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Integrated Signaling Network Involving Calcium, Nitric Oxide, and Active Oxygen Species but Not Mitogen-Activated Protein Kinases in BcPG1-Elicited Grapevine Defenses

We have already reported the identification of the endopolygalacturonase 1 (BcPG1) from Botrytis cinerea as a potent elicitor of defense responses in grapevine, independently of its enzymatic activity. The aim of the present study is the analysis of the signaling pathways triggered by BcPG1 in grapevine cells. Our data indicate that BcPG1 induces a Ca²⁺ entry from the apoplasm, which triggers a phosphorylation-dependent nitric oxide (NO) production via an enzyme probably related to a NO synthase. Then NO is involved in i) cytosolic calcium homeostasis, by activating Ca²⁺ release from internal stores and regulating Ca²⁺ fluxes across the plasma membrane, ii) plasma membrane potential variation, iii) the activation of active oxygen species (AOS) production, and iv) defense gene expression, including phenylalanine ammonia lyase and stilbene synthase, which encode enzymes responsible for phytoalexin biosynthesis. Interestingly enough, mitogen-activated protein kinase (MAPK) activation is independent of this regulation pathway that closely connects Ca²⁺, NO, and AOS.