Phytohormonal interactions are crucial for plant development. Auxin and cytokinin (CK) both play critical roles in regulating plant growth and development; however, the interaction between these two phytohormones is complex and not fully understood. Here, we isolated a wild apple (Malus sieversii Roem) GRETCHEN HAGEN3 (GH3 ) gene, MsGH3.5 , encoding an indole‐3‐acetic acid (IAA)‐amido synthetase. Overexpression of MsGH3.5 significantly reduced the free IAA content and increased the content of some IAA‐amino acid conjugates, and MsGH3.5 ‐overexpressing lines were dwarfed and produced fewer adventitious roots (ARs) than the control. This phenotype is consistent with the role of GH3 in conjugating excess free active IAA to amino acids in auxin homeostasis. Surprisingly, overexpression of MsGH3.5 significantly increased CK concentrations in the whole plant, and altered the expression of genes involved in CK biosynthesis, metabolism and signaling. Furthermore, exogenous CK application induced MsGH3.5 expression through the activity of the CK type‐B response regulator, MsRR1a, which mediates the CK primary response. MsRR1a activated MsGH3.5 expression by directly binding to its promoter, linking auxin and CK signaling. Plants overexpressing MsRR1a also displayed fewer ARs, in agreement with the regulation of MsGH3.5 expression by MsRR1a . Taken together, we reveal that MsGH3.5 affects apple growth and development by modulating auxin and CK levels and signaling pathways. These findings provide insight into the interaction between the auxin and CK pathways, and might have substantial implications for efforts to improve apple architecture.

中文翻译：

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MsGH3.5的过表达通过苹果植物中的生长素和细胞分裂素途径抑制芽和根的发育。

植物激素相互作用对植物发育至关重要。生长素和细胞分裂素（CK）在调节植物生长和发育中都起着关键作用。然而，这两种植物激素之间的相互作用是复杂的，尚未完全了解。在这里，我们分离的野苹果（野苹果香椿）GRETCHEN HAGEN3（GH3）基因，MsGH3.5，编码吲哚-3-乙酸（IAA） -酰氨基合成酶。MsGH3.5的过表达显着降低了游离IAA的含量，并增加了一些IAA-氨基酸共轭物的含量，MsGH3.5的过表达株系比对照组矮了，产生的不定根（ARs）也更少。此表型与GH3将过量的游离活性IAA与植物生长素稳态中的氨基酸缀合。出人意料的是，MsGH3.5的过表达显着增加了整个植物中的CK浓度，并改变了参与CK生物合成，代谢和信号传导的基因的表达。此外，外源CK的应用通过CK B型应答调节因子MsRR1a的活性诱导MsGH3.5表达，该调节因子介导CK的主要应答。MsRR1a通过直接与其启动子结合，连接生长素和CK信号传导来激活MsGH3.5表达。植物过表达MsRR1a也显示较少的ARS，同意与调控MsGH3.5表达MsRR1a。两者合计，我们揭示MsGH3.5通过调节生长素和CK的水平和信号传导途径影响苹果的生长发育。这些发现提供了对生长素和CK途径之间相互作用的深入了解，并且可能对改善苹果结构的努力具有实质性意义。