Wild Abortive Cytoplasmic Male Sterility Research Articles

Mapping of Rf20(t), a minor fertility restorer gene for rice wild abortive cytoplasmic male sterility in the maintainer line 'Zhenshan97B'.

Three-line hybrid rice has primarily been developed on wild abortive (WA)-type cytoplasmic male sterility (CMS) and has helped increase the yield of rice globally. The development of WA-type CMS lines and hybrids was expedited through the identification and mapping of the fertility restorer gene (Rf) in maintainers. This study observed fertile plants in WA-TianfengA/Zhenshan97B//TianfengB population, indicating that the maintainer line 'Zhenshan97B' should carry Rfs for WA-type CMS. Several advanced backcross populations were generated with the genetic background of the 'WA-TianfengA,' and the pollen fertility levels of the backcrossed individuals in BC3F1, BC4F1 and BC4F2 populations are governed by a new gene, Rf20(t), from 'Zhenshan97B.' Employing bulk segregant analysis of fertile and sterile pools from the BC4F1 population, Rf20(t) was genetically mapped to a candidate region on chromosome 10. Subsequently, Rf20(t) was located between RM24883 and RM24919 through recombination analysis of molecular markers using the BC4F2 population. Implementing a substitution mapping strategy, Rf20(t) was ultimately mapped to a 245-kb region between the molecular markers STS10-122 and STS10-126 and obtained the most likely candidate gene LOC_Os10g02650, which is predicted to encode pentatricopeptide repeat-containing (PPR) protein. These results enhance our understanding of the fertility restoration of WA-type CMS lines, facilitating the development of high-quality pairs of WA-type CMS and maintainer lines.

Identification and fine mapping of a fertility restorer gene for wild abortive cytoplasmic male sterility in the elite indica rice non-restorer line 9311

The wild abortive (WA)-type cytoplasmic male sterility (CMS) derived from the wild rice species Oryza rufipogon Griff. is used widely in three-line indica hybrids. The identification and mapping of restorer of fertility (Rf) genes aided in the development of WA-type hybrids. Here we report that testcross F1 plants from the WA-type CMS line and 9311 exhibited stainable pollen grains with no seed set, indicating that 9311 carries minor-effect Rfs for WA-type CMS. We developed an advanced backcross population consisting of plants harboring small regions of donor chromosomal segments from 9311 in the WA-TianfengA genetic background with moderate seed setting rates. Genetic analysis showed that the pollen fertility levels of the backcross individuals are governed by a single gene from 9311 that we named Rf19(t). By use of the RICE 40 K gene chip, three introduced segments were identified in the fertile lines, and a candidate region spanning 4.37–8.29 Mb on chromosome 1 was identified for Rf19(t). Finally, Rf19(t) was fine-mapped to a region of 90 kb between the DNA marker loci STS1-163 and STS1-183, in which eight ORFs were predicted. Also, using relative expression analyses, comparative sequence analyses and functional domain analyses, we identified LOC_Os01g10530 as the most likely candidate gene for Rf19(t). Furthermore, Rf19(t) was found to function in fertility restoration, most probably by regulating the degradation of mRNA transcribed from the mitochondrial gene WA352. These results increase our knowledge of fertility restoration in WA-type CMS lines and will facilitate the development of high-quality pairs of WA-type CMS and maintainer lines.

Precise genetic mapping of Rf18(t), a new fertility restorer gene from 'Nipponbare' for wild abortive cytoplasmic male sterility in rice (Oryza sativa L.).

