OsMADS1 plays a vital role in regulating floret development and grain shape, but whether it regulates rice grain quality still remains largely unknown. Therefore, we used comprehensive molecular genetics, plant biotechnology, and functional omics approaches, including phenotyping, mapping-by-sequencing, target gene seed-specific RNAi, transgenic experiments, and transcriptomic profiling to answer this biological and molecular question. Here, we report the characterization of the ‘Oat-like rice’ mutant, with poor grain quality, including chalky endosperms, abnormal morphology and loose arrangement of starch granules, and lower starch content but higher protein content in grains. The poor grain quality of Oat-like rice was found to be caused by the mutated OsMADS1^Olr allele through mapping-by-sequencing analysis and transgenic experiments. OsMADS1 protein is highly expressed in florets and developing seeds. Both OsMADS1-eGFP and OsMADS1^Olr-eGFP fusion proteins are localized in the nucleus. Moreover, seed-specific RNAi of OsMADS1 also caused decreased grain quality in transgenic lines, such as the Oat-like rice. Further transcriptomic profiling between Oat-like rice and Nipponbare grains revealed that OsMADS1 regulates gene expressions and regulatory networks of starch and storage protein metabolisms in rice grains, hereafter regulating rice quality. In conclusion, our results not only reveal the crucial role and preliminary mechanism of OsMADS1 in regulating rice grain quality but also highlight the application potentials of OsMADS1 and the target gene seed-specific RNAi system in improving rice grain quality by molecular breeding.

Salt–alkali stress threatens the resilience to variable environments and thus the grain yield of rice. However, how rice responds to salt–alkali stress at the molecular level is poorly understood. Here, we report isolation of a novel salt–alkali-tolerant rice (SATR) by screening more than 700 germplasm accessions. Using 93-11, a widely grown cultivar, as a control, we characterized SATR in response to strong salt–alkali stress (SSAS). SATR exhibited SSAS tolerance higher than 93-11, as indicated by a higher survival rate, associated with higher peroxidase activity and total soluble sugar content but lower malonaldehyde accumulation. A transcriptome study showed that cell wall biogenesis-related pathways were most significantly enriched in SATR relative to 93-11 upon SSAS. Furthermore, higher induction of gene expression in the cell wall matrix polysaccharide biosynthesis pathway, coupled with higher accumulations of hemicellulose and pectin as well as measurable physio-biochemical adaptive responses, may explain the strong SSAS tolerance in SATR. We mapped SSAS tolerance to five genomic regions in which 35 genes were candidates potentially governing SSAS tolerance. The 1,4-β-D-xylan synthase gene OsCSLD4 in hemicellulose biosynthesis pathway was investigated in details. The OsCSLD4 function-disrupted mutant displayed reduced SSAS tolerance, biomass and grain yield, whereas the OsCSLD4 overexpression lines exhibited increased SSAS tolerance. Collectively, this study not only reveals the potential role of cell wall matrix polysaccharides in mediating SSAS tolerance, but also highlights applicable value of OsCSLD4 and the large-scale screening system in developing SSAS-tolerant rice.

Background: Grain shape is a critical agronomic trait affecting grain yield and quality. Exploration and functional characterization of grain shape-related genes will facilitate rice breeding for higher quality and yield. Results: Here, we characterized a recessive mutant named Oat-like rice for its unique grain shape which highly resembles oat grains. The Oat-like rice displayed abnormal floral organs, an open hull formed by remarkably elongated leafy lemmas and paleae, occasionally formed conjugated twin brown rice, an aberrant grain shape and a low seed setting rate. By map-based cloning, we discovered that Oat-like rice harbors a novel allele of OsMADS1 gene (OsMADS1^Olr), which has a spontaneous point mutation that causes the substitution of an amino acid that is highly conserved in the MADS-box domain of the MADS-box family. Further linkage analysis indicated that the point mutation in the OsMADS1^Olr is associated with Oat-like rice phenotype, and expression analysis of the OsMADS1 by qRT-PCR and GUS staining also indicated that it is highly expressed in flower organs as well as in the early stages of grain development. Furthermore, OsMADS1^Olr-overexpressing plants showed similar phenotypes of Oat-like rice in grain shape, possibly due to the dominant negative effect. And OsMADS1-RNAi plants also displayed grain phenotypes like Oat-like rice. These results suggested that OsMADS1^Olr is responsible for the Oat-like rice phenotype including aberrant grain shape. Moreover, the expression levels of representative genes related to grain shape regulation were apparently altered in Oat-like rice, OsMADS1^Olr-overexpressing and OsMADS1-RNAi transgenic plants. Finally, compared with Oat-like rice, OsMADS1^Olr-overexpressing and OsMADS1-RNAi plants, mild phenotype of seed-specific OsMADS1-RNAi transgenic plants indicated that OsMADS1 may has has a direct regulation role in grain development and the grain phenotypes of Oat-like rice, OsMADS1^Olr-overexpressing and OsMADS1-RNAi plants are majorly caused by the abnormal lemma and palea development. Conclusions: Altogether, our results showed that grain shape and a low seed setting rate of the notable ‘Oat-like rice’ are caused by a spontaneous point mutation in the novel allele OsMADS1^Olr. Furthermore, our findings suggested that OsMADS1 mediates grain shape possibly by affecting the expression of representative genes related to grain shape regulation. Thus, this study not only revealed that OsMADS1 plays a vital role in regulating grain shape of rice but also highlighted the importance and value of OsMADS1 to improve the quality and yield of rice by molecular breeding.

