Christine Sanier

Scopoletin production and degradation in relation to resistance of Hevea brasiliensis to Corynespora cassiicola

Summary Inoculation of leaves of resistant and susceptible Hevea brasiliensis clones with Corynespora cassiicola induced foliar necrosis and biosynthesis of scopoletin (Scp), considered as a Hevea phytoalexin. Foliar symptoms appeared, as soon as 24 h after infection, and precede Scp accumulation in inoculum droplets (peak: 48 h after inoculation). Scp concentration was 5-fold higher in the susceptible than in the resistant clone 48 h after infection. Nevertheless, a fungitoxic effect of Scp on spore germination and on mycelium growth was shown in bioassays, but the efficient concentrations were greater than those found with another foliar pathogenic fungus of rubber tree such as Microcyclus ulei and Colletotrichum gloeosporioides. Bioassay showed that the low Scp fungitoxicity to C. cassiicola could also be related to the ability of fungus to detoxify Sep. Isoelectric focusing analysis of peroxidase activity in Hevea infected leaflets has shown an increase in acidic and basic isoperoxidases that are able to use Scp as substrate. The rapid increase of Scp-oxidase activity (as soon as 16 h after inoculation) was higher in the resistant than in the susceptible clone. In vitro testing of Scp peroxidation products has shown a toxic effect on C. cassiicola (conidia germination and mycelium growth), but not significantly higher than Scp. It appears that the level of Sep accumulation was a balance between its synthesis and its degradation by the pathogen or/and by foliar Scp-peroxidases. In conclusion, these results lead to the fact that Scp cannot be considered as a major defense mechanism of Hevea towards C. cassiicola disease.

Les composés phénoliques et la résistance des plantes aux agents pathogènes

Summary Plant defense responses against microorganisms require many mechanisms as constitutive phenolic compounds and phytoalexins. These secondary metabolic products can be accumulated and involved in plant defence. Their activity was related to antimicrobial properties, involvement in cell wall reinforcement, modulation and induction of plant responses. Clonage of most genes coding for enzymes of phenolic metabolism give many means for genetic manipulations aiming to improve the effectiveness of plant defences. Recent results were very interesting but do not yet allow to define the real level of the involvement of phenolic compounds in plant disease resistance.