We mapped Rf18(t), a Restorer-of-fertility gene for wild abortive cytoplasmic male sterility from the japonica maintainer 'Nipponbare', to chromosome 1. The best candidate gene, LOC_Os01g71320, is predicted to encode hexokinase. Three-line hybrid rice obtained through cytoplasmic male sterility (CMS) has helped increase the yield of rice globally, and the wild abortive (WA)-type cytoplasm from wild rice (Oryza rufipogon Griff.) is used widely in three-line indica hybrids. The identification and mapping of the Restorer-of-fertility (Rf) genes in maintainer lines aided in uncovering the genetic basis of fertility restoration of WA-type CMS and the development of WA-type hybrids. In this study, we identified a new Rf gene, Rf18(t), for WA-type CMS from the japonica maintainer line 'Nipponbare' using a chromosome segment substitution line population derived from a cross between the indica line 9311 and 'Nipponbare.' Using a substitution mapping strategy, Rf18(t) was delimited to a 48-kb chromosomal region flanked by molecular marker loci ID01M28791 and ID01M28845 on chromosome 1. By comparative sequence analyses, we propose that LOC_Os01g71320 is the most likely candidate gene for Rf18(t), and it is predicted to encode hexokinase. Furthermore, Rf18(t) was found to function in fertility restoration probably by a posttranscriptional mechanism and its function is dependent on the genetic background of 9311. These results broaden our knowledge on the mechanism of fertility restoration of WA-type CMS lines and will facilitate the development of WA-type rice hybrids.

Genetics of Pollen fertility restoration trait in rice (Oryza sativa L.) for wild abortive cytoplasmic male Sterility system

Two crosses in F2 generation viz., APMS 8A × RM 141-91-11-2-1-1 and APMS 6A × RM 71-42-2-1-1 used to study the genetics of fertility restoration, revealed that one major gene governs the fertility restoration, with dominant monogenic gene action. The crosses APMS 8A × RM 141-91-11-2-1-1 and APMS 6A × RM 71-42-2-1-1 segregated in the ratio of 3:1 in F2 generation, indicating the presence of single independently segregating dominant gene governing the fertility restoration in both the restorers under study.

Temperature stress and fertility reversibility in wild abortive CMS lines of rice for quality hybrid seed production

CMS lines are highly sensitive to temperature change for the sterility status. Determination of critical temperature for sterility breakdown in wild abortive cytoplasmic male sterile (WA-CMS) lines under field condition is essential for efficient seed production planning. CMS lines IR58025A and IR68897A were grown at different dates of sowing to give exposure to a series of temperature regime and day length at natural field condition. At panicle initiation stages, temperatures below 28.5 °C, the CMS lines were sterile and at temperatures below 30 °C, the lines became partially sterile. Above 30 °C, the CMS line reverted to fertile. The highest pollen sterility was recorded (98.3%) during the long day condition while the lowest was recorded (43.5%) during the short day condition indicating the effect of both temperature and day-length on break down of sterility. Differential expression of floral characters like panicle exertion and stigma was found with variation in temperature. This simple field-driven technique can be successfully employed to work out the functional temperature for fertility reversibility which is of utmost importance for planning seed production and maintenance of genetic purity of the hybrids. The technique will aid in designing seed production planning for hybrid rice locally in any rice-growing area of the world in the present scenario of restrictions in export and import of agricultural commodities.

Expression profiling of immature florets of IR58025A, a wild-abortive cytoplasmic male sterile line of rice and its cognate, isonuclear maintainer line, IR58025B.

Interaction between gene products encoded by the cytoplasm and nucleus form the core of wild abortive cytoplasmic male sterile (WA-CMS) system of hybrid breeding in rice. Gaining insights into such interactions can be helpful in the development of better three-line rice hybrids and also identify novel male sterility systems. In the present study, the whole transcriptome profiles of immature florets of IR58025A, a WA-CMS line and its isonuclear maintainer line, IR58025B, collected at pre-anthesis stage were compared to delineate the pathways involved in pollen abortion and male sterility. Among the 774 differentially expressed transcripts (DETs), 496 were down regulated and 278 were up regulated in IR58025A compared to IR58025B. The genes associated with oxidative stress response, defense response, etc. were significantly up-regulated, while those associated with respiration, cell wall modifications, pectinesterase activity, etc. were significantly down-regulated in the WA-CMS line. Gene ontology and pathway enrichment analyses revealed the down-regulation of both nuclear and organellar genes involved in key metabolic processes of cell respiration, photosynthesis and other energy yielding metabolites in IR58025A, relative to IR58025B, indicating a general shift toward conservation of energy and other key resources in the florets of WA-CMS line. The data derived from RNA-Seq analysis were validated through qRT-PCR analysis. Based on the results obtained, it can be hypothesized that pollen abortion principally occurs due to up-regulation of pathways leading to oxidative stress leading to energy starvation conditions in consonance with reduced expression of genes associated with the cell wall formation, respiration, and other key metabolic processes.

