Enhancing Drought Tolerance in Rice through Activation of Self-defense Genes by Eco-contributors

Abstract

In present climate scenario, water shortage is extensively damaging the grain harvest from rice fields. Paddy has semiaquatic ancestors, so it is sensitive to agricultural drought. Aridity targets the morphological, biochemical, physiological and molecular features of it. An integrated strategy is required to save rice cultivation under frequent drought stress. In the following study, four interlinked approaches have been practiced to secure rice cultivation in a dry environment. In the first experiment, diversified rice germplasm was screened at seedling stage to select 20 drought responsive genotypes including 10 drought tolerant, five moderately drought tolerant and five drought susceptible. These genotypes were selected on the basis of gain in root length, shoot length, fresh root weight, fresh shoot weight, dry root weight and dry shoot weight. Selected genotypes were further tested at germination, seedling and maturity stages to trace a marker-trait that could rapidly label drought responsive genotypes and to check the efficiency of the first experiment of germplasm screening. At germination stage, imbibition rate, the speed of germination, radical and plumule length, radical and plumule weight, proline contents, total antioxidant capacity and malondialdehyde concentration were determined from germination seeds. At seedling stage root length, shoot length, fresh root weight, fresh shoot weight, dry root weight, dry shoot weight, proline contents, total antioxidant capacity and malondialdehyde concentration were determined from the leaves. At maturity stage, the number of tillers, plant total biomass, fertile florets, 100-grain weight, proline contents, total antioxidant capacity and malondialdehyde concentration were determined from the leaves. It was observed that screening at germination stage by using proline and TAC as markers is as reliable as screening at seedling and maturity stages. The expression pattern of OsP5CS gene at germination, seedling and maturity times has confirmed the reliability of proline for screening rice germplasm at different growth stages. It was concluded that rice seed banks keep extensive genetic variability for the climate-smart breeding programs. Determination of proline and TAC from germinating seeds can speed up the process of germplasm phenotyping against drought stress. In the second experiment, selected genotypes were tested for their association with eco-contributors (Piriformospora indica, zinc, calcium and molybdenum) under optimum and deficient water supply. Genotypes were supported with P. indica and grown under control and drought environments in Zn, Ca and Mo deficient nutrition media. Factorial under CRD with three replications was performed for the completion of the experiment. Plants were evaluated on the basis of root length, shoot length, fresh root weight, fresh shoot weight, dry root weight, dry shoot weight, the concentration of phosphorus, calcium and zinc, proline contents, total antioxidant capacity and malondialdehyde concentration. Significant improvement was recorded when plants were inoculated with P. indica and maximum damage was recorded when plants were in Zn deficient environment. So, P. indica and Zn were selected as efficient eco-contributors. An experiment for the detailed study of the selected eco-contributor was performed. Three drought-responsive genotypes were used (WC-297 drought tolerant, Caawa moderately drought tolerant and IR-64 drought susceptible) experimentation has proved that both of these selected eco-contributors have strengthened rice plant and stabilized its biomass, antioxidants as well as osmolite profile, mineral uptake, expression of important drought-responsive genes, chlorophyll contents, chloroplast integrity and the activity of antenna complex of the photosystem II. In the third experiment, hybridization of drought responsive genotypes with each other and with the modern cultivars was performed by following 6 x 6 diallel scheme. The F1 plants were accessed for the change in root length, shoot length, fresh root weight, fresh shoot weight, dry root weight, dry shoot weight, proline contents, total antioxidant activity and the concentration of phosphorus, calcium and zinc in leaves. Significant heterosis for these drought-related traits was recorded in both non-inoculated and inoculated plants of the F1 populations. In the fourth experiment, the yield of the rice plant was tried to increase in such a way that if these high yielding genotypes exposed to drought stress then even after yield losses these can provide substantial grain. For this purpose, the panicle architecture of the rice plants was exploited and high yielding populations were developed by using line x tester mating design. These new phenotypes were evaluated in low watered filed following randomized complete block design. Some genotypes in F2 population had potential to grow more than 197 additional grains than their better parents. On the basis of these interlinked experiments, it was concluded that rice germplasm keeps huge genetic diversity for inducing drought tolerance in modern cultivars. The screening of rice germplasm at germination stage using proline and total antioxidant capacity as markers accelerates the process of germplasm phenotyping. The inoculation of rice genotypes with P. indica under optimum supply of Zn stabilizes rice seedlings under drought stress. Hybridization among drought responsive genotypes and modern cultivars proved that these drought tolerant genotypes have the capacity to transfer drought-responsive traits in their imminent progenies. A modified rice panicle supports the high number of grains that can compensate yield losses under drought stress. An integrated approach is the best possible solution to save rice cultivation under increasing shortage of agricultural water.