Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya, Aichi, Japan.
Plant Physiol. 2012 Jun;159(2):632-41. doi: 10.1104/pp.112.196428. Epub 2012 Apr 5.
The phytohormone auxin is a major regulator of diverse aspects of plant growth and development. The ubiquitin-ligase complex SCF(TIR1/AFB) (for Skp1-Cul1-F-box protein), which includes the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) auxin receptor family, has recently been demonstrated to be critical for auxin-mediated transcriptional regulation. Early-phase auxin-induced hypocotyl elongation, on the other hand, has long been explained by the acid-growth theory, for which proton extrusion by the plasma membrane H(+)-ATPase is a functional prerequisite. However, the mechanism by which auxin mediates H(+)-ATPase activation has yet to be elucidated. Here, we present direct evidence for H(+)-ATPase activation in etiolated hypocotyls of Arabidopsis (Arabidopsis thaliana) by auxin through phosphorylation of the penultimate threonine during early-phase hypocotyl elongation. Application of the natural auxin indole-3-acetic acid (IAA) to endogenous auxin-depleted hypocotyl sections induced phosphorylation of the penultimate threonine of the H(+)-ATPase and increased H(+)-ATPase activity without altering the amount of the enzyme. Changes in both the phosphorylation level of H(+)-ATPase and IAA-induced elongation were similarly concentration dependent. Furthermore, IAA-induced H(+)-ATPase phosphorylation occurred in a tir1-1 afb2-3 double mutant, which is severely defective in auxin-mediated transcriptional regulation. In addition, α-(phenylethyl-2-one)-IAA, the auxin antagonist specific for the nuclear auxin receptor TIR1/AFBs, had no effect on IAA-induced H(+)-ATPase phosphorylation. These results suggest that the TIR1/AFB auxin receptor family is not involved in auxin-induced H(+)-ATPase phosphorylation. Our results define the activation mechanism of H(+)-ATPase by auxin during early-phase hypocotyl elongation; this is the long-sought-after mechanism that is central to the acid-growth theory.
植物激素生长素是调节植物生长和发育各个方面的主要调节剂。泛素连接酶复合物 SCF（TIR1/AFB）（Skp1-Cul1-F-box 蛋白），其中包括 TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX（TIR1/AFB）生长素受体家族，最近被证明对生长素介导的转录调控至关重要。另一方面，早期生长素诱导的下胚轴伸长早已通过酸生长理论得到解释，其中质膜 H(+)-ATP 酶的质子外排是一个功能前提。然而，生长素介导 H(+)-ATP 酶激活的机制尚未阐明。在这里，我们通过生长素诱导早期下胚轴伸长过程中最后一个苏氨酸的磷酸化，提供了生长素直接激活拟南芥（Arabidopsis thaliana）黄化下胚轴中 H(+)-ATP 酶的直接证据。将天然生长素吲哚-3-乙酸（IAA）应用于内源生长素耗尽的下胚轴段中，诱导 H(+)-ATP 酶最后一个苏氨酸的磷酸化，并增加 H(+)-ATP 酶活性，而不改变酶的量。H(+)-ATP 酶的磷酸化水平和 IAA 诱导伸长的变化都具有相似的浓度依赖性。此外，在严重缺乏生长素介导的转录调节的 tir1-1 afb2-3 双突变体中，也发生了 IAA 诱导的 H(+)-ATP 酶磷酸化。此外，α-（苯乙基-2-酮）-IAA，一种针对核生长素受体 TIR1/AFBs 的生长素拮抗剂，对 IAA 诱导的 H(+)-ATP 酶磷酸化没有影响。这些结果表明，TIR1/AFB 生长素受体家族不参与生长素诱导的 H(+)-ATP 酶磷酸化。我们的结果定义了生长素在早期下胚轴伸长过程中激活 H(+)-ATP 酶的机制；这是长期以来寻求的机制，是酸生长理论的核心。