Molecular detection of WA‐CMS restorers from tropical japonica‐derived lines, their evaluation for fertility restoration and adaptation

AbstractHybrid rice based on wild‐abortive cytoplasmic male sterility (WA‐CMS) is important in boosting rice production, which requires diverse parents to harness heterosis. For this, exploiting the diversity of japonica through tropical japonica (TRJ) lines is an excellent route. In this study, 310 TRJ‐based new plant type (NPT) lines were developed and evaluated for Rf3 and Rf4 genes. Gene‐based (DRRM‐Rf3‐5 and DRRM‐Rf3‐10) and functional marker (RMS‐SF21‐5) targeted Rf3 locus, while gene‐linked (RM6100) and functional marker (RMS‐PPR9‐1) targeted the Rf4 locus. The frequency of the restorer allele of Rf3 gene was lower when compared to that of Rf4. Combined phenotypic and molecular screening using gene‐based and functional markers identified 42 lines that carried Rf3 and/or Rf4 genes. All the selected lines produced fertile F1s when crossed to a WA‐CMS line, “Pusa 6A”, but with varying levels of spikelet fertility. This is the first report of a marker‐cum‐phenotype‐based restorer selection using TRJ‐derived lines. Multilocation evaluation of these lines at three locations indicated better adaptation for grain yield in some of the lines.

WA352 gene indicates booming outcrossing progenies (three‐line hybrid rice × weedy rice) in weedy rice populations

AbstractWeedy rice (Oryza sativa f. spontanea) is a conspecific weedy relative of cultivated Asian rice (O. sativa L.) that occurs in paddy fields worldwide. The mechanism underlying the persistence of weedy rice in rice fields remains unclear. We tested for evidence that southern Chinese weedy rice has originated through pollen flow from weedy rice that has resulted in hybridization with three‐line hybrid rice cultivars, which have been extensively cultivated in southern China for the last 40 yr and now account for over half the total rice cultivation in China. To test the hypothesis, we screened for the presence of the maternally inherited WA352 gene in weedy rice populations in southern China. WA352 is a mitochondrial gene that controls wild abortive cytoplasmic male sterility (CMS‐WA) and is present in three‐line hybrid rice cultivars. Screening of 48 weedy rice populations (939 accessions) indicates that 36 of 48 populations (75%) and 325 of 939 accessions (34.6%) harbor the WA352 gene. The rate is significantly higher than the 1% outcrossing rate between weedy rice and rice cultivars. Further analysis indicates significance between the frequency of the WA352 gene in weedy rice populations and depth of plowing. Our results suggest that outcrossing with rice cultivars is one of the important mechanisms underlying the persistence of weedy rice in southern China and that deep plowing may be conducive to promoting the persistence of weedy rice derived from hybridization (three‐line hybrid rice × weedy rice).

Crystal structure of WA352 provides insight into cytoplasmic male sterility in rice

Plant cytoplasmic male sterility (CMS) is an important phenomenon and is widely utilized in hybrid crop breeding. The Wild Abortive CMS (CMS-WA), a well-known CMS type, has been successfully applied in the commercial production of hybrid rice seeds for more than 40 years. The CMS-WA causal gene WA352 encodes a novel transmenbrane protein and the interacts with the mitochondrial copper chaperone COX11, triggering reactive oxygen species production and resulting in male sterility in CMS-WA lines. However, the structure of WA352 is currently unknown, and the structural mechanism whereby WA352 perturbs COX11 function to cause CMS remains largely unknown. Here, we report the crystal structure of the C-terminal functional domain of WA352 at 1.3 Å resolution. This functional domain, consisting of five α helices, is spindle-shaped with a length of 42 Å, and a diameter of 28 Å. Notably, the absence of any structural similarity to a known protein structure suggests that the WA352 functional domain is a novel fold. In addition, surface conservation analysis and structural modeling of the WA352-COX11 complex revealed details about the WA352-COX11 interaction. Further structural analysis suggested that the WA352-COX11 interaction blocks the copper ion transportation activity of COX11, which is essential for the assembly of cytochrome c oxidase, resulting in male sterility in CMS-WA lines. Our study paves the way toward structural determination of the WA352-COX11 complex and provides new insight into the mechanism of plant CMS.

