The root system architecture (RSA) of crops can affect their production, particularly in abiotic stress conditions, such as with drought, waterlogging, and salinity. Salinity is a growing problem ...worldwide that negatively impacts on crop productivity, and it is believed that yields could be improved if RSAs that enabled plants to avoid saline conditions were identified. Here, we have demonstrated, through the cloning and characterization of qSOR1 (quantitative trait locus for SOIL SURFACE ROOTING 1), that a shallower root growth angle (RGA) could enhance rice yields in saline paddies. qSOR1 is negatively regulated by auxin, predominantly expressed in root columella cells, and involved in the gravitropic responses of roots. qSOR1 was found to be a homolog of DRO1 (DEEPER ROOTING 1), which is known to control RGA. CRISPR-Cas9 assays revealed that other DRO1 homologs were also involved in RGA. Introgression lines with combinations of gain-of-function and loss-of-function alleles in qSOR1 and DRO1 demonstrated four different RSAs (ultra-shallow, shallow, intermediate, and deep rooting), suggesting that natural alleles of the DRO1 homologs could be utilized to control RSA variations in rice. In saline paddies, near-isogenic lines carrying the qSOR1 loss-of-function allele had soil-surface roots (SOR) that enabled rice to avoid the reducing stresses of saline soils, resulting in increased yields compared to the parental cultivars without SOR. Our findings suggest that DRO1 homologs are valuable targets for RSA breeding and could lead to improved rice production in environments characterized by abiotic stress.
To explore the genetic resources that could be utilized to help improve root system architecture phenotypes in rice (Oryza sativa), we have conducted genome-wide association studies to investigate ...maximum root length and crown root number in 135 10-day-old Japanese rice accessions grown hydroponically. We identified a quantitative trait locus for crown root number at approximately 32.7 Mbp on chromosome 4 and designated it qNCR1 (quantitative trait locus for Number of Crown Root 1). A linkage disequilibrium map around qNCR1 suggested that three candidate genes are involved in crown root number: a cullin (LOC_Os04g55030), a gibberellin 20 oxidase 8 (LOC_Os04g55070), and a cyclic nucleotide-gated ion channel (LOC_Os04g55080). The combination of haplotypes for each gene was designated as a haploblock, and haploblocks 1, 2, and 3 were defined. Compared to haploblock 1, the accessions with haploblocks 2 and 3 had fewer crown roots; approximately 5% and 10% reductions in 10-day-old plants and 15% and 25% reductions in 42-day-old plants, respectively. A Japanese leading variety Koshihikari and its progenies harbored haploblock 3. Their crown root number could potentially be improved using haploblocks 1 and 2.
SUMMARY
Root system architecture affects plant drought resistance and other key agronomic traits such as lodging. However, although phenotypic and genomic variation has been extensively analyzed, few ...field studies have integrated phenotypic and transcriptomic information, particularly for below‐ground traits such as root system architecture. Here, we report the phenotypic and transcriptomic landscape of 61 rice (Oryza sativa) accessions with highly diverse below‐ground traits grown in an upland field. We found that four principal components explained the phenotypic variation and that accessions could be classified into four subpopulations (indica, aus, japonica and admixed) based on their tiller numbers and crown root diameters. Transcriptome analysis revealed that differentially expressed genes associated with specific subpopulations were enriched with stress response‐related genes, suggesting that subpopulations have distinct stress response mechanisms. Root growth was negatively correlated with auxin‐inducible genes, suggesting an association between auxin signaling and upland field conditions. A negative correlation between crown root diameter and stress response‐related genes suggested that thicker crown root diameter is associated with resistance to mild drought stress. Finally, co‐expression network analysis implemented with DNA affinity purification followed by sequencing analysis identified phytohormone signaling networks and key transcription factors negatively regulating crown root diameter. Our datasets provide a useful resource for understanding the genomic and transcriptomic basis of phenotypic variation under upland field conditions.
Significance Statement
Catalog of the phenomes and transcriptomes of rice cultivars grown in upland fields provides a resource for further studies toward breeding climate‐resilient crops.
Osmotic stresses, such as drought and high salinity, adversely affect plant growth and productivity. The phytohormone abscisic acid (ABA) accumulates in response to osmotic stress and enhances stress ...tolerance in plants by triggering multiple physiological responses through ABA signaling. Subclass III SNF1-related protein kinases 2 (SnRK2s) are key regulators of ABA signaling. Although SnRK2s have long been considered to be self-activated by autophosphorylation after release from PP2C-mediated inhibition, they were recently revealed to be activated by two independent subfamilies of group B Raf-like kinases, B2-RAFs and B3-RAFs, under osmotic stress conditions. However, the relationship between SnRK2 phosphorylation by these RAFs and SnRK2 autophosphorylation and the individual physiological roles of each RAF subfamily remain unknown. In this study, we indicated that B2-RAFs are constantly active and activate SnRK2s when released from PP2C-mediated inhibition by ABA-binding ABA receptors, whereas B3-RAFs are activated only under stress conditions in an ABA-independent manner and enhance SnRK2 activity. Autophosphorylation of subclass III SnRK2s is not sufficient for ABA responses, and B2-RAFs are needed to activate SnRK2s in an ABA-dependent manner. Using plants grown in soil, we found that B2-RAFs regulate subclass III SnRK2s at the early stage of drought stress, whereas B3-RAFs regulate SnRK2s at the later stage. Thus, B2-RAFs are essential kinases for the activation of subclass III SnRK2s in response to ABA under mild osmotic stress conditions, and B3-RAFs function as enhancers of SnRK2 activity under severe stress conditions.
