Summary
Post‐embryonic development depends on the activity of meristems in plants, and thus control of cell fate in the meristem is crucial to plant development and its architecture. In grasses such ...as rice and maize, the fate of reproductive meristems changes from indeterminate meristems, such as inflorescence and branch meristems, to determinate meristems, such as the spikelet meristem. Here we analyzed a recessive mutant of rice, aberrant spikelet and panicle1 (asp1), that showed pleiotropic phenotypes such as a disorganized branching pattern, aberrant spikelet morphology, and disarrangement of phyllotaxy. Close examination revealed that regulation of meristem fate was compromised in asp1: degeneration of the inflorescence meristem was delayed, transition from the branch meristem to the spikelet meristem was accelerated, and stem cell maintenance in both the branch meristem and the spikelet meristem was compromised. The genetic program was also disturbed in terms of spikelet development. Gene isolation revealed that ASP1 encodes a transcriptional co‐repressor that is related to TOPLESS (TPL) in Arabidopsis and RAMOSA ENHANCER LOCUS2 (REL2) in maize. It is likely that the pleiotropic defects are associated with de‐repression of multiple genes related to meristem function in the asp1 mutant. The asp1 mutant also showed de‐repression of axillary bud growth and disturbed phyllotaxy in the vegetative phase, suggesting that the function of this gene is closely associated with auxin action. Consistent with these observations and the molecular function of Arabidopsis TPL, auxin signaling was also compromised in the rice asp1 mutant. Taken together, these results indicate that ASP1 regulates various aspects of developmental processes and physiological responses as a transcriptional co‐repressor in rice.
In mechanically harvested soybean, green stem disorder (GSD) is an undesirable trait that causes green-stained seeds, which are graded lower in Japan. To obtain DNA markers for reduced GSD, we ...conducted a quantitative trait locus (QTL) analysis for 2 years using F4 and F5 lines from a cross between ‘Suzuotome’ (less GSD) and ‘Fukuyutaka’ (more GSD). We validated the effect of a detected QTL for GSD by first identifying F4 or F5 plants in which one or more markers in the QTL region were heterozygous. The F5 or F6 progeny of each plant was used to form a pair consisting of two groups in which the QTL region was homozygous for either the ‘Suzuotome’ or ‘Fukuyutaka’ allele in a similar genetic background, and the two groups within each pair were compared for GSD. Over 3 years of testing, the ‘Suzuotome’ allele of a QTL on chromosome 6 was found to reduce the level of GSD. This novel QTL was mapped to the region around DNA marker W06_0130, and was not closely linked to QTLs for important agronomic traits including yield components. Using this marker, the low level of GSD from ‘Suzuotome’ could be conferred to ‘Fukuyutaka’ or other high-GSD cultivars.
Although an indeterminate growth habit is attractive to develop high-yield soybean varieties with higher number of pods (Glycine max (L). Merr.), lodging in indeterminate varieties remains a problem ...in Japan. As the semi-determinate varieties have shorter main stem length than the indeterminate varieties, this trait can be useful to improve varieties with high yield and low lodging risk. We introduced the genes Dt1 and Dt2, which regulate stem growth habit, into three determinate varieties by backcrossing and evaluated the resulting effects on yield and lodging tendency under four different growing environments. The yield and lodging degree of the semi-determinate and indeterminate lines were higher and more severe than those of the determinate lines. Despite the lower overall lodging score, the semi-determinate lines had marginally lower overall yield than that of the indeterminate lines. However, the effect of introduction of semi-determinate traits on yield and lodging degree was different in the three backgrounds, with the yield of semi-determinate lines being the highest and the difference in lodging degree between the semi-determinate and determinate lines being under 1.0 in one background. Therefore, semi-determinate growth habit has potential to develop high yielding varieties with low lodging risk.
Low temperatures after flowering cause seed cracking (SC) in soybean. Previously, we reported that proanthocyanidin accumulation on the dorsal side of the seed coat, controlled by the I locus, may ...lead to cracked seeds; and that homozygous IcIc alleles at the I locus confer SC tolerance in the line Toiku 248. To discover new genes related to SC tolerance, we evaluated the physical and genetic mechanisms of SC tolerance in the cultivar Toyomizuki (genotype II). Histological and texture analyses of the seed coat revealed that the ability to maintain hardness and flexibility under low temperature, regardless of proanthocyanidin accumulation in the dorsal seed coat, contributes to SC tolerance in Toyomizuki. This indicated that the SC tolerance mechanism differed between Toyomizuki and Toiku 248. A quantitative trait loci (QTL) analysis of recombinant inbred lines revealed a new, stable QTL related to SC tolerance. The relationship between this new QTL, designated as qCS8-2, and SC tolerance was confirmed in residual heterozygous lines. The distance between qCS8-2 and the previously identified QTL qCS8-1, which is likely the Ic allele, was estimated to be 2–3 Mb, so it will be possible to pyramid these regions to develop new cultivars with increased SC tolerance.