Screening and validation of fertility restoration genes (Rf) in wild abortive CMS system of rice (Oryza sativa L.) using microsatellite markers

The present study was carried out with the objective of validating linked SSRs markers to Rf genes in diverse rice breeding lines. SSR markers enabled to determine the chromosomal locations of the two Rf genes (Rf3 and Rf4) in the wild-abortive cytoplasmic male sterility (WA-CMS) system. Forty breeding lines with no prior information on fertility restoration were screened with the help of molecular markers linked to major fertility genes Rf3 and Rf4. The breeding lines were crossed with two WA-CMS lines (IR79156A and Pusa 6A) and the 80 F1s were evaluated for pollen and spikelet fertility to identify restorers and maintainers. Four SSRs reported to be linked to Rf genes (Rf3 and Rf4) were used to detect the allelic status in 40 breeding lines. The results revealed that out of 40, eight lines found to be common effective restorers for both the CMS lines (IR 79156A and Pusa6A). Two markers (RM171 and RM6100) proved to be associated with Rf4 genes. Results demonstrated that these markers could be used to screen large number of restorers and non-restorer lines in hybrid rice breeding programs in short time.

Multi-trait improvement in rice through marker-assisted breeding

Biotechnological tools like molecular markers can add precision to breeding and accelerate breeding efforts. Towards this objective, our research team at ICAR-IIRR has applied marker-assisted breeding (MAB) for improvement of multiple traits like resistance against bacterial blight, blast, gall midge and BPH, heterosis related traits, improvement of low soil P tolerance, grain quality and yield. Through marker-assisted backcross breeding (MABB), a high-yielding, bacterial blight resistant rice variety possessing fine-grain type and low glycemic index (GI), named Improved Samba Mahsuri (ISM) has been developed and released for cultivation by farmers. MABB has also been applied for improving bacterial blight resistance of a few important traditional and evolved Basmati rice varieties and hybrid rice parental lines. A novel bacterial blight resistance gene, Xa33 has been identified from an accession of the wild rice, O. nivara, fine-mapped and transferred into several elite genetic backgrounds. Novel sources of resistance against bacterial blight and blast diseases have been identified and characterized and major blast resistance have been transferred to several elite genetic backgrounds and a major QTL associated with neck blast resistance has been identified from wild rice. Additionally, genepyramid lines possessing resistance against gall midge have also been developed and a novel and highly effective BPH resistance gene has also been identified and mapped with molecular marker and few promising donors possessing resistance against sheath blight have been identified. A molecular marker-based assay has been designed for rapid and accurate determination of impurities in seed-lots of rice hybrids and their parental lines and functional markers have been developed for the traits relevant to hybrid rice, viz., wide-compatibility, wild-abortive cytoplasmic male sterility and fertility restoration. Functional markers have also been developed for major grain quality determining genes, fgr and GS3 and a major QTL controlling gelatinization temperature has been identified through molecular mapping. The major QTL responsible for low soil phosphorus (P tolerance, viz., Pup1 has been transferred to Improved Samba Mahsuri, MTU1010 and IR64 and novel, non-Pup1 type donors have been identified for the trait. Four major yield enhancing genes, viz., Gn1a, SCM2, OsSPL14 and GW5 have been transferred to elite rice cultivars, viz., Improved Samba Mahsuri, Swarna, MTU1010 and NDR359.

