A G-protein pathway determines grain size in rice

A Gγ protein regulates alkaline sensitivity in crops

The use of alkaline salt lands for crop production is hindered by a scarcity of knowledge and breeding efforts for plant alkaline tolerance. Through genome association analysis of sorghum, a naturally high-alkaline-tolerant crop, we detected a major locus, Alkaline Tolerance 1 (AT1), specifically related to alkaline-salinity sensitivity. An at1 allele with a carboxyl-terminal truncation increased sensitivity, whereas knockout of AT1 increased tolerance to alkalinity in sorghum, millet, rice, and maize. AT1 encodes an atypical G protein γ subunit that affects the phosphorylation of aquaporins to modulate the distribution of hydrogen peroxide (H₂O₂)_. These processes appear to protect plants against oxidative stress by alkali. Designing knockouts of AT1 homologs or selecting its natural nonfunctional alleles could improve crop productivity in sodic lands.

Can the Wild Perennial, Rhizomatous Rice Species Oryza longistaminata be a Candidate for De Novo Domestication?

As climate change intensifies, the development of resilient rice that can tolerate abiotic stresses is urgently needed. In nature, many wild plants have evolved a variety of mechanisms to protect themselves from environmental stresses. Wild relatives of rice may have abundant and virtually untapped genetic diversity and are an essential source of germplasm for the improvement of abiotic stress tolerance in cultivated rice. Unfortunately, the barriers of traditional breeding approaches, such as backcrossing and transgenesis, make it challenging and complex to transfer the underlying resilience traits between plants. However, de novo domestication via genome editing is a quick approach to produce rice with high yields from orphans or wild relatives. African wild rice, Oryza longistaminata, which is part of the AA-genome Oryza species has two types of propagation strategies viz. vegetative propagation via rhizome and seed propagation. It also shows tolerance to multiple types of abiotic stress, and therefore O. longistaminata is considered a key candidate of wild rice for heat, drought, and salinity tolerance, and it is also resistant to lodging. Importantly, O. longistaminata is perennial and propagates also via rhizomes both of which are traits that are highly valuable for the sustainable production of rice. Therefore, O. longistaminata may be a good candidate for de novo domestication through genome editing to obtain rice that is more climate resilient than modern elite cultivars of O. sativa.

An endoplasmic reticulum-associated degradation-related E2-E3 enzyme pair controls grain size and weight through the brassinosteroid signaling pathway in rice

Grain size is an important agronomic trait, but our knowledge about grain size determination in crops is still limited. Endoplasmic reticulum (ER)-associated degradation (ERAD) is a special ubiquitin proteasome system that is involved in degrading misfolded or incompletely folded proteins in the ER. Here, we report that SMALL GRAIN 3 (SMG3) and DECREASED GRAIN SIZE 1 (DGS1), an ERAD-related E2-E3 enzyme pair, regulate grain size and weight through the brassinosteroid (BR) signaling pathway in rice (Oryza sativa). SMG3 encodes a homolog of Arabidopsis (Arabidopsis thaliana) UBIQUITIN CONJUGATING ENZYME 32, which is a conserved ERAD-associated E2 ubiquitin conjugating enzyme. SMG3 interacts with another grain size regulator, DGS1. Loss of function of SMG3 or DGS1 results in small grains, while overexpression of SMG3 or DGS1 leads to long grains. Further analyses showed that DGS1 is an active E3 ubiquitin ligase and colocates with SMG3 in the ER. SMG3 and DGS1 are involved in BR signaling. DGS1 ubiquitinates the BR receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) and affects its accumulation. Genetic analysis suggests that SMG3, DGS1, and BRI1 act together to regulate grain size and weight. In summary, our findings identify an ERAD-related E2-E3 pair that regulates grain size and weight, which gives insight into the function of ERAD in grain size control and BR signaling.

Genetic Localization and Homologous Genes Mining for Barley Grain Size

Grain size is an important agronomic trait determining barley yield and quality. An increasing number of QTLs (quantitative trait loci) for grain size have been reported due to the improvement in genome sequencing and mapping. Elucidating the molecular mechanisms underpinning barley grain size is vital for producing elite cultivars and accelerating breeding processes. In this review, we summarize the achievements in the molecular mapping of barley grain size over the past two decades, highlighting the results of QTL linkage analysis and genome-wide association studies. We discuss the QTL hotspots and predict candidate genes in detail. Moreover, reported homologs that determine the seed size clustered into several signaling pathways in model plants are also listed, providing the theoretical basis for mining genetic resources and regulatory networks of barley grain size.

Molecular bases of rice grain size and quality for optimized productivity

The accomplishment of further optimization of crop productivity in grain yield and quality is a great challenge. Grain size is one of the crucial determinants of rice yield and quality; all of these traits are typical quantitative traits controlled by multiple genes. Research advances have revealed several molecular and developmental pathways that govern these traits of agronomical importance. This review provides a comprehensive summary of these pathways, including those mediated by G-protein, the ubiquitin-proteasome system, mitogen-activated protein kinase, phytohormone, transcriptional regulators, and storage product biosynthesis and accumulation. We also generalize the excellent precedents for rice variety improvement of grain size and quality, which utilize newly developed gene editing and conventional gene pyramiding capabilities. In addition, we discuss the rational and accurate breeding strategies, with the aim of better applying molecular design to breed high-yield and superior-quality varieties.

Chromosomal-level genome assembly of the high-quality Xian/Indica rice (Oryza sativa L.) Xiangyaxiangzhan

The indica rice variety XYXZ carries elite traits including appearance and eating quality. Here, we report the de novo assembly of XYXZ using Illumine paired-end whole-genome shotgun sequencing and Nanopore sequencing. We annotated 39,722 protein-coding genes in the 395.04 Mb assembly. In comparison to other cultivars, XYXZ showed a larger gene size including the transcripts and introns, and more exons per gene. And hundreds of ultra-long genes were also detected. A total of 4362 complete LTRs were annotated, and among them, many were located next to or in protein-coding genes including several genes related to rice quality. We observed the different distributions of LTRs in these genes among XYXZ, Nipponbare, and R498, implying these LTRs might potentially affect expressions of the proximal genes and rice quality. Overall, This chromosome-length genome assembly of XYXZ provides a valuable resource for gene discovery, genetic variation and evolution, and the breeding of high-quality rice.

QTL cluster analysis and marker development for kernel traits based on DArT markers in spring bread wheat (Triticum aestivum L.)

Genetic dissection of yield component traits including kernel characteristics is essential for the continuous improvement in wheat yield. In the present study, one recombinant inbred line (RIL) F₆ population derived from a cross between Avocet and Chilero was used to evaluate the phenotypes of kernel traits of thousand-kernel weight (TKW), kernel length (KL), and kernel width (KW) in four environments at three experimental stations during the 2018-2020 wheat growing seasons. The high-density genetic linkage map was constructed with the diversity arrays technology (DArT) markers and the inclusive composite interval mapping (ICIM) method to identify the quantitative trait loci (QTLs) for TKW, KL, and KW. A total of 48 QTLs for three traits were identified in the RIL population on the 21 chromosomes besides 2A, 4D, and 5B, accounting for 3.00%-33.85% of the phenotypic variances. Based on the physical positions of each QTL, nine stable QTL clusters were identified in the RILs, and among these QTL clusters, TaTKW-1A was tightly linked to the DArT marker interval 3950546-1213099, explaining 10.31%-33.85% of the phenotypic variances. A total of 347 high-confidence genes were identified in a 34.74-Mb physical interval. TraesCS1A02G045300 and TraesCS1A02G058400 were among the putative candidate genes associated with kernel traits, and they were expressed during grain development. Moreover, we also developed high-throughput kompetitive allele-specific PCR (KASP) markers of TaTKW-1A, validated in a natural population of 114 wheat varieties. The study provides a basis for cloning the functional genes underlying the QTL for kernel traits and a practical and accurate marker for molecular breeding.

