Involvement of mitogen-activated protein kinase signalling in pearl millet-downy mildew interaction

Abstract

Mitogen-activated protein kinase (MAPK) cascade-mediated signalling is essential in the establishment of resistance towards pathogens. The present study compared MAPK activities in a compatible and incompatible interaction between pearl millet [Pennisetum glaucum (L.) R. Br.] and downy mildew pathogen Sclerospora graminicola. Differential expression was observed with rapid and increased activation of MAPKs, PgMPK1 (48kDa) and PgMPK2 (44kDa), in the incompatible interaction; with a weak activity of only PgMPK1 in the compatible interaction. Immunoblot analysis showed PgMPK1 and PgMPK2 to be orthologs of salicylic acid-induced protein kinase and wound-induced protein kinase, respectively. Immunocytochemical analysis revealed pathogen-induced accumulation and nuclear localisation of PgMPKs only in the incompatible interaction with highest signals in the vascular tissues. Maximum PgMPKs activation correlated with the activation of several defence-related enzymes. In addition, inhibition of MAPK-activation by kinase cascade inhibitors correlated with the suppression of defence-related enzyme activities and pathogen-induced H2O2 accumulation. Treatment of pearl millet seedlings with abiotic and biotic elicitors led to a strong early induction of only PgMPK1. β-Amino butyric acid and H2O2 were found to be best activators of PgMPK1. These results suggest that in pearl millet MAPK signalling is involved in mediating several defence mechanisms in response to pathogen infection.

Keywords: 2,6-dichloro-isonicotinic acid; BABA; Defense-related enzymes; Histochemical analysis; INA; In-gel kinase assay; Kinase inhibitors; MAPK; MBP; Mitogen-activated protein kinase; PAL; PAMP; POX; SA; SIPK; SOD; Sclerospora graminicola; WIPK; mitogen-activated protein kinase; myelin basic protein; pathogen-associated molecular pattern; peroxidase; phenylalanine ammonia lyase; salicylic acid; salicylic acid-induced protein kinase; super oxide dismutase; wound-induced protein kinase; β-amino butyric acid.