Intra-Kernel Reallocation of Proteins in Maize Depends on VP1-Mediated Scutellum Development and Nutrient Assimilation

First author: Xixi Zheng; Affiliations: CAS Institute of Plant Physiology & Ecology (中科院植物生理生态研究所): Shanghai, China

Corresponding author: Yongrui Wu

During maize (Zea mays) seed development, the endosperm functions as the major organ for storage of photoassimilate (光同化产物), serving to nourish the embryo. α-Zeins and Globulins (GLBs) predominantly accumulate in the maize endosperm and embryo, respectively. Here, we show that suppression of α-zeins by RNA interference (αRNAi) in the endosperm results in GLB1 being predominantly synthesized in the embryo, thereby markedly increasing the size and number of protein storage vacuoles. Glb genes are strongly expressed in the middle-to-upper section of the scutellum (盾片), cells of which are significantly enlarged by αRNAi induction. Elimination of GLBs caused an apparent reduction in embryo protein levels regardless of whether α-zeins were expressed or suppressed in the endosperm, indicating that GLBs represent the dominant capacity for incorporation of amino acids allocated from the endosperm. It appears that protein reallocation is mostly regulated at the transcriptional level. Differentially expressed genes between wild-type and αRNAi kernels are mainly involved in sulfur assimilation and nutrient metabolism, many of which are transactivated by VIVIPAROUS-1 (VP1). In vp1 embryos, misshaped scutellum cells contain notably less cellular contents and are unable to respond to αRNAi induction. Our results demonstrate that VP1 is essential for scutellum development and protein reallocation from the endosperm to embryo.

在玉米种子发育过程中，胚乳是贮藏光同化产物的器官，并向胚提供营养。玉米的胚乳中主要积累的是α-醇溶蛋白，而胚中主要积累的是球蛋白。本文中，作者通过RNA干扰技术抑制了胚乳中α-醇溶蛋白（αRNAi），导致胚中合成了更多的球蛋白GLB1，因此显著增加了蛋白存储液泡的大小和数量。Glb基因在盾片的中上部强烈表达，这里的细胞在αRNAi株系中显著扩大了。无论在胚乳中表达或是抑制α-醇溶蛋白，阻断球蛋白都会导致胚中的蛋白含量明显降低，说明了胚中的球蛋白是胚感知胚乳营养状态并结合来自胚乳分配的氨基酸的关键因素。很明显，胚与胚乳之间的蛋白重配很大程度上受到了转录水平的调控。野生型与αRNAi突变体籽粒中差异表达的基因主要集中在硫同化与营养代谢相关的条目，其中有很多基因都会受到了VP1的转录激活。在vp1突变体的胚胎中，盾片的形态不正常，盾片细胞具有较少的细胞内含物，并且不能响应于αRNAi的诱导。本文的结果揭示了VP1对于玉米盾片发育及胚乳向胚的蛋白重配具有重要作用。

通讯：巫永睿 (http://www.sippe.ac.cn/sourcedb_sippe/zw/zjrc/201812/t20181219_5218239.html)

个人简介：1996-2000年，兰州大学，学士；2000-2005年，中科院上海生命科学研究院生化与细胞研究所，博士；2007-2013，美国新泽西州立大学，博士后。

研究方向：1. 玉米胚乳发育的基因表达调控网络解析；2. 优质蛋白玉米（Quality Protein Maize, QPM）胚乳修饰机理研究；3. 玉米籽粒重要突变体的基因克隆和功能研究。

doi: https://doi.org/10.1105/tpc.19.00444

Journal: Plant Cell

First Published: September 17, 2019