Physiological Immune Response of Rice Plant (Oryza sativa L.) to Pyricularia oryzae Cavara

Rice blast, caused by Pyricularia oryzae Cavara, remains one of the most destructive diseases limiting rice productivity worldwide. Understanding the physiological, biochemical, molecular, and structural bases of resistance is essential for selecting durable blast-resistant genotypes. This study comprehensively evaluated these defense mechanisms in selected rice genotypes through five integrated experiments to identify stable sources of resistance and clarify the mechanisms underlying genotype-specific responses to blast infection. Biochemical analyses revealed pronounced genotype-dependent activation of defense pathways. Among eleven rice genotypes, BRRI dhan33, BRRI dhan74, and the check line IR 64 exhibited significantly higher activities of key defense enzymes, including superoxide dismutase (SOD), phenylalanine ammonia-lyase (PAL), lipoxygenase (LOX), chitinase (CHT) and β-1,3-glucanase, particularly at 120 hours after inoculation (HAI). These genotypes maintained balanced reactive oxygen species (ROS) dynamics, lower malondialdehyde (MDA) accumulation, and enhanced membrane protection. Higher levels of non-enzymatic defense compounds, such as phenolics, flavonoids, antioxidants, proteins, and soluble sugars, were also observed in these genotypes with lower disease incidence (%). In contrast, BRRI dhan31 and BRRI dhan63 showed weaker enzymatic responses, excessive ROS buildup, high lipid peroxidation and reduced metabolite accumulation. Multivariate analyses confirmed strong positive associations between disease severity and oxidative stress markers, and strong negative associations with antioxidant and defense components, highlighting the protective role of redox homeostasis. Physiological and agronomic assessments across Aman and Boro seasons demonstrated clear genotype-dependent variation in blast tolerance. BRRI dhan33 (Aman), BRRI dhan74 (Boro), and IR 64 maintained higher plant height, tillering, leaf area, chlorophyll content, membrane stability, and yield stability under infection with minimal reductions in growth traits and grain yield (9–17%). In contrast, BRRI dhan31 and BRRI dhan63 showed severe reductions in growth parameters, high electrolyte leakage, poor membrane stability, and yield losses of 30–36%. Percent Disease Index (PDI) and Stress Susceptibility Index (SSI) consistently classified BRRI dhan74 and IR 64 as resistant and BRRI dhan33 as moderately resistant, while BRRI dhan31 and BRRI dhan63 were highly susceptible. PDI values corroborated these findings, with BRRI dhan74 and BRRI dhan33 showing the lower value (33.33% and 42.6%, respectively) whereas BRRI dhan63 and BRRI dhan31 showed the highest value (85.18% and 74.07%, respectively). Multivariate and principal component analyses also clustered IR 64, BRRI dhan74 and BRRI dhan33 on the tolerant side, attributed to high physiological stability and yield performance. Molecular characterization of blast resistance genes (Pib, Pi5, Pik-h, Pik-p, Pi9, Piz-t, Piz, Pita-2) using eight gene-based molecular markers showed that BRRI dhan33, BRRI dhan74, and IR 64 harbored the highest number (four) of resistance genes, including the shared and discriminating gene Pita-2. Genetic clustering placed IR 64 and BRRI dhan74 together, supporting their unique allelic composition. Gene expression profiling through RT-qPCR analysis further validated biochemical findings, showing strong upregulation of OsCHT, OsLOX2, and OsPAL in resistant genotypes, especially in BRRI dhan74, while BRRI dhan63 exhibited downregulation of these genes with an increase in inoculation time. Genome-wide analysis identified 55 defense-related genes, including 30 chitinases, 16 lipoxygenases, and 9 PAL genes, which constituted a complex defense response system. Subcellular localization analysis revealed well-directed enzyme localization; 83% CHT proteins targeting extracellular space for direct defense and 75% LOX proteins were cytoplasmic for damage perception and integration in lipid signaling. Protein interaction and synteny analyses further demonstrated conserved and interconnected defense networks. Histopathological observations corroborated these results by demonstrating preserved tissue organization, stable stomatal density, and adaptive vascular enlargement following infection in BRRI dhan74 compared with BRRI dhan63. Overall, this study demonstrates that durable resistance to P. oryzae arises from a coordinated, multi-layered defense system integrating biochemical activation, redox balance, agronomic and physiological stability, genetic composition, defense-related gene expression regulation, and anatomical resilience. BRRI dhan33 and BRRI dhan74, together with IR 64, consistently exhibited superior resistance across all evaluated parameters and represented valuable genetic resources for blast resistance breeding. These findings provide a comprehensive framework for integrating physiological traits, molecular markers, and defense-related genes into future rice improvement programs aimed at selecting as well as developing high-yielding and blast-resilient cultivars under diverse agroecological conditions.

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