Rice Spikelet Research Articles

ATAC-Seq Data for Genome-Wide Profiling of Transcription Factor Binding Sites in the Rice False Smut Fungus Ustilaginoidea virens

Identification of transcription factor binding-sites is one of the most important steps in understanding the function of transcription factors and regulatory networks in organisms. The Assay for Transposase Accessible Chromatin Sequencing (ATAC-seq) is a simple protocol for detection of open chromatin, which could be a powerful tool to advance studies of protein:DNA interactions. Although ATAC-seq has been used in systematic identification of cis-regulatory regions in animal and plant genomes, this method has been rarely applied in fungi. Here, we describe a valuable ATAC-seq resource in the genome of an economically important phytopathogen, the rice false smut fungus, Ustilaginoidea virens. The ATAC-seq data of U. virens mycelia collected from potato sucrose broth (PSB) and PSB supplied with rice spikelet extract (RSE) were both generated. This is the first genome-wide profiling of open chromatin and transcription factor binding sites in U. virens.

Endophytic fungus Phomopsis liquidambaris B3 induces rice resistance to RSRD caused by Fusarium proliferatum and promotes plant growth.

Rice spikelet rot disease (RSRD) is an emerging disease that significantly reduces rice yield and quality. In this study, we evaluated the potential use of the broad-spectrum endophytic fungus Phomopsis liquidambaris B3 as a biocontrol agent against RSRD. We also compared the control effects of different treatments, including chemical fungicides and treatment with multiple strains and single strains in combination or individually, against RSRD. The objective of this study was to find an effective and environmentally friendly control strategy to reduce the occurrence of RSRD and improve the rice yield. In pot experiments, the effect of B3 alone was better than that of fungicide or combined measures. The results showed that root colonization by B3 significantly reduced the incidence and disease index of RSRD by 41.0% and 53.8%, respectively. This was related to enhanced superoxide dismutase (SOD), peroxidase (POD), and polyphenol oxidase (PPO) activity, and to significantly upregulated expression levels of OsAOX, OsLOX, OsPAL, and OsPR10 in rice. Moreover, B3 improved the diversity of the bacterial community rather than the fungal community in the rice rhizosphere. It also led to a decrease in Fusarium proliferatum colonization and fumonisin content in the grain. Finally, root development was markedly promoted after B3 inoculation, and the yield improved by 48.60%. The result of field experiments showed that the incidence of RSRD and the fumonisin content were observably reduced in rice receiving B3, by 24.41% and 37.87%, respectively. The endophytic fungus Phomopsis liquidambaris B3 may become an effective tool to relieve rice spikelet rot disease. © 2020 Society of Chemical Industry.

Dissection of environmental and physiological effects on the temperature difference between superior and inferior spikelets within a rice panicle

Temperature of a plant organ constitutes an integrative index to its eco-physiological properties and status. However, little attempt has been made to dissect the combined effect of ecological and physiological factors on the surface temperature of a plant organ such as the rice spikelet. In this study, using a deactivated plant as reference, we developed a novel comparison method to dissect the environmental and physiological effects on temperature of rice spikelet. Three japonica rice cultivars with contrasting canopy features were used as testing materials. Temperatures of flag leaf, superior and inferior spikelets and their diurnal rhythm during grain filling stage were precisely measured by a hand-held infrared thermometer. The results showed that the variation of environmental conditions within a panicle was relatively minor, posing a limited influence on temperature difference between the superior and inferior spikelet. On the other hand, it was the intrinsic physiological properties that considerably affected the spatial variations of spikelet temperature within a panicle. Chemical analysis of sucrose and starch in grains and bracts indicated that the superior spikelet is more physiologically active at photosynthetic assimilation and starch biosynthesis. Interestingly, sugar in bracts exhibited a pattern of diurnal changes similar to the source leaf but different from the sink grain, confirming that bracts are source organs for grain filling. Our findings yield penetrating insight into the eco-physiological foundation of spikelet temperature, thus being helpful for the application of physiological approaches in crop breeding for cooler canopy.

Field survey on rice spikelet sterility in an extremely hot summer of 2018 in Japan

Field survey on rice spikelet sterility in an extremely hot summer of 2018 in Japan

Cell cycle events and expression of cell cycle regulators are determining factors in differential grain filling in rice spikelets based on their spatial location on compact panicles.

