Induced Mutations For Enhancing Salinity Tolerance in Rice

Abstract

Salt accumulation in soil surfaces, known as soil salinity, could lead to the impairment of plant growth and development and is manifested mostly under irrigated and dryland agriculture. Excess salts in the soil affects plants through osmotic stress; accumulation to toxic levels within the cells; and through the interference with the uptake of mineral nutrients. Rice productivity in several parts of the world is therefore severely limited by salinity on account of the prevalence of irrigation in rice farming. Tolerance to salt toxicity in plants is a genetic and physiologically complex trait. Halophytes (salt tolerant plants) are different from the salt-sensitive glycophytes in terms of peculiarities in their anatomy, ability to sequester otherwise toxic ions, and other physiologic processes. It is logical therefore to infer complexity also at the genetic level on account of the several pathways involved in these mechanisms. These complexities have confounded genetic improvement strategies for salinity tolerance in plants resulting in a paucity of saline tolerant plants, with only about 30 officially released saline tolerant crop varieties world-wide. Only one saline tolerant rice variety, Bicol, has been officially released to farmers. We review strategies being currently employed in the development of saline tolerant rice varieties. These include conventional plant breeding which is hampered by the lack of suitable genetic variation for this trait; the modest progress made through doubled haploidy; and the reliance on somaclonal variation, an unsustainably unpredictable strategy. This review also posits that while genetic transformation has led to the modification of certain physiological indices implicated in salinity tolerance in rice, in isolation, these modifications have not been translated to improved yield under salt stress. A more recently adopted strategy, induced mutagenesis, has led to some promising results. We argue that the production of induced rice mutants holds the greatest promise of these strategies for mitigating the scourge of soil salinity considering the relative ease with which other traits in this crop have been modified using this methodology. The underlying principles of induced mutagenesis; the modes of action of different mutagenic agents; and procedures for the rapid production and detection of mutants are also summarised. In order to enhance efficiency in the production, detection and incorporation of induced mutants into crop improvement programmes, we suggest the coupling of in vitro (such as doubled haploidy and cell suspension cultures) and molecular genetic techniques to this methodology. It is posited also that the efficiency of this process can be greatly enhanced by marker-aided selection while high throughput reverse genetics strategies could lead to the rapid detection of mutation events in target genes. It is concluded that with the plethora of genomics resources available for rice, the use of induced mutations for improving salinity tolerance (and other traits) would rely significantly on the concerted application of efficiency enhancing in vitro techniques and functional genomics strategies (including reverse genetics)