Ministry of Agriculture, Key Laboratory of Horticultural Crop Biology and Germplasm Innovation, Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing, People's Republic of China.
Planta. 2020 Oct 7;252(5):75. doi: 10.1007/s00425-020-03467-2.
Exogenous SA treatment at appropriate concentrations promotes adventitious root formation in cucumber hypocotyls, via competitive inhibiting the IAA-Asp synthetase activity of CsGH3.5, and increasing the local free IAA level. Adventitious root formation is critical for the cutting propagation of horticultural plants. Indole-3-acetic acid (IAA) has been shown to play a central role in regulating this process, while for salicylic acid (SA), its exact effects and regulatory mechanism have not been elucidated. In this study, we showed that exogenous SA treatment at the concentrations of both 50 and 100 µM promoted adventitious root formation at the base of the hypocotyl of cucumber seedlings. At these concentrations, SA could induce the expression of CYCLIN and Cyclin-dependent Kinase (CDK) genes during adventitious rooting. IAA was shown to be involved in SA-induced adventitious root formation in cucumber hypocotyls. Exposure to exogenous SA led to a slight increase in the free IAA content, and pre-treatment with the auxin transport inhibitor 1-naphthylphthalamic acid (NPA) almost completely abolished the inducible effects of SA on adventitious root number. SA-induced IAA accumulation was also associated with the enhanced expression of Gretchen Hagen3.5 (CsGH3.5). The in vitro enzymatic assay indicated that CsGH3.5 has both IAA- and SA-amido synthetase activity and prefers aspartate (Asp) as the amino acid conjugate. The Asp concentration dictated the functional activity of CsGH3.5 on IAA. Both affinity and catalytic efficiency (K/K) increased when the Asp concentration increased from 0.3 to 1 mM. In contrast, CsGH3.5 showed equal catalytic efficiency for SA at low and high Asp concentrations. Furthermore, SA functioned as a competitive inhibitor of the IAA-Asp synthetase activity of CsGH3.5. During adventitious formation, SA application indeed repressed the IAA-Asp levels in the rooting zone. These data show that SA plays an inducible role in adventitious root formation in cucumber through competitive inhibition of the auxin conjugation enzyme CsGH3.5. SA reduces the IAA conjugate levels, thereby increasing the local free IAA level and ultimately enhancing adventitious root formation.
外源水杨酸（SA）以适当的浓度处理可以通过竞争性抑制 CsGH3.5 的吲哚-3-乙酸（IAA）-天冬氨酸合成酶的活性，以及增加局部游离 IAA 水平，从而促进黄瓜下胚轴不定根的形成。不定根的形成对于园艺植物的切割繁殖至关重要。已经表明，吲哚-3-乙酸（IAA）在调节这个过程中起着核心作用，而对于水杨酸（SA），其确切作用和调控机制尚未阐明。在这项研究中，我们表明，50 和 100 µM 的外源 SA 处理浓度可以促进黄瓜幼苗下胚轴基部不定根的形成。在这些浓度下，SA 可以诱导 CYCLIN 和细胞周期蛋白依赖性激酶（CDK）基因在不定根形成过程中的表达。表明 IAA 参与了黄瓜下胚轴中 SA 诱导的不定根形成。暴露于外源 SA 导致游离 IAA 含量略有增加，而生长素运输抑制剂 1-萘基邻氨甲酰苯甲酸（NPA）的预处理几乎完全消除了 SA 对不定根数量的诱导作用。SA 诱导的 IAA 积累也与 Gretchen Hagen3.5（CsGH3.5）的增强表达有关。体外酶促测定表明，CsGH3.5 具有 IAA 和 SA 酰胺合成酶的活性，并且偏爱天冬氨酸（Asp）作为氨基酸共轭物。Asp 浓度决定了 CsGH3.5 对 IAA 的功能活性。当 Asp 浓度从 0.3 增加到 1 mM 时，亲和力和催化效率（K/K）都增加。相比之下，CsGH3.5 在低和高 Asp 浓度下对 SA 具有相同的催化效率。此外，SA 作为 CsGH3.5 的 IAA-Asp 合成酶活性的竞争性抑制剂起作用。在不定根形成过程中，SA 的应用确实抑制了生根区的 IAA-Asp 水平。这些数据表明，SA 通过竞争性抑制生长素结合酶 CsGH3.5，在黄瓜的不定根形成中发挥诱导作用。SA 降低 IAA 结合物的水平，从而增加局部游离 IAA 水平，最终增强不定根的形成。
