We have examined molecular and physiological principles underlying the light dependency of defense activation in Arabidopsis (Arabidopsis thaliana) plants challenged with the bacterial pathogen Pseudomonas syringae. Within a fixed light/dark cycle, plant defense responses and disease resistance significantly depend on the time of day when pathogen contact takes place. Morning and midday inoculations result in higher salicylic acid accumulation, faster expression of pathogenesis-related genes, and a more pronounced hypersensitive response than inoculations in the evening or at night. Rather than to the plants' circadian rhythm, this increased plant defense capability upon day inoculations is attributable to the availability of a prolonged light period during the early plant-pathogen interaction. Moreover, pathogen responses of Arabidopsis double mutants affected in light perception, i.e. cryptochrome1cryptochrome2 (cry1cry2), phototropin1phototropin2 (phot1phot2), and phytochromeAphytochromeB (phyAphyB) were assessed. Induction of defense responses by either avirulent or virulent P. syringae at inoculation sites is relatively robust in leaves of photoreceptor mutants, indicating little cross talk between local defense and light signaling. In addition, the blue-light receptor mutants cry1cry2 and phot1phot2 are both capable of establishing a full systemic acquired resistance (SAR) response. Induction of SAR and salicylic-acid-dependent systemic defense reactions, however, are compromised in phyAphyB mutants. Phytochrome regulation of SAR involves the essential SAR component FLAVIN-DEPENDENT MONOOXYGENASE1. Our findings highlight the importance of phytochrome photoperception during systemic rather than local resistance induction. The phytochrome system seems to accommodate the supply of light energy to the energetically costly increase in whole plant resistance.

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.

Cytokinins are phytohormones that are involved in various regulatory processes throughout plant development, but they are also produced by pathogens and known to modulate plant immunity. A novel transgenic approach enabling autoregulated cytokinin synthesis in response to pathogen infection showed that cytokinins mediate enhanced resistance against the virulent hemibiotrophic pathogen Pseudomonas syringae pv tabaci. This was confirmed by two additional independent transgenic approaches to increase endogenous cytokinin production and by exogenous supply of adenine-and phenylurea-derived cytokinins. The cytokinin-mediated resistance strongly correlated with an increased level of bactericidal activities and up-regulated synthesis of the two major antimicrobial phytoalexins in tobacco (Nicotiana tabacum), scopoletin and capsidiol. The key role of these phytoalexins in the underlying mechanism was functionally proven by the finding that scopoletin and capsidiol substitute in planta for the cytokinin signal: phytoalexin pretreatment increased resistance against P. syringae. In contrast to a cytokinin defense mechanism in Arabidopsis (Arabidopsis thaliana) based on salicylic acid-dependent transcriptional control, the cytokininmediated resistance in tobacco is essentially independent from salicylic acid and differs in pathogen specificity. It is also independent of jasmonate levels, reactive oxygen species, and high sugar resistance. The novel function of cytokinins in the primary defense response of solanaceous plant species is rather mediated through a high phytoalexin-pathogen ratio in the early phase of infection, which efficiently restricts pathogen growth. The implications of this mechanism for the coevolution of host plants and cytokinin-producing pathogens and the practical application in agriculture are discussed.

SUMMARYUpon inoculation with pathogenic microbes, plants induce an array of metabolic changes that potentially contribute to induced resistance or even enhance susceptibility. When analysing leaf lipid composition during the Arabidopsis thaliana-Pseudomonas syringae interaction, we found that accumulation of the phytosterol stigmasterol is a significant plant metabolic process that occurs upon bacterial leaf infection. Stigmasterol is synthesized from b-sitosterol by the cytochrome P450 CYP710A1 via C22 desaturation. Arabidopsis cyp710A1 mutant lines impaired in pathogen-inducible expression of the C22 desaturase and concomitant stigmasterol accumulation are more resistant to both avirulent and virulent P. syringae strains than wild-type plants, and exogenous application of stigmasterol attenuates this resistance phenotype. These data indicate that induced sterol desaturation in wild-type plants favours pathogen multiplication and plant susceptibility. Stigmasterol formation is triggered through perception of pathogen-associated molecular patterns such as flagellin and lipopolysaccharides, and through production of reactive oxygen species, but does not depend on the salicylic acid, jasmonic acid or ethylene defence pathways. Isolated microsomal and plasma membrane preparations exhibited a similar increase in the stigmasterol/b-sitosterol ratio as whole-leaf extracts after leaf inoculation with P. syringae, indicating that the stigmasterol produced is incorporated into plant membranes. The increased contents of stigmasterol in leaves after pathogen attack do not influence salicylic acid-mediated defence signalling but attenuate pathogen-induced expression of the defence regulator flavin-dependent monooxygenase 1. P. syringae thus promotes plant disease susceptibility through stimulation of sterol C22 desaturation in leaves, which increases the stigmasterol to b-sitosterol ratio in plant membranes.