Abstract
Fungi employ extracellular enzymes to initiate the degradation of organic macromolecules into smaller units and to acquire the nutrients for their growth. As such, these enzymes represent important functional components in terrestrial ecosystems. While it is well-known that the regulation and efficiency of extracellular enzymes to degrade organic macromolecules and nutrient-acquisition patterns strongly differ between major fungal groups, less is known about variation in enzymatic activity and carbon/nitrogen preference in mycorrhizal fungi. In this research, we investigated variation in extracellular enzyme activities and carbon/nitrogen preferences in orchid mycorrhizal fungi (OMF). Previous research has shown that the mycorrhizal fungi associating with terrestrial orchids often differ from those associating with epiphytic orchids, but whether extracellular enzyme activities and carbon/nitrogen preference differ between growth forms remains largely unknown. To fill this gap, we compared the activities of five extracellular enzymes [cellulase, xylanase, lignin peroxidase, laccase, and superoxide dismutase (SOD)] between fungi isolated from epiphytic and terrestrial orchids. In total, 24 fungal strains belonging to Tulasnellaceae were investigated. Cellulase and xylanase activities were significantly higher in fungi isolated from terrestrial orchids (0.050 ± 0.006 U/ml and 0.531 ± 0.071 U/ml, respectively) than those from epiphytic orchids (0.043 ± 0.003 U/ml and 0.295 ± 0.067 U/ml, respectively), while SOD activity was significantly higher in OMF from epiphytic orchids (5.663 ± 0.164 U/ml) than those from terrestrial orchids (3.780 ± 0.180 U/ml). Carboxymethyl cellulose was more efficiently used by fungi from terrestrial orchids, while starch and arginine were more suitable for fungi from epiphytic orchids. Overall, the results of this study show that extracellular enzyme activities and to a lesser extent carbon/nitrogen preferences differ between fungi isolated from terrestrial and epiphytic orchids and may indicate functional differentiation and ecological adaptation of OMF to local growth conditions.
Highlights
With >26,000 species, the Orchidaceae is one of the largest and most diverse plant families in the world
A total of 135 fungal isolates was recovered from the pelotons of 90 root samples from 18 orchid species that were collected from the subtropical forest in southern China
Among the 24 taxa of Tulasnellaceae spp., half of them were isolated from terrestrial orchids, while the other half were isolated from epiphytic orchids
Summary
With >26,000 species, the Orchidaceae is one of the largest and most diverse plant families in the world. Typical orchid mycorrhizal fungi (OMF) associating with green orchids involve members of the Tulasnellaceae, Ceratobasidiaceae, and Serendipitaceae (Smith and Read, 2008; Dearnaley et al, 2012; Jacquemyn et al, 2017). Because of their dependence on mycorrhizal fungi, OMF have been regarded as one of the important factors determining whether an orchid can occur in one habitat or not (McCormick and Jacquemyn, 2014), and they have been linked to niche partitioning and coexistence of multiple orchid species (Jacquemyn et al, 2015; Xing et al, 2020)
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