Developmental Analysis of Maize Endosperm Proteome Suggests a Pivotal Role for Pyruvate Orthophosphate Dikinase

Abstract

Although the morphological steps of maize (Zea mays) endosperm development are well described, very little is known concerning the coordinated accumulation of the numerous proteins involved. Here, we present a proteomic study of maize endosperm development. The accumulation pattern of 409 proteins at seven developmental stages was examined. Hierarchical clustering analysis allowed four main developmental profiles to be recognized. Comprehensive investigation of the functions associated with clusters resulted in a consistent picture of the developmental coordination of cellular processes. Early stages, devoted to cellularization, cell division, and cell wall deposition, corresponded to maximal expression of actin, tubulins, and cell organization proteins, of respiration metabolism (glycolysis and tricarboxylic acid cycle), and of protection against reactive oxygen species. An important protein turnover, which is likely associated with the switch from growth and differentiation to storage, was also suggested from the high amount of proteases. A relative increase of abundance of the glycolytic enzymes compared to tricarboxylic acid enzymes is consistent with the recent demonstration of anoxic conditions during starch accumulation in the endosperm. The specific late-stage accumulation of the pyruvate orthophosphate dikinase may suggest a critical role of this enzyme in the starch-protein balance through inorganic pyrophosphate-dependent restriction of ADP-glucose synthesis in addition to its usually reported influence on the alanine-aromatic amino acid synthesis balance.

Functions of maize genes encoding pyruvate phosphate dikinase in developing endosperm

“…The hypothesis that PPDK regulates net division between starch and storage protein biosynthesis (3,21) also is inconsistent with the results. Endosperm starch and total N levels are not altered in transgenic segregants.…”

“…function in plant glycolysis is lacking, however, and any role in lowoxygen conditions remains to be determined. Other hypotheses for endosperm PPDK function include (i) gluconeogenesis, providing hexose for starch biosynthesis; (ii) provision of pyruvate for lipid synthesis; and (iii) control of metabolic fluxes through its contribution to PPi homoeostasis (3,19,21,23,24). The latter hypothesis considers that PPi is a product of both PPDK's gluconeogenic activity and ADP glucose pyrophosphorylase that routes glucosephosphate into starch metabolism.…”

“…Potential glycolytic function of PPDK in plants is suggested by its markedly increased expression in rice coleoptiles or seedlings subjected to anoxia (18,19). Maize endosperm is another oxygen-limited tissue with high PPDK levels (20)(21)(22), coincident with biomass accumulation and associated ATP demand. Direct evidence of PPDK…”

Maize endosperm-specific transcription factors O2 and PBF network the regulation of protein and starch synthesis

“…O2 and PBF control kernel nutritional quality by regulating the synthesis of storage zein proteins. Consistent with the temporal pattern of zein gene expression, the expression of the major starch-synthetic genes and enzymatic activities increases sharply and later decreases gradually between 10 and 35 days after pollination (DAP) (8,9,27), suggesting that the transcriptional regulation of storage protein and starch synthesis in endosperm may be coordinated temporally by some common factors. Here, we show that O2 and PBF also regulate the endosperm starch synthesis and, in turn, kernel weight.…”

“…Storage protein and starch synthesis are temporally coordinated during maize endosperm development through a transcriptional regulatory network linking the two different synthetic processes (8,9,27). This coordination is now supported by genetic and molecular evidence.…”