Rice being a staple crop for human, its production is required to be increased significantly, particularly keeping in view the expected world's population of 9.6 billion by the year 2050. In this context, although the rice breeding programs have been successful in increasing the number of spikelets per panicle, the basal spikelets remain poorly filled, undermining the yield potential. The present study also found the grain filling to bear negative correlation with the panicle grain density. The poorly filled basal spikelets of the compact-panicle cultivars showed a lower endosperm cell division rate and ploidy status of the endosperm nuclei coupled with no significant greater expression of CYCB;1 and CYCH;1 compared with the apical spikelets, unlike that observed in the lax-panicle cultivars, which might have prevented them from overcoming apical dominance. Significantly greater expression of CYCB2;2 in the basal spikelets than in the apical spikelets might also have prevented the former to enter into endoreduplication. Furthermore, expression studies of KRPs in the caryopses revealed that a higher expression of KRP;1 and KRP;4 in the basal spikelets than in the apical spikelets of the compact-panicle cultivars could also be detrimental to grain filling in the former, as KRPs form complex primarily with CDKA-CYCD that promotes S-phase activity and G1/S transition, and thus inhibits endosperm cell division. The study indicates that targeted manipulation of expression of CYCB1;1, CYCB2;2, CYCH1;1, KRP;1 and KRP4 in the basal spikelets of the compact-panicle cultivars may significantly improve their yield performance.

The Relationship Between Non-structural Carbohydrates and Rice Spikelet Formation: A Review

The Relationship Between Non-structural Carbohydrates and Rice Spikelet Formation: A Review

OsLFR is essential for early endosperm and embryo development by interacting with SWI/SNF complex members in Oryza sativa.

Rice (Oryza sativa L.) endosperm provides the developing embryo with nutrients and provides human beings with a staple food. The embryo eventually develops into a new sporophyte generation. Despite their important roles, the molecular mechanisms underlying early-stage endosperm and embryo development remain elusive. Here, we established the fundamental functions of rice OsLFR, an ortholog of the Arabidopsis SWI/SNF chromatin-remodeling complex (CRC) component LFR. OsLFR was expressed primarily in the rice spikelets and seeds, and the OsLFR protein was localized to the nucleus. We conducted genetic, cellular and molecular analyses of loss-of-function mutants and transgenic rescue lines. OsLFR depletion resulted in homozygous lethality in the early seed stage through endosperm and embryo defects, which could be successfully recovered by the OsLFR genomic sequence. Cytological observations revealed that the oslfr endosperm had relatively fewer free nuclei, had abnormal and arrested cellularization, and demonstrated premature programed cell death: the embryo was reduced in size and failed to differentiate. Transcriptome profiling showed that many genes, involved in DNA replication, cell cycle, cell wall assembly and cell death, were differentially expressed in a knockout mutant of OsLFR (oslfr-1), which was consistent with the observed seed defects. Protein-protein interaction analysis showed that OsLFR physically interacts with several putative rice SWI/SNF CRC components. Our findings demonstrate that OsLFR, possibly as one component of the SWI/SNF CRC, is an essential regulator of rice seed development, and provide further insights into the regulatory mechanism of early-stage rice endosperm and embryo development.

Seed filling determines seed vigour of superior and inferior spikelets during hybrid rice (Oryza sativa) seed production

Growth and development of spikelets are asynchronous during hybrid rice seed production. However, knowledge is limited about the variation in seed vigour between superior and inferior spikelets. The main objective of this study was to evaluate the differences in seed vigour between superior and inferior spikelets of hybrid rice, and to explore the mechanisms associated with such variations with regards to seed filling characteristics and starch accumulation. Field experiments were conducted in 2017 and 2018, and seed vigour parameters, seed filling characteristics and starch accumulation were determined. We found that significant differences were recorded in seed vigour parameters between superior and inferior spikelets. Germination percentage (GP) and vigour index (VI) of superior spikelets were higher than that of inferior spikelets, irrespective of year or variety. Moreover, significant differences were observed in seed filling characteristics and starch accumulation between superior and inferior spikelets. Lower total starch content, amylose content and amylopectin content were recorded in inferior spikelets with low seed filling rate and long seed filling duration. Superior spikelets with high seed filling rate and short seed filling duration increased final seed weight by 23.9 and 22.3% for each variety, respectively. Therefore, these results raise the possibility that seed filling influences the starch accumulation and seed weight, and is closely associated with differences in the vigour of seeds from superior and inferior spikelets during hybrid rice seed production.

