Defense response of resistant and susceptible genotypes of castor (Ricinus communis L.) to wilt disease

The biochemical basis of resistance in castor (Ricinus communis L.) to Fusarium wilt, caused by the pathogen Fusarium oxysporum f. sp. ricini, was investigated. Induction of plant defence against pathogen attack is regulated by a complex network of different signals. Thus changes in various biochemical defenses including antioxidant enzymes, phenolic compounds and pathogenesis related (PR) proteins were investigated in the roots of resistant and susceptible genotypes of castor at 0, 24, 48 and 72 h.a.i. Infection by F. oxysporum significantly increased the superoxide dismutase (SOD) and peroxidase (POX) activities in the roots of susceptible genotypes, while the catalase (CAT) activities were appreciably higher in the roots of resistant genotypes at different stages. Constitutive levels of ascorbate peroxidase (APX) and polyphenol oxidase (PPO) were higher in the resistant genotypes. Also, the activities of phenylalanine ammonia lyase (PAL) and β 1, 3 glucanase significantly increased in the roots of the resistant genotypes after infections. The rate of increment of thiobarbituric acid reactive substances (TBARS) was higher in resistant genotypes after infection. Analysis of isozyme banding pattern of SOD, POX, PPO and esterase on native PAGE electrophoresis revealed that interaction between plant and fungi invoked various isozymes at 48 h of infection. SOD 3 was observed only in resistant genotypes at 24 h.a.i. except Geeta. Similarly induction of POX 5 was observed only in resistant genotypes at 48 h of infection, though the intensity of POX 5 was very less.