Genomic footprints of Kam Sweet Rice domestication indicate possible migration routes of the Dong people in China and provide resources for future rice breeding

The Dong people are one of China's 55 recognized ethnic minorities, but there has been a long-standing debate about their origins. In this study, we performed whole-genome resequencing of Kam Sweet Rice (KSR), a valuable, rare, and ancient rice landrace unique to the Dong people. Through comparative genomic analyses of KSR and other rice landraces from south of the Yangtze River Basin in China, we provide evidence that the ancestors of the Dong people likely originated from the southeast coast of China at least 1000 years ago. Alien introgression and admixture in KSR demonstrated multiple migration events in the history of the Dong people. Genomic footprints of domestication demonstrated characteristics of KSR that arose from artificial selection and geographical adaptation by the Dong people. The key genes GS3, Hd1, and DPS1 (related to agronomic traits) and LTG1 and MYBS3 (related to cold tolerance) were identified as domestication targets, reflecting crop improvement and changes in the geographical environment of the Dong people during migration. A genome-wide association study revealed a candidate yield-associated gene, Os01g0923300, a specific haplotype in KSR that is important for regulating grain number per panicle. RNA-sequencing and quantitative reverse transcription-PCR results showed that this gene was more highly expressed in KSR than in ancestral populations, indicating that it may have great value in increasing yield potential in other rice accessions. In summary, our work develops a novel approach for studying human civilization and migration patterns and provides valuable genomic datasets and resources for future breeding of high-yield and climate-resilient rice varieties.

LG5, a Novel Allele of EUI1, Regulates Grain Size and Flag Leaf Angle in Rice

Grain size and flag leaf angle are two important traits that determining grain yield in rice. However, the mechanisms regulating these two traits remain largely unknown. In this study, a rice long grain 5 (lg5) mutant with a large flag leaf angle was identified, and map-based cloning revealed that a single base substitution followed by a 2 bp insertion in the LOC_Os05g40384 gene resulted in larger grains, a larger flag leaf angle, and higher plant height than the wild type. Sequence analysis revealed that lg5 is a novel allele of elongated uppermost internode-1 (EUI1), which encodes a cytochrome P450 protein. Functional complementation and overexpression tests showed that LG5 can rescue the bigger grain size and larger flag leaf angle in the Xiushui11 (XS) background. Knockdown of the LG5 transcription level by RNA interference resulted in elevated grain size and flag leaf angle in the Nipponbare (NIP) background. Morphological and cellular analyses suggested that LG5 regulated grain size and flag leaf angle by promoting cell expansion and cell proliferation. Our results provided new insight into the functions of EUI1 in rice, especially in regulating grain size and flag leaf angle, indicating a potential target for the improvement of rice breeding.

Artificially Selected Grain Shape Gene Combinations in Guangdong Simiao Varieties of Rice (Oryza sativa L.)

BACKGROUND

Grain shape is a key trait in rice breeding. Although many QTLs and genes of grain shape have been identified, how different combinations of alleles of these genes affect grain shape is largely unknown. It is important to understand the effects of grain shape gene combinations for breeding by design. In the present study, we performed genetic dissection of the grain shapes in Guangdong Simiao varieties, a popular kind of rice in South China, to identify the effective alleles and their combination for breeding.

RESULTS

We selected two hundred nineteen indica accessions with diverse grain shapes and fifty-two Guangdong Simiao varieties with long and slender grain shapes for genome-wide selection analysis. The results showed that four (GS3, GS5, GW5 and GL7) of the twenty grain shape genes fall into the regions selected for in Guangdong Simiao varieties. Allele analysis and frequency distribution of these four genes showed that GS3^allele3 and GW5^allele2 accounted for 96.2%, and GL7^allele2 and GS5^allele2 accounted for 76.9% and 74.5% of the Simiao varieties, respectively. Further analysis of the allelic combinations showed that 30 allelic combinations were identified in the whole panel, with 28 allelic combinations found in the international indica accessions and 6 allelic combinations found in Guangdong Simiao varieties. There were mainly three combinations (combinations 17, 18 and 19) in the Guangdong Simiao varieties, with combination 19 (GS3^allele3 + GW5^allele2 + GL7^allele2 + GS5^allele2) having the highest percentage (51.9%). All three combinations carried GS3^allele3 + GW5^allele2, while combinations 17 (GL7^allele1) and 19 (GL7^allele2) showed significant differences in both grain length and length/width ratio due to differences in GL7 alleles. Pedigree analysis of Guang8B, the maintainer of the first released Simiao male sterile line Guang8A, showed that the parent lines and Guang8B carried GS3^allele3 + GW5^allele2 + GS5^allele2, while the GL7 allele differed, resulting in significant differences in grain size.

CONCLUSION

The results suggest that specific alleles of GS3, GS5, GW5 and GL7 are the key grain shape genes used in the Guangdong Simiao varieties and selected for grain shape improvement. Combination 19 is the predominant allelic combination in the Guangdong Simiao varieties. Our current study is the first to dissect the genetics of grain shape in Guangdong Simiao varieties, and the results will facilitate molecular breeding of Guangdong Simiao varieties.

Detecting and pyramiding target QTL for plant- and grain-related traits via chromosomal segment substitution line of rice

INTRODUCTION

Plant height and grain length are important agronomic traits in rice, exhibiting a strong effect on plant architecture and grain quality of rice varieties.

METHODS

Methods: A novel rice chromosomal segment substitution line (CSSL), i.e., CSSL-Z1357, with significantly increased plant height (PH) and grain length (GL) was identified from CSSLs constructed by using Nipponbare as a receptor and a restorer line Xihui 18 as a donor. Seven agronomic traits of PH, PL, GL, GW, GPP, SPP, and TGW were phenotyped, and REML implemented in HPMIXED of SAS were used to detect the QTL for these traits. Secondary CSSLs were screened out via marker-assisted selection (MAS) to estimate the additive and epistatic effects of detected QTLs, evaluating the potential utilization of pyramiding the target QTLs for yield and quality improvement of rice varieties.

RESULTS AND DISCUSSION

Results and Discussion: CSSL-Z1357 carried nine segments from Xihui 18 with an average segment length of 4.13 Mb. The results show that the long grain of CSSL-Z1357 was caused by the increased number of surface cells and the length of the inner glume. Thirteen quantitative trait loci were identified via the F2 population of Nipponbare/CSSL-Z1357, including three each for GL (qGL-3, qGL-6, and qGL-7) and PH (qPH-1, qPH-7, and qPH-12I), among which qGL-3 increased GL by 0.23 mm with synergistic allele from CSSL-Z1357. Additionally, three single (S1 to S3), two double (D1, D2), and one triple segment (T1) substitution lines were developed in F3 via MAS. Results show that pyramiding the segments from Chr.3 (qGL-3 and qPH-3), Chr.6 (qGL-6 and qPH-6), and Chr.7 (Null and qPH-7) tended to result in better phenotype of increased GL and PH and decreased grain width, providing a potential basis for enhancing grain yield and quality in rice breeding.

gw2.1, a new allele of GW2, improves grain weight and grain yield in rice

Grain weight is an important characteristic of grain shape and a key contributing factor to the grain yield in rice. Here, we report that gw2.1, a new allele of the Grain Width and Weight 2 (GW2) gene, regulates grain size and grain weight. A single nucleotide substitution in the coding sequence (CDS) of gw2.1 resulted in the change of glutamate to lysine (E128K) in GW2.1 protein. Complementation tests and GW2 overexpression experiments demonstrated that the missense mutation in gw2.1 was responsible for the phenotype of enlarged grain size in the mutant line jf42. The large grain trait of the near-isogenic line NIL-gw2.1 was found to result from increased cell proliferation during flower development. Meanwhile, NIL-gw2.1 was shown to increase grain yield without compromising the grain quality. The GW2 protein was localized to the cell nucleus and membrane, and interacted with CHB705, a subunit of the chromatin remodeling complex. Finally, the F₁ hybrids from crosses of NIL-gw2.1 with 7 cytoplasmic male-sterile lines exhibited large grains and desirable grain appearance. Thus, gw2.1 is a promising allele that could be applied to improve grain yield and grain appearance in rice. AVAILABILITY OF DATA AND MATERIALS: The datasets generated and/or analyzed in the study are available from the corresponding author on reasonable request.

