Overproduction of an inducible extracellular serine protease improves biological control of Pythium ultimum by Stenotrophomonas maltophilia strain W81

Abstract

Stenotrophomonas maltophilia W81 can protect sugar beet against Pythiummediated damping-off disease through the production of an extracellular protease. Here, the proteolytic enzyme of W81 was purified by anion-exchange chromatography and characterized as a serine protease. The purified enzyme was fungicidal against Pythium ultimum in vitro. Its synthesis was inducible by casein in W81, and mutagenesis of this strain using the luciferase (luxAB) reporter transposon Tn5-764cd resulted in the isolation of two mutant derivatives (W81M3 and W81M4) capable of producing significantly increased levels of extracellular protease in the presence of casein. Strain W81M4 also exhibited increased chitinolytic activity. The luxAB fusions in strains W81M3 and W81M4 were highly expressed in the absence of casein but not in its presence, suggesting that the corresponding loci were involved in downregulating extracellular protease production. Extracellular protease production in the W81 wild-type strain and protease overproduction in mutants W81M3 and W81M4 were also induced in the presence of the autoclaved fungal mycelium. In soil microcosms naturally infested by Pythium spp., inoculation of sugar beet seeds with W81M3 or W81M4 resulted in improved biocontrol of Pythium-mediated damping-off disease compared with W81, and the level of protection achieved was equivalent to that conferred by chemical fungicides. The wild-type W81 and its mutant derivatives did not differ in rhizosphere colonization. Therefore, the improved biocontrol ability of W81M3 and W81M4 resulted from their capacity to overproduce extracellular serine protease.

Biocontrol potential of salinity tolerant mutants of Trichoderma harzianum against Fusarium oxysporum

“…Overall, F. oxysporum population was minimized in soil rhizosphere together with a marked increase in the plant yield in comparison with the wild isolate. Such levels of plant protection were previously achieved using mutant strains through increasing the production of the extracellular antifungal metabolites (12). Overproduction of antifungal metabolites (enzymes or antibiotics) in Trichoderma mutants resulted in a reduced growth rate of the pathogen in vitro under saline soil conditions.…”

Secondary Metabolites Help Biocontrol Strain Pseudomonas fluorescens CHA0 To Escape Protozoan Grazing

“…It has been reported that the lack of the exoprotease results in reduced biocontrol activity of CHA0 against nematodes (38). The exoprotease and DAPG also seem to complement each other, increasing the biocontrol activity against Pythium (13). It is likely that key membrane proteins, such as those involved in transport or signal transduction, are targeted by the AprA protease and result in abnormal cellular responses.…”

Stenotrophomonas maltophilia: a new potential biocontrol agent of Ralstonia solanacearum, causal agent of potato brown rot

“…Unfortunately R. solanacearum was not included in these tests. Apart from antibiotics, S. maltophilia produces lytic enzymes that have been implicated in biological control activity against fungi , for example chitinase (Zhang and Yuen 1999) and an extracellular proteolytic enzyme (Dunne et al 2000). This last enzyme may also be active against R. solanacearum, as Gram-negative bacteria contain, next to lipopolysaccharide, Braun lipoprotein in their cell walls.…”