TaIAA21 represses TaARF25-mediated expression of TaERFs required for grain size and weight development in wheat

背景回顾：Auxin signaling is essential for the development of grain size and grain weight, two important components for crop yield. 

提出问题：However, no AUX/IAA has been functionally characterized to be involved in the development of wheat grains to date. 

主要研究：Here, we identified a wheat AUX/IAA gene, TaIAA21, and studied its regulatory pathway. 

结果1-TaIAA21基因功能：We found that TaIAA21 mutation significantly increased grain length, grain width, and grain weight. Cross-section of mutant grains observed elongated outer pericarp cells than those of the wild type where the expression of TaIAA21 was detected by in situ hybridization. 

结果2-TaIAA21-TaARF25：Screening of auxin response factor (ARF) genes highly expressed in early developing grains identified TaARF25 that interacted with TaIAA21. In contrast, mutation of the tetraploid wheat (Triticum turgidum) ARF25 gene significantly reduced grain size and weight. 

结果3-TaARF25下游基因ERF3：RNA-seq analysis revealed up-regulation of several ethylene response factor genes (ERFs) in taiaa21 mutants which carried AuxRE cis-elements in their promoter. One of them, ERF3, was up-regulated in the taiaa21 mutant and down-regulated in ttarf25 mutant. Transactivation assays showed that ARF25 promoted ERF3 transcription, while mutation of TtERF3 resulted in reduced grain size and weight. 

结果4-育种选择：Analysis of natural variations identified three TaIAA21-A haplotypes with increased allele frequency in cultivars relative to landraces, a signature of breeding selection. 

结论：Our work demonstrated that TaIAA21 worked as a negative regulator in grain size and weight development via the ARF25-ERFs module and is useful for yield improvement in wheat.

 摘 要 

生长素信号转导对于籽粒大小和籽粒重量的发育是必要的，而这两个性状是作物产量的重要组成部分。但是，在小麦的籽粒研究中，还未有研究报道过AUX/IAA在该过程中的功能。本文中，作者鉴定到了一个AUX/IAA基因TaIAA21，并研究了该基因的调控网络。作者发现TaIAA21的功能缺失突变会显著增加籽粒的长度、籽粒宽度和籽粒重量。对于突变体籽粒的切片分析发现，其外果皮细胞比野生型的更长；并通过原位杂交检测到TaIAA21基因在这些细胞中表达。通过扫描在早期籽粒发育过程中高表达的生长素响应因子ARFs，发现TaARF25能够与TaIAA21互作。相反，突变四倍体小麦中的ARF25基因会显著减小籽粒大小和重量。RNA-seq分析显示ttarf25突变体中的一些乙烯响应因子ERFs出现上调，并且该基因在启动子区存在AuxRE顺式作用元件。其中一个基因，即ERF3的表达量在taiaa21突变体中上调，在ttarf25突变体中下调在。转录激活实验显示ARF25能够促进ERF3的转录，而TtERF3的突变会导致籽粒大小和重量的减小。通过对自然变异的检测，作者鉴定到了三种TaIAA21-A单倍型，其在栽培种群体中的等位基因频率要高于地方群体，说明了其受到了育种选择。本文的工作揭示了TaIAA21作为一个负调控因子，通过ARF25-ERFs模块作用于小麦的籽粒大小和重量调控，可用做未来小麦产量改良的候选靶位点。