Polygalacturonase-inhibitor proteins in pearl millet: possible involvement in resistance against downy mildew

Polygalacturonase-inhibitor protein (PGIP) is a defense protein found in plant cell walls. It prevents the degradation of pectin by modulating the endo-polygalacturonase activity. The present study has used heterologous anti-bean PGIP probes to investigate the role of PGIP in pearl millet [Pennisetum glaucum (L) R. Br.] resistance against downy mildew caused by oomycete pathogen Sclerospora graminicola (Sacc.) Schroet. Northern blot analysis using bean pgip2 DNA fragment as probe showed an early and marked induction of transcripts (∼1.2 kb) upon pathogen-inoculation in pearl millet cultivar resistant to downy mildew, with the maximum level observed at 24 and 48 h post-inoculation (h.p.i.). Western blot analysis of pearl millet total cell wall proteins using antibodies against bean PGIP showed the presence of a major band of ∼43 kDa, and several minor ones. The protein accumulation was higher in resistant seedlings than in susceptible seedlings with a differential expression observed only in the case of incompatible interaction. Immunocytochemical localization in epidermal peelings of coleoptiles and tissue-printing showed a similar trend in the PGIP accumulation. PGIP was found to localize in the epidermal as well as in the vascular regions of tissues. Higher accumulation was observed in the stomatal guard cells of resistant cultivar inoculated with the pathogen. PGIP activity of pearl millet total protein extracts when assayed against Aspergillus niger PG displayed differential PG inhibitory activities between the resistant and suceptible cultivars with resistant sample showing the highest inhibition of 16%, post-pathogen treatment. Thus, PGIP appeared to be an important player in pearl millet-S. graminicola interaction leading to host resistance.