Ethylene‐dependent salicylic acid regulates an expanded cell death response to a plant pathogen

Summary The molecular events associated with susceptible plant responses to disease‐causing organisms are not well understood. We have previously shown that ethylene‐insensitive tomato plants infected with Xanthomonas campestris pv. vesicatoria have greatly reduced disease symptoms relative to wild‐type cultivars. Here we show that salicylic acid (SA) is also an important component of the susceptible disease response. SA accumulates in infected wild‐type tissues and is correlated with necrosis but does not accumulate in ethylene‐insensitive plants. Exogenous feeding of SA to ethylene‐deficient plants restores necrosis, indicating that reduced disease symptoms are associated with failure to accumulate SA. These results indicate a mechanism for co‐ordination of phytohormone signals that together constitute a susceptible response to pathogens.