CONICET | Buscador de Institutos y Recursos Humanos

Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot of crucifers is a phytopathogen of worldwide economic relevance. Light is an important source of information for many different organisms. In plants and other photosynthetic organisms photoreceptor proteins called phytochromes regulate functions contributing to growth and development. Surprisingly photoreceptors called bacteriophytochromes are also found in non-photosynthetic bacteria, including pathogens, although their role is still poorly understood. The Xcc genome encodes one putative bacteriophytochrome (phy). Our recent results suggest that light modulates biofilm formation through Phy. Furthermore we have established that mutation of phy leads to altered levels of xanthan polysaccharide, which is a virulence factor also involved in biofilm formation. Complementation of the mutant strain with a multicopy plasmid harbouring phy restores these phenotypes. Moreover, the complemented strain produces significantly lower xanthan than the wild-type strain. Endoglucanase assays does not show differences between the wild-type an the mutant strains. However the complemented strain exhibits lower endoglucanase levels, sugesting that Phy is acting as a negative regulator. Our in planta results also show that phy mutant is more virulent that the wild-type strain. This result can be explained in part because the mutant is able to keep the stomata open for much longer times. Callose synthesis studies during infection show that phy mutant has significant reduced levels compared with wild-type strain. Finally, our results show that Xcc Phy is involved in light-sensing activities and it regulates several in vitro phenomena and it is also involved in several in planta phenotypes related to virulence. These work comprise the first evidence of an eubacterial bacteriophitochrome with light-associated phenotypes.