Expression profiles of respiratory components associated with mitochondrial biogenesis during germination and seedling growth under normal and restricted conditions in wheat.

Germination of plant embryo is a dynamic phase-changing process that is driven by a rapid increase in mitochondrial respiration. We studied the development of respiratory electron transport pathways and the profiles of their transcript and protein components during this critical period using wheat embryos. Oxygen consumption through both the cytochrome and alternative pathways increased rapidly upon imbibition. Quantitative RT-PCR analysis using specific primers and western blot analysis using specific antibodies suggested that this respiratory burst was supported both by the stored mRNA and protein components and ones synthesized de novo at least in the cytochrome pathway. Dry embryos also contained transcript and protein of alternative oxidase (AOX), but their levels remained constant during the studied period. By contrast, the alternative pathway capacity showed a marked increase when the cytochrome pathway was inhibited by antimycin A and this increase was associated with increased levels of AOX transcript and protein. Our results suggest that mitochondrial biogenesis is accompanied by sequential and differential gene expression and protein accumulation, and that AOX allows the complex I to continue to conserve energy thus to support embryo germination and initial seedling growth in wheat when the cytochrome pathway is restricted.

Mitochondrial biogenesis as revealed by mitochondrial transcript profiles during germination and early seedling growth in wheat.

Germination of imbibed embryos is the initial stage of plant development that is accompanied by the burst of mitochondrial respiration. To understand the process of mitochondrial biogenesis during this critical stage in wheat development, we monitored changes in mitochondrial transcript profiles during the first 3 days by adopting a newly devised macroarray system. The whole experiment was conducted in the dark to avoid influences of photosynthesis. Dry quiescent embryos started respiration rapidly after imbibition and the rate of oxygen uptake increased to peak at the first day followed by a continuous decrease until the third day under this condition. Both the cytochrome and alternative electron transport pathways appeared to contribute to this initial burst. Shoot and root growth was also remarkable during this period. Mitochondrial transcriptome was studied by macroarray analysis using 28 mitochondrial protein-coding genes, 4 nuclear encoded mitochondria-targeted genes and 2 nuclear genes as control. All transcripts were present in dry embryos at different initial levels, and a large variability was observed in their abundance among individual genes throughout the tested period. Gene expression was categorized into four clusters according to the profiles of individual transcript accumulation. A majority of the genes encoding subunits of the respiratory complexes belonged to two major clusters, the time course of transcript accumulation of one cluster agreeing with that of respiratory development and the other remaining at high constant levels. The macroarray system devised in this study should be useful in monitoring mitochondrial biogenesis under various growth conditions and at different developmental stages in cereals.