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Sheath blight is a serious rice disease that causes great yield losses worldwide. Sheath blight resistance is controlled by genes with minor effect. It is an urgent need to identify the resistance genes in rice germplasm and utilize them in breeding. The purpose of this study is to identify the sheath blight-resistant alleles in different rice germplasm by association mapping and explore the potential of the resistance alleles in utilization. A total of 273 rice genotypes were inoculated and evaluated for sheath blight resistance in the field for 5 years, and 158 genotypes with relatively stable disease resistance phenotype were chosen for association mapping. A wide range of phenotypic variation for sheath blight resistance was observed in the 158-genotype population. Structure analyses divided the 158 genotypes into 4 subgroups, which was consistent with principal component analysis and cluster analysis. Association mapping performed using mixed linear model with 213 markers identified 14 resistance loci, locating on all chromosomes except chromosome 6, 10 and 12. Three novel resistance loci (D704, D855, and D905) which were different from the previously reported were detected. Thirty-eight alleles were detected at the 14 loci, among which favorable alleles at 11 loci showed consistent resistance across 5 years. It was found that the favorable resistance alleles can be pyramided to improve sheath blight resistance. These results enhance the understanding of how different resistance alleles at multiple loci in regulating sheath blight resistance, and provide markers for sheath blight resistance breeding in rice.

The seed setting rate (spikelet fertility) is an important determinant of rice yield. In the past few decades, genes that control rice seed set have been cloned, and many quantitative trait loci (QTL) have been identified. However, the epistasis influencing rice seed set remains largely unclear. In this study, a recombinant inbred line population, which consisted of 219 lines developed by crossing the Lemont and Yangdao4 rice cultivars, was grown in five environments to identify the QTL and epistatic loci related to seed set. A total of 26 minor-effect QTL were detected by multiple interval mapping, which explained less than 12.7% of the phenotypic variation individually. A pair of new epistatic loci were detected and confirmed by two-way analysis of variance; the homozygous Yangdao4 allele at the qSS6.1 locus interacted with the homozygous Lemont allele at the qSS8.1 locus and resulted in a low seed setting rate. A linear regression analysis and a multiple comparison test suggested that eight alleles at four QTL (qSS1.3, qSS6.3, qSS7.1, and qSS8.1) control seed set simultaneously. Marker-assisted selection using these four loci guaranteed a greater than 70% average seed setting rate in all five environments.

Tiller angle, controlled by quantitative trait loci (QTL), is crucial for achieving ideal plant architecture in rice, which affects its final yield values. It is important to understand the genetic mechanisms underlying tiller angle when breeding new plant-type varieties. To uncover the genetic regulation mechanisms in cultivated rice, we performed QTL analysis using a recombinant inbred line (RIL) mapping population consisting of 219 lines developed by crossing two rice cultivars, Lemont and Yangdao4. The angle between the side tiller (ST) and the horizontal ground was measured as a tiller angle-related trait in QTL analysis. Twenty-three QTLs responsible for ST-ground angle were detected using multiple interval mapping in four mapping environments. A major QTL, qTA9, was detected on chromosome 9, which explained 12.9, 39.8, 37.9, and 28.3% of the phenotypic variation in the four mapping environments, respectively. Another QTL, qTA8, with a relatively large effect, was detected on chromosome 8. The other 21 QTLs explained

Background Bakanae disease, caused by the fungus Fusarium fujikuroi, occurs widely throughout Asia and Europe and sporadically in other rice production areas. Recent changes in climate and cropping patterns have aggravated this disease. To gain a better understanding of the molecular mechanisms of rice bakanae disease resistance, we employed a 6-plex tandem mass tag approach for relative quantitative proteomic comparison of infected and uninfected rice seedlings 7 days post-inoculation with two genotypes: the resistant genotype 93–11 and the susceptible genotype Nipponbare. Results In total, 123 (77.2% up-regulated, 22.8% down-regulated) and 91 (94.5% up-regulated, 5.5% down-regulated) differentially expressed proteins (DEPs) accumulated in 93–11 and Nipponbare, respectively. Only 11 DEPs were both shared by the two genotypes. Clustering results showed that the protein regulation trends for the two genotypes were highly contrasting, which suggested obviously different interaction mechanisms of the host and the pathogen between 93 and 11 and Nipponbare. Further analysis showed that a noticeable aquaporin, PIP2–2, was sharply upregulated with a fold change (FC) of 109.2 in 93–11, which might be related to pathogen defense and the execution of bakanae disease resistance. Certain antifungal proteins were regulated in both 93–11 and Nipponbare with moderate FCs. These proteins might participate in protecting the cellular integrity required for basic growth of the susceptible genotype. Correlation analysis between the transcriptome and proteome revealed that Pearson correlation coefficients of R = 0.677 (P = 0.0005) and R = − 0.097 (P = 0.702) were obtained for 93–11 and Nipponbare, respectively. Our findings raised an intriguing result that a significant positive correlation only in the resistant genotype, while no correlation was found in the susceptible genotype. The differences in codon usage was hypothesized for the cause of the result. Conclusions Quantitative proteomic analysis of the rice genotypes 93-11and Nipponbare after F. fujikuroi infection revealed that the aquaporin protein PIP2–2 might execute bakanae disease resistance. The difference in the correlation between the transcriptome and proteome might be due to the differences in codon usage between 93-11and Nipponbare. Overall, the protein regulation trends observed under bakanae disease stress are highly contrasting, and the molecular mechanisms of disease defense are obviously different between 93 and 11 and Nipponbare. In summary, these findings deepen our understanding of the functions of proteins induced by bakanae disease and the mechanisms of rice bakanae disease resistance. Electronic supplementary material The online version of this article (10.1186/s12864-019-5435-5) contains supplementary material, which is available to authorized users.

