ZmPIFs转录因子参与TAA1-YUC介导的生长素合成途径调控玉米避荫反应的分子机制

Increasing the planting density of maize per unit land is an effective measure to enhance the yield of maize. However, under high density planting, the surrounding vegetation causes a reduction in the ratio of red to far-red light, and triggers a shade avoidance response, which negatively impacts many important agronomic traits, including increased plant and ear height, increased lodging and reduced yield. Earlier studies have demonstrated that shade-induced hypocotyl elongation growth in Arabidopsis depends on auxin biosynthesis and signaling which was regulated by phytochromeB and phytochrome interaction factors (PIFs). PIFs can bind to the TAA1 and YUCCA auxin biosynthesis genes and regulate their target gene expression. Low R: FR leads to accumutation of PIF proteins and PIF-induced rapid increase of indole-3-acetic acid (IAA) production through the TAA1-YUC pathway in the cotyledons. Then IAA is transported from the cotyledons to the hypocotyl by auxin polar transport proteins, eventually leading to enhanced hypocotyl elongation. However, the mechanism of phytochrome-mediated shade avoidance response interacts with auxin biosynthesis pathway in maize seedlings is unknown so far. In this project, we aim to dissect the genetic basis and molecular mechanisms of how phytochrome B and PIFs interact with auxin biosynthesis pathway (TAA1-YUC) to regulate shade avoidance response in maize under high denstiy planting, using maize inbred lines treated with end-of-day FR. The expected results will greatly enhance our understanding of the genetic network controlling shade avoidance response in maize and provide guidance for breeding of high-density tolerant maize varieties.

合理增加玉米种植密度是提高玉米单产的有效途径之一，但密植会使得玉米群体中红光与远红光比率下降，从而激发了植物的避荫反应，导致植株和穗位增高、抗倒伏能力下降和产量降低等一系列不良后果。研究表明，遮荫诱导的拟南芥下胚轴伸长主要依赖于TAA1-YUC介导的生长素合成途径，且受光敏色素互作因子PIFs调控。PIFs能直接调节TAA1及YUC基因的表达，使子叶中生长素（IAA）水平迅速增加，再经PINs运输载体将IAA转运到下胚轴中促进其伸长生长。然而，密植条件下光质量改变如何通过ZmPIFs转录因子调控玉米避荫反应的分子机理目前尚不清楚。本项目以玉米自交系幼苗为研究材料，通过end-of-day-FR处理模拟密植引起的荫蔽环境，解析玉米密植条件下光敏色素B和PIFs因子与TAA1-YUC途径互作调控玉米避荫反应的遗传基础和分子机理。预期得到的研究结果可为耐密植、抗倒伏玉米新品种的培育提供理论依据。

在过去的几十年里，增加玉米种植密度是提高玉米单产的有效途径，但密植会使得玉米群体中红光与远红光比率下降，从而激发了植物的避荫反应，如植株和穗位增高、抗倒伏能力下降，最终导致产量降低。研究表明，遮荫诱导的拟南芥下胚轴伸长生长主要依赖于YUC介导的生长素合成途径，且受光敏色素互作因子PIFs调控。然而，密植条件下光质量改变如何通过ZmPIFs转录因子调控玉米避荫反应的分子机理目前尚不清楚。在本研究中，我们对玉米中的ZmPIF基因家族在调节光信号和光形态发生中的功能进行了初步研究。玉米基因组中共有7个不同的ZmPIF基因，可分为三个亚家族ZmPIF3s、ZmPIF4s和ZmPIF5s。与拟南芥中的PIF相似，7个ZmPIF蛋白都定位于细胞核内，而且大部分ZmPIF蛋白在光照下可形成核体。通过CRISPR/Cas9基因组编辑技术产生的ZmPIF3s-KO、ZmPIF4s-KO和ZmPIF5s-KO敲除突变体幼苗在模拟遮荫处理下，与野生型植株相比表现出减弱的避荫反应表型，表明ZmPIFs是玉米避荫反应中的正调控因子。与拟南芥pif突变体下胚轴短的表型相似，ZmPIF3s-KO突变体在黑暗中均表现出中胚轴缩短的表型。此外，我们还发现ZmPIF3.1蛋白能够结合到ZmYUC2启动子的pZmYUC2-1区段，并通过抑制ZmYUC2基因的表达来抑制中胚轴的伸长生长。外施生长素类似物NAA处理发现，随着生长素浓度的升高，玉米黄化苗的中胚轴伸长生长受到抑制。与此结果一致，ZmYUC2-KO突变体的黄化苗表现出比野生型中胚轴长的表型，表明生长素能够抑制中胚轴的伸长。综上所述，我们的研究结果揭示了ZmPIFs通过YUC介导的生长素合成途径在调节玉米幼苗避荫反应和中胚轴伸长生长中的作用，为耐密植、抗倒伏玉米新品种的培育提供理论依据，并为玉米的遗传改良提供了适应高密度种植的潜在目标。