish to publish analyses using data from unpublished CSP genomes are required to contact the PI and JGI to avoid potential conflicts in data use and coordinate other publications with the CSP master paper(s).