The tomato (Lycopersicon esculentum) resistance (R) gene Cf-9 is required for resistance to races of the fungal pathogen Cladosporium fulvum expressing the elicitor Avr9 and also confers responsiveness to Avr9 in Cf-9-containing transgenic tobacco (Nicotiana tabacum; Cf9 tobacco). Although
protein phosphorylation is required for many early Avr9/Cf-9-signaling events, so far the only phosphorylation targets known in this race-specific signaling pathway are three
kinases: the two
mitogen-activated protein kinases,
wound-induced
protein kinase and
salicylic acid-induced protein kinase, and the
calcium-dependent protein kinase NtCDPK2. Here, we provide evidence that a tobacco
syntaxin is rapidly and transiently phosphorylated after Avr9 elicitation. The
syntaxin was detected with an antibody against NtSyp121, a plasma membrane-localized
syntaxin implicated in
abscisic acid responses and secretion. Consistent with the gene-for-gene hypothesis,
syntaxin phosphorylation required the presence of both Avr9 and Cf-9. This phosphorylation event occurred either upstream of the pathway leading to
reactive oxygen species production or in a parallel pathway. Interestingly, rapid
syntaxin phosphorylation was triggered by the race-specific elicitor Avr9 but not by flg22(P.aer), a general elicitor capable of inducing other defense-related signaling events in Cf9 tobacco such as
reactive oxygen species production,
mitogen-activated protein kinase activation, and PR5 transcript up-regulation. Furthermore, NtSyp121 transcript levels were increased at 24 h after elicitation with Avr9 but not with flg22(P.aer). Because most other previously described Avr9- and flg22(P.aer)-elicited responses are similar,
syntaxin phosphorylation and NtSyp121 transcript up-regulation may serve as novel early biochemical and late molecular markers, respectively, to elucidate further differences in the signaling responses between these two elicitors.