Polyploid rice and its reverted diploid show rich phenotypic variation and strong heterosis, showing great breeding value. However, the genomic differences among tetraploids, counterpart common diploids, tetraploid-revertant diploids, and hybrid descendants are unclear. In this work, we bred a new excellent two-line hybrid rice variety, Y Liang You Duo Hui 14 (HTRM12), using Haitian tetraploid self-reverted diploid (HTRM2). Furthermore, we comparatively analyzed the important agronomic traits and genome-wide variations of those closest relatives, Haitian diploid (HT2), Haitian tetraploid (HT4), HTRM2, and HTRM12 in detail, based on multiple phenotypic investigations, genome resequencing, and bioinformatics analysis. The results of agronomic traits analysis and genome-wide variation analysis of single nucleotide polymorphism (SNP), insertion–deletion (InDel), and copy number variation (CNV) show that HT4 and HTRM2 had abundant phenotypic and genomic variations compared to HT2. HTRM2 can inherit important traits and variations from HT4. This implies that tetraploid self-reverted diploid has high potential in creating excellent breeding materials and in breeding breakthrough hybrid rice varieties. Our study verifies the feasibility that polyploid rice could be used as a mutation carrier for creating variations and provides genomic information, new breeding materials, and a new way of application for tetraploid rice breeding.

Background: Male sterility is a critical trait for large-scale hybrid rice seed production, yet the genetic and molecular regulatory networks governing tapetal degradation during anther development remain incompletely understood. This study aimed to clone the causal gene underlying a novel rice non-pollen male sterility mutant and elucidate its role in tapetal development and microsporogenesis. Methods: The nms1 (non-pollen male sterility 1) sterile mutant was screened from the ethyl methanesulfonate (EMS)-mutagenized progeny of the elite indica restorer line Shuhui 498 (R498). Map-based cloning and whole-genome resequencing-assisted bulked segregant analysis were used to identify the causal variant. Gene function was verified via cytological observation, genetic complementation testing, RNA sequencing, and quantitative real-time PCR (qRT-PCR) to profile sterility-associated transcriptional changes. Results: Gene mapping identified a T635A single-nucleotide substitution within OsR498G0305626400.01 on chromosome 3, which encodes a strictosidine synthase-like protein. This nucleotide alteration causes a Val212Glu amino acid change and is associated with delayed tapetal degradation and pollen abortion. Transgenic complementation experiments verified that functional NMS1 restores fertility in nms1 mutant plants. Spatiotemporal expression analysis showed predominant NMS1 expression in late-developing spikelets. Furthermore, combined RNA sequencing and qRT-PCR analyses demonstrated that loss of NMS1 function leads to significant transcriptional dysregulation of key regulators of programmed cell death (PCD) in the tapetum (PTC2, TIP2) and pollen wall biosynthesis genes (TIP3, OsMS2). Conclusions: This study demonstrates that NMS1 plays a crucial role in coordinating tapetal degradation and microspore development in rice. The discovered functional SNP of NMS1 provides a novel theoretical foundation and a valuable sterile genetic resource for hybrid rice breeding.