Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843, USA.
Plant Physiol. 2023 May 2;192(1):527-545. doi: 10.1093/plphys/kiac584.
The hormones salicylic acid (SA) and jasmonic acid (JA) often act antagonistically in controlling plant defense pathways in response to hemibiotrophs/biotrophs (hemi/biotroph) and herbivores/necrotrophs, respectively. Threonine deaminase (TD) converts threonine to α-ketobutyrate and ammonia as the committed step in isoleucine (Ile) biosynthesis and contributes to JA responses by producing the Ile needed to make the bioactive JA-Ile conjugate. Tomato (Solanum lycopersicum) plants have two TD genes: TD1 and TD2. A defensive role for TD2 against herbivores has been characterized in relation to JA-Ile production. However, it remains unknown whether TD2 is also involved in host defense against bacterial hemi/biotrophic and necrotrophic pathogens. Here, we show that in response to the bacterial pathogen-associated molecular pattern (PAMP) flagellin flg22 peptide, an activator of SA-based defense responses, TD2 activity is compromised, possibly through carboxy-terminal cleavage. TD2 knockdown (KD) plants showed increased resistance to the hemibiotrophic bacterial pathogen Pseudomonas syringae but were more susceptible to the necrotrophic fungal pathogen Botrytis cinerea, suggesting TD2 plays opposite roles in response to hemibiotrophic and necrotrophic pathogens. This TD2 KD plant differential response to different pathogens is consistent with SA- and JA-regulated defense gene expression. flg22-treated TD2 KD plants showed high expression levels of SA-responsive genes, whereas TD2 KD plants treated with the fungal PAMP chitin showed low expression levels of JA-responsive genes. This study indicates TD2 acts negatively in defense against hemibiotrophs and positively against necrotrophs and provides insight into a new TD2 function in the elaborate crosstalk between SA and JA signaling induced by pathogen infection.
水杨酸（SA）和茉莉酸（JA）这两种激素通常在控制植物防御途径方面表现出拮抗作用，分别对应半活体/活体（半活体/活体）和食草动物/坏死体（食草动物/坏死体）。苏氨酸脱氨酶（TD）将苏氨酸转化为α-酮丁酸和氨，作为异亮氨酸（Ile）生物合成的关键步骤，并通过产生合成生物活性 JA-Ile 缀合物所需的 Ile 来促进 JA 反应。番茄（Solanum lycopersicum）植物有两个 TD 基因：TD1 和 TD2。TD2 在与 JA-Ile 产生相关的抗食草动物方面发挥了防御作用。然而，尚不清楚 TD2 是否也参与了对细菌半活体/活体和坏死体病原体的宿主防御。在这里，我们表明，在对细菌病原体相关分子模式（PAMP）鞭毛蛋白 flg22 肽（激活 SA 防御反应的物质）的反应中，TD2 活性受到损害，可能是通过羧基末端切割。TD2 敲低（KD）植物对半活体细菌病原体丁香假单胞菌表现出更高的抗性，但对坏死体真菌病原体 Botrytis cinerea 更敏感，这表明 TD2 在对半活体和坏死体病原体的反应中发挥相反的作用。TD2 KD 植物对不同病原体的这种差异反应与 SA 和 JA 调节的防御基因表达一致。用 flg22 处理的 TD2 KD 植物表现出高水平的 SA 响应基因表达，而用真菌 PAMP 几丁质处理的 TD2 KD 植物表现出低水平的 JA 响应基因表达。这项研究表明 TD2 在防御半活体病原体方面起负作用，在防御坏死体病原体方面起正作用，并为研究病原体感染诱导的 SA 和 JA 信号之间复杂的串扰提供了一个新的 TD2 功能。
