Department of Life Sciences and Functional Genomics Center, Pohang University of Science and Technology, Pohang 790-784, Korea.
Plant Cell Physiol. 2012 Jul;53(7):1334-43. doi: 10.1093/pcp/pcs079. Epub 2012 May 28.
Cytokinins are involved in key developmental processes in rice (Oryza sativa), including the regulation of cell proliferation and grain yield. However, the in vivo action of histidine kinases (OsHks), putative cytokinin receptors, in rice cytokinin signaling remains elusive. This study examined the function and characteristics of OsHk3, 4 and 6 in rice. OsHk6 was highly sensitive to isopentenyladenine (iP) and was capable of restoring cytokinin-dependent ARR6 reporter expression in the ahk2 ahk3 Arabidopsis mutant upon treatment with 1 nM iP. OsHk4 recognized trans-zeatin (tZ) and iP, while OsHk3 scarcely induced cytokinin signaling activity. OsHk4 and OsHk6 mediated the canonical two-component signaling cascade of Arabidopsis to induce phosphorylation of ARR2. OsHk4 and OsHk6 were highly expressed in spikelets, suggesting that tZ and iP might play key roles in grain development. OsHk6 formed a self-interacting homomer in rice protoplasts, although the trans-phosphorylation activity between subunits was much lower than the intra-molecular trans-phosphorylation activity. This indicates that the action mechanism of OsHks is evolutionarily diverged from bacterial histidine kinases. Ectopic expression of OsHk6 in rice calli promoted green pigmentation and subsequent shoot induction, further supporting an OsHk6 in planta function as a cytokinin receptor. From the results of this study, OsHks are homomeric cytokinin receptors with distinctive cytokinin preferences in rice.
细胞分裂素参与水稻（Oryza sativa）的关键发育过程，包括细胞增殖和籽粒产量的调节。然而，组氨酸激酶（OsHks）作为潜在的细胞分裂素受体，在水稻细胞分裂素信号转导中的体内作用仍不清楚。本研究研究了 OsHk3、4 和 6 在水稻中的功能和特性。OsHk6 对异戊烯腺嘌呤（iP）高度敏感，并且能够在 1 nM iP 处理后恢复 ahk2 ahk3 拟南芥突变体中依赖细胞分裂素的 ARR6 报告基因的表达。OsHk4 识别玉米素（tZ）和 iP，而 OsHk3 几乎不诱导细胞分裂素信号活性。OsHk4 和 OsHk6 介导拟南芥的经典双组分信号级联反应，诱导 ARR2 的磷酸化。OsHk4 和 OsHk6 在小穗中高表达，表明 tZ 和 iP 可能在籽粒发育中起关键作用。OsHk6 在水稻原生质体中形成自相互作用的同源二聚体，尽管亚基之间的转磷酸化活性远低于分子内转磷酸化活性。这表明 OsHks 的作用机制在进化上与细菌组氨酸激酶不同。OsHk6 在水稻愈伤组织中的异位表达促进了绿色素的形成和随后的芽诱导，进一步支持 OsHk6 在植物体内作为细胞分裂素受体的功能。本研究结果表明，OsHks 是水稻中具有独特细胞分裂素偏好的同源细胞分裂素受体。
