No abstract

Mitochondrial functions are potential targets of abiotic stresses that are major environmental factors limiting plant development and productivity. To evaluate mitochondrial responses to abiotic stresses we studied mitochondrial transcriptome profiles at the early stages of wheat development after imbibition under normal and induced stress conditions. Three stresses given were low temperature (4°C), high salinity (0.2 M NaCl) and high osmotic potential (0.3 M mannitol). All these stresses greatly reduced growth but dramatically increased respiration both via the cytochrome and alternative pathways. Macroarray analysis of the mitochondrial transcriptome revealed that most of the changes in transcript levels were stress specific but groups of genes responded commonly to different stresses. Under 3-days continuous stresses, 13 genes showed low temperature specific responses with either up-or down-regulation, while 14 and 23 genes showed responses specific to high salinity and high osmotic potential, respectively. On the other hand, 13 genes showed common responses, among which cob and ccmFn increased their transcript levels while transcripts of the other genes including nad6, atp4 and atp9 decreased. The differential profiles of mitochondrial transcriptome revealed by the macroarray analysis were verified by the quantitative reverse transcriptase PCR analysis. Taken together, three among five nuclearencoded mitochondria-targeted genes included in the array showed decreases under the stresses, while MnSOD and AOX increased their transcript amounts. Our results indicated the existence of common and different regulatory mechanisms that can sense different abiotic stresses and modulate both nuclear and mitochondrial gene expression in germinating wheat embryos and seedlings.

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.

Introgression lines derived from Oryza minuta and O. sativa subsp. japonica var. Junambyeo were crossed for a mapping of the population composed of 112 recombinant lines to identify putative QTLs against rice blast disease using the percentage of diseased leaf area. By using 148 Sequence Tagged Site (STS) and Single Sequence Repeat (SSR) markers, five QTLs on chromosomes 6, 7, 9 and 11 and seven epistatic QTLs were identified against two blast isolates (KI307 and KI209). Of them two QTLs (qKI307-2 and qKI209-3) shared a similar position on chromosome 11. O. minuta introgression contributed the resistance allele for all of these QTLs. Combined phenotypic variations by QTL and (E-QTL) accounted for 56.9% against KI307, and 53.4% against KI209. Each QTL could account for the resistance variation between 11 and 24.6%. The resistance from wild introgressions was attributable to a combination of QTLs and epistatic effects between different loci, capable of inducing hypersensitive reactions. Our findings are in support of the strategy of pyramiding major QTLs to develop improved rice varieties with durable broad spectrum resistance against the blast fungus.