生长素极性运输对棉花纤维起始细胞中IAA积累的调控研究

Cotton, a major fiber crop in the world, plays an important role in China's domestic economy. Recently, we found IAA, the predominant auxin in plant, was highly accumulated in fiber initials, while not in non-fiber cells of ovule epidermis. In a fiberless mutant (Xuzhou 142), there is no such phenomenon observed in ovule epidermal cells. According to these findings, we enhanced the IAA level in ovule epidermal cells specifically through spatiotemporally manipulating auxin biosynthesis in cotton. This promoted more fiber cells produced, resulting improved fiber yield and quality in the transgenic cotton. This suggested that it's practically valuable to study the mechanism of auxin promoting fiber initiation. Based on above works, we plan to elucidate the mechanism of the IAA distribution in ovule epidermal layers further. . At present, two major reasons might be responsible for IAA accumulation in fiber initials. One is local auxin synthesis. But previous studies weakened its importance. Comparing to pre- and post-anthesis ovules, none of auxin synthesis related genes was detected up-regulated in ovules on the day of anthesis. Moreover, in contrast to non-fiber cells of ovule epidermis, the expression levels of these genes were not higher in fiber cells. .Another is polar auxin transport (PAT). It was regulated by direction transport of auxin between cells. Up to date, three proteins (PIN, PGP and AUX1) were characterized involving in auxin distribution within plant tissue as auxin carriers. Among them, the polar subcellular localization of the PIN auxin efflux carriers has been shown to be more important for the directionality of auxin fluxes. In addition, it is also proved that PIN proteins perform a rate-limiting function in cellular auxin efflux. Hence, we propose that PIN proteins are required to establish auxin gradient in ovules during fiber initiation, finally resulting the IAA accumulation in fiber cells. .Based on our findings, we plan to clarify the role of PIN-mediated polar auxin transport playing in IAA accumulation of cotton initiated fibers, and to investigate the regulation of some key factors which influence PIN's function, such as PP2A phosphatase, PINOID kinase and phospholipase A2, in this process. This will further elucidate the mechanism of cotton fiber initiation. Moreover, it could find some useful genes to improve fiber traits through modulating IAA accumulation.

棉花是世界上重要的天然纤维作物，棉花生产在我国国民经济中的地位举足轻重。申请人和课题组研究发现，生长素在棉花纤维起始细胞中高浓度积累，而非纤维细胞和无纤维突变体的胚珠表皮细胞则没有这种现象。根据此生长素极性分布的特点，我们通过对生长素生物合成基因的时空调控，特异性提高棉花胚珠外表皮的生长素水平，促进更多的表皮细胞发育成纤维，实现了纤维产量和品质的同步改良。这证明研究生长素促进纤维起始的作用机理在改良棉花纤维性状上的价值。结合前人的报道和我们的工作基础推测，载体蛋白PIN介导的生长素极性运输可能参与纤维起始细胞中IAA积累的调控。为进一步弄清生长素在棉花胚珠表皮中极性分布的分子机理，申请人以生长素运输为重点，拟研究PIN蛋白的分布、调控及其磷酸化状态等与生长素在纤维起始细胞中特异积累的关系。本研究对揭示棉花纤维细胞起始分化的分子机理有理论价值，对棉花高产优质的分子育种也有指导意义。

棉纤维是由胚珠外表皮细胞发育分化而来的单细胞，是棉花区别于其它植物的特征细胞结构。在开花当天，纤维细胞中积累大量的生长素IAA，用于促进纤维的起始发育。迄今为止，生长素促进纤维发育的分子机制尚有很多不明确的地方。我们利用生长素响应启动子DR5融合GUS报告基因在棉花中表达用以展示在纤维分化和起始阶段胚珠中生长素的含量分布。生长素主要集中在胚珠的珠心部位和胚珠表皮的纤维细胞中。在珠心部位的生长素积累始于开花前1天（-1DPA），而在胚珠表皮纤维细胞中的积累始于0DPA。生长素的极性运输而非原位合成在此过程中发挥着主要的作用，将棉铃以外的生长素运进胚珠。在胚珠中通过RNAi特异下调棉花中生长素运输蛋白基因GhPINs的表达发现，纤维起始延迟，伸长受到抑制。这表明GhPINs介导的生长素运输在调控胚珠表皮生长素积累以及纤维发挥过程中发挥着重要的作用。此外胚珠表皮特异调控GhPIN1a上调和下调，以及超量表达PIN蛋白磷酸化基因AtPID和去磷酸化基因AtPP2A转基因棉花均表现出纤维起始无明显变化，纤维伸长受抑制的表型，暗示棉花胚珠内生长素的运输路径复杂。通过RNA原位和蛋白原位杂交，我们发现GhPIN3a在胚珠外种皮表皮表达，并在胚珠外表皮纤维细胞和非纤维细胞中分别呈现极性和非极性的定位方式，暗示出GhPIN3a可能是调控胚珠表皮生长素积累的关键运输蛋白。