Ca2+ transport in Pseudomonas savastanoi pv: savastanoi, the causal agent of olive knot disease

Pseudomonas savastanoi pv. savastanoi (Psv) has the capacity to leave as epiphyte on phyllosphere and to penetrate host tissues through wounds. Once inside plant tissue, Psv colonize the apoplast and, due to his ability to produce auxins and cytokinins, give raise to knots formation. Little is known about the molecular signals that are perceived in the apoplast by the plant pathogenic bacteria, which probably are involved in the adaptation to this environment very poor in nutrients. Through the use of fluorescent probes FURA 2AM we observed that Psv (strain DAPP-PG 722) communicates in vitro with the extracellular medium through the variations of the cytosolic Ca2+ levels, an important secondary messenger involved in various cellular activities and in plant-pathogen interaction. Preliminary results indicate a relationship between a possible state of energy deficiency in the cells and Ca2+ signals. In fact, when the bacterial cells were incubated in the absence of glucose, fructose, and sorbitol, which are present at very low concentrations in the apoplast, we observed an increase in the cytosolic Ca2+ due to the entry of this ion from the extracellular medium. We have also demonstrated that the channel for the entry of extracellular Ca2+ in the bacterium is: i) a Na+/Ca2+ exchanger, since it is inhibited by Li+ ions employed in place of Na+; ii) antagonized by nifedipine, inhibitor of the L-voltage channels responsible for the entry of the extracellular Ca2+ in mammals. In silico analysis of the genome of Psv strain DAPP-PG 722 revealed the presence of a gene coding for a Na+/Ca2+ exchanger, belonging to the Na+/Ca2+ exchanger family protein integrated membrane. Studies are in progress to obtain and characterise Psv mutants unable to produce such Na+/Ca2+ exchanger, to verify if the mutation affects the virulence of the bacterium.