No abstract

An incompletely dominant gene Ur1 is characterized by undulation of primary and secondary rachis branches. The gene increases spikelet number per panicle owing to increase of secondary branches. The Ur1/Ur1, Ur1/+ and +/+ genotypes with sd1 (denoted by u, h and d, respectively, viz. three dwarfs) and those without sd1 (U, H and T, respectively, viz. three talls) were produced under the common genetic background of a japonica variety Taichung 65. The six genotypes were grown in 2000. Yield and its related traits were measured. Analysis of variance on yield indicated that the effect of Ur1 was significant whereas the effects of sd1 and the Ur1 × sd1 interaction were nonsignificant. Genotypes u and h had significantly higher yields than d; h showed the highest yield of all genotypes. The three talls had yields in the order of H ≥ U ≥ T (H > T). Regarding spikelet number per panicle, the genotypes were in the order of u = U > h ≥ H > T > d, reflecting the principal effect of Ur1. Regarding panicle number per m 2 , the effect of Ur1 was not significant. The ripened-grain percentages of the six genotypes were almost in the reverse order of spikelet number per panicle. The 1000 grain weight of the genotypes was in the order of U ≅ u ≤ h ≤ H ≤ d < T. Sink size-2 (single grain weight × fertilized-spikelet number per m 2 ) showed a high positive correlation with yield. As for LAI, there were no significant differences among the six genotypes. The total weight per m 2 at maturity for the three dwarf genotypes were in the order of h ≥ u ≥ d (h > d). As for harvest index, u and h were higher than d. Similar results were obtained for the three dwarf genotypes under three fertilizer levels in 1998. Thus, Ur1 enlarged sink size through increase of spikelet number per m 2 resulting in higher yield. Consequently, Ur1 may be utilized for developing high yielding inbred varieties and F 1 varieties at the Ur1/+ genotype.

Effects of the dwarfing gene sd1 originating from the Taiwanese rice variety 'Dee-geo-woo-gen' on yield, yield components and other traits were investigated using two isogenic lines, T d and S d , and their respective parental varieties, 'Taichung 65' (T65) and 'Shiokari'. At Kochi University, T d and T65 were grown at three fertilizer levels in 1998, and at one fertilizer level in 1995. S d and 'Shiokari' were grown at one fertilizer level at Hokkaido University in 1996. Yields of T d and T65 increased as the fertilizer level was increased in 1998. The yield of T d was lower than that of T65 under all four growing conditions. Similarly, the yield of S d was lower than that of 'Shiokari'. In spikelet number per panicle, T d and S d were significantly smaller than their respective parental varieties under all growing conditions. In panicle number per m 2 , differences between T d and T65 were nonsignificant under all growing conditions. In this trait, S d was larger than 'Shiokari'. In ripenedgrain percentage, T d was higher than T65. In 1000 grain weight, on the other hand, T d was smaller than T65 under all growing conditions except that in 1995. In these two traits, however, S d and 'Shiokari' were similar to each other. T d and T65 were similar to each other in length and width of spikelet. To sum up, the lower yields of T d and S d were mainly due to their small numbers of spikelets per panicle in comparison with their respective parental varieties. In spikelet number per m 2 and in sink size, T d and S d were smaller than their respective parental varieties, indicating that sd1 decreases sink size. In LAI and leaf weight per m 2 at the 80 %-heading stage, T d and T65 were similar to each other, indicating that sd1 has almost no effect on the total amount of photosynthetic organ.

Inflorescence architecture is diverse in angiosperms, and is mainly determined by the arrangement of the branches and flowers, known as phyllotaxy. In rice (Oryza sativa), the main inflorescence axis, called the rachis, generates primary branches in a spiral phyllotaxy, and flowers (spikelets) are formed on these branches. Here, we have studied a classical mutant, named verticillate rachis (ri), which produces branches in a partially whorled phyllotaxy. Gene isolation revealed that RI encodes a BELL1-type homeodomain transcription factor, similar to Arabidopsis PENNYWISE/BELLRINGER/REPLUMLESS, and is expressed in the specific regions within the inflorescence and branch meristems where their descendant meristems would soon initiate. Genetic combination of an ri homozygote and a mutant allele of RI-LIKE1 (RIL1) (designated ri ril1/+ plant), a close paralog of RI, enhanced the ri inflorescence phenotype, including the abnormalities in branch phyllotaxy and rachis internode patterning. During early inflorescence development, the timing and arrangement of primary branch meristem (pBM) initiation were disturbed in both ri and ri ril1/+ plants. These findings suggest that RI and RIL1 were involved in regulating the phyllotactic pattern of the pBMs to form normal inflorescences. In addition, both RI and RIL1 seem to be involved in meristem maintenance, because the ri ril1 double-mutant failed to establish or maintain the shoot apical meristem during embryogenesis.

A dwarfing gene (allele) sd1-d has been intensively utilized to develop short-culm indica varieties in southeast Asia up to now. Before the first sd1-d-carrying variety IR8 was released, rice researchers had recognized the general tendency that culm length is higher in indica varieties than in temperate-japonica ones. Inter-subspecific difference of the tall (wild-type) allele SD1 at the sd1 locus was examined on the common genetic background, using five isogenic lines developed by substituting sd1-d of the recurrent parent IR36 by SD1s of two indica varieties, two temperate-japonica varieties and one tropical-japonica variety. The two indica -donor isogenic lines had longer culms than the three japonica-donor isogenic lines consistently in two different environmental conditions. Moreover, nonsynonymous single-nucleotide polymorphism between the two subspecies was detected at two sites in Exon 1 and Exon 3 of the sd1 locus. It is demonstrated that the inter-subspecific differentiation of SD1 contributes height difference between indica and japonica. The indica-originating and japonica-originating alleles at the sd1 locus were designated as SD1-in(t) and SD1-ja(t), respectively.