MORE FLORET1 Encodes a MYB Transcription Factor That Regulates Spikelet Development in Rice.

Rice (Oryza sativa) spikelets have a unique inflorescence structure, and the mechanisms regulating their development are not yet fully understood. Moreover, approaches to manipulate spikelet development have the potential to increase grain yield. In this study, we identified and characterized a recessive spikelet mutant, namely more floret1 (mof1). The mof1 mutant has a delayed transition from the spikelet to the floral meristem, inducing the formation of extra lemma-like and palea-like organs. In addition, the main body of the palea was reduced, and the sterile lemma was enlarged and partially acquired hull (lemma and/or palea) identity. We used map-based cloning to identify the MOF1 locus and confirmed our identification by complementation and by generating new mof1 alleles using CRISPR-Cas9 gene editing. MOF1 encodes a MYB domain protein with the typical ethylene response factor-associated amphiphilic repression motifs, is expressed in all organs and tissues, and has a strong repression effect. MOF1 localizes to the nucleus and interacts with TOPLESS-RELATED PROTEINs to possibly repress the expression of downstream target genes. Taken together, our results reveal that MOF1 plays an important role in the regulation of organ identity and spikelet determinacy in rice.

Endophytic Bacillus megaterium triggers salicylic acid-dependent resistance and improves the rhizosphere bacterial community to mitigate rice spikelet rot disease

Endophytes have been demonstrated to trigger salicylic acid-dependent resistance and alter rhizosphere bacterial communities. However, few comprehensive studies have been conducted to elucidate the interactions between the direct (systemic resistance) and indirect (rhizosphere microbiome) defenses triggered by endophytes. The present study aimed to illuminate the possible roles of endophytic Bacillus megaterium in rice resistance against rice spikelet rot disease (SRD), and to make inferences on the potential interactions between plant defenses and the rhizosphere microenvironment. An outdoor pot experiment was conducted to explore the degree to which endophytes mitigate disease in rice, where four combination treatments were designed (inoculation and non-inoculation with B. megaterium, and infection with and without Fusarium proliferatum). Rice plants and rhizosphere soil, at the heading stage, were sampled to measure the accumulation of defense-related enzymes, including L-phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), β-1,3-glucanase and chitinase, and to perform 16S rRNA Illumina sequencing. Defense-related enzyme activities in rice plants inoculated with B. megaterium showed local or systemic increases. Plant-beneficial bacteria were found to be enriched when rice plants were treated with the endophyte. A correlation between plant resistance and the rhizosphere microbiome was also observed. The present study highlights the significance of B. megaterium for plant defenses against SRD through dual mechanism — systemic resistance and rhizosphere microbiome. Inferences of this study elucidate correlations between plant defenses and the rhizosphere microenvironment, which were attributed to the presence of B. megaterium.

Ustiloxin A is Produced Early in Experimental Ustilaginoidea virens Infection and Affects Transcription in Rice.

Ustiloxin is a kind of 13-membered cyclic peptides found in mature rice false smut generated by Ustilaginoidea virens infecting rice spikelet. So far, six kinds of ustiloxins have been identified from false smut balls (FSBs) in which ustiloxin A is the main component. The toxins can not only inhibit the growth of rice, wheat, and corn, but also poison people and animals. However, so far, there have been few studies of the content of ustiloxin except that in mature FSB. The effect of ustiloxins on the process of infection has not been clarified. In this study, the technique of artificial inoculation coupled with UPLC-ESI-MS was introduced to investigate the content of ustiloxins in the course of infection. The initial formation time of ustiloxin A, B, C, D, F, and G was no later than 5, 5, 9, 7, 7, and 9days post inoculation (dpi) prior to FSB's formation, respectively. The content of ustiloxin A per spikelet was increased rapidly from 6.0ng at 5 dpi to 14,157.1ng at 25 dpi. Meanwhile, the content of ustiloxin A per dry weight (DW) of the FSBs also peaked at 1321.2μg/g at 25 dpi. Interestingly,both the contents of ustiloxin A per dry weight and per spikelet were significantly reduced from 25 to 30 dpi. Transcriptome sequencing revealed that a total of 146 transcripts (103 upregulated and 43 downregulated) were significantly changed in rice spikelets after 3-h acute exposure to 100ng ustiloxin A. In addition, several of the significantly altered genes were validated by RT-qPCR.