QTL Detection for Rice Grain Length and Fine Mapping of a Novel Locus qGL6.1

BACKGROUND

Grain length (GL) that is directly associated with appearance quality is a key target of selection in rice breeding. Although abundant quantitative trait locus (QTL) associated with GL have been identified, it was still relatively weak to identify QTL for GL from japonica genetic background, as the shortage of japonica germplasms with long grains. We performed QTLs analysis for GL using a recombinant inbred lines (RILs) population derived from the cross between japonica variety GY8 (short grains) and LX1 (long grains) in four environments.

RESULTS

A total of 197 RILs were genotyped with 285 polymorphic SNP markers. Three QTLs qGL5.3, qGL6.1 and qGL11 were detected to control GL by individual environmental analyses and multi-environment joint analysis. Of these, a major-effect and stable QTL qGL6.1 was identified to be a novel QTL, and its LX1 allele had a positive effect on GL. For fine-mapping qGL6.1, a BC₁F₂ population consisting of 2,487 individuals was developed from a backcross between GY8 and R176, one line with long grain. Eight key informative recombinants were identified by nine kompetitive allele specific PCR (KASP) markers. By analyzing key recombinants, the qGL6.1 locus was narrowed down to a 40.41 kb genomic interval on chromosome 6. One candidate gene LOC_Os06g43304.1 encoding cytochrome P450 (CYP71D55) was finally selected based on the difference in the transcriptional expression and variations in its upstream and downstream region.

CONCLUSIONS

Three QTLs qGL5.3, qGL6.1 and qGL11 were identified to control grain length in rice. One novel QTL qGL6.1 was fine mapped within 40.41 kb region, and LOC_Os06g43304.1 encoding cytochrome P450 (CYP71D55) may be its candidate gene. We propose that the further cloning of the qGL6.1 will facilitate improving appearance quality in japonica varieties.

Genome-wide association study reveals a GLYCOGEN SYNTHASE KINASE 3 gene regulating plant height in Brassica napus

Rapeseed (Brassica napus) is an allotetraploid crop that is the main source of edible oils and feed proteins in the world. The ideal plant architecture breeding is a major objective of rapeseed breeding and determining the appropriate plant height is a key element of the ideal plant architecture. Therefore, this study aims to improve the understanding of the genetic controls underlying plant height. The plant heights of 230 rapeseed accessions collected worldwide were investigated in field experiments over two consecutive years in Wuhan, China. Whole-genome resequencing of these accessions yielded a total of 1,707,194 informative single nucleotide polymorphisms (SNPs) that were used for genome-wide association analysis (GWAS). GWAS and haplotype analysis showed that BnaA01g09530D, which encodes BRASSINOSTEROID-INSENSITIVE 2 and belongs to the GLYCOGEN SYNTHASE KINASE 3 (GSK3) family, was significantly associated with plant height in B. napus. Moreover, a total of 31 BnGSK3s with complete domains were identified from B. napus genome and clustered into four groups according to phylogenetic analysis, gene structure, and motif distribution. The expression patterns showed that BnGSK3s exhibited significant differences in 13 developmental tissues in B. napus, suggesting that BnGSK3s may be involved in tissue-specific development. Sixteen BnGSK3 genes were highly expressed the in shoot apical meristem, which may be related to plant height or architecture development. These results are important for providing new haplotypes of plant height in B. napus and for extending valuable genetic information for rapeseed genetic improvement of plant architecture.

Fine mapping of qTGW2b and qGL9, two minor QTL conferring grain size and weight in rice

Rice grain size is a key determinant of both grain yield and quality. In this study, we conducted QTL mapping on grain size using a recombinant inbred line (RIL) population derived from a cross between japonica variety Beilu130 (BL130) and indica variety Jin23B (J23B). A total of twenty-two QTL related to grain length (GL), grain width (GW), grain length-to-width ratio (LWR), grain thickness (GT), and thousand grain weight (TGW) were detected under two environments, and 14 of them were repeatedly detected. Two minor QTL, qTGW2b and qGL9, were validated and further delimited to regions of 631 kb and 272 kb, respectively. Parental sequence comparison of genes expressed in inflorescence in corresponding candidate regions identified frameshifts in the exons of LOC_Os02g38690 and LOC_Os02g38780, both of which encode protein phosphatase 2C-containing protein, and LOC_Os09g29930, which encodes a BIM2 protein. Scanning electron microscopy (SEM) analysis revealed that the increase of cell size rather than cell number caused the differences in grain size between NILs of qTGW2b and qGL9. Quantitative RT-PCR analysis showed that the expression levels of EXPA4, EXPA5, EXPA6, EXPB3, EXPB4, and EXPB7 were significantly different in both qTGW2b NILs and qGL9 NILs. Our results lay the foundation for the cloning of qTGW2b and qGL9, and provide genetic materials for the improvement of rice yield and quality.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01328-2.

Genome-wide association studies provide genetic insights into natural variation of seed-size-related traits in mungbean

Although mungbean (Vigna radiata (L.) R. Wilczek) is an important legume crop, its seed yield is relatively low. To address this issue, here 196 accessions with 3,607,508 SNP markers were used to identify quantitative trait nucleotides (QTNs), QTN-by-environment interactions (QEIs), and their candidate genes for seed length (SL), seed width, and 100-seed weight (HSW) in two environments. As a result, 98 QTNs and 20 QEIs were identified using 3VmrMLM, while 95, >10,000, and 15 QTNs were identified using EMMAX, GEMMA, and CMLM, respectively. Among 809 genes around these QTNs, 12 were homologous to known seed-development genes in rice and Arabidopsis thaliana, in which 10, 2, 1, and 0 genes were found, respectively, by the above four methods to be associated with the three traits, such as VrEmp24/25 for SL and VrKIX8 for HSW. Eight of the 12 genes were significantly differentially expressed between two large-seed and two small-seed accessions, and VrKIX8, VrPAT14, VrEmp24/25, VrIAR1, VrBEE3, VrSUC4, and Vrflo2 were further verified by RT-qPCR. Among 65 genes around these QEIs, VrFATB, VrGSO1, VrLACS2, and VrPAT14 were homologous to known seed-development genes in A. thaliana, although new experiments are necessary to explore these novel GEI-trait associations. In addition, 54 genes were identified in comparative genomics analysis to be associated with seed development pathway, in which VrKIX8, VrABA2, VrABI5, VrSHB1, and VrIKU2 were also identified in genome-wide association studies. This result provided a reliable approach for identifying seed-size-related genes in mungbean and a solid foundation for further molecular biology research on seed-size-related genes.

Rice co-expression network analysis identifies gene modules associated with agronomic traits

Identifying trait-associated genes is critical for rice (Oryza sativa) improvement, which usually relies on map-based cloning, quantitative trait locus analysis, or genome-wide association studies. Here we show that trait-associated genes tend to form modules within rice gene co-expression networks, a feature that can be exploited to discover additional trait-associated genes using reverse genetics. We constructed a rice gene co-expression network based on the graphical Gaussian model using 8,456 RNA-seq transcriptomes, which assembled into 1,286 gene co-expression modules functioning in diverse pathways. A number of the modules were enriched with genes associated with agronomic traits, such as grain size, grain number, tiller number, grain quality, leaf angle, stem strength, and anthocyanin content, and these modules are considered to be trait-associated gene modules. These trait-associated gene modules can be used to dissect the genetic basis of rice agronomic traits and to facilitate the identification of trait genes. As an example, we identified a candidate gene, OCTOPUS-LIKE 1 (OsOPL1), a homolog of the Arabidopsis (Arabidopsis thaliana) OCTOPUS gene, from a grain size module and verified it as a regulator of grain size via functional studies. Thus, our network represents a valuable resource for studying trait-associated genes in rice.

