Salicylic acid (SA) is an important phytohormone mediating both local and systemic defense responses in plants. Despite over half a century of research, how plants biosynthesize SA remains unresolved. In Arabidopsis, a major part of SA is derived from isochorismate, a key intermediate produced by the isochorismate synthase, which is reminiscent of SA biosynthesis in bacteria. Whereas bacteria employ an isochorismate pyruvate lyase (IPL) that catalyzes the turnover of isochorismate to pyruvate and SA, plants do not contain an IPL ortholog and generate SA from isochorismate through an unknown mechanism. Combining genetic and biochemical approaches, we delineated the SA biosynthetic pathway downstream of isochorismate in Arabidopsis. We found that PBS3, a GH3 acyl adenylase-family enzyme important for SA accumulation, catalyzes ATP- and Mg²⁺-dependent conjugation of L-glutamate primarily to the 8-carboxyl of isochorismate and yields the key SA biosynthetic intermediate, isochorismoyl-glutamate A. Moreover, we discovered that EPS1, a BAHD acyltransferase-family protein with a previously implicated role in SA accumulation upon pathogen attack, harbors a noncanonical active site and an unprecedented isochorismoyl-glutamate A pyruvoyl-glutamate lyase activity that produces SA from the isochorismoyl-glutamate A substrate. Together, PBS3 and EPS1 form a two-step metabolic pathway to produce SA from isochorismate in Arabidopsis, which is distinct from how SA is biosynthesized in bacteria. This study closes a major knowledge gap in plant SA metabolism and would help develop new strategies for engineering disease resistance in crop plants.

水杨酸（SA）是一种重要的植物激素，可调节植物的局部和全身防御反应。尽管进行了半个多世纪的研究，植物如何生物合成SA仍未解决。在拟南芥中，SA的主要部分来自等渗酸，这是一种由等渗酸合酶产生的关键中间体，使人想起细菌中SA的生物合成。细菌使用等渗酸丙酮酸裂解酶（IPL）来催化等渗酸转化为丙酮酸和SA，而植物不含IPL直系同源物，并通过未知机制从等渗酸中生成SA。结合遗传和生化方法，我们描绘了拟南芥中等渗油的SA生物合成途径。。我们发现PBS3是一种对SA积累很重要的GH3酰基腺苷酶家族酶，催化L-谷氨酸的ATP-和Mg ²⁺依赖性结合主要是异麦草酸酯的8-羧基，并产生了关键的SA生物合成中间体，异鞘氨酰基-谷氨酸酯答：此外，我们发现EPS1是一种BAHD酰基转移酶家族蛋白，在病原体侵袭时与SA积累有关，它具有非规范的活性位点和空前的异麦芽糖氨酸-谷氨酸酯。 -谷氨酸盐底物。PBS3和EPS1共同形成了一个两步代谢途径，从拟南芥中的等渗油中产生SA，这与SA在细菌中的生物合成方式不同。这项研究弥补了植物SA代谢方面的主要知识空白，将有助于开发新的策略来改造农作物的抗病性。