Search

Major nutritional and agronomical issues relating to maize (Zea mays) grains depend on the vitreousness/hardness of its endosperm. To identify the corresponding molecular and cellular mechanisms, most studies have been conducted on opaque/floury mutants, and recently on Quality Protein Maize, a reversion of an opaque2 mutation by modifier genes. These mutant lines are far from conventional maize crops. Therefore, a dent and a flint inbred line were chosen for analysis of the transcriptome, amino acid, and sugar metabolites of developing central and peripheral endosperm that is, the forthcoming floury and vitreous regions of mature seeds, respectively. The results suggested that the formation of endosperm vitreousness is clearly associated with significant differences in the responses of the endosperm to hypoxia and endoplasmic reticulum stress. This occurs through a coordinated regulation of energy metabolism and storage protein (i.e., zein) biosynthesis during the grain-filling period. Indeed, genes involved in the glycolysis and tricarboxylic acid cycle are up-regulated in the periphery, while genes involved in alanine, sorbitol, and fermentative metabolisms are up-regulated in the endosperm center. This spatial metabolic regulation allows the production of ATP needed for the significant zein synthesis that occurs at the endosperm periphery; this finding agrees with the zein-decreasing gradient previously observed from the sub-aleurone layer to the endosperm center. The massive synthesis of proteins transiting through endoplasmic reticulum elicits the unfolded protein responses, as indicated by the splicing of bZip60 transcription factor. This splicing is relatively higher at the center of the endospermthan at its periphery. The biological responses associated with this developmental stress, which control the starch/protein balance, leading ultimately to the formation of the vitreous and floury regions of mature endosperm, are discussed. [ABSTRACT FROM AUTHOR]

Relationships between kernel vitreousness and proteins and starch partitioning to the floury and vitreous regions of the endosperm were monitored in a set of 13 maize inbred lines. Decrease of protein contents from the vitreous to the floury endosperms were mainly assigned to α-zeins. Using Raman microspectroscopy, we observed a protein gradient from the periphery to the center of endosperms that well fitted with the inverse relationships between vitreousness and protein content of the vitreous and floury regions. In addition, Raman microspectroscopy highlighted an increase of starch crystallinity from the periphery to the center of the maize endosperms. This agrees with the higher amylose and associated lipid contents within starches of vitreous than in those of floury endosperms. Finally, starch granules from vitreous regions displayed more channels than the floury ones. These channels contain proteins that might favor adhesion of proteins to starch granules or granule–granule contacts to form the close packing of the vitreous endosperm. Therefore transition from vitreous to floury endosperm is at least the result of both protein and starch gradients. These gradients are probably associated with metabolic gradients that have been observed during endosperm development. [ABSTRACT FROM AUTHOR]