Low grain weight, a new allele of BRITTLE CULM12, affects grain size through regulating GW7 expression in rice

Grain weight is a major determinant in rice yield, which is tightly associated with grain size. However, the underlying molecular mechanisms that control this trait remain unclear. Here, we report a rice (Oryza sativa) mutant, low grain weight (lgw), which shows that reduced grain length is caused by decreased cell elongation and proliferation. Map-based cloning revealed that all mutant phenotypes resulted from a nine-base pair (bp) deletion in LGW, which encodes the kinesin-like protein BRITTLE CULM12 (BC12). Protein sequence alignment analysis revealed that the mutation site was located at the nuclear localization signal (NLS) of LGW/BC12, resulting in the lgw protein not being located in the nucleus. LGW is preferentially expressed in both culms and roots, as well as in the early developing panicles. Overexpression of LGW increased the grain length, indicating that LGW is a positive regulator for regulating grain length. In addition, LGW/BC12 is directly bound to the promoter of GW7 and activates its expression. Elevating the GW7 expression levels in lgw plants rescued the small grain size phenotype. We conclude that LGW regulates grain development by directly binding to the GW7 promoter and activating its expression. Our findings revealed that LGW plays an important role in regulating grain size, and manipulation of this gene provides a new strategy for regulating grain weight in rice.

Discovery and Validation of Grain Shape Loci in U.S. Rice Germplasm Through Haplotype Characterization

Rice grain shape is a major determinant of rice market value and the end-use. We mapped quantitative trait loci (QTL) for grain shape traits in a bi-parental recombinant inbred line population (Trenasse/Jupiter) and discovered two major grain length QTLs—qGL3.1 and qGL7.1. Previously, a major grain shape gene GS3 was reported in the qGL3.1 region and grain length gene GL7 was reported to be encompassing qGL7.1 locus. The re-sequencing SNP data on the International Rice Research Institute (IRRI) 3K Rice Genome Project (RGP) panel were obtained from the IRRI SNP-Seek database for both genes and haplotype diversity was characterized for each gene in this diverse panel. United States rice germplasm was not well represented in the IRRI 3K RGP database. Therefore, a minimum SNP set was identified for each gene that could differentiate all the characterized haplotypes. These haplotypes in the 3K RGP panel were screened across 323 elite U.S. genotypes using the minimum SNP set. The screening of haplotypes and phenotype association confirmed the role of GS3 under qGL3.1. However, screening of the GL7 haplotypes in the U.S. germplasm panel showed that GL7 did not play a role in qGL7.1, and in addition, GL7.1 did not segregate in the Trenasse/Jupiter RIL population. This concluded that qGL7.1 is a novel QTL discovered on chr7 for grain shape in the Trenasse/Jupiter RIL population. A high-throughput KASP-based SNP marker for each locus (GS3 and qGL7.1) was identified and validated in elite U.S. rice germplasm to be used in an applied rice breeding program.

OsBSK2, a putative brassinosteroid-signalling kinase, positively controls grain size in rice

Grain size is an important trait that directly affects grain yield in rice; however, the genetic and molecular mechanisms regulating grain size remain unclear. In this study, we identified a mutant, grain length and grain weight 10 (glw10), which exhibited significantly reduced grain length and grain weight. Histological analysis demonstrated that GLW10 affects cell expansion, which regulates grain size. MutMap-based gene mapping and transgenic experiments demonstrated that GLW10 encodes a putative brassinosteroid (BR) signalling kinase, OsBSK2. OsBSK2 is a plasma membrane protein, and an N-myristoylation site is needed for both membrane localization and function. OsBSK2 directly interacts with the BR receptor kinase OsBRI1; however, genetic experiments have demonstrated that OsBSK2 may regulate grain size independent of the BR signalling pathway. OsBSK2 can form a homodimer or heterodimer with OsBSK3 and OsBSK4, and silencing OsBSK2, OsBSK3, and OsBSK4 reduce grain size. This indicates that OsBSKs seem to function as homodimers or heterodimers to positively regulate grain size in rice. OsBSK2/3/4 are all highly expressed in young panicles and spikelet hulls, suggesting that they control grain size. In summary, our results provide novel insights into the function of BSKs in rice, and identify novel targets for improving grain size during crop breeding.

Emerging roles of the ubiquitin-proteasome pathway in enhancing crop yield by optimizing seed agronomic traits

Ubiquitin-proteasome pathway has the potential to modulate crop productivity by influencing agronomic traits. Being sessile, the plant often uses the ubiquitin-proteasome pathway to maintain the stability of different regulatory proteins to survive in an ever-changing environment. The ubiquitin system influences plant reproduction, growth, development, responses to the environment, and processes that control critical agronomic traits. E3 ligases are the major players in this pathway, and they are responsible for recognizing and tagging the targets/substrates. Plants have a variety of E3 ubiquitin ligases, whose functions have been studied extensively, ranging from plant growth to defense strategies. Here we summarize three agronomic traits influenced by ubiquitination: seed size and weight, seed germination, and accessory plant agronomic traits particularly panicle architecture, tillering in rice, and tassels branch number in maize. This review article highlights some recent progress on how the ubiquitin system influences the stability/modification of proteins that determine seed agronomic properties like size, weight, germination and filling, and ultimately agricultural productivity and quality. Further research into the molecular basis of the aforementioned processes might lead to the identification of genes that could be modified or selected for crop development. Likewise, we also propose advances and future perspectives in this regard.

Wheat genomic study for genetic improvement of traits in China

Bread wheat (Triticum aestivum L.) is a major crop that feeds 40% of the world's population. Over the past several decades, advances in genomics have led to tremendous achievements in understanding the origin and domestication of wheat, and the genetic basis of agronomically important traits, which promote the breeding of elite varieties. In this review, we focus on progress that has been made in genomic research and genetic improvement of traits such as grain yield, end-use traits, flowering regulation, nutrient use efficiency, and biotic and abiotic stress responses, and various breeding strategies that contributed mainly by Chinese scientists. Functional genomic research in wheat is entering a new era with the availability of multiple reference wheat genome assemblies and the development of cutting-edge technologies such as precise genome editing tools, high-throughput phenotyping platforms, sequencing-based cloning strategies, high-efficiency genetic transformation systems, and speed-breeding facilities. These insights will further extend our understanding of the molecular mechanisms and regulatory networks underlying agronomic traits and facilitate the breeding process, ultimately contributing to more sustainable agriculture in China and throughout the world.

GLW7.1, a Strong Functional Allele of Ghd7, Enhances Grain Size in Rice

Grain size is a key determinant of both grain weight and grain quality. Here, we report the map-based cloning of a novel quantitative trait locus (QTL), GLW7.1 (Grain Length, Width and Weight 7.1), which encodes the CCT motif family protein, GHD7. The QTL is located in a 53 kb deletion fragment in the cultivar Jin23B, compared with the cultivar CR071. Scanning electron microscopy analysis and expression analysis revealed that GLW7.1 promotes the transcription of several cell division and expansion genes, further resulting in a larger cell size and increased cell number, and finally enhancing the grain size as well as grain weight. GLW7.1 could also increase endogenous GA content by up-regulating the expression of GA biosynthesis genes. Yeast two-hybrid assays and split firefly luciferase complementation assays revealed the interactions of GHD7 with seven grain-size-related proteins and the rice DELLA protein SLR1. Haplotype analysis and transcription activation assay revealed the effect of six amino acid substitutions on GHD7 activation activity. Additionally, the NIL with GLW7.1 showed reduced chalkiness and improved cooking and eating quality. These findings provide a new insight into the role of Ghd7 and confirm the great potential of the GLW7.1 allele in simultaneously improving grain yield and quality.

