Morphological variations of starch grains

Starch is synthesized in plant storage organs and forms transparent grains inside cells, which are referred to as starch granules or starch grains (SGs). SGs exhibit different morphologies and sizes depending on the species and are prominent in Poaceae endosperm. Comprehensive observations indicate that SG morphologies can be classified into four types: compound grains, bimodal simple grains, uniform simple grains, and a mixed configuration containing compound and simple grains in the same cells. Phylogenetic evaluation of SG morphological diversity indicates that the compound grain type is the ancestral SG morphology in Poaceae, and the bimodal simple grain type is only observed in specific phylogenetic groups that include barley and wheat. Starch morphology and size are important characteristics for industrial applications. However, the molecular mechanisms that determine SG morphology and size are not completely understood. This review summarizes starch grain morphological characteristics and phylogenetic information about SG morphological diversity. It also discusses methods for cytological observation of SGs and recently identified genes that control SG size.