Identification of restorers and maintainers for WA based Indica CMS lines of rice

Among the innovative genetic options available for enhancing the rice production hybrid rice is one of the technologies which is feasible and readily adoptable. In the early stage of the hybrid rice breeding programme, breeders identified restorer lines by testcross screening from rice germplasm pools. The availability of stable cytoplasmic male sterility and fertility restoring system is vital for commercial exploitation of heterosis in rice. In the present experiment, seven cms lines of divergent origin were studied for their pollen fertility and spikelet fertility in first year at DRR, Hyderabad. Out of them top five were reevaluated in next year at Varanasi location. These five stable cytoplasmic male sterile lines (CMS) of rice having wild abortive (WA) cytoplasmic male sterility source were crossed with 30 genotypes to identify their restorer/maintainer nature. Most of the genotypes expressed differential fertility reactions when cross with cms lines. Among the genotypes tested, no one was found to be common as effective restorer for all the five cms lines. Genotypes found to be effective restorers for four CMS lines, Sarju 52, Malviya 36, HUR 105, Narendra 359, Pusa Sugandh 3, Pusa Sugandh 4 and NDR 118 were the promising effective restorers for most of the CMS lines.

Molecular stacking of wide compatibility gene, S5 n and elongated uppermost internode (eui) gene into IR 58025B, an elite maintainer line of rice

One of the most widely used and stable wild-abortive cytoplasmic male sterile (WA-CMS) line is IR 58025A. We had earlier transferred the gene (eui1) for elongated uppermost internode into the elite WA-CMS line named IR 58025eB with the help of markers. To overcome incomplete panicle exsertion in the WA-CMS line and better exploitation of indica/japonica heterosis, we have stacked a major gene for wide-compatibility (S5 n ) into IR 58025eB through marker-assisted breeding. SSR markers (RM5970) tightly linked to eui1 gene and (S5-InDel) functional maker of S5 n gene were used for foreground selection, while ninety-one microsatellite markers polymorphic between IR 58025eB and donor Dular (S5 n ) were utilized for background selection at each backcross generation and backcrossing was continued till BC3 generation. Phenotypic observation previous to background selection at each generation provided supplementary information for identifying the novel recombinants and the fast-tracked recurrent parent genome recovery which ranged from 94.50 to 96.70% in selected BC3F4 plants. At BC3F5 generation, seven stable, backcross-derived lines possessing S5 n and eui genes, high yield, and long-slender grain type have been identified; their maintainer ability was confirmed through test crosses. These improved lines are being converted to CMS lines by marker-assisted backcrossing.

Development and validation of candidate gene-specific markers for the major fertility restorer genes, Rf4 and Rf3 in rice

Two major nuclear genes, Rf3 and Rf4, are known to be associated with fertility restoration of wild-abortive cytoplasmic male sterility (WA-CMS) in rice. In the present study, through a comparative sequence analysis of the reported putative candidate genes, viz. PPR9-782-(M,I) and PPR762 (for Rf4) and SF21 (for Rf3), among restorer and maintainer lines of rice, we identified significant polymorphism between the two lines and developed a set of PCR-based codominant markers, which could distinguish maintainers from restorers. Among the five markers developed targeting the polymorphisms in PPR9-782-(M,I), the marker RMS-PPR9-1 was observed to show clear polymorphism between the restorer (n = 120) and maintainer lines (n = 44) analyzed. Another codominant marker, named RMS-PPR762 targeting PPR762, displayed a lower efficiency in identification of restorers and maintainers, indicating that PPR9-782-(M,I) is indeed the candidate gene for Rf4. With respect to Rf3, a codominant marker, named RMS-SF21-5 developed targeting SF21, displayed significantly lower efficiency in identification of restorers and non-restorers as compared to the Rf4-specific markers. Validation of these markers in a F2 mapping population segregating for fertility restoration indicated that Rf4 has a major influence on fertility restoration and Rf3 is a minor gene. Further, the functional marker RMS-PPR9-1 was observed to be very useful in identification of impurities in a seed lot of the popular hybrid, DRRH3. Interestingly, when RMS-PPR9-1 and RMS-SF21-5 were considered in conjunction with analysis, near-complete, marker–trait co-segregation was observed, indicating that deployment of the candidate gene-specific markers both Rf4 and Rf3, together, can be helpful in accurate identification of fertility restorer lines and can facilitate targeted transfer of the two restorer genes into elite varieties through marker-assisted breeding.