Sheath blight, caused by Rhizoctonia solani, is one of the most serious diseases of rice. Among 33 rice accessions, mainly from National Institute of Agrobiological Sciences (NIAS) Core Collection, ...we found three landraces from the Himalayas—Jarjan, Nepal 555 and Nepal 8—with resistance to sheath blight in 3 years’ field testing. Backcrossed inbred lines (BILs) derived from a cross between Jarjan and the leading Japanese cultivar Koshihikari were used in QTL analyses. Since later-heading lines show fewer lesions, we used only earlier-heading BILs to avoid association with heading date. We detected eight QTLs; the Jarjan allele of three of these increased resistance. Only one QTL, on chromosome 9 (between markers Nag08KK18184 and Nag08KK18871), was detected in all 3 years. Chromosome segment substitution lines (CSSLs) carrying it showed resistance in field tests. Thirty F2 lines derived from a cross between Koshihikari and one CSSL supported the QTL.
Target-site and non-target-site herbicide tolerance are caused by the prevention of herbicide binding to the target enzyme and the reduction to a nonlethal dose of herbicide reaching the target ...enzyme, respectively. There is little information on the molecular mechanisms involved in non-target-site herbicide tolerance, although it poses the greater threat in the evolution of herbicide-resistant weeds and could potentially be useful for the production of herbicide-tolerant crops because it is often involved in tolerance to multiherbicides. Bispyribac sodium (BS) is an herbicide that inhibits the activity of acetolactate synthase. Rice (Oryza sativa) of the indica variety show BS tolerance, while japonica rice varieties are BS sensitive. Map-based cloning and complementation tests revealed that a novel cytochrome P450 monooxygenase, CYP72A31, is involved in BS tolerance. Interestingly, BS tolerance was correlated with CYP72A31 messenger RNA levels in transgenic plants of rice and Arabidopsis (Arabidopsis thaliana). Moreover, Arabidopsis overexpressing CYP72A31 showed tolerance to bensulfuron-methyl (BSM), which belongs to a different class of acetolactate synthase-inhibiting herbicides, suggesting that CYP72A31 can metabolize BS and BSM to a compound with reduced phytotoxicity. On the other hand, we showed that the cytochrome P450 monooxygenase CYP81A6, which has been reported to confer BSM tolerance, is barely involved, if at all, in BS tolerance, suggesting that the CYP72A31 enzyme has different herbicide specificities compared with CYP81A6. Thus, the CYP72A31 gene is a potentially useful genetic resource in the fields of weed control, herbicide development, and molecular breeding in a broad range of crop species.
In soybean Glycine max (L.) Merrill, the genetic analysis of seed yield is important to aid in the breeding of high-yielding cultivars. Seed yield is a complex trait, and the number of quantitative ...trait loci (QTL) involved in seed yield is high. The aims of this study were to identify QTL associated with seed yield and validate their effects on seed yield using near-isogenic lines. The QTL analysis was conducted using a recombinant inbred line population derived from a cross between Japanese cultivars ‘Toyoharuka’ and ‘Toyomusume’, and eight seed yield-associated QTL were identified. There were significant positive correlations between seed yield and the number of favorable alleles at QTL associated with seed yield in the recombinant inbred lines for three years. The effects of qSY8-1, a QTL promoting greater seed yield, was validated in the Toyoharuka background. In a two-year yield trial, the 100-seed weight and seed yield of Toyoharuka-NIL, the near-isogenic line having the Toyomusume allele at qSY8-1, were significantly greater than those of Toyoharuka (106% and 107%, respectively) without any change for days to flowering and maturity. Our results suggest that qSY8-1 was not associated with maturity genes, and contributed to the 100-seed weight.
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.
The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is a devastating pest of soybean (Glycine max (L.) Merr.) in the world. Three soybean QTLs for resistance to SCN race 1 were detected ...through QTL analyses using recombinant inbred lines (RILs) derived from a cross between ‘Tokei 758’ (susceptible) and ‘To-8E’ (resistant to races 1 and 3, derived from ‘PI 84751’ and ‘Gedenshirazu’). Two of the three QTLs appear to be rhg1 and Rhg4 from their locations on the linkage map. The third QTL, detected around Satt359 on chromosome 11, was tentatively identified as rhg2. All RILs resistant to race 1 had all three QTLs. We developed lines carrying the three loci in various combinations, including all and none, from descendants of a cross between ‘NIL-SCN’ (with resistance derived from ‘PI 84751’ in the ‘Natto-shoryu’ background) and ‘Natto-shoryu’. Evaluating these lines in a race 1-infected field in Mito, Ibaraki, showed that resistance to race 1 required all three loci. Through field evaluation of 10 recombinant fixed pairs that we developed, we located the rhg2 locus to an 821 kb-region between SSR markers Sat_123 (=WGSP11_0140) and BARCSOYSSR11_1420 on chromosome 11.
Leaf color is an important indicator when evaluating plant growth and responses to biotic/abiotic stress. Acquisition of images by digital cameras allows analysis and long-term storage of the ...acquired images. However, under field conditions, where light intensity can fluctuate and other factors (shade, reflection, and background, etc.) vary, stable and reproducible measurement and quantification of leaf color are hard to achieve. Digital scanners provide fixed conditions for obtaining image data, allowing stable and reliable comparison among samples, but require detached plant materials to capture images, and the destructive processes involved often induce deformation of plant materials (curled leaves and faded colors, etc.). In this study, by using a lightweight digital scanner connected to a mobile computer, we obtained digital image data from intact plant leaves grown in natural-light greenhouses without detaching the targets. We took images of soybean leaves infected by Xanthomonas campestris pv. glycines, and distinctively quantified two disease symptoms (brown lesions and yellow halos) using freely available image processing software. The image data were amenable to quantitative and statistical analyses, allowing precise and objective evaluation of disease resistance.
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