| Summary: | Bacterial fruit blotch caused by <i>Acidovorax</i><i>citrulli</i> is a serious disease of cucurbit crops. Here we report characterization of a mutant strain of <i>A. citrulli</i> M6 defective in <i>lip1</i>, a gene encoding a lipolytic enzyme. The M6-<i>lip1<sup>-</sup></i> mutant was detected in a mutant library screen aimed at identifying M6 mutants with altered levels of twitching motility. In this screen M6-<i>lip1<sup>-</sup></i> was the only mutant that showed significantly larger twitching motility haloes around colonies than wild-type M6. Sequence analyses indicated that <i>lip1</i> encodes a member of the GDSL family of secreted lipolytic enzymes. In line with this finding, lipolytic assays showed that the supernatants of M6-<i>lip1<sup>-</sup></i> had lower lipolytic activity as compared with those of wild-type M6 and a <i>lip1</i>-complemented strain. The mutant was also affected in swimming motility and had compromised virulence on melon seedlings and on <i>Nicotiana benthamiana</i> 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 <i>A. citrulli</i>. To the best of our knowledge this is the first study reporting contribution of lipolytic activity to virulence of a plant-pathogenic <i>Acidovorax</i> species.
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