To clarify the effect of deep rooting on grain yield in rice (Oryza sativa L.) in an irrigated paddy field with or without fertilizer, we used the shallow-rooting IR64 and the deep-rooting Dro1-NIL ...(a near-isogenic line homozygous for the Kinandang Patong allele of DEEPER ROOTING 1 (DRO1) in the IR64 genetic background). Although total root length was similar in both lines, more roots were distributed within the lower soil layer of the paddy field in Dro1-NIL than in IR64, irrespective of fertilizer treatment. At maturity, Dro1-NIL showed approximately 10% higher grain yield than IR64, irrespective of fertilizer treatment. Higher grain yield of Dro1-NIL was mainly due to the increased 1000-kernel weight and increased percentage of ripened grains, which resulted in a higher harvest index. After heading, the uptake of nitrogen from soil and leaf nitrogen concentration were higher in Dro1-NIL than in IR64. At the mid-grain-filling stage, Dro1-NIL maintained higher cytokinin fluxes from roots to shoots than IR64. These results suggest that deep rooting by DRO1 enhances nitrogen uptake and cytokinin fluxes at late stages, resulting in better grain filling in Dro1-NIL in a paddy field in this study.
BACKGROUND: The functional allele of the rice gene DEEPER ROOTING 1 (DRO1) increases the root growth angle (RGA). However, wide natural variation in RGA is observed among rice cultivars with the ...functional DRO1 allele. To elucidate genetic factors related to such variation, we quantitatively measured RGA using the basket method and analyzed quantitative trait loci (QTLs) for RGA in three F₂mapping populations derived from crosses between the large RGA–type cultivar Kinandang Patong and each of three accessions with varying RGA: Momiroman has small RGA and was used to produce the MoK-F2 population; Yumeaoba has intermediate RGA (YuK-F2 population); Tachisugata has large RGA (TaK-F2 population). All four accessions belong to the same haplotype group of functional DRO1 allele. RESULTS: We detected the following statistically significant QTLs: one QTL on chromosome 4 in MoK-F2, three QTLs on chromosomes 2, 4, and 6 in YuK-F2, and one QTL on chromosome 2 in TaK-F2. Among them, the two QTLs on chromosome 4 were located near DRO2, which has been previously reported as a major QTL for RGA, whereas the two major QTLs for RGA on chromosomes 2 (DRO4) and 6 (DRO5) were novel. With the LOD threshold reduced to 3.0, several minor QTLs for RGA were also detected in each population. CONCLUSION: Natural variation in RGA in rice cultivars carrying functional DRO1 alleles may be controlled by a few major QTLs and by several additional minor QTLs.
To clarify the inter- and intraspecific variations of floral traits in rice, we investigated eleven traits related to pistil, stamen and glume using 128 Asian cultivated rice (Oryza sativa L.) ...accessions including 72 Indica and 56 Japonica type accessions, and 53 wild rice (O. rufipogon Griff.) accessions including 32 perennial and 21 annual ecotype accessions. We examined the results from three specific levels: intraspecific variation between perennial and annual ecotypes of wild rice, interspecific variation between cultivated rice and wild rice, and intraspecific variation between Indica and Japonica type cultivars. The annual wild rice accessions exhibited a shorter stigma and anther than the perennial wild rice ones. The cultivated rice accessions showed a lower stigma exsertion, shorter stigma, shorter anther, and thicker and wider lemma and palea than the wild rice accessions. These floral traits of cultivated rice are considered to play an important role in selfing and high seed production. None of the floral traits showed distinct differences between the Indica and Japonica type accessions in cultivated rice, although the Indica type accessions tended to display a slenderer stigma and glume than the Japonica type accessions. It is suggested that the varietal differentiation into the Indica and Japonica types is not likely to be related to genetic variations of floral traits. We clarified the differences in floral traits between annual and perennial ecotypes of wild rice, between cultivated rice and wild rice, and between the Indica and Japonica type cultivars, and discussed the relationship between genetic variations of floral traits and domestication and varietal differentiation in rice.
Improving crop yield potential through an enhanced response to rising atmospheric CO
levels is an effective strategy for sustainable crop production in the face of climate change. Large-sized ...panicles (containing many spikelets per panicle) have been a recent ideal plant architecture (IPA) for high-yield rice breeding. However, few breeding programs have proposed an IPA under the projected climate change. Here, we demonstrate through the cloning of the rice (Oryza sativa) quantitative trait locus for MORE PANICLES 3 (MP3) that the improvement in panicle number increases grain yield at elevated atmospheric CO
levels. MP3 is a natural allele of OsTB1/FC1, previously reported as a negative regulator of tiller bud outgrowth. The temperate japonica allele advanced the developmental process in axillary buds, moderately promoted tillering, and increased the panicle number without negative effects on the panicle size or culm thickness in a high-yielding indica cultivar with large-sized panicles. The MP3 allele, containing three exonic polymorphisms, was observed in most accessions in the temperate japonica subgroups but was rarely observed in the indica subgroup. No selective sweep at MP3 in either the temperate japonica or indica subgroups suggested that MP3 has not been involved and utilized in artificial selection during domestication or breeding. A free-air CO
enrichment experiment revealed a clear increase of grain yield associated with the temperate japonica allele at elevated atmospheric CO
levels. Our findings show that the moderately increased panicle number combined with large-sized panicles using MP3 could be a novel IPA and contribute to an increase in rice production under climate change with rising atmospheric CO
levels.
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