PS 14-186 - Chitinase variation in Stenotrophomonas and its inhibitory effect on Batrachochytrium dendrobatidis and Fusarium spp.

Abstract: Infectious diseases are a leading cause of death globally, across plants, animals, and other taxa. These diseases are caused by an array of organisms, including viruses, bacteria, fungi, and other species. While SARS-CoV-2 currently draws the focus of the media, the global spread of other pathogens is having devastating effects on global biodiversity and productivity. To use two fairly disparate examples, hundreds of salamander species are threatened with extinction by a pathogenic fungus, Batrachochytrium dendrobatidis (Bd), while Fusarium species are important fungal pathogens on crop plants. A critical factor in the susceptibility of individuals to infection and disease is the resident microbial community that lives symbiotically on, and in, an individual. The diversity of these microbes has been shown to affect the health of organisms in many species of plants and animals. Yet, the taxonomic diversity of microbes on an organism is an imperfect predictor of protective effects against fungal pathoges, due in part to variation in antifungal metabolites. Accordingly, our goal here was to examine the relationship between variation in chitinase, a key antifungal metabolite, and overall antifungal activity of isolates of two Stenotrophomonas species. To address these issues, chitinase variation was analyzed in S. rhizophila and S. maltophilia isolates collected from Plethodon cinereus salamanders in the New York City metropolitan area. Species identification of each isolate was confirmed using 16S sequencing. Chitinase sequencing involved the use of novel primers to sequence the gene. Differences among isolates in nucleotide and amino acid sequences were then compared with differences among isolates’ antifungal abilities against Bd in functional assays, and against Fusarium in six tomato cultivars grown under greenhouse conditions. Ultimately, the complete chitinase gene was sequenced for four S. rhizophila and two S. maltophilia isolates. Despite this sample size, substantial variation was observed among the chitinases, with nucleotide sequences differing by up to 25% among isolates, and amino acid sequences diverging by up to 29%. However, nucleotide and amino acid differences among isolates did not correlate with differences in antifungal ability against Bd. Similarly, while these six isolates differed by up to 20% in their effects on plant growth, genetic differences did not correlate with differences among isolates in plant growth promotion. Taken together, these results suggest substantial genetic and amino acid variation in chitinase exists within and among Stenotrophomonas species, yet other metabolites likely play a larger role in the beneficial effects of Stenotrophomonas against fungal pathogens on hosts.