Signals are exchanged at all stages of the arbuscular mycorrhizal (AM) symbiosis between fungi and their host plants. Root-exuded strigolactones are well-known early symbiotic cues, but the role of other phytohormones as interkingdom signals has seldom been investigated. Here we focus on ethylene and cytokinins, for which candidate receptors have been identified in the genome of the AM fungus Rhizophagus irregularis. Ethylene is known from the literature to affect asymbiotic development of AM fungi, and in the present study, we found that three cytokinin forms could stimulate spore germination in R. irregularis. Heterologous complementation of a Saccharomyces cerevisiae mutant strain with the candidate ethylene receptor RiHHK6 suggested that this protein can sense and transduce an ethylene signal. Accordingly, its N-terminal domain expressed in Pichia pastoris displayed saturable binding to radiolabeled ethylene. Thus, RiHHK6 displays the expected characteristics of an ethylene receptor. In contrast, the candidate cytokinin receptor RiHHK7 did not complement the S. cerevisiae mutant strain or Medicago truncatula cytokinin receptor mutants and seemed unable to bind cytokinins, suggesting that another receptor is involved in the perception of these phytohormones. Taken together, our results support the hypothesis that AM fungi respond to a range of phytohormones and that these compounds bear multiple functions in the rhizosphere beyond their known roles as internal plant developmental regulators. Our analysis of two phytohormone receptor candidates also sheds new light on the possible perception mechanisms in AM fungi. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

中文翻译：

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丛枝菌根真菌的组氨酸激酶是否参与对乙烯和细胞分裂素的反应？

真菌与其寄主植物之间的丛枝菌根（AM）共生的所有阶段都会交换信号。根部分泌的独脚金内酯是众所周知的早期共生信号，但其他植物激素作为界间信号的作用很少被研究。在这里，我们重点关注乙烯和细胞分裂素，它们的候选受体已在 AM 真菌不规则根噬菌的基因组中被发现。从文献中已知乙烯会影响 AM 真菌的非共生发育，在本研究中，我们发现三种细胞分裂素形式可以刺激不规则红霉的孢子萌发。酿酒酵母突变株与候选乙烯受体 RiHHK6 的异源互补表明该蛋白可以感知并转导乙烯信号。因此，其在毕赤酵母中表达的 N 端结构域表现出与放射性标记乙烯的可饱和结合。因此，RiHHK6 显示出乙烯受体的预期特征。相比之下，候选细胞分裂素受体RiHHK7不与酿酒酵母突变株或蒺藜苜蓿细胞分裂素受体突变体互补，并且似乎无法结合细胞分裂素，这表明另一种受体参与了这些植物激素的感知。总而言之，我们的结果支持这样的假设：AM 真菌对一系列植物激素做出反应，并且这些化合物在根际中具有多种功能，超出了其作为植物内部发育调节剂的已知作用。我们对两种候选植物激素受体的分析也为 AM 真菌可能的感知机制提供了新的线索。[公式：见文字] 版权所有 © 2023 作者。这是一篇根据 CC BY-NC-ND 4.0 国际许可证分发的开放获取文章。