Keisuke Nemoto

Rice phloem thioredoxin h has the capacity to mediate its own cell-to-cell transport through plasmodesmata

Abstract. Rice (Oryza sativa L.) phloem sieve tubes contain RPP13-1, a thioredoxin h protein that moves around the plant via the translocation stream. Such phloem-mobile proteins are thought to be synthesized in the companion cells prior to being transferred, through plasmodesmata, to the enucleate sieve-tube members. In this study, in-situ hybridization experiments confirmed that expression of RPP13-1 is restricted to companion cells within the mature phloem. To test the hypothesis that RPP13-1 enters the sieve tube, via plasmodesmata, recombinant RPP13-1 was expressed in Escherichia coli, extracted, purified and fluorescently labeled with fluorescein isothiocyanate (FITC) for use in microinjection experiments into tobacco (Nicotiana tabacum L.) mesophyll cells. The FITC-RPP13-1 moved from the injected cell into surrounding cells, whereas the E. coli thioredoxin, an evolutionary homolog of RPP13-1, when similarly labeled and injected, failed to move in this same experimental system. In addition, co-injection of RPP13-1 and FITC-dextrans established that RPP13-1 can induce an increase in plasmodesmal size exclusion limit to a value greater than 9.4 but less than 20 kDa. Nine mutant forms of RPP13-1 were constructed and tested for their capacity to move from cell to cell; two such mutants were found to be incapable of movement. Crystal-structure prediction studies were performed on wild-type and mutant RPP13-1 to identify the location of structural motifs required for protein trafficking through plasmodesmata. These studies are discussed with respect to plasmodesmal-mediated transport of macromolecules within the companion cell-sieve tube complex.

Promotion of leaf sheath growth by gibberellic acid in a dwarf mutant of rice

Abstract. The mechanism of gibberellin (GA)-induced leaf sheath growth was examined using a dwarf mutant of rice (Oryza sativa L. cv. Tan-ginbozu) treated in advance with an inhibitor of GA biosynthesis. Gibberellic acid (GA3) enhanced the growth of the second leaf sheath, but auxins did not. Measurement of the mitotic index and cell size revealed that cell elongation rather than cell division is promoted by GA3. Gibberellic acid increased the extensibility of cell walls in the elongation zone of the leaf sheath. It also increased the total amount of osmotic solutes including sugars in the leaf sheath, but did not increase the osmotic concentration of the cell sap, due to an accompanying increase in cell volume by water absorption. In the later stage of GA3-induced growth, starch granules completely disappeared from leaf sheath cells, whereas dense granules remained in control plants. These findings indicate that GA enhances cell elongation by increasing wall extensibility, osmotic concentration being kept unchanged by starch degradation.

QTLs for branching, floret formation, and pre-flowering floret abortion of rice panicle in a temperate japonica × tropical japonica cross

A large panicle with numerous florets is essential for improving rice (Oryza sativa L.) yield. Rice panicle size is determined by such underlying morphogenetic processes as: (1) primary branch formation on the panicle axis; (2) floret formation on the primary branches (mainly determined by the secondary branch formation); and (3) pre-flowering abortion of florets in the panicle. We examined QTLs for these processes to understand how they are integrated into panicle size. We developed 106 backcross-inbred lines (BC1F4) from a cross between ‘Akihikari’ (a temperate japonica) and ‘IRAT109’ (a tropical japonica) and constructed a genetic map. One QTL detected on chromosome 2, with a large effect (R=0.30) on the number of florets per panicle, affected both primary branch formation on the panicle axis and floret formation on the primary branches. In addition, three QTLs that affect only one of these two processes were identified on chromosomes 4, 9, and 11, each having a subsidiary effect on the number of florets per panicle (R2=0.04–0.07). QTLs for pre-flowering floret abortion were detected at three different regions of the genome (chromosomes 1, 10, and 11). This is the first report on QTLs for pre-flowering floret abortion in grasses. The absence of a co-location between QTLs suggests that floret formation and abortion are not directly linked causally. These results demonstrate that studying the partitioning of panicle size into these underlying morphogenetic components would be helpful in understanding the complicated genetic control of panicle size.

Inheritance of early elongation ability in floating rice revealed by diallel and QTL analyses

In floating rice, stem elongation begins much earlier than in non-floating rice, which is the major survival mechanism for flooding. Inheritance of this early elongation ability was studied using diallel and quantitative trait locus (QTL) analyses. The diallel analysis was undertaken using a set of 6×6 half-diallel crosses involving four floating (‘Goai’, ‘Habiganj Aman VIII’, ‘Badal 106’, and Oryza rufipogon strain W120) and two non-floating (‘Latisail’ and ‘Patnai 23’) parents. The additive gene effects were higher than the dominant effects. The dominant alleles were concentrated in the cultivated floating parents (‘Goai’, ‘Habiganj ‘Aman VIII’, and ‘Badal 106’), whereas the recessive alleles were in the wild floating parent (W120). A QTL analysis using a ‘Patnai 23’ × ‘Goai’ F2 population detected two putative QTLs. Of these QTLs, the one on chromosome 12 behaved as a partially dominant major gene that explained more than half of the total genetic variation.