G-protein couples MAPK cascade through maize heterotrimeric Gβ subunit

G protein couples MAPK cascade through maize heterotrimeric Gβ subunit MGB1. Heterotrimeric G protein Gβ interacts with Gγ subunit to generate Gβγ dimer in modulation of various biological processes. The modulatory events at transcriptome scale of plant Gβ subunit remain largely unknown. To reveal the regulatory basis of Gβ subunit at transcriptome level, we first identified a canonical maize Gβ subunit MGB1 that physically interacted with Type C Gγ protein MGG4. For transcriptome analysis, two independent CRISPR/Cas9-edited MGB1 lines were generated, which all exhibited growth arrest, suggestive of MGB1 essential for maize seedling establishment. Transcriptomic outcomes showed that MGB1 knockout resulted in elevated transcriptional abundance of plant immune response marker PR and immune receptor RPM1. Integrated GO, KEGG, and GSEA analyses pinpointed the enrichment of differentially expressed genes in defense response pathway. Functional association network construction revealed MAPK cascade components and PR protein as hub regulators of MGB1-mediated immune signaling. MGB1 and scaffold protein ZmRACK1 together with MAPK cascade components coordinately modulated maize immune responses. We built a modulatory hierarchy of Gβ subunit at transcriptome and interacting scales, which is informative for our understanding of the regulatory basis of G protein signaling.

Fine Mapping of qTGW7b, a Minor Effect QTL for Grain Weight in Rice (Oryza sativa L.)

Grain weight is a key trait that determines rice quality and yield, and it is primarily controlled by quantitative trait loci (QTL). Recently, attention has been paid to minor QTLs. A minor effect QTL qTGW7 that controls grain weight was previously identified in a set of chromosomal fragment substitution lines (CSSLs) derived from Nipponbare (NPB)/93-11. Compared to NPB, the single segment substitution line (SSSL) N83 carrying the qTGW7 introgression exhibited an increase in grain length and width and a 4.5% increase in grain weight. Meanwhile, N83 was backcrossed to NPB to create a separating population, qTGW7b, a QTL distinct from qTGW7, which was detected between markers G31 and G32. Twelve near-isogenic lines (NILs) from the BC₉F₃ population and progeny of five NILs from the BC₉F_3:4 population were genotyped and phenotyped, resulting in the fine mapping of the minor effect QTL qTGW7b to the approximately 86.2-kb region between markers G72 and G32. Further sequence comparisons and expression analysis confirmed that five genes, including Os07g39370, Os07g39430, Os07g39440, Os07g39450, and Os07g39480, were considered as the candidate genes underlying qTGW7b. These results provide a crucial foundation for further cloning of qTGW7b and molecular breeding design in rice.

Effects of Wx Genotype, Nitrogen Fertilization, and Temperature on Rice Grain Quality

Quality is a complex trait that is not only the key determinant of the market value of the rice grain, but is also a major constraint in rice breeding. It is influenced by both genetic and environmental factors. However, the combined effects of genotypes and environmental factors on rice grain quality remain unclear. In this study, we used a three-factor experimental design to examine the grain quality of different Wx genotypes grown under different nitrogen fertilization and temperature conditions during grain development. We found that the three factors contributed differently to taste, appearance, and nutritional quality. Increased Wx function and nitrogen fertilization significantly reduced eating quality, whereas high temperature (HT) had almost no effect. The main effects of temperature on appearance quality and moderate Wx function at low temperatures (LTs) contributed to better appearance, and higher nitrogen fertilization promoted appearance at HTs. With regard to nutritional quality, Wx alleles promoted amylose content (AC) as well as starch-lipids content (SLC); nitrogen fertilization increased storage protein content (PC); and higher temperature increased lipid content but decreased the PC. This study helps to broaden the understanding of the major factors that affect the quality of rice and provides constructive messages for rice quality improvement and the cultivation of high-quality rice varieties.

The phosphoinositide-specific phospholipase C1 modulates flowering time and grain size in rice

Preferential expression of OsPLC1 is detected at the heading stage of rice, OsPLC1 overexpression results in early flowering, increased-grain size and yield; however, opposing phenotypes produced in the osplc1 mutants. Abstract: The importance of phospholipase C (PLC) in plant development has been demonstrated in several studies. OsPLC1, a member of PI-PLC in rice, although its role in the response to salt stress of rice seedlings has been reported, its functions in the growth and development of rice is elusive. Here, we report that OsPLC1 expression could be detectable in various tissues throughout the developmental stages of rice, and the highest expression level of OsPLC1 was detected at the heading stage. OsPLC1 overexpression (OE) produced rice plants with early flowering, whereas OsPLC1 loss-of-function led to delay in flowering. The expression levels of subset genes, which are involved in the control of flowering time in rice, were altered in the plants of OE and osplc1. In addition, the enlargement of grain size was observed in OE plants, however, the reduction of grain size was noticed in osplc1 mutants. The increase in the grain size and the grain yield of OE lines were associated with the improvement of cell length and expression levels of a set of genes related to cell expansion, contrarily, the decrease in osplc1 mutant grain size and yield were linked to declined cell length and expression levels of related genes. No significant differences, in terms of the grain quality of mature seeds, were found in OE and osplc1 mutants, with compared to those in Nipponbare (Nip). In summary, our study suggests that OsPLC1 could modulate rice flowering time and grain size.

Natural variation of the BRD2 allele affects plant height and grain size in rice

The zqdm1 identified from a rice mutant is a novel allele of BRD2 and is responsible for regulating rice plant height, grain size and appearance, which has possibilities on improving rice quality. Plant height is an important agronomic trait related to rice yield, and grain size directly determines grain yield in rice (Oryza sativa L.). With the development of molecular biotechnology and genome sequencing technology, more and more key genes associated with plant height and grain size have been cloned and identified in recent years. This study identified the zqdm1 gene from a mutant with reduced plant height and grain size. The zqdm1 gene was revealed to be a new allele of BRASSINOSTEROID DEFICIENT DWARF 2 (BRD2), encoding a FAD-linked oxidoreductase protein involved in the brassinosteroid (BR) biosynthesis pathway, and regulates plant height by reducing cell number of longitudinal sections of the internode and regulates grain size by altering cell expansion. A 369-bp DNA fragment was found inserted at the first exon, resulting in protein-coding termination. This mutation has not been discovered in previous studies. Complementation tests have confirmed that 369-bp insertion in BRD2 was responsible for the plant height and grain size changing in the zqdm1 mutant. Over-expression of BRD2 driven by different promoters into indica rice variety Jiafuzhan (JFZ) results in slender grains, suggesting its function on regulating grain shape. In summary, the current study has identified a new BRD2 allele, which facilitated the further research on the molecular mechanism of this gene on regulating growth and development.

Regulation of OsPIL15 on rice quality

The phytochrome-interacting factor-like gene OsPIL15 negatively regulates grain size and 1000-grain weight, but its regulatory effect on rice quality traits is unknown. Here, knock-down, knock-out, and over-expression of OsPIL15 transgenic rice lines were used to investigate the effects of OsPIL15 on rice yield and quality traits. The results showed that knock-down or knock-out of OsPIL15 increased grain length and width, chalkiness, amylose content, glutenin and globulin content, and total protein content but reduced amylopectin content, total starch content, prolamin and albumin content, and gel consistency. Over-expression of OsPIL15 showed the opposite results, except for the reduction of prolamin content. Although OsPIL15 changed the grain size and weight, it had no effect on grain length/width ratio, brown rice rate, and milled rice rate. KEGG pathway enrichment analysis of differentially expressed genes between transgenic lines and wild type showed that OsPIL15 mainly regulated genes related to ribosome, metabolic pathways, and biosynthesis of secondary metabolites. Gene expression analysis showed that RNAi transgenic lines decreased OsCIN2 and OsSUS1 expression and increased OsGBSSI, OsSSI, OsAPGL2, and OsAPGL3 expression level, while over-expression of OsPIL15 increased OsCIN2, OsSUS1, OsSUS6, and OsSSI and decreased OsSSIIa, OsSSIIc, and OsAPGL2 expression level. These results revealed that OsPIL15 plays an important role in rice grain development. In addition to grain shape, OsPIL15 also regulates chalkiness, starch content, protein content, and gel consistency.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01311-x.

Genetic control of grain appearance quality in rice

Grain appearance, one of the key determinants of rice quality, reflects the ability to attract consumers, and is characterized by four major properties: grain shape, chalkiness, transparency, and color. Mining of valuable genes, genetic mechanisms, and breeding cultivars with improved grain appearance are essential research areas in rice biology. However, grain appearance is a complex and comprehensive trait, making it challenging to understand the molecular details, and therefore, achieve precise improvement. This review highlights the current findings of grain appearance control, including a detailed description of the key genes involved in the formation of grain appearance, and the major environmental factors affecting chalkiness. We also discuss the integration of current knowledge on valuable genes to enable accurate breeding strategies for generation of rice grains with superior appearance quality.

