National Key Laboratory of Wheat and Maize Crops Science, CIMMYT-China (Henan) Joint Center of Wheat and Maize, Collaborative Innovation Center of Henan Grain Crops, College of Agronomy, Henan Agricultural University, Zhengzhou 450002, China.
School of Environmental and Rural Science, University of New England, Armidale, New South Wales, Australia.
Plant Sci. 2023 Jul;332:111727. doi: 10.1016/j.plantsci.2023.111727. Epub 2023 May 5.
Rerouting the starch biosynthesis pathway in maize can generate specialty types, like sweet corn and waxy corn, with a drastically increasing global demand. Hence, a fine-tuning of starch metabolism is relevant to create diverse maize cultivars for end-use applications. Here, we characterized a new maize brittle endosperm mutant, referred to as bt1774, which exhibited decreased starch content but a dramatic increase of soluble sugars at maturity. Both endosperm and embryo development was impaired in bt1774 relative to the wild-type (WT), with a prominently arrested basal endosperm transfer layer (BETL). Map-based cloning revealed that BRITTLE ENDOSPERM2 (Bt2), which encodes a small subunit of ADP-glucose pyrophosphorylase (AGPase), is the causal gene for bt1774. A MuA2 element was found to be inserted into intron 2 of Bt2, leading to a severe decrease of its expression, in bt1774. This is in line with the irregular and loosely packed starch granules in the mutant. Transcriptome of endosperm at grain filling stage identified 1,013 differentially expressed genes in bt1774, which were notably enriched in the BETL compartment, including ZmMRP1, Miniature1, MEG1, and BETLs. Gene expression of the canonical starch biosynthesis pathway was marginally disturbed in bt1774. Combined with the residual 60 % of starch in this nearly null mutant of Bt2, this data strongly suggests that an AGPase-independent pathway compensates for starch synthesis in the endosperm. Consistent with the BETL defects, zein accumulation was impaired in bt1774. Co-expression network analysis revealed that Bt2 probably has a role in intracellular signal transduction, besides starch synthesis. Altogether, we propose that Bt2 is likely involved in carbohydrate flux and balance, thus regulating both the BETL development and the starchy endosperm filling.
重新路由玉米中的淀粉生物合成途径可以产生特殊类型，如甜玉米和蜡质玉米，全球需求急剧增加。因此，精细调节淀粉代谢对于创造用于最终用途的不同玉米品种是相关的。在这里，我们描述了一种新的玉米脆性胚乳突变体，称为 bt1774，其表现出淀粉含量降低，但成熟时可溶性糖含量急剧增加。与野生型（WT）相比，bt1774 的胚乳和胚胎发育均受损，基础胚乳转移层（BETL）明显受阻。基于图谱的克隆表明，BRITTLE ENDOSPERM2（Bt2），它编码 ADP-葡萄糖焦磷酸化酶（AGPase）的小亚基，是 bt1774 的原因基因。在 bt1774 中发现 MuA2 元件插入 Bt2 的内含子 2 中，导致其表达严重下降。这与突变体中不规则且松散堆积的淀粉颗粒一致。在灌浆阶段的胚乳转录组中，在 bt1774 中鉴定出 1013 个差异表达基因，这些基因在 BETL 隔室中显著富集，包括 ZmMRP1、Miniature1、MEG1 和 BETLs。bt1774 中经典淀粉生物合成途径的基因表达受到轻微干扰。结合这个 Bt2 近空突变体中残留的 60%淀粉，这一数据强烈表明 AGPase 独立途径在胚乳中补偿淀粉合成。与 BETL 缺陷一致，bt1774 中的 zein 积累受损。共表达网络分析表明，Bt2 除了参与淀粉合成外，可能还在细胞内信号转导中发挥作用。总的来说，我们提出 Bt2 可能参与碳水化合物通量和平衡，从而调节 BETL 发育和淀粉胚乳填充。
