Xanthomonas citri subsp. citri (
Xcc) is a bacterial pathogen that causes citrus canker in susceptible Citrus spp. The
Xcc genome contains genes encoding
enzymes from three separate pathways of
trehalose biosynthesis. Expression of genes encoding
trehalose-6-phosphate synthase (otsA) and
trehalose phosphatase (otsB) was highly induced during canker development, suggesting that the two-step pathway of
trehalose biosynthesis via
trehalose-6-phosphate has a function in pathogenesis. This pathway was eliminated from the bacterium by deletion of the otsA gene. The resulting XccΔotsA mutant produced less
trehalose than the wild-type strain, was less resistant to
salt and oxidative stresses, and was less able to colonize plant tissues. Gene expression and proteomic analyses of infected leaves showed that
infection with XccΔotsA triggered only weak defence responses in the plant compared with
infection with
Xcc, and had less impact on the host plant's metabolism than the wild-type strain. These results suggested that
trehalose of bacterial origin, synthesized via the otsA-otsB pathway, in
Xcc, plays a role in modifying the host plant's metabolism to its own advantage but is also perceived by the plant as a sign of pathogen attack. Thus,
trehalose biosynthesis has both positive and negative consequences for
Xcc. On the one hand, it enables this bacterial pathogen to survive in the inhospitable environment of the leaf surface before
infection and exploit the host plant's resources after
infection, but on the other hand, it is a tell-tale sign of the pathogen's presence that triggers the plant to defend itself against
infection.