Differential response of Brn1 at varied thermal regimes and its relation with virulence and extracellular release of cellulase and amylase in Alternaria brassicicola

Investigations were undertaken to identify the effect of temperature on the expression of a gene involved in melanin biosynthesis ( Brn1 ) in the dark spored fungus Alternaria brassicicola causal agent of black spot of mustard and its association with the biology, virulence, and release of extracellular enzymes such as cellulase and amylase. The expression of genes Brn1, AbCbh7, Glycosyl hydrolase , responsible for melanin biosynthesis, cellulase, and amylase production, respectively was assessed after exposing the pathogen to 15, 25, and 35 °C temperature. Results revealed a reduction in both melanin biosynthesis and expression of Brn1 at 35 °C when compared to those at 25 °C. The expression of Glycosyl hydrolase also decreased at 35 °C, whereas AbCbh7 expression remained unaffected. However, with decrease in gene expression; extracellular release of cellulase and amylase by A. brassicicola increased at 35 °C, compared to 25 °C. Additionally, reduced mycelial growth and sporulation at warmer conditions were correlated with an increase in occurrence of multiple germ tube per spore. Such changes might represent a mechanism to compensate for the reduced radial growth as well as sporulation at warmer temperatures. Artificial inoculations of leaves of the susceptible variety Brassica juncea cv. Varuna with A. brassicicola at 35 °C also resulted in larger chlorotic zones than inoculations at cooler temperatures, indicating an increase in virulence of the pathogen possibly due to the enhanced release of extracellular enzymes as a result of reduced melanin production in the cell walls.