Genome-Wide Comprehensive Survey of the Subtilisin-Like Proteases Gene Family Associated With Rice Caryopsis Development

Subtilisin-like proteases (SUBs), which are extensively distributed in three life domains, affect all aspects of the plant life cycle, from embryogenesis and organogenesis to senescence. To explore the role of SUBs in rice caryopsis development, we recharacterized the OsSUB gene family in rice (Oryza sativa ssp. japonica). In addition, investigation of the SUBs was conducted across cultivated and wild rice in seven other Oryza diploid species (O. brachyantha, O. glaberrima, O. meridionalis, O. nivara, O. punctata, O. rufipogon, and O. sativa ssp. indica). Sixty-two OsSUBs were identified in the latest O. sativa ssp. japonica genome, which was higher than that observed in wild species. The SUB gene family was classified into six evolutionary branches, and SUB1 and SUB3 possessed all tandem duplication (TD) genes. All paralogous SUBs in eight Oryza plants underwent significant purifying selection. The expansion of SUBs in cultivated rice was primarily associated with the occurrence of tandem duplication events and purifying selection and may be the result of rice domestication. Combining the expression patterns of OsSUBs in different rice tissues and qRT-PCR verification, four OsSUBs were expressed in rice caryopses. Moreover, OsSUBs expressed in rice caryopses possessed an earlier origin in Oryza, and the gene cluster formed by OsSUBs together with the surrounding gene blocks may be responsible for the specific expression of OsSUBs in caryopses. All the above insights were inseparable from the continuous evolution and domestication of Oryza. Together, our findings not only contribute to the understanding of the evolution of SUBs in cultivated and wild rice but also lay the molecular foundation of caryopsis development and engineering improvement of crop yield.

Identification of qGL3.5, a Novel Locus Controlling Grain Length in Rice Through Bulked Segregant Analysis and Fine Mapping

Grain length (GL) directly affects the yield and quality of rice. Very few cloned GL-related genes are applied in production because their yield-increasing effects are not obvious, and the overall regulatory networks underlying the associated processes remain poorly understood. DNA samples from two bulk DNA pools (L-pool and S-pool) and their parents (KJ01 and Huaye 3) were subjected to high-throughput sequencing. Using bulked segregant analysis (BSA), qGL3.5 was mapped to a 0.34-Mb "hotspot" region on chromosome 3 that contains 37 genes related to various traits. Then, qGL3.5 was mapped to the genomic interval between the flanking markers M2 and M3 using 2786 BC₄F₂ individuals. Because the region from B5 to B6 was not the associated region under BSA-seq analysis, qGL3.5 was narrowed down to the interval between B6 and M3, which spanned 24.0-kb. Of all 37 genes with non-synonymous single-nucleotide polymorphisms (SNPs) between KJ01 and Huaye 3 based on BSA-seq analysis, only one complete annotated gene, ORF18 (Gene ID: LOC_Os03g42790.1) was found. ORF18 encodes an IBR-RING zinc-finger-related protein, with one really interesting new gene (RING) and two in between ring finger (IBR) domains. The knockout of ORF18 derived from Huaye 3 using clustered, regularly interspaced, short palindromic repeat (CRISPR)/CRISPR-associated 9 (Cas9) editing technology increased the GL of the mutant by approximately 2.2 mm. The novel locus qGL3.5 negatively regulated GL by promoting elongation of the longitudinal cell of the grain outer glume. These results provide a new genetic resource for rice grain shape breeding and a starting point for the functional characterization of the wild rice GL gene.

Verifying the Breeding Value of A Rare Haplotype of Chalk7, GS3, and Chalk5 to Improve Grain Appearance Quality in Rice

Grain quality is a key determinant of commercial value in rice. Efficiently improving grain quality, without compromising grain yield, is a challenge in rice breeding programs. Here we report on the identification and application of a grain quality gene, Chalk7, which causes a slender shape and decreases grain chalkiness in rice. Three allele-specific markers for Chalk7, and two other grain genes (GS3 and Chalk5) were developed, and used to stack the desirable alleles at these loci. The effects of individual or combined alleles at the loci were evaluated using a set of near-isogenic lines, each containing one to three favorable alleles in a common background of an elite variety. We found that the favorable allele combination of the three loci, which rarely occurs in natural rice germplasm, greatly reduces chalky grains without negatively impacting on grain yield. The data for newly developed allele-specific markers and pre-breeding lines will facilitate the improvement of grain appearance quality in rice.

QTL Analysis of Z414, a Chromosome Segment Substitution Line with Short, Wide Grains, and Substitution Mapping of qGL11 in Rice

Most agronomic traits of rice (Oryza sativa), such as grain length, are complex traits controlled by multiple genes. Chromosome segment substitution lines (CSSLs) are ideal materials for dissecting these complex traits. We developed the novel rice CSSL 'Z414', which has short, wide grains, from progeny of the recipient parent 'Xihui 18' (an indica restorer line) and the donor parent 'Huhan 3' (a japonica cultivar). Z414 contains four substitution segments with an average length of 3.04 Mb. Z414 displays seven traits that significantly differ from those of Xihui 18, including differences in grain length, width, and weight; degree of chalkiness; and brown rice rate. We identified seven quantitative trait loci (QTL) that are responsible for these differences in an F₂ population from a cross between Xihui 18 and Z414. Among these, six QTL (qPL3, qGW5, qGL11, qRLW5, qRLW11, and qGWT5) were detected in newly developed single-segment substitution lines (SSSLs) S1-S6. In addition, four QTL (qGL3, qGL5, qCD3, and qCD5) were detected in S1 and S5. Analysis of these SSSLs attributed the short, wide grain trait of Z414 to qGL11, qGL3, qGL5, and qGW5. Substitution mapping delimited qGL11 within an 810-kb interval on chromosome 11. Sequencing, real time quantitative PCR, and cell morphology analysis revealed that qGL11 might be a novel QTL encoding the cyclin CycT1;3. Finally, pyramiding qGL3 (a = 0.43) and qGL11 (a =  - 0.37) led to shorter grains in the dual-segment substitution line D2 and revealed that qGL11 is epistatic to qGL3. In addition, S1 and D2 exhibited different grain sizes and less chalkiness than Z414. In conclusion, the short grain phenotype of the CSSL Z414 is controlled by qGL11, qGL3, and qGL5. qGL11 might be a novel QTL encoding CycT1;3, whose specific role in regulating grain length was previously unknown, and qGL11 is epistatic to qGL3. S1 and D2 could potentially be used in hybrid rice breeding.

KNOX II transcription factor HOS59 functions in regulating rice grain size

Plant Knotted1-like homeobox (KNOX) genes encode homeodomain-containing transcription factors. In rice (Oryza sativa L.), little is known about the downstream target genes of KNOX Class II subfamily proteins. Here we generated chromatin immunoprecipitation (ChIP)-sequencing datasets for HOS59, a member of the rice KNOX Class II subfamily, and characterized the genome-wide binding sites of HOS59. We conducted trait ontology (TO) analysis of 9705 identified downstream target genes, and found that multiple TO terms are related to plant structure morphology and stress traits. ChIP-quantitative PCR (qPCR) was conducted to validate some key target genes. Meanwhile, our IP-MS datasets showed that HOS59 was closely associated with BELL family proteins, some grain size regulators (OsSPL13, OsSPL16, OsSPL18, SLG, etc.), and some epigenetic modification factors such as OsAGO4α and OsAGO4β, proteins involved in small interfering RNA-mediated gene silencing. Furthermore, we employed CRISPR/Cas9 editing and transgenic approaches to generate hos59 mutants and overexpression lines, respectively. Compared with wild-type plants, the hos59 mutants have longer grains and increased glume cell length, a loose plant architecture, and drooping leaves, while the overexpression lines showed smaller grain size, erect leaves, and lower plant height. The qRT-PCR results showed that mutation of the HOS59 gene led to upregulation of some grain size-related genes such as OsSPL13, OsSPL18, and PGL2. In summary, our results indicate that HOS59 may be a repressor of the downstream target genes, negatively regulating glume cell length, rice grain size, plant architecture, etc. The identified downstream target genes and possible interaction proteins of HOS59 improve our understanding of the KNOX regulatory networks.

