Linkage mapping and association analysis to identify a reliable QTL for stigma exsertion rate in rice

Improvement of the outcrossing ability of CMS lines by pyramiding QTLs for SER

The stigma exsertion rate (SER) is a key factor in improving the outcrossing ability of cytoplasmic male sterility (CMS) lines in rice. In previous studies, we identified 18 SER-QTLs and developed some SER-QTL pyramiding lines (PLs). In this study, 4QL-1 and 4QL-2 were selected from these PLs and crossed with CMS maintainer lines H211B and H212B, respectively, to develop two new CMS maintainer lines, H221B and H222B, and their CMS lines H221A and H222A. The SER of H221B and H222B were 74.7% and 73.1%, respectively, reaching a high SER level. Compared with CMS maintainer lines, the CMS lines consistently exhibited higher SER, which may be related to the delayed flowering time of the CMS lines. Filed experiments showed that outcrossing seed-setting rates of H221A and H222A were significantly higher than those of the original CMS lines, which meets the requirements for hybrid rice seed production. These results confirm that SER is a key factor in enhancing rice outcrossing ability. Our findings demonstrate that pyramiding SER-QTLs is an effective strategy for improving rice SER and increasing outcrossing seed-setting rate.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-025-01545-5.

Genetic Analysis and Fine Mapping of QTLs for Stigma Exsertion Rate in Rice

Stigma exsertion rate (SER) is a crucial trait that influences the seed production of hybrid rice by determining the outcrossing ability of male sterile lines (MSLs). However, the molecular genetic mechanisms underlying SER are still poorly understood. In this study, we identified 14 quantitative trait loci (QTLs) using a recombinant inbred line (RIL) population derived from B805D-MR-16-8-3 (B805D) and Hua6S. Two major QTLs, qSE1 and qSE9, were validated for their effects in the residual heterozygous line (RHL) background. The RHL carrying homozygous qSE1 region from Hua6S increased dual stigma exsertion rate (DSE) by 14.67% and 15.04%, and increased total stigma exsertion rate (TSE) by 11.73% and 13.04%, in F10 and F11 progeny, respectively. Conversely, the RHL carrying homozygous qSE9 region from B805D showed a substantial increase of 22.72% and 14.45% in single stigma exsertion rate (SSE), an increase of 13.46% and 8.30% in TSE, and an increase in percentage of spikelets with exserted stigma (PSE) by 24.82% and 15.57%, respectively, in F10 and F11 progeny. Furthermore, examination of floral organ traits revealed that both the Hua6S allele of qSE1 and the B805D allele of qSE9 increased pistil size to improve SER, but they had contrasting effects on spikelet shape. Subsequently, qSE1 and qSE9 were fine-mapped to intervals of 246.5 kb and 341.4 kb, respectively. A combination of sequencing, expression and haplotype analysis revealed that a single nucleotide variation (T to C) in the 5'UTR region of LOC_Os01g72020 (OsBOP1) was likely to be the functional variation for qSE1. Collectively, our work has laid a foundation for cloning the genes responsible for SER, and demonstrated that the Hua6S allele of qSE1 and the B805D allele of qSE9 can effectively increase SER, which could make important contributions to the genetic improvement of MSLs aimed at improving hybrid seed production.

Research Progress on the Trait of Stigma Exsertion in Rice

As global food demand continues to grow, enhancing rice seed-setting rate and yield has emerged as a crucial research topic. The stigma exsertion rate in rice, a pivotal determinant of the outcrossing seed-setting rate in sterility lines, is essential for facilitating the propagation and efficient seed production of hybrid rice varieties. This article reviews the research progress on stigma exertion rate in rice, systematically analyzing the latest molecular biology and genetics findings to uncover the key genes and molecular mechanisms regulating stigma exertion. Furthermore, it explores the application of molecular marker-assisted selection technology in rice breeding, aiming to optimize stigma exertion traits to enhance the stigma exertion rate and outcrossing habits of rice sterility lines. By integrating existing research outcomes, this article not only provides researchers with a theoretical foundation for a deeper understanding of the regulatory mechanisms of stigma exertion but also offers practical strategies for rice breeding practices.

Four QTLs control stigma exsertion rate by changing stigma size in rice

The stigma exsertion rate (SER) is a key trait for the outcrossing ability of hybrid rice, which directly affects the yield of hybrid seeds in hybrid seed production. In previous studies, we have located 18 QTLs for SER using single-segment substitution lines in rice. In this study, we found that 4 of 18 QTLs for SER controlled stigma size (SS). On chromosome 1, a QTL qSL-1 controlling stigma length (SL) was located at the same interval of qSER-1b. On chromosome 2, two QTLs for SS, qSS-2a and qSS-2b, linked closely within a 1288.0 kb region, were at the same positions of qSER-2a and qSER-2b, respectively. A QTL qSL-12 controlling SL on chromosome 12 was at the same location of qSER-12. Additive effects of four QTLs for SS ranged from 0.12 mm to 0.38 mm, showing significant effects on SS. In pyramiding lines of QTLs for SS, SS enlarged with the increase of QTLs. The effect of QTLs on SER was consistent with their effect on SS, and SL had a greater positive effect on SER than the stigma width. Our findings demonstrate that SS is one of the important factors affecting SER in rice.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01499-0.

