A Phylogenetic Re-evaluation of Morphological Variations of Starch Grains among Poaceae Species

Abstract

Starch forms transparent grains, referred to as starch grains (SGs), in amyloplasts. Despite the simple glucose polymer composition of starch, SGs exhibit different morphologies depending on plant species, especially in the endosperm of the family Poaceae. In previous comprehensive work, SG morphologies of Poaceae endosperm were classified into four types: compound grains, bimodal simple grains, uniform simple grains and a mixed configuration comprised of compound and simple grains. Despite these observations, the phylogenetic relationships among the SG types remained unclear. In this report, we analyzed the previous observations with respect to current knowledge of the molecular phylogeny of Poaceae to determine how SG morphological types are clustered and scattered in the phylogenetic tree. We also obtained clear images of SGs of 26 species belonging to the four genera Hordeum, Elymus, Triticum and Bromus. Previously published work indicated that the SG morphologies of these genera were clustered in the phylogenetic tree and specific for the bimodal simple grain type. However, our work identified novel morphological types of SGs within Bromus in addition to the known bimodal simple grain type. A discussion of the molecular phylogenetic analysis and the remarkable intrageneric variation found in this study will contribute to a greater understanding of SG morphological diversity.