ABSTRACT:
Rice is one of the most important food crops in the world and feeds more people than any other crop. Rice belongs to the genus Oryza which includes approximately 24 species. They are widely distributed growing in different habitats and different soil types. They show differences in plant growth, yield, pest and discase resistance, stress tolerance and water requirement. Rice grain is classified as short, medium or long grain sizes and exhibits colours like brown, black, purple, and red. The genus Oryza has ten recognized genome types. They are AA, BB, CC, BBCC, CCDD, EE, HHKK, HHJJ, FF, and GG. There is a fivefold difference in genome size among the species with diploid species having 2n = 24 and tetraploid species having 2n = 48 chromosomes. The cultivated species are 0. glaberrima, in West and Central Africa, and O. sativa with three cultivars, japonica, indica, and javanica. O. sativa L. is the most consumed rice species and is the staple food for more than half of the world's population. It grows throughout the world and can be found in all kinds of climates. It is diploid with AA genome type. O. sativa L.ssp japonica is the rice variety grown in subtropics and temperate climates. It prefers heavy, relatively impervious soil and is tolerant to flooding. The grains are generally rounder and shorter. It has a genome size of 389 Mb. It has been sequenced to 10x coverage by the members of the International Rice Genome Sequencing Project (IRGSP). O. sativa Lssp indica is a rice variety grown throughout the world. It has an estimated genome size of 466 Mb. It generally bears long. slender grains. A WGS project has developed an improved version of this at 6.28x coverage.