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

Chiaraluce Moretti,Alberto Marco Del Pino,Giulia Devescovi,Simone Trabalza,Letizia Granieri,Harrold A Van Den Burg,Cayo Ramos,Eloy Caballo‐Ponce,Roberto Buonaurio,Carlo Alberto Palmerini,Vittorio Venturi

doi:10.1111/mpp.12787

Abstract

SummaryIn 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.

Highlights

Cytosolic calcium (Ca2+) has essential functions in eukaryotic signalling as secondary messenger
Ca2+ entry in pv. savastanoi (Psav) cells is promoted under starvation conditions and is not influenced by exogenous indole3-acetic acid
Since indole-3-acetic acid (IAA) is produced by Psav to stimulate plant cell proliferation and knot formation, we investigated whether IAA or its precursor (L-tryptophan) influences Ca2+ entry

Summary

Introduction

Cytosolic calcium (Ca2+) has essential functions in eukaryotic signalling as secondary messenger. Ca2+ acts both as a messenger and cofactor to coordinate many intracellular signalling pathways (Rajagopal and Ponnusamy, 2017) It can already activate different cellular responses only by differences in the amplitude, frequency and duration of the intracellular Ca2+ concentration (Rajagopal and Ponnusamy, 2017). Located predominantly in the extracellular environment, Ca2+ entry relies in animals on membrane depolarization resulting from action potentials, where it can perform its regulatory functions In these eukaryotes, most ion channels as well as transporters, pumps, binding proteins and L-type voltage-dependent calcium channels have the capacity to transport Ca2+ across the depolarized membrane (Cai and Lytton, 2004; Carafoli, 1987; Norris et al, 1996; Tsien and Tsien, 1990). Ca2+ signalling plays an essential role in pollen tube elongation, seed germination, hyperosmotic and oxidative stresses (Sanders et al, 1999; White and Broadley, 2003)

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