Microsatellite-Aided Screening for Fertility Restoration Genes (Rf) Facilitates Hybrid Improvement

DNA markers enabled to determine the chromosomal locations of the two Rf genes (Rf3 and Rf4) in the wild-abortive cytoplasmic male sterility (WA-CMS) system. Four simple sequence repeats (SSRs) RM171, RM258, RM315 and RM443 were used to detect the allelic status with respect to the fertility restoration genes (Rf3 and Rf4) in 300 rice cultivars or breeding lines. The results revealed that out of 300 lines, 90 lines screened had Rf3, 65 lines had Rf4, and 45 lines had Rf3 and Rf4 alleles. Furthermore, 45 lines selected using SSR markers were mated with a CMS line (IR58025A) to analyze their restoring ability. Offspring of all the test lines except HHZ8-SAL9DT1-Y1, HHZ5-SAL9-Y3-1 and IDSA77 exhibited higher pollen and spikelet fertility (> 80%), thus confirming they bear the Rf alleles. The hybrid offspring of ARH12-6-1-1-B-3-1, IR32307-10-3-2-1 and Sahel 329 had the highest pollen fertility (97.39%, 98.30% and 97.10%, respectively) and spikelet fertility (95.10%, 97.07% and 96.10%, respectively).

Use of Hybrid Rice and the Proliferation of Weedy Rice: A Case in Jiangsu, China

Jiangsu Province, located between northern China and southern China, is a typically mixed area of conventional and three‐line hybrid rice without Oryza rufipogon Griff. and Oryza nivara S. D. Sharma & Shastry. In the recent decade, occurrence of weedy rice rapidly increased year by year since it was found and reported from 2000. The recent cloning of the CMS‐WA gene (wild abortive cytoplasmic male sterility) makes it possible to design a marker specific for the gene for testing large quantities of weedy rice samples in a single assay by polymerase chain reaction (PCR). Through performing a BLAST search in the National Center for Biotechnology Information (NCBI) database using the sequence of the CMS‐WA gene cloned from ZS97A as a reference, the major difference between three‐line hybrid rice and conventional rice cultivars was found in the 287 to 439 bp region of the CMS‐WA gene. A specific marker, WA7, was designed and evaluated for this region of the CMS‐WA gene. This marker, in combination with RED4, a specific marker for the Rc red pericarp gene, was used to distinguish weedy rice, three‐line hybrid rice, and conventional rice in a single PCR assay. A total of 1232 accessions of weedy rice collected from 63 locations in Jiangsu Province, China, were evaluated using the red pericarp and CMS‐WA DNA markers. Only four accessions from three locations were found positive for the CMS‐WA gene, accounting for just 4.76 and 0.32% of all locations and accessions, respectively. These results indicate that the three‐line hybrid is likely a source but not the major source of weedy rice in Jiangsu Province.

Transgenic expression of an unedited mitochondrial orfB gene product from wild abortive (WA) cytoplasm of rice (Oryza sativa L.) generates male sterility in fertile rice lines.

Over-expression of the unedited mitochondrial orfB gene product generates male sterility in fertile indica rice lines in a dose-dependent manner. Cytoplasmic male sterility (CMS) and nuclear-controlled fertility restoration are widespread developmental features in plant reproductive systems. In self-pollinated crop plants, these processes often provide useful tools to exploit hybrid vigour. The wild abortive CMS has been employed in the majority of the "three-line" hybrid rice production since 1970s. In the present study, we provide experimental evidence for a positive functional relationship between the 1.1-kb unedited orfB gene transcript, and its translated product in the mitochondria with male sterility. The generation of the 1.1-kb unedited orfB gene transcripts increased during flowering, resulting in low ATP synthase activity in sterile plants. Following insertion of the unedited orfB gene into the genome of male-fertile plants, the plants became male sterile in a dose-dependent manner with concomitant reduction of ATPase activity of F1F0-ATP synthase (complex V). Fertility of the transgenic lines and normal activity of ATP synthase were restored by down-regulation of the unedited orfB gene expression through RNAi-mediated silencing. The genetic elements deciphered in this study could further be tested for their use in hybrid rice development.