Spermidine Enhanced Free Polyamine Levels and Expression of Polyamine Biosynthesis Enzyme Gene in Rice Spikelets under Heat Tolerance before Heading

High temperatures (HT) before heading strongly inhibit the development of spikelets in rice. Spermidine (Spd) can improve rice’s resistance to HT stress; however, the mechanism underlying this effect has not been elucidated. This study investigated several parameters, including yield, superoxide anion (O2.-), protective enzyme activities, and polyamine content, in a heat-sensitive genotype, Shuanggui 1. The yield and yield components decreased dramatically when subjected to HT stress, while this reduction could be partially recovered by exogenous Spd. Spd also slowed the generation rate of O2.- and increased protective enzyme, superoxide dismutase (SOD) and catalase (CAT) activities both under normal and high temperatures, which suggested that Spd may participate in the antioxidant system. Furthermore, genes involved in polyamine synthesis were analyzed. The results show that HT before heading significantly increased the expression of arginine decarboxylase OsADC1, Spd synthase OsSPDS1 and OsSPDS3 and had little effect on the expression of the S-adenosylmethionine decarboxylase OsSAMDC2 and ornithine decarboxylase OsODC1. In addition, exogenous Spd considerably reduced the expression of OsSAMDC2, OsSPDS1 and OsSPDS3 under HT but not the expression of OsADC1. The above mentioned results indicate that the exogenous Spd could help young rice spikelets to resist HT stress by reducing the expression of OsSAMDC2, OsSPDS1 and OsSPDS3, resulting in higher levels of endogenous Spd and Spm, which were also positively correlated with yield. In conclusion, the adverse effect of HT stress on young spikelets seems to be alleviated by increasing the amounts of Spd and Spm, which provides guidance for adaptation to heat stress during rice production.

Global analysis of differentially expressed genes between two Japonica rice varieties induced by low temperature during the booting stage by RNA-Seq.

As one of the abiotic stresses, low temperature severely threatens rice production during its entire growth period, especially during the booting stage. In the present study, transcriptome analysis was performed comparing Longjing (LJ) 25 (chilling-tolerant) and LJ 11 (chilling-sensitive) rice varieties to identify genes associated with chilling tolerance in rice spikelets. A total of 23 845 expressed genes and 13 205 differentially expressed genes (DEGs) were identified, respectively. Gene ontology (GO) enrichment analyses revealed ‘response to cold’ (containing 180 DEGs) as the only category enriched in both varieties during the entire cold treatment period. Through MapMan analysis, we identified nine and six DEGs related to the Calvin cycle and antioxidant enzymes, respectively, including OsRBCS3, OsRBCS2, OsRBCS4, OsAPX2 and OsCATC, that under chilling stress were markedly downregulated in LJ11 compared with LJ25. Furthermore, we predicted their protein–protein interaction (PPI) network and identified nine hub genes (the threshold of co-expressed gene number ≥ 11) in Cytoscape, including three RuBisCO-related genes with 14 co-expressed genes. Under chilling stress, antioxidant enzyme activities (peroxidase (POD) and catalase (CAT)) were downregulated in LJ11 compared with LJ25. However, the content of malondialdehyde (MDA) was higher in LJ11 compared with LJ25. Collectively, our findings identify low temperature responsive genes that can be effectively used as candidate genes for molecular breeding programmes to increase the chilling tolerance of rice.

A Novel Variation in the FRIZZLE PANICLE (FZP) Gene Promoter Improves Grain Number and Yield in Rice.

Secondary branch number per panicle plays a crucial role in regulating grain number and yield in rice. Here, we report the positional cloning and functional characterization for SECONDARY BRANCH NUMBER7 (qSBN7), a quantitative trait locus affecting secondary branch per panicle and grain number. Our research revealed that the causative variants of qSBN7 are located in the distal promoter region of FRIZZLE PANICLE (FZP), a gene previously associated with the repression of axillary meristem development in rice spikelets. qSBN7 is a novel allele of FZP that causes an ∼56% decrease in its transcriptional level, leading to increased secondary branch and grain number, and reduced grain length. Field evaluations showed that qSBN7 increased grain yield by 10.9% in a temperate japonica variety, TN13, likely due to its positive effect on sink capacity. Our findings suggest that incorporation of qSBN7 can increase yield potential and improve the breeding of elite rice varieties.

