The THOUSAND-GRAIN WEIGHT 6 genes (TaTGW6 and OsTGW6) are reported to result in larger grains of wheat and rice by reducing production of indole-3-acetic acid (IAA) in developing grains. However, a critical comparison of data on TaTGW6 and OsTGW6 with other reports on IAA synthesis in cereal grains requires that this hypothesis be reinvestigated. Here, we show that TaTGW6 and OsTGW6 are members of a large gene family that has undergone major, lineage-specific gene expansion. Wheat has nine genes, and rice three genes encoding proteins with more than 80% amino acid identity with TGW6 making it difficult to envisage how a single inactive allele could have a major effect on IAA levels. TGW6 is proposed to affect auxin levels by catalysing the hydrolysis of IAA-glucose (IAA-Glc). However, we show that developing wheat grains contain undetectable levels of ester IAA in comparison to free IAA and do not express an IAA-glucose synthase. Previous work on TGW6, reported maximal expression at 20 days after anthesis (DAA) in wheat and 2 DAA in rice. However, we show that neither gene is expressed in developing grains. Instead, TaTGW6, OsTGW6 and their close homologues are exclusively expressed in pre-emergence inflorescences; TaTGW6 is expressed particularly in microspores prior to mitosis. This combined with evidence for high levels of IAA production from tryptophan in developing grains demonstrates TaTGW6 and OsTGW6 cannot regulate grain size via the hydrolysis of IAA-Glc. Instead, their similarity to rice strictosidine synthase-like (OsSTRL2) suggests they play a key role in pollen development.

中文翻译：

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对谷粒重量基因TaTGW6和OsTGW6的重新研究使人们怀疑它们在生长素调控谷物生长中的作用

据报道，千粒重6基因（TaTGW6和OsTGW6）通过减少发育中谷物中吲哚-3-乙酸（IAA）的产生而导致小麦和大米的谷物增加。但是，将TaTGW6和OsTGW6的数据与谷物中IAA合成的其他报告进行严格的比较需要重新研究这一假设。在这里，我们显示TaTGW6和OsTGW6是一个大型基因家族的成员，该家族已经进行了主要的，谱系特异性的基因扩增。小麦有9个基因，而水稻的3个基因编码的蛋白质与TGW6具有80％的氨基酸同一性，因此很难设想单个无活性等位基因如何对IAA水平产生重大影响。提出TGW6通过催化IAA-葡萄糖（IAA-Glc）的水解来影响植物生长素水平。然而，我们显示，与游离IAA相比，发育中的小麦籽粒含有不可检测的酯IAA，并且不表达IAA-葡萄糖合酶。关于TGW6的先前研究报道了小麦花后20天（DAA）和水稻中2 DAA的最大表达。但是，我们表明这两个基因均未在发育中的谷物中表达。相反，TaTGW6，OsTGW6及其紧密同源物仅在芽前花序中表达。TaTGW6特别在有丝分裂之前在小孢子中表达。这与发育中谷物中色氨酸中高水平IAA产生的证据相结合，表明TaTGW6和OsTGW6无法通过IAA-Glc的水解来调节晶粒尺寸。相反，它们与水稻类固醇合酶类似物（OsSTRL2）的相似性表明它们在花粉发育中起关键作用。