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The plant hormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) play crucial roles in the signaling network that regulates induced defense responses against biotic stresses. Antagonism between SA and JA operates as a mechanism to finetune defenses that are activated in response to multiple attackers. In Arabidopsis (Arabidopsis thaliana), NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) was demonstrated to be required for SA-mediated suppression of JA-dependent defenses. Because ET is known to enhance SA/NPR1-dependent defense responses, we investigated the role of ET in the SA-JA signal interaction. Pharmacological experiments with gaseous ET and the ET precursor 1-aminocyclopropane-1-carboxylic acid showed that ET potentiated SA/NPR1-dependent PATHOGENESIS-RELATED1 transcription, while it rendered the antagonistic effect of SA on methyl jasmonate-induced PDF1.2 and VSP2 expression NPR1 independent. This overriding effect of ET on NPR1 function in SA-JA cross talk was absent in the npr1-1/ein2-1 double mutant, demonstrating that it is mediated via ET signaling. Abiotic and biotic induction of the ET response similarly abolished the NPR1 dependency of the SA-JA signal interaction. Furthermore, JA-dependent resistance against biotic attackers was antagonized by SA in an NPR1-dependent fashion only when the plant-attacker combination did not result in the production of high levels of endogenous ET. Hence, the interaction between ET and NPR1 plays an important modulating role in the fine tuning of the defense signaling network that is activated upon pathogen and insect attack. Our results suggest a model in which ET modulates the NPR1 dependency of SA-JA antagonism, possibly to compensate for enhanced allocation of NPR1 to function in SA-dependent activation of PR genes.

Durante o processo evolucionário, as plantas adquiriram uma sofisticada estratégia defensiva para ´perceber´ os ataques de patógenos e insetos, traduzindo essa percepção em uma resposta apropriada e de forma adaptativa. A imunidade inata da planta é baseada surpreendemente na resposta complexa que é altamente flexível nas suas capacidades de reconhecer e responder aos mais diversos invasores. Já faz muito tempo que a planta Arabidopsis tem sido utilizada como modelo para estudos moleculares de resistência sistêmica induzida por rizobactérias (RSI). Atualmente, nossa pesquisa é focada em uma questão: Como as plantas são capazes de integrar sinais induzidos por insetos e microorganismos em respostas específicas contra esses invasores? Os sinais de alarmes de ataque, ácido salicílico, jasmônico e etileno são os principais reguladores da defesa das plantas. As rotas de sinalização se intercalam, fornecendo as plantas um grande potencial de regulação para uma defesa refinada. Recentemente, nós descobrimos que a proteína NPR1 funciona com um modulador da comunicação entre as rotas de sinalização do ácido salicílico e do ácido jasmônico, e desse modo, ajuda a planta a ´decidir´ qual estratégia de defesa tomar, dependendo do tipo de invasor encontrado. Aqui, nós apresentamos uma revisão de nossa pesquisa nos últimos anos.

Plant defenses against pathogens and insects are regulated differentially by cross-communicating signal transduction pathways in which salicylic acid (SA) and jasmonic acid (JA) play key roles. In this study, we investigated the molecular mechanism of the antagonistic effect of SA on JA signaling. Arabidopsis plants unable to accumulate SA produced 25-fold higher levels of JA and showed enhanced expression of the JA-responsive genes LOX2, PDF1.2, and VSP in response to infection by Pseudomonas syringae pv tomato DC3000, indicating that in wild-type plants, pathogen-induced SA accumulation is associated with the suppression of JA signaling. Analysis of the Arabidopsis mutant npr1, which is impaired in SA signal transduction, revealed that the antagonistic effect of SA on JA signaling requires the regulatory protein NPR1. Nuclear localization of NPR1, which is essential for SA-mediated defense gene expression, is not required for the suppression of JA signaling, indicating that cross-talk between SA and JA is modulated through a novel function of NPR1 in the cytosol.

Plants possess inducible defense mechanisms to effectively combat invasion by microbial pathogens or attack by herbivorous insects. Research on defense signaling pathways revealed that induced defenses against pathogens and herbivores are regulated by a network of interconnecting signaling pathways in which the plant signal molecules salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) play a dominant role. In many cases, attack by pathogens or herbivores is associated with enhanced production of these hormones and a concomitant activation of distinct sets of defense-related genes. Moreover, exogenous application of SA, JA or ET often results in an enhanced level of resistance.