Rice bakanae disease, caused by the pathogen Fusarium fujikuroi, is becoming severely detrimental to rice production worldwide. To understand the interaction between rice and F. fujikuroi, a moderate resistant genotype, 93-11 (Oryza sativa indica), and a susceptible genotype, Nipponbare (Oryza sativa japonica), were used for transcriptome analysis. Several cDNA libraries were constructed using mRNA isolated from the leaves of non-stressed 93-11 and NIPPONBARE plants and from the leaves of both genotypes inoculated with F. fujikuroi. In total, 1152 and 1052 transcripts were differentially expressed between the controls and the treatments for 93-11 and NIPPONBARE, respectively. Comparative transcriptome analysis revealed different expression patterns for the two genotypes. Although some common defense-related enriched GO terms were shared in both genotypes, specific defense-related terms were enriched exclusively in 93-11. A detailed comparison of defense-related differentially expressed genes revealed that certain WRKYs, WAK and MAP3Ks were responsible for the bakanae disease resistance in 93-11. The OsWAK112d gene was up-regulated in the resistant genotype. The POEI gene response to abiotic stress was modulated in Nipponbare. Further analysis suggested that the defense-related genes WRKYs and MARKs on chromosome 1 that are modulated in 93-11 upon infection might play a crucial role in the rice–F. fujikuroi interaction. Further characterization of these resistance genes may provide candidate genes for the development of molecular markers for rice bakanae resistance breeding programs. Transcriptomic dissection of the rice–F. fujikuroi interaction provides valuable information for future studies on the molecular mechanisms of rice bakanae resistance.

Rice sheath blight (ShB), caused by Rhizoctonia solani, leads to severe yield losses in many rice production areas worldwide. Resistance to rice ShB is a typical quantitative trait controlled by quantitative trait loci (QTLs). To identify the QTLs responsible for ShB resistance, phenotypes for ShB resistance have been surveyed in different individuals/lines in various mapping populations. The 0–9 rating system, based on the percentage of plant height (PH) above the water line with ShB symptoms, is extensively used in the evaluation of ShB in rice communities. By reviewing previous ShB-QTL-mapping studies, we found that the ShB disease score attained using the 0–9 system was inevitably affected by PH and resulted in the colocalization of QTLs for PH and ShB resistance, i.e., some of the PH-QTLs were mistaken as “ShB-QTLs” and had pleiotropic effects. These colocalizing “ShB-QTLs” are irrelevant to physiological ShB resistance and should not be targeted for utilization. We analyzed the ways in which PH affects ShB resistance through the 0–9 rating system and suggest solutions for improving the accuracy of QTL mapping for rice ShB resistance. We also developed the first physical map of the ShB-QTLs and PH-QTLs detected since 1995. This map will be useful in the marker-assisted selection of ShB-resistant QTLs. Under current circumstances, we believe that conventional breeding remains an effective approach for developing ShB-resistant varieties.

Hybrid rice has contributed greatly to the self-sufficiency of the food supply in China. However, bacterial blight is a major disease that limits hybrid rice production in China. The study was conducted to develop an efficient breeding technique to improve the bacterial blight resistance in hybrid rice. A marker-assisted backcross breeding technique was adopted to improve HN189, an elite restorer line containing the Pi1 gene. This breeding technique was simplified to foreground selection with only one generation of backcrossing and background selection based on phenotypic selection. A novel bacterial blight resistance gene, Xa23, was introgressed into HN189. Two improved restorer lines, HBH145 (with one generation of backcrossing) and HBH146 (with two generations of backcrossing), were obtained that had a significant bacterial blight resistance advantage over HN189. The corresponding hybrid combination Tianyou H145 (Tianfeng A / HBH145) was certified one year earlier than Qianyou H146 (Qianjiang 1A / HBH146). The use of the marker-assisted backcross breeding technique with one generation of backcrossing and without background selection in rice breeding programs shortened the breeding period of the rice.

