1. Genetic regulatory network of storage proteins in maize seed.

Endosperm is a terminal organ for storage of proteins and carbohydrates. In maize, the major storage proteins are alcohol-soluble prolamines called zeins. Zeins are divided into four classes, i.e., α (19- and 22-kDa), γ (50-, 27- and 16-kDa), β (15-kDa) and δ (18- and 10-kDa). Opaque 2 (O2) and P-box Binding Factor (PBF) are two functionally conserved storage-protein TFs in grass crops belonging to the bZIP and Dof TF family, respectively. O2 and PBF begin to express at 10 DAP, concomitant with activation of all zein genes. O2 regulates the expression of most zein genes, while PBF by itself has limited transactivation activity, but it promotes the binding of O2 to O2 motifs, resulting in the synergistic transactivation. OHP1 and OHP2 are two homologues of O2. They are generally expressed in all tissues and primarily regulate the expression of the 27-kDa γ-zein gene through interaction with PBF as well. Currently, other TFs regulating storage-protein gene expression are under investigation.

Figure 1. The working model for transcriptional regulation of storage protein genes in maize endosperm and embryo

2. Mechanisms of endosperm modification in Quality Protein Maize (QPM)

Maize is one of the most important feed and food crops in China. However, essential amino acids like lysine and tryptophan are deficient in its seed, which substantially compromises its nutritional quality. The poor protein quality of maize can be improved by the o2 mutation, which increases the lysine and tryptophan levels by decreasing the synthesis of zeins and compensatorily increasing other seed proteins. But unfortunately, the chalky and soft texture of o2 kernels limits its practical use. Genetic selection for o2 modifiers can convert the normally chalky endosperm of the mutant into a hard, vitreous phenotype, yielding what is known as Quality Protein Maize (QPM). QPM contains nearly doubled levels of lysine, therefore has significantly higher nutrition value than the regular maize. However, lack of knowledge in identities of o2 modifiers, and their acting mechanisms has greatly impeded the development of new QPM varieties. We are actively addressing two basic questions: What are the genetic identities of the quantitative o2 modifiers? What are the mechanisms by which the o2 modifiers convert the soft endosperm into a hard version? Our long-term goal is to facilitate development of new maize varieties with high yield and nutritional quality.

Figure 2. Segregation of endosperm modification in QPM Pool 42. Because QPM Pool 42 is not homozygous for o2 modifiers, it segregates progeny ears and kernels with differential degrees of modification. From left to right, ear 1 is totally unmodified and therefore all kernels are opaque; ear 2 is partially modified and only a few kernels are vitreous; ear 3 is mostly modified, with sporadic kernels that are mosaics of vitreousness and opaque endosperm; ear 4 is almost completely modified. The asterisk, arrow and arrowhead in ears 2 and 3 indicate kernels with vitreous, mosaic and opaque phenotypes

3. Endosperm development and filling

Angiosperm seeds result from double fertilization, which gives rise to the embryo and endosperm. The embryo is diploid, while the endosperm is triploid, two doses of the nuclear genome coming from the female and one from the male. Arabidopsis and maize are two model plants for dicot and monocot, respectively. Arabidopsis develops a transient endosperm, while maize has a persistent one. Endosperm is the largest portion of the maize seed, so a better understanding of endosperm development especially the filling stage is critical for its genetic improvement. The endosperm is a highly differentiated organ that contains four cell types: the aleurone cell layer, starchy endosperm cells, basal endosperm, transfer layers (BETL), and embryo surrounding cells; these cell types perform specific functions during endosperm development. Since maize has abundant different endosperm mutants and well established genetic analysis systems, it is no doubt the best model for study of endosperm development in grass crops.

Figure 3. Comparison of seed development in Maize and Arabidopsis.