Oryza

ABSTRACT Rice (Oryza sativa L) is an important cereal for one-third of the world’s population. Its productivity is continually threatened by a series of biotic and abiotic stresses. There is thus an urgent need to broaden the gene pool of rice for developing varieties with high yield and resistance to several stresses. Wild species of Oryza are a reservoir of genes for tolerance to various biotic and abiotic stresses and also possess yield-enhancing loci. Morphological, cytological, and molecular divergence studies have been used to identify ten genome types in 22 Oryza species. Interspecific hybrids and alien introgression lines have been produced. Genes for resistance to grassy stunt virus, tungro virus, brown planthopper, bacterial blight, blast, tolerance to soil toxicity, and cytoplasmic male sterility have been transferred from wild species and varieties released. Many of the genes/QTL introgressed from wild species have been tagged with DNA markers. Xa21, a gene for bacterial blight resistance has been pyramided along with other genes through MAS and 12 varieties/hybrids released. Molecular analysis has shown limited introgression of short segments from distant genomes. Parental genomes and alien chromosomes segments have been characterized using genomic in situ hybridization. BAC libraries from different wild species have been established. BAC end sequence, physical framework of different genomes of Oryza, and alignment with the rice reference genome have provided valuable information on evolutionary mechanisms and on genome organization, structural rearrangements, repeats, transposable elements, SSR, miRNA, and single nucleotide variation. Long terminal repeated (LTR)-retrotransposons are considered as the predominant class of repeat elements that increase the genome size in some species. Introgression of genes with broad spectrum of resistance to stresses, yield-enhancing loci/QTLs, and identification of genes controlling homoeologous pairing to introgress wild species genes from distant genomes need to be explored. BAC libraries and CSSL of wild relatives are important genetic resource for functional genomics and for mapping and cloning of agronomically important genes/QTLs harbored in wild species of Oryza. With the advances in tissue culture techniques, molecular markers, and genomics, the scope for utilization of wild species in genetic enhancement of rice seems more promising than before.