A rice lesion mimic mutant, lm3, was obtained by the mutagenesis of an indica cultivar, 93-11, using γ-ray radiation. Brownish lesions appeared on the leaves of lm3 at the young seedling stage and persisted until the ripening stage. The lm3 mutant was characterised by a shorter plant height and delayed heading compared with the wild-type 93-11. A genetic analysis indicated that the lesion mimic phenotype was controlled by a single recessive gene. Using simple sequence repeat (SSR) markers, the target gene LM3 was first located between marker RM5748 and RM14906 on chromosome 3. We then developed Insertion-Deletion (InDel) markers to fine-map LM3, and the locus was localised to a 29 kb region defined by two InDel markers, In12571 and In12600. Five ORFs were predicted in the candidate region, and DNA sequencing detected a single-nucleotide polymorphism (SNP) in the coding region of LOC Os03g21900. The SNP in the fourth exon (C in 93-11; T in lm3) of LOC_Os03g21900 results in the substitution of a proline (P) with a serine (S) at the 140th amino acid of the deduced uroporphyrinogen decarboxylase protein. We did not detect polymorphisms in the other predicted ORF regions between lm3 and 93-11. These results suggest that LOC_Os03g21900 is the most likely candidate gene for LM3.

Uniformity in the height of main stem and tillers is a key factor affecting ideal plant type, a key component in super high-yielding rice breeding. An understanding of the genetic basis of the panicle layer uniformity may thus contribute to breeding varieties with good plant type and high yield. In the present study, a doubled haploid (DH) population, derived from a cross between indica rice variety Zhai-Ye-Qing 8 (ZYQ8) and japonica rice variety Jing-Xi 17 (JX17) was used to analyze quantitative trait loci (QTL) for panicle layer uniformity related traits. Six, four and three QTL were detected for the highest panicle height (HPH), lowest panicle height (LPH) and panicle layer dis-uniformity (PLD), respectively. qHPH-1-1 and qPLD-1 were located at the same interval on chromosome 1. The JX17 allele(s) of these QTL increased HPH and PLD by 2.57 and 1.26 cm, respectively. Similarly, qPLD-7 and qHPH-7 were located at the same interval on chromosome 7, where the ZYQ8 allele(s) increased HPH and PLD by 3.74 and 1.96 cm, respectively. These four QTL were unfavourable for panicle layer uniformity improvement because a decrease of the PLD was accompanied by decrease of the plant height. qPLD-6 and qLPH-6-1 were located at the same interval on chromosome 6, however here the JX17 allele(s) increased LPH, but decreased PLD, suggesting that this QTL was favourable for improvement of panicle layer uniformity. The markers identified in this study are potential for marker assisted breeding for the improvement of the panicle layer uniformity and ideal plant type.

Pre-harvest sprouting (PHS) in rice production is usually caused by high temperature and humidity or continuous rains. It frequently happens in F1 in hybrid rice seed production. The PHS or “Physiologically germinated” seeds are of lower quality, by which the hybrid rice seed production is badly affected every three years at a loss of 20% or even 50% yield in seed production over the vast Yangtse River Valley and Southwest China. It is estimated that PHS causes an average decrease of seed activity by 10%. A recombinant inbred line population including 304 lines, derived from a cross between Indica rice (Oryza sativa L.) cultivars Zhong-156 and Gumei-2, was used to study the PHS physiology. Based on the data of sprouting rate in panicles and sprouting rate in grains, two kinds of lines, namely easy-to-sprout lines and hard-to-sprout lines, were selected to investigate their physiological differences when PHS happened. The experiment was conducted in a special field with a microclimate of higher temperature and humidity. The results indicated that it was easier to produce PHS from the female parent GM-2 than the male parent ZH-156, besides, the GA1 content and amylase activity in GM-2 grains were higher than those in ZH-156. However, the abscisic acid (ABA) content in GM-2 grains was lower than that in ZH-156. Higher temperature and higher humidity facilitated the GA1 increment from milk ripe stage to yellow ripe stage. GM-2 and the easily-sprouting lines showed an even higher increase in GA1 than ZH-156 and the hardly-sprouting lines, which enhanced the amylase activity and induced pre-harvest sprouting. This may be the physiological basis for pre-harvest sprouting induced by higher temperature and higher humidity, and these special characteristics must be inherited from their parents.