Differential grain filling in apical and basal spikelets of compact panicle rice is associated with difference in expression of miRNAs targeting gene products involved in grain filling

Development of rice cultivars bearing numerous spikelets by breeding approach to increase production of the crop to feed the ever increasing human population accompanies poor filling of grains in their basal spikelets preventing achievement of the yield potential. To understand the reason of such poor grain filling at the level of the initial biochemical events, 5 RACE library was prepared taking RNA extracted from the spikelets of two indica rice cultivars, a compact-panicle Mahalaxmi bearing numerous spikelets per panicle (>350) and a lax-panicle Upahar bearing fewer spikelets (<250) per panicle, and the presence of the miRNA cleaved gene products involved in grain filling was identified by running PCR using gene-specific primers. Bioinformatics analysis was conducted to find if the grain filling enzymes were targeted by miRNAs. The effort led to identification of three enzymes, namely granule bound starch synthase, invertase and sucrose phosphate synthase, the transcripts of which were targeted by the miRNAs osa-miR160a-5p, osa-miR166e-3p and osa-miR156b-5p, respectively. The expressions of the three miRNAs were confirmed by Northern in the apical and basal spikelets of the two cultivars. The expression analysis of the three miRNAs further revealed that these were expressed significantly more in the basal spikelets than in the apical ones of Mahalaxmi, but not of Upahar. Real time PCR of the target genes showed an opposite result, i.e., these were expressed significantly less in the basal spikelets than in the apical ones of Mahalaxmi, but mostly not of Upahar. The finding led to the conclusion that the grain filling in rice could be regulated at the level of miRNAs, and manipulation of their expression could be proved an important tool in improving grain filling in compact-panicle rice cultivars.

The C2H2 zinc-finger protein LACKING RUDIMENTARY GLUME 1 regulates spikelet development in rice

Rice (Oryza sativa) spikelets are a unique inflorescence structure and their development directly determines grain size and yield. Although many genes related to spikelet development have been reported, the molecular mechanisms underlying this process have not been fully elucidated. In this study, we identified a new recessive rice mutant, lacking rudimentary glume 1 (lrg1). The lrg1 spikelets only formed one rudimentary glume, which, along with the sterile lemmas, was homeotically transformed into lemma-like organs and acquired lemma identity. The transition from the spikelet to the floral meristem was delayed in the lrg1 mutant, resulting in the formation of an ectopic lemma-like organ between the sterile lemma and the terminal floret. In addition, we found that the abnormal lrg1 grain phenotype resulted from the alteration of cell numbers and the hull size. LRG1 encodes a ZOS4-06-C2H2 zinc-finger protein with the typical EAR motifs, and is expressed in all organs and tissues. LRG1 localizes to the nucleus and can interact with the TOPLESS-RELATED PROTEINs (TPRs) to repress the expressions of their downstream target genes. Taken together, our results reveal that LRG1 plays an important role in the regulation of spikelet organ identity and grain size.

Response of brassinosteroids to nitrogen rates and their regulation on rice spikelet degeneration during meiosis

AbstractThe plant steroid hormones brassinosteroids (BRs) play pivotal roles in modulating flower and fruit production. Nitrogen (N) is a key factor affecting rice (Oryza sativa L.) production. We hypothesized that BRs would respond to N application rates at spikelet differentiation (SD) and regulate spikelet degeneration in rice. Three rice cultivars were field‐grown and treated with five N rates at SD. Plant N and BRs contents, antioxidant capacity, and energy status were observed during meiosis in young panicles, and their relationships with spikelet degeneration were evaluated. In all the N treatments, the BRs, adenosine triphosphate, and energy charge levels, activities of the enzymes involved in energy metabolism, including cytochrome C oxidase and succinate dehydrogenase, total antioxidant capacity, and grain yield, were the highest, whereas hydrogen peroxide (H2O2) content and vacuolar processing enzymes (VPE) genes expression levels, and spikelet degeneration rate were the lowest when plant N content was 2.6% during meiosis. The rice BRs‐deficient mutants showed substantial reduction in BRs levels, antioxidant capacity, and energy status, but great increase in H2O2 content, VPE genes expression levels, and spikelet degeneration rate than the wild type (WT); moreover, these parameters in WT panicles could be regulated, whereas in BRs‐deficient mutant panicles, they were not distinctly affected by N application. The results indicated that BRs mediate the effects of N rates on spikelet degeneration, and elevated BRs levels in rice panicles effectively inhibit spikelet degeneration when plant N content is 2.6% during meiosis by elevating antioxidant capacity and energy status.

