Differential biochemical and gene expression responses to UVB radiation in ten Southern Indian rice (Oryza sativa L. indica) varieties

Developing rice varieties with enhanced UVB stress tolerance is crucial for ensuring global food security in the face of changing environmental conditions. The present study aimed to screen UVB tolerant rice varieties by evaluating ten rice cultivars (O.sativa) under UVB irradiation. Five days old seedlings were subjected to UVB stress for 30 min per day (8.5 kJ m^-2 day^-1) for 7 days and samples were collected for phenotypic traits, biochemical and gene expression analysis. Different biochemical parameters including Chlorophyll, H₂O₂, MDA, phenol, flavonoids, total free amino acids, total sugar and anti-oxidative enzyme activities (Catalase, Ascorbic peroxidase, Superoxide dismutase and peroxidase) were measured using spectrophotometric analysis. UVB mediated changes in expression pattern of mRNA encoding CA, WRKY89, PHT2, pZIP48, UVR8a, URP and RLCK were studied using quantitative real time PCR (RT-qPCR). The cumulative stress response index (CSRI) were calculated based on the results from phenotypic traits, biochemical and gene expression studies and categorized IR20 as the most UVB tolerant and RNR as most UVB susceptible rice cultivars. In response to UVB irradiation, root length, root area, H₂O₂ content and expression of mRNAs encoding CA, WRKY89, PHT2, UVR8a, URP and RLCK were increased significantly in IR20. In addition, the total free amino acids content and peroxidase activity in IR20 declined in response to UVB treatment. Whereas, in RNR UVB treatment increased the activity of catalase and peroxidase, which subsequently decreased H₂O₂ content without affecting the expression of mRNAs encoding CA, WRKY89, PHT2, UVR8a, URP and RLCK. Multivariate statistical analysis, Principal component analysis (PCA) and hierarchical clustering, was used to classify the cultivars based on their UVB stress responses. PCA revealed a strong positive correlation between CSRI and UVB-tolerance genes (UVR8a, RLCK) and hierarchical clustering revealed the tolerant cultivars as separate cluster. In this study, findings demonstrate that UVB tolerance is genetically based, and IR20's resilience may be due to biochemical modulation and gene regulation. It offers valuable insights for breeding UVB-resistance rice cultivars to enhance crop resilience in UVB- at risk regions.