Isoleucic acid (ILA), a branched-chain amino acid-related 2-hydroxycarboxylic acid, occurs ubiquitously in plants. It enhances pathogen resistance and inhibits root growth of Arabidopsis. The salicylic acid (SA) glucosyltransferase UGT76B1 is able to conjugate ILA. Here, we investigate the role of ILA in planta in Arabidopsis and reveal a triad of distinct responses to this small molecule. ILA synergistically co-operates with SA to activate SA-responsive gene expression and resistance in a UGT76B1-dependent manner in agreement with the observed competitive ILA-dependent repression of SA glucosylation by UGT76B1. However, ILA also shows an SA-independent stress response. Nitroblue tetrazolium staining and pharmacological experiments indicate that ILA induces superoxide formation of the wild type and of an SA-deficient (NahG sid2) line. In contrast, the inhibitory effect of ILA on root growth is independent of both SA and superoxide induction. These effects of ILA are specific and distinct from its isomeric compound leucic acid and from the amino acid isoleucine. Leucic acid and isoleucine do not induce expression of defense marker genes or superoxide production, whereas both compounds inhibit root growth. All three responses to ILA are also observed in Brassica napus.

中文翻译：

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异亮氨酸三联体：对植物防御，根系生长和活性氧形成的独特影响。

异氰酸（ILA）是一种支链氨基酸相关的2-羟基羧酸，广泛存在于植物中。它增强了病原体的抗性并抑制了拟南芥的根生长。水杨酸（SA）葡萄糖基转移酶UGT76B1能够缀合ILA。在这里，我们调查了ILA在植物中的作用并在拟南芥中显示出对这个小分子的独特反应。ILA与SA协同合作，以UGT76B1依赖性方式激活SA应答基因表达和耐药性，这与观察到的UGT76B1竞争性ILA依赖性SA糖基化抑制作用相一致。但是，ILA也显示出独立于SA的应激反应。氮蓝四唑染色和药理实验表明，ILA会诱导野生型和SA缺陷型（NahG sid2）行。相反，ILA对根生长的抑制作用不依赖于SA和超氧化物诱导。ILA的这些作用是特异的，不同于其同分异构的化合物亮氨酸和氨基酸异亮氨酸。白氨酸和异亮氨酸不诱导防御标记基因的表达或超氧化物的产生，而两种化合物均抑制根的生长。在甘蓝型油菜中也观察到对ILA的三种反应。