A serralysin-like protein of Liberibacter asiaticus modulates components of the bacterial extracellular matrix.

Huanglongbing (HLB), the current major threat for species, is caused by intracellular alphaproteobacteria of the genus Liberibacter (L), with L asiaticus (Las) being the most prevalent species. This bacterium inhabits phloem cells and is transmitted by the psyllid . A gene encoding a putative serralysin-like metalloprotease (CLIBASIA_01345) was identified in the Las genome. The expression levels of this gene were found to be higher in citrus leaves than in psyllids, suggesting a function for this protease in adaptation to the plant environment. Here, we study the putative role of Las-serralysin (Las1345) as virulence factor. We first assayed whether Las1345 could be secreted by two different surrogate bacteria, bv. A34 () and . The protein was detected only in the cellular fraction of and expressing Las1345, and increased protease activity of those bacteria by 2.55 and 4.25-fold, respectively. In contrast, Las1345 expressed in leaves did not show protease activity nor alterations in the cell membrane, suggesting that Las1345 do not function as a protease in the plant cell. Las1345 expression negatively regulated cell motility, exopolysaccharide production, and biofilm formation in pv. (). This bacterial phenotype was correlated with reduced growth and survival on leaf surfaces as well as reduced disease symptoms in and These results support a model where Las1345 could modify extracellular components to adapt bacterial shape and appendages to the phloem environment, thus contributing to virulence.