We characterized a rice monoculm mutant moc2, which showed significantly reduced tiller numbers, pale-green leaves, a reduced growth rate, and a consequent dwarf phenotype. The monoculm feature was attributed to a deficiency in the efficient outgrowth of tiller buds, although the moc2 mutant produced tiller buds. Inconsistent change was observed in the expression of genes involved in tiller bud outgrowth, suggesting that the moc2 mutant has a defective function necessary for the tiller bud outgrowth. The gene responsible for the moc2 mutant was mapped to a locus encoding cytosolic fructose-1,6-bisphosphatase 1 (FBP1), in which a Tos17 retrotransposon was inserted in exon 4. Reverse-transcription PCR for the FBP1 gene amplified a shorter transcript from the moc2 mutant than from the wild-type plant. The sequence of the shorter transcript revealed a deletion of exon 4 by abnormal splicing, and the resulting frameshift generated a new translation termination signal. The moc2 mutant showed a very low level of FBPase activity, suggesting that it involves a loss-of-function mutation of FBP1. Cytosolic FBPase is considered a key enzyme in the sucrose biosynthesis pathway. Defective FBPase activity is anticipated to lead a shortage of sucrose supply, which probably causes the inhibition of tiller bud outgrowth in the moc2 mutant. The monoculm phenotype of the moc2 mutant supports the idea that sucrose supply may be an important cue to outgrow tiller buds.

Male sterility induced by low temperatures (LTs) during the reproductive stage is a major constraint for temperate zone rice. To detect physiological quantitative trait loci (QTLs), we modeled genotypic variation in the physiological processes involved in low temperature spikelet sterility on the basis of anther length (AL), a proxy for microspore and pollen grain number per anther. The model accounted for 83% of the genotypic variation in potential AL at normal temperature and the ability to maintain AL at LT. We tested the model on 208 recombinant inbred lines of cold-tolerant 'Tohoku-PL3' (PL3) × cold-sensitive 'Akihikari' (AH) for 2 years. QTLs for spikelet fertility (FRT) at LT were detected on chromosomes 5 (QTL for Cold Tolerance at Reproductive stage, qCTR5) and 12 (qCTR12). qCTR12 was annotated with the ability to maintain AL under LTs. qCTR5 was in a region shared with QTLs for culm length and heading date. Genome-wide expression analysis showed 798 genes differentially expressed in the spikelets between the parents at LTs. Of these, 12 were near qCTR5 and 23 were near qCTR12. Gene expression analysis confirmed two candidate genes for qCTR5 (O-methyltransferase ZRP4, Os05g0515600; beta-1,3-glucanase-like protein, Os05g0535100) and one for qCTR12 (conserved hypothetical protein, Os12g0550600). Nucleotide polymorphisms (21 deletions, 2 insertions and 10 single nucleotide polymorphisms) in PL3 were found near the candidate conserved hypothetical protein (Os12g0550600) and upstream in PL3, but not in AH. Haplotype analysis revealed that this gene came from 'Kuchum'. The combination of mapping physiological QTLs with gene expression analysis can be extended to identify other genes for abiotic stress response in cereals.

High temperature stress during seed development can reduce the rice grain yield and lead to poor milling quality because of inadequate grain filling. We tested the high temperature stress tolerance of eight rice cultivars and fluctuations in the ATP content during seed development. The phenotype of Norin-22 was very sensitive to high temperature conditions, which decreased the grain yield and produced defective characteristics. A significant reduction in the ATP content was also detected in the developing seeds. Koshihikari and Nipponbare, which are progenies derived from Norin-22, are known to be sensitive to high temperature stress. It is suggested that the high temperature stress sensitivity of Norin-22 was inherited by these cultivars. Shinriki was tolerant of high temperature stress, and it produced a large proportion of normal-shaped grains under high temperature stress conditions, with only a moderate decrease in its ATP content. These results suggest that there was a relationship between ATP deficiency and defective endosperm characteristics. Therefore, it is expected that identifying a rice cultivar that produces an adequate amount of ATP during seed development will be useful for breeding a cultivar that tolerates high temperature stress.

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We tested whether exposing rice plants to abiotic stress (salt or shade) during vegetative growth affects the chilling tolerance of reproductive organs, which is one of the most important traits for rice growing in a cool climate; we used two rice cultivars with different tolerance in two growing seasons. We divided the vegetative growth into three phases to clarify the most sensitive period: 7-22 days after transplanting (DAT), 23-38 DAT and 39-54 DAT. Chilling tolerance of the pre-stressed plants was based on the male sterility induced by low temperatures. Shade and salt stress during all three vegetative growth phases significantly reduced stomatal conductance. Shade decreased the specific leaf weight and the leaf sugar and starch contents, but salt had no significant effect, despite causing leaf damage. Low temperatures during the reproductive stage induced spikelet sterility in all plants, but the magnitude was greater in the salt-and shade-stressed plants of both cultivars, especially those stressed late during vegetative growth. The increased spikelet sterility caused by chilling was closely related to the reduction of the total spikelet number per panicle. This is the first study to show that salt and low light stress during vegetative growth increased the susceptibility of rice plants to chilling damage during panicle development.