Functional analysis of 3-Oxacyl-Acyl carrier protein reductase homolog (FabG3) in Xanthomonas arboricola pv. juglandis

Abstract

Most of the pathogenic microorganisms that cause serious production and economic losses for the agricultural industry are members of the genus Xanthomonas. The main bacterial pathogen causing walnut black spot is Xanthomonas arboricola pv. juglandis (Xaj). Members of the genus Xanthomonas produce a yellow pigment called xanthomonadin, which is membrane-bound and protects the bacteria against photobiological and peroxidative damage. Here, we report the characterization of a gene named XajfabG3 (a homologous gene of fabG3 in Xanthomonas campestris pv. campestris), demonstrating that it is responsible for xanthomonadin synthesis. Expression of XajfabG3 in Escherichia coli fabG temperature-sensitive mutant CL104 restored the growth of CL104 under nonpermissive temperature, although XajFabG3 had a lower activity than XajFabG1 or FabG from Escherichia coli. In vitro assays demonstrated that XajFabG3 could reduce 3-oxoacyl-acyl carrier proteins (-ACPs) of fatty acid synthesis reactions and showed that although deletion of XajfabG3 did not affect bacterial growth and fatty acid composition, xanthomonadin production was abolished in a XajfabG3-deficient strain. Xanthomonadin production could be restored only by wild-type fabG3 and not by XajfabG1, another 3-oxoacyl-ACP reductase-encoding gene in Xaj, indicating that XajFabG3 specifically participates in the biosynthesis of xanthomonadin. Moreover, our study showed that the XajfabG3 knockout mutant affected virulence in the host Juglans regia L. Our elucidation of the biological function of the fabG3 gene in Xaj provides a target for the development of prevention and treatment agents for walnut blight.