背景回顾：Seedling emergence is critical for food security. It requires rapid hypocotyl elongation and apical hook formation, both of which are mediated by regulated cell expansion. 

提出问题：How these events are coordinated in etiolated seedlings is unclear. 

主要发现：Here, we show that biphasic control of cell expansion by the phytohormone auxin underlies this process. 

第一阶段-抑制下胚轴伸长：Shortly after germination, high auxin levels restrain elongation. This provides a temporal window for apical hook formation, involving a gravity-induced auxin maximum on the eventual concave side of the hook. 

第二阶段-顶钩发育：This auxin maximum induces PP2C.D1 expression, leading to asymmetrical H+-ATPase activity across the hypocotyl that contributes to the differential cell elongation underlying hook development. 

第三阶段-促进下胚轴伸长：Subsequently, auxin concentrations decline acropetally and switch from restraining to promoting elongation, thereby driving hypocotyl elongation. 

结论：Our findings demonstrate how differential auxin concentrations throughout the hypocotyl coordinate etiolated development, leading to successful soil emergence.

 摘 要 

种子出苗对粮食安全具有至关重要的作用。该过程需要快速的下胚轴伸长和顶钩形成，这两者都是由受调控的细胞扩张所介导的。但是，这些事件在黄化幼苗中是如何相互协调的尚不清楚。本文中，作者发现植物激素生长素对细胞扩张的双相控制是这一过程的分子基础。在种子萌发后不久，高水平的生长素抑制了细胞伸长。这为顶端弯钩的形成提供了一个时间窗口，主要涉及到一个由重力诱导的、在最终所形成的顶钩的凹侧形成的生长素极大值点。这种生长素极大值点会进一步诱导PP2C.D1基因的表达，导致下胚轴中不对称的氢离子ATP酶活性，最终作用于顶钩发育过程中差异化的细胞伸长。随后，生长素的浓度开始出现向顶性下降趋势，并从抑制转变为促进细胞伸长，从而驱动下胚轴的伸长。本文的研究结果证明了在整个下胚轴中，差异性的生长素浓度是如何协调黄化苗发育的，从而最终作用于种苗的成功破土而出。