Research Progress on the Function and Synthesis of Plant Salicylic Acid

水杨酸(Salicylic Acid, SA)作为重要的植物激素，在植物生命活动中扮演关键角色。它广泛参与植物生长发育进程，如调控种子萌发，促使种子在适宜条件下打破休眠、启动萌发；影响根系生长，对根的形态建成和生长速率发挥调节作用；在开花诱导方面，参与植物从营养生长到生殖生长的转变过程，影响花期。同时，SA是植物应对生物和非生物胁迫的核心信号分子。在生物胁迫中，病原菌入侵时，SA能诱导植物产生局部和系统获得性抗性，增强植物对病原菌的防御能力；面对非生物胁迫，如干旱、低温、盐碱时，SA可通过调节植物体内的生理生化过程，提升植物的抗逆性。植物中SA的合成主要有两条途径。异分支酸合成酶(ICS)途径起始于叶绿体，分支酸在ICS1催化下形成异分支酸，经EDS5转运至细胞质，再由PBS3催化生成异分支酸–谷氨酸加合物，进而生成SA。苯丙氨酸解氨酶(PAL)途径则以苯丙氨酸为底物，经反式肉桂酸、苯甲酸等中间产物合成SA，但将苯甲酸转化为SA的部分基因尚未明确。不同植物中，这两条途径对SA合成的贡献存在差异。Salicylic acid, as an important plant hormone, plays a key role in plant life activities. It is widely involved in plant growth and development, such as the regulation of seed germination, to promote the seeds to break dormancy and initiate germination under suitable conditions; affecting root growth, and regulating the morphology and growth rate of roots; in the induction of flowering, it is involved in the transition from nutrient growth to reproductive growth, and affects the flowering period. Meanwhile, SA is a core signaling molecule for plants to cope with biotic and abiotic stresses. In biotic stress, when pathogenic bacteria invade, SA can induce local and systemic acquired resistance and enhance the defense ability of plants against pathogenic bacteria; in the face of abiotic stress, such as drought, low temperature, salinity and alkalinity, SA can enhance the plant’s resilience by regulating the physiological and biochemical processes in the body of the plant. There are two main pathways for SA synthesis in plants. The isobranched acid synthase (ICS) pathway begins in chloroplasts, where branch acids are catalyzed by ICS1 to form isobranched acids, which are transported to the cytoplasm by EDS5 and then catalyzed by PBS3 to form isobranched acid-glutamic acid adducts, which in turn generate SA. The phenylalanine ammonia-lyase (PAL) pathway, on the other hand, synthesizes SA using phenylalanine as a substrate and intermediates, such as trans-cinnamic acid and benzoic acid, but some genes for the conversion of benzoic acid to SA have not yet been developed. The partial genes that convert benzoic acid to SA have not been identified. The contribution of these two pathways to SA synthesis varies among plants.