Kasem, S 2012, 'Evaluating grain food potential of wild relatives of rice', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright S Kasem 2012
Cultivated rice, Oryza sativa, is the major calorie provider for two thirds of the world population and in developing countries is a major source of proteins and some micro nutrients. Rice is mainly favoured for its starch content which has a myriad of applications in food and non-food industries. Rice comes from just two species of the Oryza genus which consists of 23 species. A number of these have contributed to the production of a commercial species with increased pest and disease resistance and male sterility. Despite this the starch properties and the cropping potential of many these varieties are largely unknown. Rice and its wild relatives grow in a diverse range of habitats and climates and it is believed their adaptations and genetic diversity may be harnessed for crop improvement programmes.
This study was undertaken as an exploration of the wild species of Oryza that might prove to possess comparable starch characteristics with that of cultivated rice. Analysis of the whole grain morphology of 17 wild species of rice by light microscopy found a diverse array of grain sizes and shapes which could be defined by the "standard rice breeders’ classification scale" that is routinely used for rice breeding evaluation. The range of colours of the grains of these species might also indicate their potential nutritional value as carriers of antioxidants in their pericarp.
The endosperm morphology of the wild species was subjected to an evaluation and comparison with domesticated rice. Wild rice species starch granule morphology, protein body shape and distribution and endosperm cell shape were similar to domesticated species, suggesting their starch has similar functional properties. The wild species exhibited pinholes, channels and internal cavities which had not been previously reported in rice. The presence of pinholes and channels in other cereal starches are favourable for starch processing quality, indicating these species might possess potential benefits in starch processing. The existence of distinctive sub aleurone layers are associated with high protein grains in other cereals; two of the wild species O. longiglumis and O. spontanea exhibited a distinctive sub aleurone layer, indicating that they are potential high protein species.
Evaluation of the physicochemical properties of starch of the wild species found they expanded greater than that of domesticated rice after cooking. Additionally, they all had intermediate apparent amylose content with a higher percentage of longer branch chain amylopectin which imparted a high gelatinization temperature but low viscosity parameters.
A comparative study of three different starch genes in domesticated and wild species of rice found a reasonable conservation in the coding regions across the species. Shared non-synonymous changes between non AA genome wild Oryza species and other cereals were observed, suggesting a probable parallel evolution of the genes. A concatenated dataset of the conserved regions produced a topology reflecting gene evolution and more or less the established phylogeny of wild species of Oryza genus. In combination, these studies suggest these wild species have potential to be a crop of their own right and/or can be used in O. sativa breeding programmes.