ABSTRACT
Understanding and improving protein traits in maize Yongrui Wu, National Key Laboratory of Plant Molecular Genetics, CAS Center for Excellence in Molecular Plant Sciences, Institute of Plant Physiology & Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, China; and Joachim Messing, Waksman Institute of Microbiology, Rutgers University, USA
1 Introduction
2 Storage proteins in maize seed
3 Regulation of storage protein genes
4 Synthesis and deposition of zein proteins
5 Improving protein quality in maize seed
6 Maintaining sulphur storage in maize seed
7 Future trends
8 Acknowledgements
9 References
Global maize (Zea mays, commonly known as corn) production yields approximately 1 billion tons of seeds per year. Given that seeds comprise about 10% protein, corn supplies 100 million tons of protein for feed and food annually. Here, we review several parameters that contribute to the supply and quality of corn protein. For instance, kernel hardness is critical for mechanic strength during harvest, transportation and storage. On the other hand, kernel hardness depends on the composition of storage proteins, with zeins as the major component. Zeins are extremely biased in amino acid composition. In contrast to a-zeins, b-zein, g-zeins and d-zeins are rich in sulphur-rich amino acids, but all of them are deficient in the essential amino acids lysine and tryptophan. Because of the preponderance of zeins, which account for 60% of total seed protein, maize is nutritionally inferior and needs costly dietary supplements. Three transcription factors (TFs), opaque2 (O2), prolamin-box binding factor (PBF) and opaque2 heterodimerizing proteins (OHPs), regulate 90% of zein transcription. Misfolded or low levels of proteins result in irregular protein body (PB) shapes and opaque, non-vitreous seed phenotypes. Reduction of a-zeins causes an increase in lysine levels because of the compensatory up-regulation of other proteins in the seed. Moreover, o2 modifiers can overcome the unfavourable
concomitant defect in the loss of kernel hardness, which is known today as Quality Protein Maize (QPM). Still, QPM alone cannot alleviate the methionine deficiency in maize seed, which is mainly caused by limited availability of reduced sulphur. Therefore, increasing the capacity of sulphur reduction during photosynthesis should enhance the methionine level in addition to the overexpression of the methionine-rich 10-kD d-zein gene.
