Yang Kezhen, Wang Hongzhe, Xue Shan, Qu Xiaoxiao, Zou Junjie, Le Jie
Key Laboratory of Plant Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Beijing 100093, China.
Key Laboratory of Plant Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Beijing 100093, China
J Exp Bot. 2014 Jun;65(9):2449-61. doi: 10.1093/jxb/eru139. Epub 2014 Mar 31.
The Arabidopsis stoma is a specialized epidermal valve made up of a pair of guard cells around a pore whose aperture controls gas exchange between the shoot and atmosphere. Guard cells (GCs) are produced by a symmetric division of guard mother cells (GMCs). The R2R3-MYB transcription factor FOUR LIPS (FLP) and its paralogue MYB88 restrict the division of a GMC to one. Previously, the upstream regions of several core cell cycle genes were identified as the direct targets of FLP/MYB88, including the B-type cyclin-dependent kinase CDKB1;1 and A2-type cyclin CYCA2;3. Here we show that CDKA;1 is also an immediate direct target of FLP/MYB88 through the binding to cis-regulatory elements in the CDKA;1 promoter region. CDKA;1 activity is required not only for normal GMC divisions but also for the excessive cell overproliferation in flp myb88 mutant GMCs. The impaired defects of GMC division in cdkb1;1 1;2 mutants could be partially rescued by a stage-specific expression of CDKA;1. Although targeted overexpression of CDKA;1 does not affect stomatal development, ectopic expression of the D3-type cyclin CYCD3;2 induces GC subdivision, resulting in a stoma with 3-4 GCs instead of the normal two. Co-overexpression of CDKA;1 with CYCD3;2, but not with CYCA2;3, confers a synergistic effect with respect to GC subdivision. Thus, in addition to a role in stomatal formative asymmetric divisions at early developmental stages, CDKA;1 is needed in triggering GMC symmetric divisions at the late stage of stomatal development. However, timely down-regulation of CDKA;1-CYCD3 activity is required for restriction of GC proliferation.
拟南芥气孔是一种特殊的表皮瓣膜，由围绕气孔的一对保卫细胞组成，气孔的孔径控制着地上部分与大气之间的气体交换。保卫细胞（GCs）由保卫母细胞（GMCs）的对称分裂产生。R2R3-MYB转录因子四唇（FLP）及其同源物MYB88将GMC的分裂限制为一次。此前，几个核心细胞周期基因的上游区域被确定为FLP/MYB88的直接靶标，包括B型细胞周期蛋白依赖性激酶CDKB1;1和A2型细胞周期蛋白CYCA2;3。在这里，我们表明CDKA;1也是FLP/MYB88的直接靶标，通过与CDKA;1启动子区域的顺式调控元件结合。CDKA;1的活性不仅是正常GMC分裂所必需的，也是flp myb88突变体GMC中细胞过度增殖所必需的。cdkb1;1 1;2突变体中GMC分裂的受损缺陷可以通过CDKA;1的阶段特异性表达得到部分挽救。虽然靶向过表达CDKA;1不影响气孔发育，但D3型细胞周期蛋白CYCD3;2的异位表达会诱导GC细分，导致形成具有3-4个GC的气孔，而不是正常的两个。CDKA;1与CYCD3;2共同过表达，但与CYCA2;3共同过表达则不会，在GC细分方面具有协同作用。因此，除了在早期发育阶段的气孔形成不对称分裂中发挥作用外，CDKA;1在气孔发育后期触发GMC对称分裂中也是必需的。然而，限制GC增殖需要及时下调CDKA;1-CYCD3的活性。
