异源六倍化诱导小麦表型变异的生理学解析

Common wheat is a young allohexaploid species that arisen through hybridization and chromosome doubling of cultivated tetraploid wheat (Triticum turgidum, 4n=28, AABB genome) with the wild grass Aegilops tauschii Coss. (2n=14, DD genome). Physiological systems of cultivated tetraploid wheat and wild Ae. tauschii independently evolve, undergoing distinct selective pressures for several hundred thousand years. Differentiated physiological metabolism traits have probably been sculpted. It is interesting that, after merging AABB and DD genomes, how physiological metabolism traits respond and behave and how the altered traits will evolve during its subsequent evolution process. It is important for understanding of wheat evolution. Some synthetic allohexaploid wheats produced by crossing cultivated tetraploid wheat with wild diploid grass ( Ae. tauschii, genome DD), natural hexaploid bread wheats, and a ploidy-reversed wheat (extracted tetraploid wheat) were used in main experiments of the present work. We compared the metabolism traits of synthetic allohexaploid wheats with their parental genotypes to understand the instantaneous effects of allohexaploidization. Also, we compared the synthetic allohexaploid wheats with natural hexaploid wheats to get insights into the evolution and domestication. To construct extracted tetraploid wheat, the AABB component from a common wheat line were extracted by hybridization to a tetraploid wheat, followed by repeated backcrossing to this common wheat as the recurrent parent. In theory, “extracted” tetraploid wheat with a genomic composition of AABB that is virtually identical to the BBAA subgenomes of its common wheat donor. It is ideal mode for investigating how AABB component of common wheat behaves during its long term evolution process comparing “extracted” tetraploid wheat with its common wheat donor and natural tetraploids. In this work, we plan to focus on two key phenotypes, root biomass and nitrogen use efficiency, because we observed that both root biomass and nitrogen use efficiency were enhanced after allohexaploidization in our previous research. We will investigate the physiological mechanisms of enhancements of root biomass and nitrogen use efficiency during wheat allohexaploidization, and also will consider whether the change of nitrogen use efficiency is related to root trait evolution.

普通小麦是由二倍体节节麦（DD genome）和栽培四倍体小麦 (AABB genome)经杂交和染色体加倍形成的异源六倍体。在小麦形成前，其二倍体和四倍体祖先已独立进化数十万年，选择压力的巨大差异可能已经将二者雕琢成完全不同的生理系统。异源六倍化后两个完全不同的生理系统融合，生理代谢特点将如何变化？进化过程中又将如何演化？对这些问题的解析无疑对理解小麦进化具有重要意义。本项目拟用小麦的二倍体和四倍体祖先人工合成六倍体小麦来模拟普通小麦的瞬时形成过程；通过比较合成小麦与自然六倍体小麦的差别来分析长期进化的影响，也使用抽提四倍体小麦（将六倍体的DD染色体抽离）与其六倍体供体、自然四倍体相比，来探讨长期进化过程中AB亚基因组发生哪些变化。我们前期工作已经证明，六倍化后小麦根生物量和氮利用效率都明显增加。本项目拟利用上述材料，研究小麦六倍化过程中，这两个关键表型变异的生理生化机制，二者是否关联。

目前全世界广泛种植的小麦（普通小麦）是一个仅形成不到一万年的年轻异源六倍体物种。六倍体小麦形成后，随即取代其四倍体祖先，迅速从它的诞生地“美索不达米亚”扩散到全世界。然而时至今日，人们对小麦进化和驯化过程所知甚少，特别是，几乎无人涉及六倍体小麦形成过程中表型变异的生理生化机制。本项目使用小麦的二倍体和四倍体祖先人工合成六倍体小麦来模拟普通小麦的瞬时形成过程；通过比较合成小麦与自然六倍体小麦的差别来分析长期进化的影响，也使用抽提四倍体小麦（将六倍体的DD染色体抽离）与其六倍体供体、自然四倍体相比，来探讨长期进化过程中AB亚基因组发生哪些变化。本项目利用上述材料，研究了小麦六倍化过程中，表型变异的生理生化机制。我们获得了三方面的研究结果：..1..项目研究发现在低氮条件下，合成六倍体小麦茎叶氮含量明显高于其二倍体和四倍亲本。合成六倍体小麦在低氮条件下具有较高的根冠比和质子外排速度。更高的质子外排速度可能促进合成六倍体小麦吸收更多的氮。更高的根冠比使合成六倍体小麦用更多的根供应更少的地上部分，这可能是合成六倍体小麦的重要适应机制。此外，我们也发现NPF可能是小麦氮利用效率进化的关键基因。.2..项目研究结果表明小麦从四倍体到六倍体的进化过程中抗旱性没有变化，也就是说小麦高抗旱性可能形成于四倍体时期 (三十万年前)，人类的驯化并未增加其抗旱性。.3..课题组使用倍性反转的抽提四倍体小麦材料的研究表明，抽提四倍体小麦糖和氨基酸代谢出现明显的异常，分析原因可能是莽草酸和蔗糖代谢异常引起的。这直接表明小麦驯化过程对BBAA基因组产生了巨大影响。. 总的来说，本项目的研究结果极大的丰富了小麦适应性进化理论，阐明了小麦几个关键表型进化的生理过程。本项目的研究结果还可为进一步的小麦基因工程育种提供启示。