The GDSL-Lipolytic Enzyme Lip1 Is Required for Full Virulence of the Cucurbit Pathogenic Bacterium Acidovorax citrulli .

Bacterial fruit blotch caused by Acidovorax citrulli is a serious disease of cucurbit crops. Here we report characterization of a mutant strain of A. citrulli M6 defective in lip1 , a gene encoding a lipolytic enzyme. The M6- lip1 - mutant was detected in a mutant library screen aimed at identifying M6 mutants with altered levels of twitching motility. In this screen M6- lip1 - was the only mutant that showed significantly larger twitching motility haloes around colonies than wild-type M6. Sequence analyses indicated that lip1 encodes a member of the GDSL family of secreted lipolytic enzymes. In line with this finding, lipolytic assays showed that the supernatants of M6- lip1 - had lower lipolytic activity as compared with those of wild-type M6 and a lip1 -complemented strain. The mutant was also affected in swimming motility and had compromised virulence on melon seedlings and on Nicotiana benthamiana leaves relative to wild-type and complemented strains. Lip1 contains a predicted N-terminal signal sequence for type II secretion. Evidence from our study confirms Lip1 is indeed secreted in a type II secretion-dependent manner, and this is required for full virulence of A. citrulli . To the best of our knowledge this is the first study reporting contribution of lipolytic activity to virulence of a plant-pathogenic Acidovorax species.