A Na⁺ /Ca2⁺ exchanger of the olive pathogen Pseudomonas savastanoi pv. savastanoi is critical for its virulence

In a number of compatible plant‐bacterium interactions, a rise in apoplastic Ca²⁺ levels is observed, suggesting that Ca²⁺ represents an important environmental clue, as reported for bacteria infecting mammalians. We demonstrate that Ca²⁺ entry in Pseudomonas savastanoi pv. savastanoi (Psav ) strain DAPP‐PG 722 is mediated by a Na⁺/Ca²⁺ exchanger critical for virulence. Using the fluorescent Ca²⁺ probe Fura 2‐AM, we demonstrate that Ca²⁺ enters Psav cells foremost when they experience low levels of energy, a situation mimicking the apoplastic fluid. In fact, Ca²⁺ entry was suppressed in the presence of high concentrations of glucose, fructose, sucrose or adenosine triphosphate (ATP). Since Ca²⁺ entry was inhibited by nifedipine and LiCl, we conclude that the channel for Ca²⁺ entry is a Na⁺/Ca²⁺ exchanger. In silico analysis of the Psav DAPP‐PG 722 genome revealed the presence of a single gene coding for a Na⁺/Ca²⁺ exchanger (cneA ), which is a widely conserved and ancestral gene within the P. syringae complex based on gene phylogeny. Mutation of cneA compromised not only Ca²⁺ entry, but also compromised the Hypersensitive response (HR) in tobacco leaves and blocked the ability to induce knots in olive stems. The expression of both pathogenicity (hrpL , hrpA and iaaM ) and virulence (ptz ) genes was reduced in this Psav ‐cneA mutant. Complementation of the Psav ‐cneA mutation restored both Ca²⁺ entry and pathogenicity in olive plants, but failed to restore the HR in tobacco leaves. In conclusion, Ca²⁺ entry acts as a ‘host signal’ that allows and promotes Psav pathogenicity on olive plants.