National Institute of Plant Genome Research, New Delhi 110067, India.
Plant Physiol. 2011 Aug;156(4):1851-66. doi: 10.1104/pp.111.175794. Epub 2011 Jun 10.
Optimal root architecture is established by multiple intrinsic (e.g. hormones) and extrinsic (e.g. gravity and touch) signals and is established, in part, by directed root growth. We show that asymmetrical exposure of cytokinin (CK) at the root tip in Arabidopsis (Arabidopsis thaliana) promotes cell elongation that is potentiated by glucose in a hexokinase-influenced, G protein-independent manner. This mode of CK signaling requires the CK receptor, ARABIDOPSIS HISTIDINE KINASE4 and, at a minimum, its cognate type B ARABIDOPSIS RESPONSE REGULATORS ARR1, ARR10, and ARR11 for full responsiveness, while type A response regulators act redundantly to attenuate this CK response. Ethylene signaling through the ethylene receptor ETHYLENE RESISTANT1 and its downstream signaling element ETHYLENE INSENSITIVE2 are required for CK-induced root cell elongation. Negative and positive feedback loops are reinforced by CK regulation of the expression of the genes encoding these elements in both the CK and ethylene signaling pathways. Auxin transport facilitated by PIN-FORMED2 as well as auxin signaling through control of the steady-state level of transcriptional repressors INDOLE-3-ACETIC ACID7 (IAA7), IAA14, and IAA17 via TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEIN are involved in CK-induced root cell elongation. This action lies downstream of ethylene and CK induction. Intrinsic signaling in this response operates independently of the extrinsic signal touch, although actin filament organization, which is important in the touch response, may be important for this response, since latrunculin B can induce similar growth. This root growth response may have adaptive significance, since CK responsiveness is inversely related to root coiling and waving, two root behaviors known to be important for fitness.
最优的根系结构是由多种内在（如激素）和外在（如重力和触摸）信号建立的，部分是通过定向的根生长建立的。我们表明，拟南芥（Arabidopsis thaliana）根尖处细胞分裂素（CK）的不对称暴露促进了细胞伸长，这种伸长作用在葡萄糖存在的情况下被六磷酸激酶（hexokinase）影响、G 蛋白非依赖的方式增强。这种 CK 信号转导模式需要 CK 受体、拟南芥组氨酸激酶 4（Arabidopsis HISTIDINE KINASE4，AHK4）以及至少其同源的类型 B 拟南芥响应调节剂（ARR1、ARR10 和 ARR11）才能充分响应，而类型 A 响应调节剂则以冗余方式减弱这种 CK 反应。乙烯通过乙烯受体 ETHYLENE RESISTANT1（ETR1）及其下游信号元件 ETHYLENE INSENSITIVE2（EIN2）对 CK 诱导的根细胞伸长起作用。通过 CK 调节 CK 和乙烯信号通路中这些元件的基因表达，形成了正反馈和负反馈回路。PIN-FORMED2 促进的生长素运输以及通过控制转录抑制因子 INDOLE-3-ACETIC ACID7（IAA7）、IAA14 和 IAA17 的稳态水平的生长素信号，通过 TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEIN（TIR1/AFB）参与 CK 诱导的根细胞伸长。这种作用位于乙烯和 CK 诱导之后。该反应中的内在信号独立于外在信号触摸而运作，尽管在触摸反应中很重要的肌动蛋白丝组织可能对该反应很重要，因为拉他丁 B 可以诱导类似的生长。这种根生长反应可能具有适应性意义，因为 CK 响应性与根卷曲和摆动呈负相关，这两种根行为对适应性很重要。
