Proteomics in Identifying New Regulatory Mechanisms Involved in Seed Development and Ultimately Seed Quality

Abstract

Many changes in gene expression occur during maize kernel development. Most changes are associated with the three main developmental stages: lag phase, grain filling, and grain maturation. Proteomics approaches dedicated to non storage proteins coupled with measurements of enzyme activities and metabolite levels provide a way to get an overview of the main metabolic changes and to look at the coordinated variation of the enzymes involved in the main biosynthetic pathways. It also enables to single out specific variation, which may have been overlooked by targeted approaches. Identification of proteins during maize kernel development not only uncovers physiologically consistent protein patterns associated with each stage but also reveals the unexpected importance of some pathways. A major modification occurs at the transition from lag phase (establishment of potential grain size) to grain-filling phase, where starch and proteins are accumulated in the endosperm storage tissue. Although the expression of enzymes involved in the biosynthetic pathway for storage product is dominant in the accumulation phase, the proportion of protein destination (mainly, chaperonins) and protein synthesis machinery is still important. Detailed proteomics analysis of metabolism shows a surprising upsurge of the pyruvate-Pi-dikinase (PPDK) at the late grain-filling period (21 DAP onwards), and that is interpreted as a switch in the starch/protein balance. This hypothesis is based on biochemical arguments involving the negative effect of PPi generated by PPDK activity on the cytosolic ADP-glucose pyrophosphorylase, a key-enzyme in starch synthesis, and the role of phosphoenolpyruvate in aromatic amino acid synthesis. It is substantiated by the molecular genetic data on the O2 gene, which encodes a transcription factor with pleiotropic effects on lysine content, carbohydrate metabolism, and expression of the cyPPDK1 gene. One way to test this hypothesis is to use association genetics on a large panel covering most of the genetic variability in tropical, American, and European maize. After genotyping of 375 lines for O2 and cyPPDK1 genes, a polymorphism in the O2 coding sequence and several polymorphisms along the cyPPDK1 promoter have been identified which are associated in anon-additive way to both the lysine content and the protein versus starch balance. These and other findings are the subject of discussion of this chapter.