UvCom1 Is an Important Regulator Required for Development and Infection in the Rice False Smut Fungus Ustilaginoidea virens.

Ustilaginoidea virens is an economically important biotrophic fungal pathogen that causes rice false smut worldwide. However, the regulatory mechanisms of smut ball formation under U. virens infection remain unclear. Here, we identified an important transcription factor, UvCom1, from this pathogen, which regulates the formation of smut balls on rice spikelets. UvCom1 contains two conserved internal repeat 1 (RPT) domains and is found only in fungi, with specific conservation in species of Basidiomycetes and Ascomycetes. UvCom1 protein N- or C-terminal fusion vectors both showed transactivation activity in yeast. Deletion of UvCom1 significantly affected the vegetative growth and conidiation of U. virens. UvCom1 negatively regulated the responses to oxidative, osmotic, and cell wall stresses. Remarkably, UvCom1 was found to be essential for the formation of rice smut balls, and UvCom1 deletion mutants lost the ability to stably utilize nutrients from the rice host. UvCom1 was also highly expressed at the mycelial expansion stage. Transcriptomic analysis and quantitative real-time PCR revealed that UvCom1 could affect the expression of genes significantly enriched in transmembrane transport. This study demonstrates that UvCom1 is a key transcription factor governing smut ball formation of this biotrophic fungus.

Effects of Late Sowing on Differentiation and Degeneration of Branches and Spikelets of <i>Indica</i> Rice under Different Ecological Conditions

In rice (Oryza sativa L.), yield is related to characteristics of branches and spikelets. To investigate the effects of late sowing date on differentiation and degeneration of spikelets in rice, field experiments were conducted in Chongzhou and Hanyuan, China. Differentiation and survival of branches and spikelets in Hanyuan were lower than that of Chongzhou, whereas degeneration was greater than that of Chongzhou. In Chongzhou, sowing date affected differentiation and survival of primary, secondary, and total branches, as well as differentiation and degeneration of secondary and total spikelets. In Hanyuan, sowing date affected degeneration of secondary and total branches, and the survival and degeneration of primary spikelets. Late-sown plants experienced higher temperatures in the jointing to heading period, and there were higher sunshine hours. Rainfall and humidity were higher in Chongzhou, but lower in Hanyuan. Late sowing increased differentiation, degeneration, and survival number of branches and spikelets in Chongzhou. However, in Hanyuan, late sowing increased differentiation and degeneration of branches, but decreased survival of branches, which reduced spikelet differentiation and degeneration, resulting in fewer branches. Thus, the key to higher yield in Chongzhou was to increase differentiation of spikelets, whereas in Hanyuan, it was to reduce degeneration of branches and spikelets.

Gene mapping and candidate gene analysis ofmulti‐grains 1(Mg1) in rice

AbstractAs the basic unit of rice (Oryza sativaL.) inflorescence, the spikelet has a profound influence on grain size, weight, and yield. However, the molecular mechanism that underlies the development of spikelets has never been fully elucidated. In this study, we characterized a rice spikelet mutant,multi‐grains 1(mg1), which was derived from treatment of ‘Xinong 1B’ with ethyl methanesulfonate. In themg1mutant, the spikelet developed two florets with an extra lemma‐like organ and an increased number of inner floral organs. Notably, the spikelet could produce multi‐grains. These phenotypes reflect the considerable potential of usingmg1as a means of increasing rice yield. Genetic analysis revealed that themg1phenotype is controlled by a single recessive gene. TheMG1gene was mapped to the interval between simple sequence repeat (SSR) markers RM20754 and RM494 on chromosome 6 within a physical region of 121.57 kb. Sequencing analysis identified a single‐nucleotide substitution from G to A withinOs06g0717200(which corresponds toFLORAL ORGAN NUMBER 1[FON1]), causing an amino acid mutation of Gly‐869 to Asp‐869 in themg1mutant. This finding suggests thatMG1may be a novel allele ofFON1.MG1is involved in the development of spikelet determinacy and floral organs by regulating the expression of genes associated with these characteristics. The present results detail a novel function ofMG1, which may facilitate the breeding of rice cultivars with increased numbers of grains per panicle.