Teff, an orphan cereal in the chloridoideae, provides insights into the evolution of storage proteins in grasses

Abstract

Seed storage proteins (SSP) in cereals provide essential nutrition for humans and animals. Genes encoding these proteins have undergone rapid evolution in different grass species. To better understand the degree of divergence, we analyzed this gene family in the subfamily Chloridoideae, where the genome of teff (Eragrostis tef) has been sequenced. We find gene duplications, deletions, and rapid mutations in protein-coding sequences. The main SSPs in teff, like other grasses, are prolamins, here called eragrostins. Teff has g- and d-prolamins, but has no b-prolamins. One d-type prolamin (d1) in teff has higher methionine (33%) levels than in maize (23-25%). The other d-type prolamin (d2) has reduced methionine residues (<10%) and is phylogenetically closer to a prolamins. Prolamin d2 in teff represents an intermediate between d and a types that appears to have been lost in maize and other Panicoideae, and was replaced by the expansion of a-prolamins. Teff also has considerably larger numbers of a-prolamin genes, which we further divide into five sub-groups, where a2 and a5 represent the most abundant a-prolamins both in number and in expression. In addition, indolines that determine kernel softness are present in teff and the panicoid cereal called foxtail millet (Setaria italica) but not in sorghum or maize, indicating that these genes were only recently lost in some members of the Panicoideae. Moreover, this study provides not only information on the evolution of SSPs in the grass family but also the importance of a-globulins in protein aggregation and germplasm divergence.