Twenty years of rice genomics research: From sequencing and functional genomics to quantitative genomics

Since the completion of the rice genome sequencing project in 2005, we have entered the era of rice genomics, which is still in its ascendancy. Rice genomics studies can be classified into three stages: structural genomics, functional genomics, and quantitative genomics. Structural genomics refers primarily to genome sequencing for the construction of a complete map of rice genome sequence. This is fundamental for rice genetics and molecular biology research. Functional genomics aims to decode the functions of rice genes. Quantitative genomics is large-scale sequence- and statistics-based research to define the quantitative traits and genetic features of rice populations. Rice genomics has been a transformative influence on rice biological research and contributes significantly to rice breeding, making rice a good model plant for studying crop sciences.

Overexpression of TaSTT3b-2B improves resistance to sharp eyespot and increases grain weight in wheat

STAUROSPORINE AND TEMPERATURE SENSITIVE3 (STT3) is a catalytic subunit of oligosaccharyltransferase, which is important for asparagine-linked glycosylation. Sharp eyespot, caused by the necrotrophic fungal pathogen Rhizoctonia cerealis, is a devastating disease of bread wheat. However, the molecular mechanisms underlying wheat defense against R. cerealis are still largely unclear. In this study, we identified TaSTT3a and TaSTT3b, two STT3 subunit genes from wheat and reported their functional roles in wheat defense against R. cerealis and increasing grain weight. The transcript abundance of TaSTT3b-2B was associated with the degree of wheat resistance to R. cerealis and induced by both R. cerealis and exogenous jasmonic acid (JA). Overexpression of TaSTT3b-2B significantly enhanced resistance to R. cerealis, grain weight, and JA content in transgenic wheat subjected to R. cerealis stress, while silencing of TaSTT3b-2B compromised resistance of wheat to R. cerealis. Transcriptomic analysis showed that TaSTT3b-2B affected the expression of a series of defense-related genes and JA biosynthesis-related genes, as well as genes coding starch synthase and sucrose synthase. Application of exogenous JA elevated expression levels of the abovementioned defense- and grain weight-related genes, and rescuing the resistance of TaSTT3b-2B-silenced wheat to R. cerealis, while pretreatment with sodium diethyldithiocarbamate, an inhibitor of JA synthesis, attenuated the TaSTT3b-2B-mediated resistance to R. cerealis, suggesting that TaSTT3b-2B played critical roles in regulating R. cerealis resistance and grain weight via JA biosynthesis. Altogether, this study reveals new functional roles of TaSTT3b-2B in regulating plant innate immunity and grain weight, and illustrates its potential application value for wheat molecular breeding.

Rice extra-large G proteins play pivotal roles in controlling disease resistance and yield-related traits

To better explore the potential of rice extra-large G (XLG) proteins in future breeding, we characterised the function of OsXLG1, OsXLG2 and OsXLG3 in disease resistance. Loss-of-function Osxlg2 and Osxlg3 mutants showed reduced resistance to the fungal pathogen Magnaporthe oryzae, whereas Osxlg1 mutants were specifically compromised in resistance to the bacterial pathogen Xanthomonas oryzae pv oryzae. Consistent with their effects on rice blast resistance, mutations in OsXLG2 and OsXLG3 caused greater defects than did mutations in OsXLG1 for chitin-induced defence responses. All three OsXLGs interacted with components of a surface immune receptor complex composed of OsCERK1, OsRLCK176 and OsRLCK185. Further characterisation of yield-related traits showed that the Osxlg3 mutants displayed reduced plant height, panicle length and 1000grain weight, whereas Osxlg1 mutants exhibited increased plant height, panicle length and 1000-grain weight. Together the study shows the differential contributions of the three OsXLG proteins to disease resistance to fungal and bacterial pathogens, their yield-related traits and provides insights for future improvement of rice production.

Identification, pyramid, and candidate gene of QTL for yield-related traits based on rice CSSLs in indica Xihui18 background

Chromosome segment substitution line (CSSL) is important for functional analysis and design breeding of target genes. Here, a novel rice CSSL-Z431 was identified from indica restorer line Xihui18 as recipient and japonica Huhan3 as donor. Z431 contained six segments from Huhan3, with average substitution length of 2.12 Mb. Compared with Xihui18, Z431 increased panicle number per plant (PN) and displayed short-wide grains. The short-wide grain of Z431 was caused by decreased length and increased width of glume cell. Then, thirteen QTLs were identified in secondary F2 population from Xihui18/Z431. Again, eleven QTLs (qPL3, qPN3, qGPP12, qSPP12, qGL3, qGW5, qRLW2, qRLW3, qRLW5, qGWT3, qGWT5-2) were validated by six single-segment substitution lines (SSSLs, S1-S6) developed in F3. In addition, fifteen QTLs (qPN5, qGL1, qGL2, qGL5, qGW1, qGW5-1, qRLW1, qRLW5-2, qGWT1, qGWT2, qYD1, qYD2, qYD3, qYD5, qYD12) were detected by these SSSLs, while not be identified in the F2 population. Multiple panicles of Z431 were controlled by qPN3 and qPN5. OsIAGLU should be the candidate gene for qPN3. The short-wide grain of Z431 was controlled by qGL3, qGW5, etc. By DNA sequencing and qRT-PCR analysis, two best candidate genes for qGL3 and qGW5 were identified, respectively. In addition, pyramid of different QTLs in D1-D3 and T1-T2 showed independent inheritance or various epistatic effects. So, it is necessary to understand all genetic effects of target QTLs for designing breeding. Furthermore, these secondary substitution lines improved the deficiencies of Xihui18 to some extent, especially increasing yield per plant in S1, S3, S5, D1-D3, T1, and T2.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01284-x.

A rice single cell transcriptomic atlas defines the developmental trajectories of rice floret and inflorescence meristems

Rice inflorescence development determines yield and relies on the activity of axillary meristems (AMs); however, high-resolution analysis of its early development is lacking. Here, we have used high-throughput single-cell RNA sequencing to profile 37 571 rice inflorescence cells and constructed a genome-scale gene expression resource covering the inflorescence-to-floret transition during early reproductive development. The differentiation trajectories of florets and AMs were reconstructed, and discrete cell types and groups of regulators in the highly heterogeneous young inflorescence were identified and then validated by in situ hybridization and with fluorescent marker lines. Our data demonstrate that a WOX transcription factor, DWARF TILLER1, regulates flower meristem activity, and provide evidence for the role of auxin in rice inflorescence branching by exploring the expression and biological role of the auxin importer OsAUX1. Our comprehensive transcriptomic atlas of early rice inflorescence development, supported by genetic evidence, provides single-cell-level insights into AM differentiation and floret development.

The multifaceted roles of heterotrimeric G-proteins: lessons from models and crops

The review summarizes our advanced understanding of the heterotrimeric G-protein research from model plants and their emerging roles in modulating various plant architecture and agronomical traits in crop species. Heterotrimeric G-proteins (hereafter G-proteins), consisting of G-alpha (Gα), G-beta (Gβ) and G-gamma (Gγ) subunits, are key signal transducers conserved across different forms of life. The discovery of plant lineage-specific G-protein components (extra-large G-proteins and type-C Gγ subunits), inherent polyploidy in angiosperms, and unique modes of G-protein cycle regulation in plants pointed out to a few fundamental differences of plant G-protein signaling from its animal counterpart. Over the last 2 decades, extensive studies in the model plant Arabidopsis thaliana have confirmed the involvement of G-proteins in a wide range of plant growth and development, and stress adaptation processes. The G-protein research in crop species, however, is still in its infancy, and a handful of studies suggest important roles of G-proteins in regulating plant architectural and key agronomical traits including plant's response to abiotic and biotic factors. We propose that the advancement made in plant G-proteins research will facilitate the development of novel approaches to manage plant yield and fitness in changing environments.

