Roles of six Hsp70 genes in virulence, cell wall integrity, antioxidant activity and multiple stress tolerance of Beauveria bassiana

Abstract

Functions of most Hsp70-coding genes remain unknown in filamentous fungi. Here, we show important roles of six Hsp70 homologs (Hsp70a-f) in Beauveria bassiana, a filamentous fungal insect pathogen that serves as a main source of wide-spectrum fungal insecticides. In phylogeny, Hsp70b-e, Hsp70g and Hsp70h are distinct from eight other partners that share the same clade with 14 Hsp70 proteins in model yeast. Among the 14 Hsp70-coding genes in B. bassiana, eight (hsp70g/hsp70h/ssa/ssb/ssc/kar/sse/ssz) were individually undeletable perhaps due to an essentiality of each for the fungal viability. Moderate defects in growth and conidiation occurred in the absence of hsp70e or hsp70f (lsh1). The disruption of hsp70f resulted in a reduction of 43% in conidial heat (45 °C) tolerance and of 97% in virulence, and the two reductions decreased to 19–27% and 18–45% in the disruption mutants of hsp70a to hsp70d respectively. Conidial UVB resistance decreased more in the absence of hsp70e (33%) than of hsp70f (25%), hsp70c (19%) or hsp70d (15%). All of these disruption mutants were differentially compromised in both cell wall integrity and antioxidant activity and also in cellular tolerance to Na+, Zn2+, Mn2+, Cu2+ and/or Fe2+. Delayed acidification of submerged cultures and reduced secretion of organic acids occurred only in the absence of hsp70b or hsp70f. Our findings indicate that hsp70a-f are important for both host infection and multiple stress tolerance of B. bassiana, and functionally similar to or beyond the lsh1, ssb and ssz homologs characterized previously in some plant-pathogenic fungi.