The use of fertilizer results in tall rice plants that are susceptible to lodging and results in reduced plant yields. In this study, using chromosome segment substitution lines, we identified an effective quantitative trait loci (QTL) for culm strength, which was designated STRONG CULM2 (SCM2). Positional cloning of the gene revealed that SCM2 was identical to ABERRANT PANICLE ORGANIZATION1 (APO1), a gene previously reported to control panicle structure. A near-isogenic line carrying SCM2 showed enhanced culm strength and increased spikelet number because of the pleiotropic effects of the gene. Although SCM2 is a gain-of-function mutant of APO1, it does not have the negative effects reported for APO1 overexpression mutants, such as decreased panicle number and abnormal spikelet morphology. The identification of lodging-resistance genes by QTL analysis combined with positional cloning is a useful approach for improving lodging resistance and overall productivity in rice.

A pressure chamber and a root pressure probe technique have been used to measure hydraulic conductivities of rice roots (root Lp(r) per m(2) of root surface area). Young plants of two rice (Oryza sativa L.) varieties (an upland variety, cv. Azucena and a lowland variety, cv. IR64) were grown for 31-40 d in 12 h days with 500 micromol m(-2) s(-1) PAR and day/night temperatures of 27 degrees C and 22 degrees C. Root Lp(r) was measured under conditions of steady-state and transient water flow. Different growth conditions (hydroponic and aeroponic culture) did not cause visible differences in root anatomy in either variety. Values of root Lp(r) obtained from hydraulic (hydrostatic) and osmotic water flow were of the order of 10(-8) m s(-1) MPa(-1) and were similar when using the different techniques. In comparison with other herbaceous species, rice roots tended to have a higher hydraulic resistance of the roots per unit root surface area. The data suggest that the low overall hydraulic conductivity of rice roots is caused by the existence of apoplastic barriers in the outer root parts (exodermis and sclerenchymatous (fibre) tissue) and by a strongly developed endodermis rather than by the existence of aerenchyma. According to the composite transport model of the root, the ability to adapt to higher transpirational demands from the shoot should be limited for rice because there were minimal changes in root Lp(r) depending on whether hydrostatic or osmotic forces were acting. It is concluded that this may be one of the reasons why rice suffers from water shortage in the shoot even in flooded fields.

Improvement of leaf photosynthesis is an important strategy for greater crop productivity. Here we show that the quantitative trait locus GPS (GREEN FOR PHOTOSYNTHESIS) in rice (Oryza sativa L.) controls photosynthesis rate by regulating carboxylation efficiency. Map-based cloning revealed that GPS is identical to NAL1 (NARROW LEAF1), a gene previously reported to control lateral leaf growth. The high-photosynthesis allele of GPS was found to be a partial loss-of-function allele of NAL1. This allele increased mesophyll cell number between vascular bundles, which led to thickened leaves, and it pleiotropically enhanced photosynthesis rate without the detrimental side effects observed in previously identified nal1 mutants, such as dwarf plant stature. Furthermore, pedigree analysis suggested that rice breeders have repeatedly selected the high-photosynthesis allele in high-yield breeding programs. The identification and utilization of NAL1 (GPS) can enhance future high-yield breeding and provides a new strategy for increasing rice productivity.

Lodging has been a major roadblock to attaining increased crop productivity. In an attempt to understand the mechanism for culm strength in rice, we isolated an effective quantitative trait locus (QTL), STRONG CULM3 (SCM3), the causal gene of which is identical to rice TEOSINTE BRANCHED1 (OsTB1), a gene previously reported to positively control strigolactone (SL) signaling. A near-isogenic line (NIL) carrying SCM3 showed enhanced culm strength and increased spikelet number despite the expected decrease in tiller number, indicating that SL also has a positive role in enhancing culm strength and spikelet number. We produced a pyramiding line carrying SCM3 and SCM2, another QTL encoding APO1 involved in panicle development. The NIL-SCM2+SCM3 showed a much stronger culm than NIL-SCM2 and NIL-SCM3 and an increased spikelet number caused by the additive effect of these QTLs. We discuss the importance of utilizing suitable alleles of these STRONG CULM QTLs without inducing detrimental traits for breeding.

An indica variety Takanari is known as one of the most productive rice varieties in Japan and consistently produces 20–30% heavier dry matter during ripening than Japanese commercial varieties in the field. The higher rate of photosynthesis of individual leaves during ripening has been recognized in Takanari. By using pot-grown plants under conditions of minimal mutual shading, it was confirmed that the higher rate of leaf photosynthesis is responsible for the higher dry matter production after heading in Takanari as compared with a japonica variety, Koshihikari. The rate of leaf photosynthesis and shoot dry weight became larger in Takanari after the panicle formation and heading stages, respectively, than in Koshihikari. Roots grew rapidly in the panicle formation stage until heading in Takanari compared with Koshihikari. The higher rate of leaf photosynthesis in Takanari resulted not only from the higher content of leaf nitrogen, which was caused by its elevated capacity for nitrogen accumulation, but also from higher stomatal conductance. When measured under light-saturated conditions, stomatal conductance was already decreased due to the reduction in leaf water potential in Koshihikari even under conditions of a relatively small difference in leaf–air vapour pressure difference. In contrast, the higher stomatal conductance was supported by the maintenance of higher leaf water potential through the higher hydraulic conductance in Takanari with the larger area of root surface. However, no increase in root hydraulic conductivity was expected in Takanari. The larger root surface area of Takanari might be a target trait in future rice breeding for increasing dry matter production.

Dry seeds contain translatable, long-lived mRNAs that are stored during seed maturation. Early studies using transcriptional inhibitors supported the view that protein synthesis during the initial phase of germination occurs on long-lived mRNA templates. Rice seeds were treated with the transcriptional inhibitor actinomycin D (Act D), and the embryonic proteins translated from long-lived mRNAs during germination were identified using a proteomic analysis. De novo transcription was not required for germination of rice seeds, since >80% of seeds germinated when transcription was prevented by treatment with Act D. In contrast, germination was completely inhibited in the presence of cycloheximide, an inhibitor of translation. Thus, de novo protein synthesis is necessary for germination of rice seeds. The proteomic analysis revealed that 20 proteins are up-regulated during germination, even after Act D treatment. Many of the up-regulated proteins are involved in carbohydrate metabolism and cytoskeleton formation. These results indicate that some of the germination-specific proteins involved in energy production and maintenance of cell structure in rice seeds are synthesized from long-lived mRNAs. The timing of translation of eight up-regulated proteins was clearly later than that of the other up-regulated proteins under conditions in which transcription was inhibited by Act D, suggesting that translation of long-lived mRNAs in rice seeds is regulated according to the germination phase.

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The depths of cure and the distributions of degree of conversion (DC), polymerization conversion (PC) and percent pendant double bonds (PDB) of light-cured composite resins cured under various intensities of light were investigated. When the total amount of exposure, represented by the product of the light intensity and the irradiation time, was kept constant, each of the depth of cure and the distributions of DC, PC and PDB were the same for each material regardless of the light intensity and irradiation time. The depth of cure could be expressed as a logarithmic function of the total amount of exposure. From regression analysis, the attenuation coefficient and the critical total amount of exposure capable of initiating polymerization of each composite resin could be determined.