Genes and Their Molecular Functions Determining Seed Structure, Components, and Quality of Rice

With the improvement of people's living standards and rice trade worldwide, the demand for high-quality rice is increasing. Therefore, breeding high quality rice is critical to meet the market demand. However, progress in improving rice grain quality lags far behind that of rice yield. This might be because of the complexity of rice grain quality research, and the lack of consensus definition and evaluation standards for high quality rice. In general, the main components of rice grain quality are milling quality (MQ), appearance quality (AQ), eating and cooking quality (ECQ), and nutritional quality (NQ). Importantly, all these quality traits are determined directly or indirectly by the structure and composition of the rice seeds. Structurally, rice seeds mainly comprise the spikelet hull, seed coat, aleurone layer, embryo, and endosperm. Among them, the size of spikelet hull is the key determinant of rice grain size, which usually affects rice AQ, MQ, and ECQ. The endosperm, mainly composed of starch and protein, is the major edible part of the rice seed. Therefore, the content, constitution, and physicochemical properties of starch and protein are crucial for multiple rice grain quality traits. Moreover, the other substances, such as lipids, minerals, vitamins, and phytochemicals, included in different parts of the rice seed, also contribute significantly to rice grain quality, especially the NQ. Rice seed growth and development are precisely controlled by many genes; therefore, cloning and dissecting these quality-related genes will enhance our knowledge of rice grain quality and will assist with the breeding of high quality rice. This review focuses on summarizing the recent progress on cloning key genes and their functions in regulating rice seed structure and composition, and their corresponding contributions to rice grain quality. This information will facilitate and advance future high quality rice breeding programs.

Comprehensive Transcriptome Analysis of GS3 Near-Isogenic Lines During Panicle Development in Rice (Oryza sativa L.)

Panicle architecture is an important agronomic trait in rice that affects rice yields and quality. The GRAIN SIZE 3 (GS3) locus has been identified as a major quantitative trait locus (QTL) affecting grain length and weight. The current understanding of the function of the GS3 gene, especially concerning the regulatory mechanism of panicle development, is still in its infancy. In this study, we generated GS3 near-isogenic lines (NILs) by successive crossing and backcrossing of TD70 (large grain) with Kasalath (small grain), using Kasalath as the recurrent parent. To identify potential transcription dynamic changes in rice panicle formation and grain shape, we deeply analyzed transcriptional profiles for the NILs (NIL-GS3 and NIL-gs3) at three different panicle developmental stages (S, M, and L). A total of 887, 1,768, and 1,478 differentially expressed genes (DEGs) were identified at stages S, M, and L, respectively. We also found 542 differential expressed long non-coding RNAs (lncRNAs). Co-expression analysis further revealed significant clusters associated with different development periods in NIL-gs3 lines. Gene Ontology and KEGG enrichment analysis revealed G-protein signaling and hormones pathway were successively activated at the M and L stages of NIL-gs3, which indicated activation of the G-protein signaling pathway might trigger the down-streaming hormone signaling transduction. we found that other hormones such ABA, Auxin, CK were significantly enriched in the L stage in the NIL-gs3. We highlighted the synergistic interplay of G-protein and multiple hormones signaling pathways and their essential roles in regulating rice panicle formation and the grain shape. Our study provides an invaluable resource for further molecular mechanistic studies that affect rice grain size and provide new insight for directed selection by marker-assisted backcross breeding.

Natural variation in Glume Coverage 1 causes naked grains in sorghum

One of the most critical steps in cereal threshing is the ease with which seeds are detached from sticky glumes. Naked grains with low glume coverage have dramatically increased threshing efficiency and seed quality. Here, we demonstrate that GC1 (Glume Coverage 1), encoding an atypical G protein γ subunit, negatively regulates sorghum glume coverage. Naturally truncated variations of GC1 C-terminus accumulate at higher protein levels and affect the stability of a patatin-related phospholipase SbpPLAII-1. A strong positive selection signature around the GC1 genic region is found in the naked sorghum cultivars. Our findings reveal a crucial event during sorghum domestication through a subtle regulation of glume development by GC1 C-terminus variation, and establish a strategy for future breeding of naked grains.

Low glume coverage is the preferred for easy threshing in grain production, but the genetic basis remains unclear. Here, the authors report the gene GC1, which encodes an atypical G protein γ subunit, negatively regulates sorghum glume coverage and the naturally truncated alleles can be useful in the naked grain breeding.

Genetic Architecture of Grain Yield-Related Traits in Sorghum and Maize

Grain size, grain number per panicle, and grain weight are crucial determinants of yield-related traits in cereals. Understanding the genetic basis of grain yield-related traits has been the main research object and nodal in crop science. Sorghum and maize, as very close C4 crops with high photosynthetic rates, stress tolerance and large biomass characteristics, are extensively used to produce food, feed, and biofuels worldwide. In this review, we comprehensively summarize a large number of quantitative trait loci (QTLs) associated with grain yield in sorghum and maize. We placed great emphasis on discussing 22 fine-mapped QTLs and 30 functionally characterized genes, which greatly hinders our deep understanding at the molecular mechanism level. This review provides a general overview of the comprehensive findings on grain yield QTLs and discusses the emerging trend in molecular marker-assisted breeding with these QTLs.

A cryptic inhibitor of cytokinin phosphorelay controls rice grain size

Plant hormone cytokinin signals through histidine-aspartic acid (H-D) phosphorelay to regulate plant growth and development. While it is well known that the phosphorelay involves histidine kinases, histidine phosphotransfer proteins (HPs), and response regulators (RRs), how this process is regulated by external components remains unknown. Here we demonstrate that protein phosphatase with kelch-like domains (PPKL1), known as a signaling component of steroid hormone brassinosteroid, is actually a cryptic inhibitor of cytokinin phosphorelay in rice (Oryza sativa). Mutation at a specific amino acid D364 of PPKL1 activates cytokinin response and thus enlarges grain size in a semi-dominant mutant named s48. Overexpression of PPKL1 containing D364, either with the deletion of the phosphatase domain or not, rescues the s48 mutant phenotype. PPKL1 interacts with OsAHP2, one of authentic HPs, and D364 resides in a region resembling the receiver domain of RRs. Accordingly, PPKL1 can utilize D364 to suppress OsAHP2-to-RR phosphorelay, whereas mutation of D364 abolishes the effect. This function of PPKL1 is independent of the phosphatase domain that is required for brassinosteroid signaling. Importantly, editing of the D364-residential region produces a diversity of semi-dominant mutations associated with variously increased grain sizes. Further screening of the edited plants enables the identification of two genotypes that confer significantly improved grain yield. Collectively, our study uncovers a noncanonical cytokinin signaling suppressor and provides a robust tool for seed rational design.

OsGGC2, Gγ Subunit of Heterotrimeric G Protein, Regulates Plant Height by Functionally Overlapping with DEP1 in Rice

Plant heterotrimeric G proteins have been shown to regulate the size of various organs. There are three types of Gγ subunits in plants: type A, consisting of a canonical Gγ domain; type B, possessing a plant-specific domain at the N-terminus of the Gγ domain; and type C, possessing a plant-specific domain at the C-terminal of the Gγ domain. There is one type A, one type B, and three type C of the five γ-subunits in the rice genome. In type C Gγ subunits, GS3, which controls grain size; DEP1, which controls plant height and panicle branching; and their homolog OsGGC2, which affects grain size, have been reported; however, the function of each gene, their interactions, and molecular mechanisms for the control of plant height have not yet been clarified. In this study, we generated loss-of-function mutants of DEP1 and OsGGC2, which have high homology and similar expression, and investigated their phenotypes. Since both dep1 and osggc2 mutants were dwarfed and the double mutants showed a synergistic phenotype, we concluded that both DEP1 and OsGGC2 are positive regulators of plant height and that their functions are redundant.