Morphological Structure and Genetic Mapping of New Leaf-Color Mutant Gene in Rice (Oryza sativa)

Leaf-color mutations are a widely-observed class of mutations, playing an important role in the study of chlorophyll biosynthesis and plant chloroplast structure, function, genetics and development. A naturally-occurring leaf-color rice mutant, Baihuaidao 7, was analyzed. Mutant plants typically exhibited a green-white-green leaf-color progression, but this phenotype was only expressed in the presence of a stress signal induced by mechanical scarification such as transplantation. Prior to the appearance of white ~eaves, mutant plant growth, leaf color, chlorophyll content, and chloroplast ultrastructure appeared to be identical to those of the wild type. After the changeover to white leaf color, an examination of the mutated leaves revealed a decrease in total chlorophyll, chlorophyll a, chlorophyll b, and carotenoid content, a reduction in the number of chloroplast grana lamella and grana, and a gradual degradation of the thylakoid lamellas. At maturity, the mutant plant was etiolated and dwarfed compared with wild-type plants. Genetic analysis indicated that the leaf mutant character is controlled by a recessive nuclear gene. Genetic mapping of the mutant gene was performed using an F2 population derived from a Baihuaidao 7 ~ Jiangxi 1587 cross. The mutant gene was mapped to rice chromosome 11, positioned between InDel markers L59.2-7 and L64.8-11, which are separated by approximately 740.5 kb. The mutant gene is believed to be a new leaf-color mutant gene in rice, and is tentatively designated as gwgl.

iTRAQ-based quantitative proteomics analysis of defense responses triggered by the pathogen Rhizoctonia solani infection in rice

The soil-borne necrotrophic fungus Rhizoctonia solani is one of destructive fungi causing severe yield losses in various important crops. However, the host defense mechanisms against the invasion of this pathogen are poorly understood. In this study, we employed an i TRAQ-based quantitative proteomic approach to investigate host proteins responsive to R. solani using the resistant rice cultivar YSBR1. As a whole, we identified 319 differentially accumulated proteins(DAPs) after inoculation of rice plants with R. solani. Functional categorization analysis indicates that these DAPs cover a broad range of functions. Notably, a substantial portion of the DAPs are involved in cell redox homeostasis, carbohydrate metabolism, and phenylpropanoid biosynthesis, or belong to pathogenesis-related proteins, indicating that these processes/proteins play important roles in host defense against R. solani. Interestingly, all of the DAPs involved in photosynthesis and chlorophyll biosynthetic processes, and part of the DAPs involved in phenylpropanoid biosynthesis, show reduced accumulation after R. solani infection, suggesting that R. solani probably inhibits host photosynthetic system and phenylpropanoid biosynthesis to facilitate infection and colonization. In conclusion, our results provide both valuable resources and new insights into the molecular mechanisms underlying rice and R. solani interaction.

Improvement of Rice Resistance to Sheath Blight by Pyramiding QTLs Conditioning Disease Resistance and Tiller Angle

Rice resistance to sheath blight （SB） is controlled by polygenes or quantitative trait loci （QTLs） and affected by plant morphological traits, qSB-9^TQ and TAG1^TQ, which control SB resistance and tiller angle （TA）, respectively, were employed to test whether the combination of the two genes will further improve rice SB resistance and reduce yield loss rather than only one of them or neither. Using two pairs of near isogenic lines （NILs）, TAC1^TQ was confirmed to contribute to SB resistance. However, its effect was less considerable than that of qSB-9^TQ. Subsequently, the two genes were introduced into two commercial rice varieties to develop a series of NILs. The NILs carrying both TAG1^TQ and qSB-9^TQ showed more resistance than the NILs containing only one of them. Comparing the grain yield per plant （GYP） under the three different SB disease conditions, namely slight, moderate and severe conditions, NILs carrying both genes apparently lost lower GYP than the NILs without the two genes and the NILs carrying only TAC1^TQ. Under slight disease condition, no significant differences were found on morphology, development and GYP associated traits except for TA between the NILs carrying both genes and either of them, indicating that the two genes have no inferior effect on rice agronomic traits. Results indicated that mAC1^TQ and qSB-9^TQ have high breeding potential, and pyramiding SB resistance QTL and morphological trait QTL is a potential approach in improving rice SB resistance.

Genome-Wide Association Studies Reveal New Genetic Targets for Five Panicle Traits of International Rice Varieties

Narrow genetic background is a key limiting factor in breeding stable high-yielding rice. The introduction and utilization of international rice core germplasm is an important way to increase the genetic diversity of domestic rice varieties. We conducted a genome-wide association study on 5 panicle traits of 315 rice accessions introduced from the international rice micro-core germplasm bank. Based on the tests from Yangzhou of China and Arkansas of American, environment exhibited a significant impacts on panicle length and primary branch number, while grain length, grain width and grain length/width ratio were insensitive to environment changes. We discovered a total of 7, 5, 10, 8 and 6 chromosomal regions or single nucleotide polymorphism marker loci that were significantly associated with primary branch number, panicle length, grain length, grain width and grain length/width ratio, respectively. Among them, eleven regions were associated with grain shape and one region associated with primary branch number, showing the good consistence in two different environments. Significant linear correlation was discovered between the average trait value and the number of favorable alleles carried by the varieties in all associated loci. Among the associated loci, varieties in aromatic and tropical japonica sub-groups possessed most favorable alleles, while those in temperate japonica sub-group contained the least. The domestic varieties mainly harbored unfavorable alleles in six of the associated loci being detected. On the contrary, 15 varieties from 11 different countries harbored more favorable alleles （as many as 30 or more） than the others. Remarkably, all these 15 varieties belonged to the tropical japonica sub-group. In conclusion, our study demonstrates that varieties in the tropical japonica sub-group had high potentials for breeding stable high-yielding rice. Based on this discovery, we proposed a new approach for improving the panicle traits of domestic rice by using tropical japonica varieties.

Fine mapping of a novel wax crystal-sparse leaf3 gene in rice

Cuticular wax plays an important role in protecting plants against water loss and pathogen infection and in the adaptations to environmental stresses. The genetic mechanism of the biosynthesis and accumulation of epicuticular wax in rice remains largely unknown. Here, we show a spontaneous mutant displaying wax crystal-sparse leaves and decreased content of epicuticular wax that was derived from the cytoplasmic male sterility （CMS） restorer line Zhenhui 714. Compared with the wild type Zhenhui 714, the mutant exhibited hydrophilic features on leaf surface and more sensitivity to drought stress. The mutation also caused lower grain number per panicle and thousand grain weight, leading to the decline of yield. Genetic analysis indicates that the mutation is controlled by a single recessive gene, named wax crystal-sparse leaf3 （wsl3）. Using segregation populations derived from crosses of mutant/Zhendao 88 and mutant/Wuyujing 3, respectively, the wsl3 gene was fine-mapped to a 110-kb region between markers c3-16 and c3-22 on chromosome 3. According to the rice reference genome and gene analysis, we conclude that a novel gene/mechanism involved in regulation of rice cuticular wax formation.