Stigma and Glume Characteristics Synergistically Determine the Stigma Exsertion Rate in Thermo-Photo-Sensitive Genic Male Sterile Wheat

Wheat hybrids have been widely demonstrated to have remarkable heterosis or hybrid vigor in increasing yield potential and stability since the 1960s. Two-line hybrid wheat can achieve higher yields than local varieties, especially in marginal environments. However, the commercial application of hybrid wheat is hindered by higher seed costs, primarily due to lower yields in hybrid seed production. Stigma exsertion has been verified as a decisive factor in increasing rice's hybrid seed yield, but more investigation is needed in hybrid wheat breeding and production. In this study, four thermo-photo-sensitive genic male sterile lines, including K41S, K64S, K66S, and K68S, with different stigma exsertion rates (SERs) were used to compare the differences in floral architecture relative to stigma exsertion over two growing seasons. The results revealed that the K41S and K64S exhibited a relatively higher SER at 21.87% and 22.81%, respectively. No exserted stigma was observed in K66S, and K68S had an SER of only 0.82%. This study found that the stigma length, glume width and the length-width ratio of the glume were significantly correlated with the SER, with correlation coefficients of 0.46, -0.46 and 0.60, respectively. Other stigma features such as the branch angle, stretch width and hairbrush length, as well as the glume length, also had a weakly positive correlation with SER (r = 0.09-0.27). For K41S and K64S, the SER was significantly affected by the differences in the stigma branch angle and stigma stretch width among florets. A cross-pollination survey showed that the out-crossing ability of florets with an exserted stigma was about three times as high as that of florets with a non-exserted stigma. As a result, the stigma-exserted florets that accounted for 21.87% and 22.81% of the total florets in K41S and K64S produced 46.80% and 48.53% of the total cross-pollinated seeds in both sterile lines. These findings suggest that a longer stigma combined with a slender glume appears to be the essential floral feature of stigma exsertion in sterile wheat lines. It is expected that breeding and utilizing sterile lines with a higher SER would be a promising solution to cost-effective hybrid wheat seed production.

Hybrid Rice Production: A Worldwide Review of Floral Traits and Breeding Technology, with Special Emphasis on China

Rice is an important diet source for the majority of the world's population, and meeting the growing need for rice requires significant improvements at the production level. Hybrid rice production has been a significant breakthrough in this regard, and the floral traits play a major role in the development of hybrid rice. In grass species, rice has structural units called florets and spikelets and contains different floret organs such as lemma, palea, style length, anther, and stigma exsertion. These floral organs are crucial in enhancing rice production and uplifting rice cultivation at a broader level. Recent advances in breeding techniques also provide knowledge about different floral organs and how they can be improved by using biotechnological techniques for better production of rice. The rice flower holds immense significance and is the primary focal point for researchers working on rice molecular biology. Furthermore, the unique genetics of rice play a significant role in maintaining its floral structure. However, to improve rice varieties further, we need to identify the genomic regions through mapping of QTLs (quantitative trait loci) or by using GWAS (genome-wide association studies) and their validation should be performed by developing user-friendly molecular markers, such as Kompetitive allele-specific PCR (KASP). This review outlines the role of different floral traits and the benefits of using modern biotechnological approaches to improve hybrid rice production. It focuses on how floral traits are interrelated and their possible contribution to hybrid rice production to satisfy future rice demand. We discuss the significance of different floral traits, techniques, and breeding approaches in hybrid rice production. We provide a historical perspective of hybrid rice production and its current status and outline the challenges and opportunities in this field.

Grain shape is a factor affecting the stigma exsertion rate in rice

Stigma exsertion rate (SER) is an index of outcrossing ability in rice and is a key trait of male sterile lines (MSLs) in hybrid rice. In this study, it was found that the maintainer lines carrying gs3 and gs3/gw8 showed higher SER. Single-segment substitution lines (SSSLs) carrying gs3, gw5, GW7 or gw8 genes for grain shape and gene pyramiding lines were used to reveal the relationship between grain shape and SER. The results showed that the grain shape regulatory genes had pleiotropic effects on SER. The SERs were affected by grain shapes including grain length, grain width and the ratio of length to width (RLW) not only in low SER background, but also in high SER background. The coefficients of determination (R²) between grain length and SER, grain width and SER, and grain RLW and SER were 0.78, 0.72, and 0.91 respectively. The grain RLW was the most important parameter affecting SER, and a larger grain RLW was beneficial to stigma exsertion. The pyramiding line PL-gs3/GW7/gw8 showed the largest grain RLW and the highest SER, which will be a fine breeding resource. Further research showed that the grain shape regulatory genes had pleiotropic effects on stigma shape, although the R² values between grain shape and stigma shape, and stigma shape and SER were lower. Our results demonstrate that grain shape is a factor affecting SER in rice, in part by affecting stigma shape. This finding will be helpful for breeding MSLs with high SER in hybrid rice.

Potential roles of stigma exsertion on spikelet fertility in rice (Oryza sativa L.) under heat stress

Heat stress during the flowering stage induces declining spikelet fertility in rice plants, which is primarily attributed to poor pollination manifesting as insufficient pollen deposited on the stigma. Plant pollination is associated with anther dehiscence, pollen dispersal characteristics, and stigma morphology. The mechanisms underlying the responses of spikelet fertility to heat stress have been clarified in depth in terms of the morphological and behavioral characteristics of the male reproductive organs in rice. However, the roles of female reproductive organs, especially the stigma, on spikelet fertility under heat conditions are unclear. The present study reviews the superiority of stigma exsertion on pollen receptivity under heat during the flowering stage and discusses the variations in the effects of exserted stigma on alleviating injury under asymmetric heat (high daytime and high nighttime temperatures). The pollination advantages of exserted stigmas seem to be realized more under high nighttime temperatures than under high daytime temperatures. It is speculated that high stigma exsertion is beneficial to spikelet fertility under high nighttime temperatures but detrimental under high daytime temperatures. To cope with global warming, more attention should be given to rice stigma exsertion, which can be manipulated through QTL pyramiding and exogenous hormone application and has application potential to develop heat-tolerant rice varieties or innovate rice heat-resistant cultivation techniques, especially under high nighttime temperatures.