Brachypodium Seed: A Potential Model for Studying Grain Development of Cereal Crops

Abstract

Seeds of small grains are important resources for human and animal food. The understanding of seed biology is essential for crop improvement by increasing grain yields and nutritional value. In the last decade, Brachypodium distachyon has been developed as a model plant for temperate cereal grasses. Recently, several studies have been published that compare Brachypodium seed anatomy and grain development to those of wheat and barley. While seeds of these three species share many properties, distinct features were identified in Brachypodium, including relatively smaller endosperms with thick cell walls and irregularities in cell sizes in the aleurone layer. Brachypodium seeds have lower starch and higher (1,3;1,4)-β-glucan content, and lower prolamin and higher globulin protein contents, compared to its domesticated relatives. The sequences and expression of genes involved in starch biosynthesis are conserved between Brachypodium and domesticated cereals, but the expression of certain genes in Brachypodium seeds is earlier and at much lower levels, providing a possible explanation for their relatively low starch content. Proteomics analyses indicated that the predominant globulins in the storage proteins were the 11S type and that they are encoded by a multi-gene family. Less than 12 % of the storage proteins were of the prolamin class. Annotation of the Brachypodium genome revealed it contains much fewer prolamin genes than the genomes of wheat and maize, suggesting that the high levels of seed globulins in the former are at the expense of prolamins. The demonstration that expression of a wheat High-Molecular-Weight glutenin gene promoter was endosperm-specific in Brachypodium transgenic plants opens the way for analyses of other gene promoters from cereal crop species that are difficult to transform. The ease of obtaining transgenic Brachypodium plants and the relatively large size of its seeds compared to its small stature make it an ideal system for research of seed properties including dormancy, germination and maturation.