A transposable element insertion within ZmGE2 gene is associated with increase in embryo to endosperm ratio in maize

Most of the maize kernel oil is located in the embryo while the majority of starch is located in the endosperm. Maize kernel composition and value are affected significantly by the ratio of the embryo size to the endosperm size; however, the genetic regulation of embryo to endosperm ratio (EER) in maize is unknown. Here we identified ZmGE2 gene, which encodes a cytochrome p450 protein, as a gene associated with EER variation in maize. We first expressed rice Giant Embryo (GE) gene driven by oleosin promoter in maize and detected a 23.2 % reduction in EER in transgenic seeds, demonstrating the existence of evolutionarily conserved mechanisms for EER determination in rice and maize. We next identified maize GE2, a homolog of rice GE sharing 70 % identity in amino sequence, as a candidate based on the similar expression pattern and co-localization with a previously detected QTL for EER. Followed by linkage and association mapping, a 247-bp transposable element (TE) insertion in 3'-untranslated region of ZmGE2 gene was identified to be associated with increase in EER and kernel oil content. Expression level of the favorable ZmGE2 allele containing the 247-bp TE insertion was strongly reduced. In addition, the 247-bp TE insertion site was a selection target during the artificial long-term selection for the high EER trait in a high oil population. This is the first report that demonstrates an association of ZmGE2 with EER variation in maize and identifies ZmGE2 gene as a promising target for manipulation of EER and grain composition by either transgenic approach or molecular breeding in maize.

玉米籽粒的大部分油脂位于胚中，而大部分淀粉位于胚乳。胚与胚乳大小的比例显著影响玉米籽粒的成分和价值；然而，玉米中胚与胚乳比例（EER）的遗传调控机制尚不清楚。在此我们鉴定出ZmGE2基因，它编码一种细胞色素P450蛋白，是与玉米EER变异相关的基因。我们首先在玉米中表达由油质蛋白启动子驱动的水稻巨胚（GE）基因，并检测到转基因种子的EER降低了23.2%，这表明水稻和玉米中存在进化上保守的EER决定机制。接下来，我们根据相似的表达模式以及与先前检测到的EER数量性状位点（QTL）的共定位，将玉米GE2（其与水稻GE在氨基酸序列上有70%的一致性，是水稻GE的同源基因）鉴定为候选基因。通过连锁和关联作图，在ZmGE2基因的3' - 非翻译区鉴定出一个247bp的转座元件（TE）插入，它与EER和籽粒含油量的增加相关。含有247bp TE插入的有利ZmGE2等位基因的表达水平显著降低。此外，在一个高油群体对高EER性状的长期人工选择过程中，247bp的TE插入位点是一个选择目标。这是首次报道表明ZmGE2与玉米EER变异相关，并将ZmGE2基因确定为通过转基因方法或分子育种手段调控玉米EER和籽粒成分的一个有潜力的目标。