The plant seed is not only an organ of propagation and dispersal but also the major plant tissue harvested and used either directly as part of the human diet or as feed for animals. At the present time there is concern over long term food security and the impact of the move towards meat-based diets that will lead to a significant increase in the demand for plant protein for animal feed . The amount of protein present in plant seeds varies from ~10% of the dry weight in most monocot (e.g. O. sativa, S. bicolor, S. italica, Z. mays and B. distachyon) to more than 30% in most dicots (e.g. G. max, R. communis, C. sativus and A. thaliana), and forms a major source of dietary protein [2–7]. To determine whether differences in evolutionary patterns may explain the phenotypic differences observed, a comparative investigation of evolutionary divergence in genes underlying protein synthesis in these two groups of plants is thus warranted.
Seed storage proteins can be classified into four groups: albumins, globulins, prolamins and glutelins . Albumins and globulins comprise the storage proteins of dicots, whereas prolamins and glutelins are the major proteins in monocots [4, 9, 10]. 2S albumins, a major class of dicot seed storage proteins, have been most widely studied in the Cruciferae, notably B. napus and A. thaliana [9, 10]. Prolamins, the major endosperm storage proteins of all cereal grains, with the exceptions of oats and rice, can be classified into many subgroups, e.g. sulphur-rich (S-rich), sulphur-poor (S-poor) and high molecular weight (HMW) prolamins . The globulins, the most widely distributed group of storage proteins, are part of the cupin superfamily  and are evolved from bacterial enzymes. The globulins are present not only in dicots but also in monocots  and can be divided into 7S vicilin-type and 11S legumin-type globulins according to their sedimentation coefficients. It should be noted that the genes encoding the 11S-type globulins in monocots are the same gene family, 11S globulin family, as those in dicots; whereas the genes encoding the 7S-type globulins in monocots are not evolutionarily related to those in dicots . Thus, we focus on the genes encoding the 11S-type globulins in this study.
Many efforts have been made to describe the gene families encoding seed storage proteins. For albumins, they are encoded by multi-gene families in many dicots (e.g., A. thaliana and B. napus); and evolutionary research into the gene families suggests that the albumin genes were duplicated prior to the Brassiceae-Sysimbrieae split, and gene duplication has played a role in their evolution [12, 13]. For prolamins, they are the major seed storage proteins in most grass species (e.g., Z. mays and S. bicolor); and studies have suggested that the prolamin gene families have undergone many rounds of gene duplication [14, 15]. For globulins, eight, four, eleven and fourteen gene have been identified and classified in G. max, A. thaliana, R. communis and O. sativa, respectively [16–24].
From the research described above, it can be concluded that the seed storage protein gene families have expanded in a lineage-specific manner through gene duplication. As a major process in the evolution, gene duplications can provide raw material for evolution by producing new copies. The human globin gene family is a representative example: several globin genes have arisen from a single ancestral precursor, thus making individual genes available to take on specialized roles, with some genes becoming active during embryonic and fetal development, and others becoming active in the adult organism . Gene duplications may also affect phenotype by altering gene dosage: the amount of protein synthesized is often proportional to the number of gene copies present, so extra genes can lead to excess proteins. This applies to many kinds of genes, such as rRNAs, tRNA and histones [26, 27]. A critical question thus can be asked: does gene duplication contribute to the higher levels of protein synthesis in dicots than in monocots? On the other hand, gene duplication usually brings variation in evolutionary rate , and such variation has been predicted to be associated with phenotypic differences. For example, Hunt et al.  investigated the evolution of genes associated with phenotypically plastic castes, sexes, and developmental stages of the fire ant Solenopsis invicta, and argued that an elevated rate is a precursor to the evolution of phenotypic differences. Thus, we are also interested in another question: does an accelerated evolutionary rate play a role in the evolution of storage protein content?
To shed light on the two questions above, we investigated the process of molecular evolution of the 11S globulin gene family, which is widely distributed in dicot and monocot species, by comparing the differing evolutionary patterns in the two groups. Our analyses suggested that gene duplication and an accelerated evolutionary rate in 11S globulin genes may be associated with higher protein synthesis in dicots than in monocots.