Physiological and anatomical plasticity of root traits under water stress and molecular characterization using root specific genes in rice

Abstract

The study entitled “Physiological and anatomical plasticity of root traits under water stress and molecular characterization using root specific genes in rice (Oryza sativa L.)” was conducted in the Department of Plant Physiology, College of Agriculture, Vellayani during October – January, 2017-19 with the objective to quantify the adaptive plasticity in root-shoot morphology, physiology and root anatomical plasticity under water stress in selected rice genotypes and molecular characterization using root specific genes. The extent of variation for various physiological, biochemical and anatomical characters were assessed as an indicator of water stress from six selected genotypes collected from RARS Pattambi and N-22 from IIRR, Hyderabad. Plants were maintained under 100% and 50% field capacity (FC) soil moisture in a rain out shelter. A set of five replications were maintained and observations were made at booting stage on root, physiological, biochemical and anatomical parameters and significant variations for these traits were noticed for tolerant and susceptible genotypes. The study revealed that parameters such as relative water content (RWC) (%), specific leaf area (cm2 g-1), and cell membrane stability index (%) were found to be decreasing but not significant under stress condition whereas root parameters, biochemical and biomass partitioning were found to be increasing among the tolerant and susceptible genotypes. The highest RWC was recorded from N-22 (85.37%) under stress condition whereas least from Ptb 35 (71.96%). N-22 showed the highest reduction in specific leaf area with 219.9 cm2 g-1 whereas Ptb 39 showed an increasing trend in specific leaf area by 1.5% with 183.73 cm2 g-1 under stress. Cell membrane stability index (%) was highest in Ptb 30 (97.10%) under stress whereas least was recorded from Ptb 39 (83.11%). Carbon isotope discrimination (Δ13C)(mil-1) was least for N-22 (21.84) (Δ13C)(mil-1) and highest in Ptb 39 (23.49) (Δ13C)(mil-1) at panicle initiation. Study on root parameters of tolerant and susceptible genotypes at two FC levels exhibited significant variation among root parameters. Root length was highest in Ptb 29 (38.46 cm) and least in Ptb 35 (20.66 cm) under water stress. Among the genotypes Ptb 29 was found to be performing better for other root characters viz., root volume, root dry weight, root/shoot ratio and specific root length whereas least performance was noticed from susceptible genotypes Ptb 35 and Ptb 39. A significant differences in biomass partitioning was noticed among the genotypes, for characters such as leaf weight ratio, stem weight ratio and root weight ratio. Under stress root weight ratio was highest in Ptb 29 (0.21) and lowest in Ptb 35 (0.106). Anatomical studies revealed significant effects at both genotype and treatment levels. Tolerant genotypes were found to be more responsive under water stress for anatomical traits. N-22 and Ptb 29 exhibited an increase in root diameter (1.55mm and 1.796 mm), stele diameter (0.49 and 0.31 mm), late metaxylem number and late metaxylem diameter (5.6, 0.069 mm and 5.6, 0.076 mm respectively. Early metaxylem number found to be increasing in tolerant genotypes N-22 (30.66) whereas susceptible genotypes exhibited declining trend. Sclerenchymatous tissue was found to be highest in N-22 (0.024mm) whereas Ptb 35 a susceptible genotype exhibited lowest value for sclerenchyma with 0.012 mm. Yield attributes were found to vary significantly among genotypes. Spikelet fertility percentage and yield per plant was highest in N-22 with 85.66% and 24.66 g respectively. 1000 grain weight was highest in Ptb 30 (27.23 g) and lowest in Ptb 39 (22.5 g). Genotyping of the selected tolerant and susceptible rice genotypes using available DEEPER ROOTING QTL specific primers and other available drought specific SSR primers was done from seedlings raised in a petri dish. It was found that DRO1 specific microsatellite markers did not exhibited polymorphism among tolerant and susceptible genotypes but another drought related SSR primer RM 518 showed polymorphism for tolerant and susceptible genotypes. Expression studies were done between one tolerant and one susceptible genotypes i.e., Ptb 29 and Ptb 35 with DEEPER ROOTING QTL specific primers and EST- SSR RM 518. Results of expression studies using RM 518 exhibited differential expression under 100% FC and 50% FC condition and also among the genotypes Ptb 29 and Ptb 35. Significant variation was observed for physio-morphological and yield components among rice genotypes under 100% FC and water 50% FC conditions. Genotypes with better root traits such as root length, root shoot ratio and root anatomical plasticity exhibited more tolerance towards drought. The tolerant genotypes i.e., N-22. Ptb 29 and Ptb 30 can be used as donor plants in breeding programs for trait introgression for developing drought tolerant cultivars. Microsatellite marker RM 518 which could distinguish drought tolerant and susceptible genotype can be used for marker assisted selection for drought tolerance in rice. A differential expression of drought related genes was seen in tolerant and susceptible genotypes under water stress condition.