Resistance to downy mildew in pearl millet is associated with increased phenylalanine ammonia lyase activity
N. P. Geetha A , K. N. Amruthesh A , R. G. Sharathchandra A and H. S. Shetty A BA Downy Mildew Research Laboratory, Department of Studies in Applied Botany, Seed Pathology and Biotechnology, University of Mysore, Manasagangotri, Mysore-570 006, India.
B Corresponding author. Email: hss_uom@hotmail.com
Functional Plant Biology 32(3) 267-275 https://doi.org/10.1071/FP04068
Submitted: 5 April 2004 Accepted: 11 February 2005 Published: 5 April 2005
Abstract
Phenylalanine ammonia lyase (PAL) activity was studied in pearl millet cultivars with different levels of resistance to the downy mildew disease caused by Sclerospora graminicola, an important oomycete pathogen. PAL activity was elevated in resistant host cultivar and decreased in susceptible cultivars following downy mildew pathogen infection. The enzyme activation varied between cultivars and was correlated with the degree of resistance to downy mildew disease. The induction of PAL as a response to pathogen inoculation was further corroborated by a time-course study in seedlings and cultured cells of pearl millet. The level of PAL activity was highest at 1.5 h in cultured cells and 4 h in seedlings of resistant host cultivar after inoculation with Sclerospora graminicola. Further studies on PAL activity in different tissues of seedlings showed highest enzyme activity in the young growing region of the root of the resistant host cultivars. The accumulation of wall-bound phenolics and lignin was higher in the resistant cultivar seedlings as evidenced by phloroglucinol–HCl staining and p-coumaric acid assay. The temporal changes in lignin concentration and the concentration of soluble phenolics were greater in root tissues of resistant cultivars than in those of susceptible cultivars. Treatment of resistant seedlings with a PAL inhibitor, α-aminooxy-β-phenylpropionic acid, resulted in the enhancement of the enzyme activity, whereas in the presence of 1 mm trans-cinnamic acid the pathogen-induced PAL was completely inhibited. Treatment of pearl millet seedlings with exogenously applied PAL inhibitors induced downy mildew disease susceptibility in the resistant pearl millet cultivar, consistent with direct involvement of PAL in downy mildew resistance. Results are discussed with respect to the presumed importance of host phenolic compounds and lignin accumulation and its relation to PAL activation as a response to the pathogen infection.
Keywords: disease resistance, downy mildew, enzyme inhibitors, lignins, pearl millet, phenylalanine ammonia lyase, soluble phenolics.
Acknowledgments
This work was conducted as a part of the project ‘Systemic Acquired Resistance — an Eco-friendly Strategy for Managing Diseases in Rice and Pearl Millet’, funded by the Danish International Agency under Enhancement of Research Capacity Program (DANIDA–ENRECA). The access to the facilities provided by the Indian Council of Agricultural Research (ICAR), Government of India, New Delhi, through the All India Coordinated Pearl Millet Improvement Project is highly acknowledged. NP Geetha is grateful to the Council of Scientific and Industrial Research (CSIR), Government of India, New Delhi, for financial support.
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