Analysis of grain characters in temperate grasses reveals distinctive patterns of endosperm organization associated with grain shape

Abstract

Members of the core pooids represent the most important crops in temperate zones including wheat, barley, and oats. Their importance as crops is largely due to the grain, particularly the storage capabilities of the endosperm. In this study, a comprehensive survey of grain morphology and endosperm organization in representatives of wild and cultivated species throughout the core pooids was performed. As sister to the core pooid tribes Poeae, Aveneae, Triticeae, and Bromeae within the Pooideae subfamily, Brachypodium provides a taxonomically relevant reference point. Using macroscopic, histological, and molecular analyses distinct patterns of grain tissue organization in these species, focusing on the peripheral and modified aleurone, are described. The results indicate that aleurone organization is correlated with conventional grain quality characters such as grain shape and starch content. In addition to morphological and organizational variation, expression patterns of candidate gene markers underpinning this variation were examined. Features commonly associated with grains are largely defined by analyses on lineages within the Triticeae and knowledge of grain structure may be skewed as a result of the focus on wheat and barley. Specifically, the data suggest that the modified aleurone is largely restricted to species in the Triticeae tribe.

Fig. 1.

Selected taxa based on published phylogenies; schematic of grain cross-section, based on wheat, indicating grain features in maternal and endosperm tissues analysed in this study.

Fig. 2.

Vital staining of grains from several genera with tetrazolium chloride (TZ). Scale bars 400 µm. Double-ended arrow indicates bridge depth in spelt wheat.

Fig. 3.

SEM of selected grains showing peripheral aleurone and underlying central endosperm cells. The aleurone area in each species is indicated by the double-ended arrows. Scale bars (A, B, G, H) 10 µm; (C, D,E, F) 20 µm.

Fig. 4.

SEM of modified aleurone region in selected grains. ce, central endosperm; ma, modified aleurone; al, aleurone; np, nucellar projection. Arrows in (B) indicate internal surfaces of modified aleurone cells. Scale bars (A, D) 50 µm; (B, E) 10 µm; (C, F) 5 µm.

Fig. 5.

Expression analysis of Brachypodium orthologues of key aleurone/endosperm transcription factor genes in developing grain tissues. (A) RT-PCR in B. distachyon tissues and (B) RT-PCR in H. vulgare tissues. In, inflorescence; yg, young grain; mg, mid-length grain; fl, full-length grain; dg, dry grain; gDNA genomic DNA.

Fig. 6.

Toluidine blue staining of grains focusing on the aleurone cells (modified and peripheral) juxtaposed with the central endosperm and endosperm cavity. (A) Brachypodium (sylvaticum). (B) Bromus (sterilis). (C) Elymus (repens). (D–F) Triticum (uratu, tauschii, speltoides). ne, Nucellar epidermis; ce, central endosperm; al, aleurone; ca, cavity. Scale bars (A, B, D, E, F) 20 µm; (C) 50 µm.

Fig. 7.

ISH of domain-specific endosperm markers on cultivated and primitive (uncultivated) wheats. (A–E, H) peripheral aleurone marker; (I–L, N, P) modified aleurone marker; (F, G, M, O) central endosperm marker. Species are arranged vertically as labelled except where indicated. ne, Nucellar epidermis; ce, central endosperm. Scale bars (A–H) 50 µm, (I–P) 200 µm.

Fig. 8.

Schematic diagram summarizing trends in grain characteristics in terms of basic phylogeny. NOT to scale. Green, pericarp; yellow, nucellar tissues; light red, aleurone; red, central endosperm.