QTL Analysis For Percentage Of Grains With Chalkiness(PGWC)of Brown Rice And A Preliminary Study Of Mutants With High PGWC

Chalky endosperm in rice refers to white opaque part. The formation of chalky endosperm starch is due to an abnormal accumulation starch in rice. Percentage of grains with chalkiness (PGWC) is important to the appearance quality of milled rice. PGWC is a physical characteristic which negatively affects not only the appearence and milling quality but also affects the cooking texture and palatability. Chalkiness grain is a complicated quantitative trait and its molecular mechanisms of formation are still poorly understood. This largely retarded the genetic improvement of rice quality In this study, a chromosome segment substhitute lines (CSSL) population derived from the cross of Asominori (Japonica) and IR24(Indica) were used to detect stable QTL analysis for grain length(GL), width(GW) and PGWC of brown rice. CSSL carried overlapping chromosome segments of Asominori in a genetic background of IR24. One chromosome segmental substitution line (CSSL64) with7segments derived from IR24was significantly higher PGWC than its parent Asominori. A secondary F2and F3populations derived from CSSL64crossed with Asominori were used to detect QTLs and to further explore the genetics of CSSL64with high PGWC. Two mutants with chalky endosperm were identified from a T-DNA insertion population derived from a Japonica rice variety (Nipponbare). The results would be useful for the simultaneous improvement of PGWC and map-based cloning of target QTL. The main conclusions are as follows:1. A chromosome segment substitution line (CSSL)population, derived from the cross of Asominori and IR24with IR24as the recurrent parent, was phenotyped for GW, GL and PGWC of brown rice in three sites. The population was used to detect the correlations and stable QTLs on GW, GL and PGWC. Correlation analysis showed that there was a significantly positive correlation between GW and PGWC among the CSSL population in3sites. A total of7,2,4QTLs for GW, GL, PGWC were detected at3sites in2007, respectively. Moreover, phenotypic values were different significantly (P<0.05) between IR24and the target CSSLs harboring qGW-1b, qGL-3, qGL-5, qPGWC-5, qPGWC-7QTL alleles. One main-effect QTL simultaneously controlling GW and PGWC were detected and their alleles increasing GW and PGWC were from the same parent, IR24. Two main-effect QTLs simultaneously controlling GW and GL were detected. There is no QTL simultaneously controlling GL and PGWC. Some main-effect QTLs, controlling grain shape but not PGWC, such as qGW-1a、qGW-1b、qGW-3、qGW-5b、qGW-7, qGL-5,�'�qGL-3are useful for breeding.2. To further explore the genetics of CSSL64with high PGWC, we constructed an F2secondary population derived from Asominori×CSSL64. qPGWC-9was identified repeatedly in continuous three years (2005-2007), and qPGWC-9with an average PVE of19.4%was identified in the F2secondary population in all three years (2005-2007), and was finally located in the interval of RM23958-RM1328on chromosome9by using the Asominori×CSSL64F3population. These results should be useful for fine-mapping and map-based cloning of the qPGWC-9allele and for marker-assisted transfer of the allele in rice breeding programs.3. In this study, genetic characterization of two mutants with chalky endosperm in rice was conducted by the mutant and its related genetic populations with Nipponbare. The results showed that there was a significant difference between T4420mutant and Nipponbare (wild-type) in PGWC, GL, GW, HGW and plant height. Genetic analysis of the mutant showed that two kinds of phenotype, such as high and lower PGWC in the segregating population derived from the cross of T4420with Nipponbare. They were fit for the ratio of3(high PGWC):1(lower type), indicating the high PGWC gene is a dominant mutation. The high PGWC gene was further mapped on rice chromosome2using PCR-WALKING. Test for HPT resistance showed the high PGWC plants were resistant while the lower PGWC plants were susceptive, and the ratio of resistance and sensitive plants was3:1, indicating that the mutation was co-segregating with HPT resistance. These data showed that the rice high PGWC mutant phenotype is controlled by a dominant gene mutation, which is caused by T-DNA insertion. T4420from Nipponbare enhancer trap mutant library, the analysis of flanking sequence showed that exogenous gene inserted into the non-coding regions of chromosome2. According to the Real-time RT-PCR analysis of upstream and downstream genes of the insertion site with10days seedlings, there was little difference in gene expression. There is no significant difference in grain weight between T2915mutation and the wild-type with chalky endosperm. The genetic analysis showed that a single recessive locus was responsible for chalky phenotype, which is located on the long arm of chromosome1. Exogenous T-DNA insertion caused the mutation. The Real-time RT-PCR analysis showed that the expression of the gene which locates on the upstream of the insertion site was significantly higher than in Nipponbare, up to about36times. The results indicate that T-DNA insertion enhanced expression of the gene, resulted in mutant phenotypes.