Phenylalanine Ammonia Lyase Activity in Pearl Millet Seedlings and its Relation to Downy Mildew Disease Resistance

Phenylalanine ammonia lyase (PAL) activity was studied in different genotypes of pearl millet with varying degrees of susceptibility to downy mildew disease, after inoculating with Pathotype 1 of Sclerospora graminicola. In resistant genotypes, the enzyme activity significantly increased 24 h after fungal inoculation while in the susceptible genotypes, the activity decreased. The increase or decrease in enzyme activity was well-correlated with the degree of host resistance to the pathogen. A time-course of change in activity of PAL after inoculation showed a considerable difference between resistant and susceptible genotypes. Studies on the activity of PAL in different parts of pearl millet seedlings revealed that in the resistant genotype, enzyme activity significantly increased at 24 h post-inoculation only in the shoot portion, whereas in mesocotyl and root the activity decreased. In susceptible seedlings, enzyme activity decreased at 24 h post-inoculation in shoot, mesocotyl and root. The activity of PAL was also found to be pathotype-specific. Histochemical tests for lignin were positive in infected cells in the resistant genotypes. The role of PAL in imparting resistance to pearl millet against downy mildew disease is discussed.