Morphology and anatomy of rice roots with special reference to coordination in organo- and histogenesis

Rice belongs to monocotyledon which is characterized by having a so-called fibrous root system. Such root system is built up with seminal and nodal roots with numerous lateral roots. Morphology and anatomy of rice roots which is fundamentally the same as other cereal crops, has been relatively well-described. Rice roots, however, have their own characteristics including well-developed air space in the matured cortex. In addition, many examples of coordination were shown in root organo- and histogenesis as well as root function. Morphometric analysis on anatomy of matured rice roots including vascularization suggests that differentiation and maturation of each tissue in roots proceed quite integratedly. Because vascular system is quite important with reference to root function, many studies have been done. However, following aspects of vascularization have rarely been examined; (a) connections between vascular systems of stem and nodal roots, (b) changing patterns of vascular system along the root axes, (c) connections between vascular systems of the parental roots and their lateral roots and (d) connections among different kinds of vascular elements in the same roots. Morphology and anatomy with reference to coordination in organo- and histogenesis should be further studied in order to understand both structure and function of rice roots.

Size and activity of shoot apical meristems as determinants of floret number in rice panicles

Abstract The sink capacity (floret number per unit land area) is currently a serious constraint to grain yield production in japonica rice. The size and activity of the early reproductive shoot apex (incipient panicle) are potential determinants of the number of florets generated on the panicle. This hypothesis was tested using eight field-grown japonica rice cultivars (IR65598-112-2, IR65564-44-51, Nipponbare, Akenohoshi, Dobashi 1, Koshihikari, Kochihibiki and Nakateshinsenbon). Using morphometric microscopy, we found that the initial size of the reproductive shoot apex was highly correlated with the number of primary branches, but not with the number of florets per primary branch. The cell division activity of the early reproductive apex examined by in situ hybridization analyses using the histone H4 gene probe as a marker for DNA-replicating cells varied with the cultivar. Akenohoshi had twice as many DNA-replicating cells as Nipponbare and the cell division activity was highly correlated with the number of florets per primary branch, but not with the primary branch number. We concluded that the primary branch number was determined by the initial size of the reproductive apex, and that the floret number per primary branch was determined by the cell division activity in the following apex growth. This result provides the first evidence of a relationship between cell division activity and floret formation in the rice panicle.

Radiocesium Absorption by Rice in Paddy Field Ecosystems

Although most of the radiocesium fallout that deposited in paddy fields after the Fukushima nuclear disaster in March 2011 was expected to be bound to clay in the soil resulting in a very low soil-to-plant transfer function, a radiocesium contamination level of >500 Bq/kg was detected in brown rice grown in several hilly areas of Fukushima Prefecture in the autumn of the same year. The likely source of the radiocesium was fallout deposited on organic matter in the paddy fields and litter in mountain forests, from which runoff water flowed into irrigation channels that ultimately lead to the paddy fields. This problem appears to have been caused by conditions specific to lowland rice paddy fields, which are wetland ecosystems. Integrated studies of the soil, water, and plants from an ecological viewpoint are necessary to understand the mechanism of radiocesium absorption by rice before commercial rice production in the affected areas can be resumed.

Diversity of the rachis-branching system in a panicle in Japonica rice

Abstract Recently, rice varieties having large panicles with many spikelets are expected to produce high yield. However, the ripening, growth pattern, priority to photoassimilate partitioning and final grain weight in each spikelet vary with the position of the spikelet in a panicle. Therefore, not only the panicle size but also rachis-branching system in a panicle is an important factor determining the yield. In this report, we performed principal component analysis to characterize the rachis-branching system in 65 japónica varieties. In the principal component analysis, the proportions of variability explained were 49.6 and 22.2% for the first two attribute components. The first principal component was assumed as the factor of size in number, and the second principal component was assumed as the factor of shape of panicle. The distribution of the scores for each variety in the scatter diagram showed a large diversity in panicle characteristics in japónica varieties. A small number of varieties had scores distributed in the first quadrant of the scatter diagram. These varieties would be high-yielding because the number of spikelets is high in the panicle having a relatively large number of primary rachis-branches compared with the secondary rachis-branches. Some clusters in the scatter diagram were related with their origins.

Tillering Behavior of the Rice fine culm 1 Mutant

(*Graduate School of Agricultural Science, Tohoku Univ. Tsutsumidori-Amamiyamachi, Sendai 981-8555, Japan; **Graduate School of Agricultural and Life Sciences, The Univ. of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan; ***Faculty of Agriculture, Kyushu Univ. Hakozaki, Fukuoka 812-8581, Japan; ****Asian Natural Environmental Science Center, The Univ. of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan)

QTL Analysis for Traits Associated with Feeding Value of Straw in Rice (Oryza sativa L.)

Genetic improvement of the digestibility of rice straw for increasing the utilization of the whole rice plant as feedstuffs is an important way to solve the feedstuffs shortage in southeastern China. To elucidate the genetic basis of the traits affecting the digestibility of rice straw, a rice population of 111 recombinant inbred lines (RILs) derived from a cross of Nekken 2×Gaya was used to map the quantitative traits loci (QTLs) for in vitro dry matter digestion (IVDMD), the percentages of nonstructural carbohydrate (NSC) and acid detergent fiber (exclusive of residual ash, ADFom) in 2005 and 2006. IVDMD was positively correlated with NSC, and negatively correlated with ADFom. A total of 16 QTLs were detected in the two years, and the amounts of variation explained by individual QTLs ranged from 6.9% to 15.5%. Some QTLs for IVDMD and ADFom on chromosome 2 were detected in the two years. On chromosome 2, the interval RM475–RM3515 contained QTLs for IVDMD in the two years and for NSC in 2006 only, while the interval RM3874–RM5305 influenced ADFom in the two years and NSC in 2005 only. At those loci, Gaya contributed favorable alleles to IVDMD and NSC, while Nekken 2 contributed positive alleles to ADFom.