Development and Properties of Starches in Vitreous and Floury Endosperm of Maize

Starches from vitreous and floury endosperm in mature maize kernels exhibit significantly different properties, yet the developmental basis for the differences remains unclear. In this research, inner endosperm (IE) and outer endosperm (OE) regions, which develop into floury and vitreous endosperm, respectively, were separated from developing maize kernels. Their starch development and properties were investigated using morphological observation, physicochemical characterization, transcriptome analysis, and biochemical assays. The IE contained small, spherical starch granules with loose arrangement, ultimately forming floury endosperm, whereas the OE displayed large, polygonal starch granules packed tightly, contributing to vitreous endosperm formation. The OE exhibited a higher starch filling degree compared to the IE. Throughout endosperm development, amylose content progressively increased in both regions, but was consistently higher in OE starch than in IE starch. The relative crystallinity and lamellar peak intensity of starch decreased gradually during endosperm development; however, at later stages, both parameters were higher in IE starch than in OE starch. Transcriptome analysis revealed that processes such as anaerobic respiration, glycolysis, and response to hypoxia were more enriched in IE compared to OE. Nearly all genes associated with glycolysis and ethanol fermentation pathways were upregulated in IE. Although no significant difference was observed in the activity of granule-bound starch synthase I between IE and OE, the activity of pyruvate orthophosphate dikinase was higher in OE than in IE. These findings suggest that the insufficient nutrient supply and pronounced hypoxic conditions in the IE reduced the availability of carbon substrates for starch synthesis, thereby impairing starch development and accumulation. In contrast, the larger granule size of OE starch facilitates higher amylose accumulation, leading to distinct physicochemical properties between IE and OE starches.