Functional analysis of a sensor kinase gene family implicated in cytokinin signaling in higher plants

我们鉴定了一组结构上相关的基因，AHK2，3和4，每个基因都编码拟南芥中的一个感受器组氨酸激酶。我们发现了AHK4基因的一个功能缺失突变。该突变体不仅表现为细胞分裂素抑制根的生长，而且还表现为愈伤组织的绿化和不定芽诱导。此外，在芽殖酵母中表达的AHK4表现出依赖于细胞分裂素存在的组氨酸激酶活性。这些结果有力地表明，AHK4作为直接受体参与了细胞分裂素信号通路。然后我们筛选和分离了AHK2和AHK3基因的功能丧失突变。AHK3突变的两个独立等位基因在愈伤组织的绿化和芽诱导中表现出细胞分裂素不敏感的表型，表明AHK3也是细胞分裂素受体。虽然ahk2的两个独立等位基因表现出正常生长和细胞分裂素敏感性，但ahk2/3双突变体在地上部分组织中表现为半矮化表型。这些结果表明，AHK2和AHK3也具有细胞分裂素受体的功能，细胞分裂素在体内的细胞分裂活动中起着积极的调节作用。

We identified a set of structurally related genes, AHK2 3 and 4, each encoding a sensor histidine kinase in Arabidopsis thaliana. We identified a loss-of-function mutation of the AHK4 gene. The mutant exhibited the cytokinin-resistant phenotype not only in inhibition of root growth by cytokinin but also in greening and shoot induction of calli. Moreover, AHK4 expressed in budding yeast showed histidine kinase activity in a manner dependent on the presence of cytokinin. These results strongly suggested that AHK4 is involved in the cytokinin-signaling pathway, as a direct receptor. We then screened and isolated loss-of function mutations of the AHK2 and 3 genes. Two independent alleles of ahk3 mutations exhibited the cytokinin-insensitive phenotype in greening and shoot induction of calli, indicating that AHK3 is also a cytokinin receptor. Although two independent alleles of ahk2 showed normal growth and cytokinin sensitivity, the ahk2/ 3 double mutant exhibited semi-dwarf phenotype in the above-ground tissues. These results suggest that AHK2 and 3 also function as cytokinin receptors and that cytokinins act as positive regulators in cell division activity in vivo.