Functional analysis of the FORKED and FORKED-LIKE proteins and their role in the environmental control of leaf vein pattern

植物暴露在不断变化的环境中，无法逃脱。相反，它们改变发展，以产生一种更适应新环境的形式。生长素控制着植物发育过程中的许多重要事件，如决定叶片的形成位置、芽和根的分支位置以及使根朝向重力方向。生长素在细胞间的定向运输依赖于PINFORMED （PIN）转运蛋白的极性定位到细胞膜的特定一侧。植物生长的许多环境反应是通过改变PIN定位，从而改变细胞极性，组织生长和组织来实现的。***我对叶脉的形成和组织感兴趣，这也取决于PIN在细胞内的极性定位，这些细胞将成为静脉。我的团队发现了FORKED1 （FKD1）基因，它对这些细胞中PIN的定位很重要。如果基因发生突变，PIN就不会定位于细胞的顶部，而不是形成正常的闭合环，静脉形成开放的“分叉”静脉模式。我们发现FKD1在分泌途径中起作用，将PIN蛋白带到质膜。我建议使用FKD1来研究将蛋白质引导到细胞顶部的其他因素，这是一种之前提出的鲜为人知的途径。*** FKD1是一个植物特有的基因家族，存在于所有植物基因组中。该基因首先出现在无血管的肝植物中，表明其功能早于静脉。一种突变基因敲除了拟南芥基因家族的四个成员，它的根短，对重力没有反应，叶子畸形、块状。显微镜观察显示，肿块和空洞包含随机方向的细胞斑块。叶片中的细胞通常呈极化，其长端平行于中脉。虽然植物学家认为生长素可能使叶片内的细胞极化，但没有人能够通过实验来验证这一想法。突变体为探索叶细胞极化的机制提供了一个独特的工具。静脉形态随环境变化而变化。例如，干旱条件会导致更高的静脉密度和更多的关节，这种适应有助于植物维持光合作用。有趣的是，基因家族成员的序列表明受干旱和应激激素ABA的调节，我的学生已经证明FKD1对ABA有反应。此外，FKD1或其他基因家族成员的突变消除了静脉模式对干旱的反应。因此，我提出基因家族允许静脉模式适应压力，通过包括ABA在内的压力信号通路的调节，并设计了实验来验证这一想法。***通过揭示实现PIN定位和细胞极性的新过程，并定义环境如何影响这些过程，我的研究将允许操纵更适合环境的植物形态。

Plants are exposed to a constantly changing environment from which they cannot escape. Instead, they alter development to produce a form better adapted to the new environment. The hormone auxin controls many important events in plant development such as defining where leaves form, where shoots and root branch and turning roots towards gravity. Auxin is directionally transported from cell to cell dependent upon the polar localization of PINFORMED (PIN) transport proteins to a particular side of the cell membrane. Many of the environmental responses of plant growth are achieved by changing PIN localization and hence cell polarity, tissue growth and organization.*** I am interested in the formation and organization of leaf veins, which also depends on polar localization of PIN within cells that will become veins. My group discovered the FORKED1 (FKD1) gene, that is important for localizing PIN within these cells. If the gene is mutated, PIN is never localized to the top side of cells, and rather than meeting to form the normal closed loops, the veins form an open, “forking” vein pattern. We found that FKD1 works in the secretory pathway, taking the PIN protein to the plasma membrane. I propose to use FKD1 to study other factors that direct proteins to the top side of cells, a previously proposed pathway that is little understood. *** FKD1 is in a gene family unique to plants and found in all plant genomes. The gene first appears in non-vascular liverworts, indicating that its function predates veins. A mutant that knocks out four members of the Arabidopsis gene family has short roots that do not respond to gravity and misshapen, lumpy leaves. Microscopic observation reveals that the bumps and hollows contain patches of randomly oriented cells. Cells in leaves are normally polarized with their long ends parallel to the midvein. While botanists have suggested that auxin might polarize the cells within the leaf, no one has been able to test the idea experimentally. The mutants provide a unique tool to explore the mechanism by which leaf cells are polarized.*** Vein pattern changes in response to the environment. For example, drought conditions cause a higher vein density with more joints, an adaptation that helps plants maintain photosynthesis. Interestingly, members of the gene family have sequences that suggest regulation by drought and the stress hormone ABA, and my students have shown that FKD1 responds to ABA. As well, mutation to FKD1 or other gene family members eliminates the vein pattern response to drought. Therefore, I propose that the gene family allows vein pattern adaptation to stress, through its regulation by stress-signaling pathways including ABA, and have designed experiments to test this idea. *** By revealing novel processes that achieve PIN localization and cell polarity and defining how the environment influences these processes, my research will allow manipulation of plant form better suited to the environment.