控制授粉技术挽回干旱诱导玉米籽粒败育的分子机制

As global warming increases the prevalence of drought stress, drought-induced kernel abortion and kernel number reduction have become one of the main limitation to a high and stable yield. However, the molecular mechanism of drought-induced abortion is still unclear. Especially, the timepoint for abortion program to initiate is unknown, which results into an argument about whether sugar depletion or these changes in hormones and other factors are causes or consequences of kernel abortion. Besides, the relationships between sugar depletion with drought-induced abortion is vague since drought simultaneously affects sugar metabolism and other regulation factors. To address these questions, the initiation timepoint of kernel abortion will be examined by a continuous detection of programmed cell death indicators under different degrees of drought treatments accompanied with a synchronous pollination method that unifies the fertilization time of kernels within the ear. Then, by a controlled pollination approach, the apical kernels could be reversed from abortion to normal set under drought condition. Under this scenario, the set and aborted kernels from the same the ear region will be separately sampled and investigated. Specifically, these molecular differences between set and aborted apical kernels that occur before abortion program initiated will be systematically compared and analyzed, in terms of (a) these transporters and key enzymes in sugar metabolism and sugar unloading process and (b) these metabolisms in ethylene, polyamines and other candidate regulation factors. Based on these analyses, we aim to provide solid experimental evidence to (a) demonstrate how sugar metabolism and transport process and other selected regulation factors are influenced by drought, and (b) investigate their relationships in the process of drought-induced maize kernel abortion. Finally, a comprehensive model for drought-induced kernel abortion will be proposed.

随着全球气候变暖，干旱引起的籽粒败育及穗粒数降低已经成为玉米高产和稳产的重要限制因素。然而，干旱诱导玉米籽粒败育的分子机制仍不明确，尤其是败育程序启动的关键时间节点并未确定，导致无法判断糖、激素或其他调控因素究竟是籽粒败育的结果还是诱因；并且，干旱胁迫下糖供应降低的同时也诱导产生逆境信号，导致糖分匮缺与籽粒败育的关系难以厘清。针对这些问题，本研究通过设置不同程度干旱胁迫处理，利用人工授粉精确控制授粉时间，于籽粒建成期连续监测籽粒中程序性死亡相关指标动态，以明确籽粒败育启动时间节点。在此基础上，通过控制授粉技术阻止干旱诱导的顶部籽粒败育，在分子等水平上系统比较败育程序启动前相同位置不同育性籽粒间（1）糖代谢和糖卸载相关的载体和酶类，以及（2）乙烯、多胺和其他调控因子变化特性，明确干旱胁迫对糖代谢和糖卸载过程的影响以及其他候选调控因子在籽粒败育中的作用关系，提出干旱诱导玉米籽粒败育的分子机制。

干旱引起的玉米籽粒败育及穗粒数降低已经成为玉米高产稳产的主要限制因素。然而，干旱诱导果穗籽粒败育的分子机制仍不明确。本研究首先系统性总结了玉米花期干旱导致穗粒数降低的潜在机制：花期干旱胁迫会影响多个不同的生理过程，包括抽雄、花丝吐出、抽雄吐丝间隔期、授粉受精、以及受精后的籽粒建成过程，导致花期干旱致灾机制的复杂性。本研究拟排除对其他生理过程的影响，仅针对受精后的籽粒建成过程，设置不同程度的干旱胁迫处理，同时，利用人工控制授粉精确控制授粉时间，于籽粒建成期连续监测籽粒发育动态，找到籽粒败育程序的关键时间节点。在败育程序启动前，通过比对分析干旱胁迫诱导的败育粒与不同建成粒间的生理代谢差异，发现蔗糖向淀粉或海藻糖合成代谢平衡与乙烯-多胺代谢平衡可能参与了籽粒败育诱导过程。在后续试验中，通过设置不同程度穗三叶遮荫试验，与干旱胁迫处理效应进行比较，发现在两种胁迫下果穗顶端籽粒都发生不同程度败育，两种胁迫诱导的败育籽粒中单糖与乙烯含量都存在拮抗关系，说明单糖与乙烯平衡与籽粒败育诱导有关，并且在不同胁迫中具有保守性。. 此外，为深入解析籽粒糖卸载机制（籽粒内糖分代谢的上游过程），我们系统分析了高分辨率玉米籽粒时空转录图谱，绘制了玉米籽粒糖转运蛋白及糖代谢关键酶家族基因的时空转录图谱，从而筛选并鉴定出参与玉米籽粒糖卸载的关键因子。其中，蔗糖转运蛋白ZmSWEET11/13b将蔗糖由PC运输至母体-子代交界处的质外体空间，随后BETL通过两种途径吸收质外体空间的糖分：（1）蔗糖转运蛋白ZmSUT1和ZmSWEET11/13a驱动下吸收蔗糖；（2）质外体蔗糖由细胞壁酸性转化酶水解为单糖，随后由单糖转运蛋白ZmSTP3和ZmSWEET3a/4c驱动下以单糖形式吸收。同时，ZmSWEET14a/15a和ZmSUT4分别负责蔗糖由胚周边区域（胚乳）向胚中的转运。玉米籽粒糖卸载分子机制解析为将来进一步研究籽粒发育（败育）调控奠定了重要的基础，为通过栽培调控或育种抑制籽粒败育，增加玉米产量提供了理论依据。