Distribution of gluten proteins in bread wheat (Triticum aestivum) grain

TLDR

Gluten proteins, the main storage proteins confined to the starchy endosperm, form a polymeric network that gives dough its viscosity and elasticity, and their protein, starch, and cell wall gradients—especially in gluten composition—affect grain processing quality. The study mapped gluten protein gradients by immunolocalizing specific gluten types in developing grains and performing western blots on flour fractions obtained through sequential pearling to isolate tissue layers. High‑molecular‑weight glutenins and γ‑gliadins were concentrated in inner endosperm layers, whereas low‑molecular‑weight glutenins, ω‑ and α‑gliadins predominated in the subaleurone, with segregation occurring between and within protein bodies and gradients established during development, likely driven by subaleurone origin or maternal regulatory signals.

Abstract

Gluten proteins are the major storage protein fraction in the mature wheat grain. They are restricted to the starchy endosperm, which forms white flour on milling, and interact during grain development to form large polymers which form a continuous proteinaceous network when flour is mixed with water to give dough. This network confers viscosity and elasticity to the dough, enabling the production of leavened products. The starchy endosperm is not a homogeneous tissue and quantitative and qualitative gradients exist for the major components: protein, starch and cell wall polysaccharides. Gradients in protein content and composition are the most evident and are of particular interest because of the major role played by the gluten proteins in determining grain processing quality. Protein gradients in the starchy endosperm were investigated using antibodies for specific gluten protein types for immunolocalization in developing grains and for western blot analysis of protein extracts from flour fractions obtained by sequential abrasion (pearling) to prepare tissue layers. Differential patterns of distribution were found for the high-molecular-weight subunits of glutenin (HMW-GS) and γ-gliadins when compared with the low-molecular-weight subunits of glutenin (LMW-GS), ω- and α-gliadins. The first two types of gluten protein are more abundant in the inner endosperm layers and the latter more abundant in the subaleurone. Immunolocalization also showed that segregation of gluten proteins occurs both between and within protein bodies during protein deposition and may still be retained in the mature grain. Quantitative and qualitative gradients in gluten protein composition are established during grain development. These gradients may be due to the origin of subaleurone cells, which unlike other starchy endosperm cells derive from the re-differentiation of aleurone cells, but could also result from the action of specific regulatory signals produced by the maternal tissue on specific domains of the gluten protein gene promoters.