Rice (Oryza sativa) is a monocot plant having 2n = 24 and it belongs to a grass family Poaceae. It is consumed as a staple food by a vast majority of the human population, especially in Asia. Global rice stocks have declined from 147 million tons in 2001 to 82 million tons in 2008 and to sustain the rising world population, world rice production must increase from 440 million tons at present to 475 million tons in 2020 at the rate of 2 million tons per year (Jeon et al., 2011). Grain size is one of the three components (number of panicles per plant, number of grains per panicle and grain weight) of grain yield and is evaluated by grain weight, which is positively correlated with several characteristics including grain length, grain width and grain weight. Grain yield can be potentially increased by identification and introgression of alleles for yield related genes from diverse germplasm of a species and even from wild germplasm.
In this study, two methods, inter-specific hybridization and allele mining were employed as a tool to increase grain yield. Wide hybridization was performed between Oryza sativa (CAU R-1, Pant dhan-4, Pant dhan-10, Pant dhan-12, Pant dhan-16, Pant dhan-18) (female parents) and O. nivara (RWR-19 and RWR-125) (pollen parents) species to transfer functionally useful alleles for yield related grain traits and allele mining was performed in a panel of 41 genotypes including the two O. nivara accessions for previously reported genes/ QTLs viz. Rdd 1 (grain length), gw 7, gt 7, gw 8.1 (1000 grain weight) and GS 5 and GW 8 (grain width) (Aluko, 2003; Yoon et al., 2006; Xie et al., 2006; Talukdar and Zhang, 2008; Iwamoto et al., 2009; Li et al., 2011 and Xue et al., 2014) with their associated markers (RDD 1-1, RM 234, RM 478, RM 23201, RM 502, RM 574-1, 2 and RM 593-1, 2) to identify superior alleles for grain length, width and 1000 grain weight among the genotypes. Four hybrids were confirmed with SSR 288 primer, out of which two plants were obtained. Analysis of variance for phenotypic traits for grain length and grain width between 41 genotypes revealed significant differences at 5 % level. The genotypes consisted of 19 % large (9.18 - 10.31), 56 % medium (7.36 - 8.74) and 25 % small (5.48 - 6.82) grain length (mm); 44 % high (23 - 36), 34 % medium (19.5 - 22.8) and 22 % low (17 - 19.45) 1000 grain weight (mm) and 39 % large(2.49 - 3.62), 51 % medium (2.01 - 2.44) and 10 % small (1.85 - 1.97) grain width (mm).
Χ2 analysis was performed to study association of the markers with grain traits. In this study, it was observed that for grain length marker RDD 1-2, two alleles designated as A (600 bp) and B (620 bp) were found to have significant association with grain length. Allele A (Χ2 = 3.94, p = 0.05) was found to be associated with large grain length and allele B (Χ2 = 4.51, p = 0.03) with short grain length. For grain weight marker RM 478, allele A (205 bp) was found to be significantly associated (Χ2 = 5.08, p = 0.02) with lower grain weight. Another marker RM 574-2 for grain width also showed significant association (Χ2 = 4.3, p = 0.08) of the B allele (240 bp) with lower grain width. From this study, it is anticipated that the hybrids can be used for back crossing in future breeding programmes and marker RDD 1-2 can be used for identifying alleles for enhancing grain length from diverse genomic sources for crop improvement. The genotypes viz. Hans raj, Pant dhan-18, Pant dhan-10, Narendra, Pant dhan-12, Pant dhan-4, ARR-09, Shahsarang, IVT-ASG-2712, IVT-ASG-2701, AVT-IASG-2602, AVT-IASG-2609 and IVT-ASG-2705 identified carrying the allele can be used as a source of A allele for large grain length in future breeding programmes. Alleles, B, A and B for primers, RDD 1-2, RM 478 and RM 574-2, which showed significant negative associations for grain length, grain weight and grain width, respectively can be used for rejection at seedling stage in marker assisted breeding programme. This study also identified novel alleles (alleles distinct from those reported in biparental programme); which could be significant in a larger panel of rice genotypes.