Accumulation of hydroxyproline-rich glycoproteins in pearl millet seedlings in response to Sclerospora graminicola infection

Abstract

Hydroxyproline-rich glycoproteins (HRGPs) are important plant cell wall structural components that are known to be induced in response to pathogen attack. In the present study, accumulation of HRGPs in pearl millet seedlings in response to Sclerospora graminicola, the incitant of downy mildew disease was monitored by estimation of hydroxyproline (Hyp) in the hydrolyzed cell wall samples. In 2-day-old seedlings of a highly resistant cultivar of pearl millet inoculated with S. graminicola, a marked increase in Hyp content was observed in the coleoptiles as early as 2 h compared to that in a highly susceptible cultivar. A correlation was observed between the amount of Hyp induced in coleoptiles of different cultivars of pearl millet to S. graminicola infection and the host resistance. Electrophoretic separation of the acid-ethanol extracted proteins from these coleoptiles followed by periodic acid Schiff staining revealed three glycoproteins of molecular masses 27, 17 and 14 kDa. These three glycoproteins also reacted positively on western blots with MAC 265, a monoclonal antibody against pea HRGP, indicating that they are HRGPs. The 17 kDa HRGP was markedly increased in highly resistant cultivar to S. graminicola infection compared to highly susceptible cultivar. The 14 kDa HRGP was exclusively present in the highly resistant cultivar and not in highly susceptible cultivar. Histological studies showed cross-linking of proteins in the cell walls of only highly resistant cultivar against S. graminicola infection. In addition, immunocytochemical studies using MAC 265 antibodies recognized these cross-linked proteins to be HRGPs. These findings indicated that accumulation of HRGPs in cell walls of highly resistant pearl millet seedlings is a component of defense reaction against downy mildew pathogen S. graminicola.