Mapping of minor quantitative trait loci (QTLs) conferring fertility restoration of wild abortive cytoplasmic male sterility and QTLs conferring stigma exsertion in rice

AbstractCytoplasmic male sterility (CMS) hybrid rice has made a great contribution to the increase of rice yield globally. To facilitate the development of high‐quality pairs of the wild abortive (WA) male sterile and maintainer lines, the genetic basis of fertility restoration of WA‐CMS and stigma exsertion was investigated in this study using a testcross population with the WA‐CMS background. Seed‐setting rate and stigma exsertion rate were used as the indicators of the two traits, respectively. Results showed that four minor QTL regions from 9311 were responsible for the variation of seed‐setting rate, while a few minor QTLs and epistatic QTL pairs influenced stigma exsertion rate. These results would be of great use in the development of high‐quality pairs of WA male sterile and maintainer lines in rice.

Comparative analysis of fertility restoration genes for WA, Y, and DA cytoplasmic male sterility in rice

Rice chromosome single segment substitution line (SSSL) W23-19-06-06-11 with the genotype Rf3Rf3/Rf4Rf4, a strong restorer line for wild-abortive (WA) cytoplasmic male sterility (CMS), was recently identified from the SSSL library. To investigate the genetic mode of Rf genes and the genetic relationship among WA, yegong (Y), and dwarf-wildabortive (DA) CMS systems, the plants derived from three BC3F2 populations involving W23-19-06-06-11 and the three CMS lines, that carried the Rf3Rf3/Rf4Rf4, Rf3Rf3/rf4rf4, and rf3rf3/Rf4Rf4 genotypes and WA-, Y-, and DA-CMS cytoplasm, were selected and their pollen and spikelet fertility were evaluated. The results show that the genetic effect displayed a trend of Y-CMS > WA-CMS > DA-CMS in the genetic background of W23-19-06-06-11, the effect of Rf4 appeared to be slightly larger than that of Rf3, and their effects were additive for the three CMS systems. Two pairs of dominant genes governed the fertility restoration in pollen and spikelet in the W23-19-06-06-11 which indicates that the genetic mode of the Rf genes was a qualitative character for the three CMS systems.

Marker‐assisted introgression of bacterial blight and blast resistance into IR 58025B, an elite maintainer line of rice

AbstractIR 58025A is a very popular wild‐abortive cytoplasmic male sterile (WA‐CMS) line of rice and is extensively used for hybrid rice breeding. However, IR 58025A and many hybrids derived from it possess mild aroma (undesirable in some parts of India) and are highly susceptible to bacterial blight (BB) and blast diseases. To improve IR 58025A for BB and blast resistance, we have introgressed a major dominant gene conferring resistance against BB (i.e. Xa21) and blast (i.e. Pi54) into IR 58025B, the maintainer line of IR 58025A. An introgression line of Samba Mahsuri (i.e. SM2154) possessing Xa21 and Pi54 genes in homozygous condition and fine‐grain type was used as donor parent, and backcross breeding strategy was adopted for targeted introgression of the resistance genes. PCR‐based molecular markers tightly linked to Xa21 and Pi54 were used for selection of BB‐ and blast‐resistant lines, while closely linked markers were used for identification of backcross‐derived plants devoid of Rf4 and aroma. At BC2F5, four backcross‐derived lines possessing resistance against BB and blast, devoid of aroma, high yield, short plant stature, long‐slender grain type and with recurrent parent genome recovery ranging from 88.8% to 98.6% were selected and advanced for further evaluation. The improved versions of IR 58025B, viz. SB54‐11‐143‐9‐44‐5, SB54‐11‐143‐9‐44‐98, SB54‐11‐143‐9‐44‐111 and SB54‐11‐143‐9‐44‐171, behaved as perfect maintainers when test‐crossed with WA‐CMS lines. Agronomically superior lines of improved IR 58025B are being converted to CMS line through backcrossing for developing high‐yielding and biotic stress‐resistant rice hybrids.