Abstract
BackgroundThe changes in storage reserve accumulation during maize (Zea mays L.) grain maturation are well established. However, the key molecular determinants controlling carbon flux to the grain and the partitioning of carbon to starch and protein are more elusive. The Opaque-2 (O2) gene, one of the best-characterized plant transcription factors, is a good example of the integration of carbohydrate, amino acid and storage protein metabolisms in maize endosperm development. Evidence also indicates that the Opaque-7 (O7) gene plays a role in affecting endosperm metabolism. The focus of this study was to assess the changes induced by the o2 and o7 mutations on maize endosperm metabolism by evaluating protein and amino acid composition and by transcriptome profiling, in order to investigate the functional interplay between these two genes in single and double mutants.ResultsWe show that the overall amino acid composition of the mutants analyzed appeared similar. Each mutant had a high Lys and reduced Glx and Leu content with respect to wild type. Gene expression profiling, based on a unigene set composed of 7,250 ESTs, allowed us to identify a series of mutant-related down (17.1%) and up-regulated (3.2%) transcripts. Several differentially expressed ESTs homologous to genes encoding enzymes involved in amino acid synthesis, carbon metabolism (TCA cycle and glycolysis), in storage protein and starch metabolism, in gene transcription and translation processes, in signal transduction, and in protein, fatty acid, and lipid synthesis were identified. Our analyses demonstrate that the mutants investigated are pleiotropic and play a critical role in several endosperm-related metabolic processes. Pleiotropic effects were less evident in the o7 mutant, but severe in the o2 and o2o7 backgrounds, with large changes in gene expression patterns, affecting a broad range of kernel-expressed genes.ConclusionAlthough, by necessity, this paper is descriptive and more work is required to define gene functions and dissect the complex regulation of gene expression, the genes isolated and characterized to date give us an intriguing insight into the mechanisms underlying endosperm metabolism.
Highlights
The changes in storage reserve accumulation during maize (Zea mays L.) grain maturation are well established
The highly variable gene expression patterns they obtained made it difficult to identify common pathways that lead to soft endosperm texture. Our study extends their analysis by including the o7 mutation and the double o2o7 mutant, that appear useful in conjunction with the o2 mutation to i) identify and catalogue in endosperm the changes of genes involved in several metabolic pathways underlying the synthesis of storage reserves, ii) give new information about the effects of the O7 gene in endosperm metabolism in order to better understand its function in carbohydrate and protein syntheses, and iii) gain an insight into the complex gene system that integrates C and N metabolism in the developing endosperm
Effect of o2, o7, and o2o7 mutations on protein and amino acid compositions To verify whether the mutants analyzed exhibited qualitative and quantitative differences in protein composition compared to wild-type, we evaluated the protein and amino acid compositions of mature endosperm of the nearly isogenic A69Ywt, o2, o7, and o2o7 inbreds
Summary
The changes in storage reserve accumulation during maize (Zea mays L.) grain maturation are well established. The changes in storage reserve accumulation during maize grain maturation are well established, identifying key molecular determinants controlling carbon (C) flux to the grain and the partitioning of C to starch and protein remain elusive [1]. The most abundant protein storage component in developing endosperms, are alcohol-soluble compounds with a characteristic amino acid composition, being rich in glutamine, proline, alanine, and leucine, and almost completely devoid of lysine and tryptophan [3] Based on their solubility, genetic properties, and apparent molecular masses, zeins were classified into a- (19- and 22-kDa), b- (15-kDa), g- (16-, 27-, and 50-kDa), and δ-zeins (10- and 18-kDa) that are encoded by distinct classes of structural genes [4]. The large a-zein component, accounting for > 70% of all zein proteins, is encoded by multiple active genes clustered in several chromosomal locations [5]
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