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

During maize (Zea mays) seed development, the endosperm functions as the major organ for storage of photoassimilate, serving to nourish the embryo. alpha-Zeins and globulins (GLBs) predominantly accumulate in the maize endosperm and embryo, respectively. Here, we show that suppression of alpha-zeins by RNA interference (alpha RNAi) in the endosperm results in more GLB1 being 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, cells of which are significantly enlarged by alpha RNAi induction. Elimination of GLBs caused an apparent reduction in embryo protein level, regardless of whether alpha-zeins were expressed or suppressed in the endosperm, indicating that GLBs represent the dominant capacity for storage of amino acids allocated from the endosperm. It appears that protein reallocation is mostly regulated at the transcriptional level. Genes differentially expressed between wild-type and alpha RNAi kernels are mainly involved in sulfur assimilation and nutrient metabolism, and many are transactivated by VIVIPAROUS1 (VP1). In vp1 embryos, misshapen scutellum cells contain notably less cellular content and are unable to respond to alpha RNAi induction. Our results demonstrate that VP1 is essential for scutellum development and protein reallocation from the endosperm to embryo.