Salicylic acid (SA) plays an important role in plant immune response, including resistance to pathogens and systemic acquired resistance. Two major components, NONEXPRESSOR OF PATHOGENESIS-RELATED GENES (NPRs) and TGACG motif-binding transcription factors (TGAs), are known to mediate SA signaling, which might also be orchestrated by other hormonal and environmental changes. Nevertheless, the molecular and functional interactions between SA signaling components and other cellular signaling pathways remain poorly understood. Here we showed that the steroid plant hormone brassinosteroid (BR) promotes SA responses by inactivating BR-INSENSITIVE 2 (BIN2), which inhibits the redox-sensitive clade I TGAs in Arabidopsis. We found that both BR and the BIN2 inhibitor bikinin synergistically increase SA-mediated physiological responses, such as resistance to Pst DC3000. Our genetic and biochemical analyses indicated that BIN2 functionally interacts with TGA1 and TGA4, but not with other TGAs. We further demonstrated that BIN2 phosphorylates Ser-202 of TGA4, resulting in the suppression of the redox-dependent interaction between TGA4 and NPR1 as well as destabilization of TGA4. Consistently, transgenic Arabidopsis overexpressing TGA4-YFP with a S202A mutation displayed enhanced SA responses compared to the wild-type TGA4-YFP plants. Taken together, these results suggest a novel crosstalk mechanism by which BR signaling coordinates the SA responses mediated by redox-sensitive clade I TGAs.

水杨酸（SA）在植物免疫反应中发挥重要作用，包括对病原体的抗性和系统获得性抗性。已知两个主要成分，即发病相关基因的非表达因子 (NPR) 和 TGACG 基序结合转录因子 (TGA)，可介导 SA 信号传导，这也可能与其他激素和环境变化有关。然而，SA 信号成分与其他细胞信号通路之间的分子和功能相互作用仍然知之甚少。在这里，我们发现类固醇植物激素油菜素内酯 (BR) 通过灭活 BR-INSENSITIVE 2 (BIN2) 来促进 SA 反应，BR-INSENSITIVE 2 (BIN2) 抑制了拟南芥中氧化还原敏感的进化枝 I TGA. 我们发现 BR 和 BIN2 抑制剂 bikinin 协同增加 SA 介导的生理反应，例如对Pst DC3000 的抗性。我们的遗传和生化分析表明 BIN2 在功能上与 TGA1 和 TGA4 相互作用，但与其他 TGA 不相互作用。我们进一步证明 BIN2 磷酸化 TGA4 的 Ser-202，导致 TGA4 和 NPR1 之间的氧化还原依赖性相互作用受到抑制以及 TGA4 的不稳定。一致地，与野生型TGA4-YFP相比，过表达具有 S202A 突变的TGA4-YFP的转基因拟南芥显示出增强的 SA 反应植物。总之，这些结果表明了一种新的串扰机制，BR 信号通过该机制协调由氧化还原敏感的进化枝 I TGA 介导的 SA 反应。