PHISIOLOGICAL RESPONSES IN PEARL MILLET DURING THE INDUCTION OF SYSTEMIC RESISTANCE AGAINST DOWNY MILDEW DISEASE
PHYSIOLOGICAL RESPONSES IN PEARL MILLET DURING THE INDUCTION OF SYSTEMIC RESISTANCE AGAINST DOWNY MILDEW DISEASE
VU. KUMAR, PH THEJASWINI and HS SHETTY
Department of Studies in Applied Botany, University of Mysore, Mysore- 570 006, INDIA
Background and objectives
Pearl millet is a staple food and fodder crop in the Semi-Arid Tropics around the world. Downy mildew disease caused by the biotrophic, oomycetous fungus Sclerospora graminicola reduces pearl millet production by 30% amounting to US $270 millions during epidemics. Strategies like the use of resistant cultivars and systemic fungicides for disease management are associated with problems of pollution and destruction of biological communities that otherwise support crop protection. These together with strong motivation to lower pearl millet production costs suggested the need for alternative strategy that complements resistant breeding and pesticide use for managing downy mildew disease. In this laboratory, acquired systemic resistance (SAR) was successfully developed as a method of pearl millet protection against downy mildew (1). Seed treatments offer economical and less polluting delivery system and induction of resistance by seed treatment is a practical way to deliver benefits of systemic acquired resistance (2). Induction of resistance factors and associated host responses in induced resistant pearl millet plants were also analysed.
Pearl millet seedlings/seeds were induced for resistance against downy mildew by treatment with suboptimal concentration of the same pathogen and known SAR inducing compounds like salicylic acid and acetyl salicylic acid. Treated seedlings showed 70% protection. The resistance induced was systemic and protected the tillers and floral buds as well from subsequent infection. However, it was not transferred to the next generation via seeds of protected plants. ELISA studies using antibodies raised against the components of pathogen cell wall showed reduced fungal colonization in the protected plants. This suggested operative host defense. Biochemical studies indicated hypersensitive response and lignification through activation of peroxidase, b-1,3-glucanase, phenylalanine ammonia lyase and lipoxygenase. Induced resistant seedlings showed early hypersensitive response and accumulation of more lignin at the site of infection. Correlation was observed between activities of the defense related enzymes and induction of resistance. Within 24h of treatment, all enzyme activities were significantly enhanced in pearl millet seedlings raised from inducer treated seeds. Upon infection with the pathogen, further increase in the activity of all the enzymes was recorded. Results also indicated tissue-specific expression of these enzymes. Induced resistant plants showed increased reproductive and vegetative growth. Studies also showed that the levels of SAR that was achieved in different pearl millet cultivars with varying resistance to S. graminicola were independent of their constitutive resistance.
SAR is a safe effective, inexpensive, durable and eco-friendly strategy for pearl millet downy mildew management and finds its niche in sustainable agriculture. It also restricts the use of pesticides and advocates increased integration into other control practices. Induction of SAR by seed treatment with known inducers offers immediate prospects of their use in pearl millet protection. It also suggests the possibility of return to use some of the older cultivars that had desirable characters apart from their susceptibility to downy mildew disease.
1. Kumar VU, Meera MS, Hindumathy CK, Shetty HS, 1993. Crop Protection 12, 458-452
2. Wei G, Kloepper JW, Tuzan S, 1996. Phytopathology 86, 221-224