Polyphenol oxidase (PPO) activity was analysed in seedlings of resistant and susceptible pearl millet [Pennisetum glaucum (L.) R.Br] cultivars with or without inoculation of the downy mildew pathogen Sclerospora graminicola (Sacc.) Schroet. Seedlings of resistant varieties had greater PPO activity than susceptible seedlings, and inoculated seedlings had significantly higher PPO levels than uninoculated seedlings. Temporal accumulation of PPO showed a maximum activity at 24 h post-inoculation in resistant seedlings, whereas in susceptible seedlings it peaked at 48 h. PPO activity was positively correlated with levels of downy mildew resistance in different pearl millet cultivars under field conditions. Native PAGE staining showed four isoforms of PPO, which were differentially induced in relation to the time of appearance and intensities in the uninoculated seedlings, whereas a fifth PPO isoform appeared after inoculation with S. graminicola. PPO activity was significantly higher in the shoot and leaves of pearl millet than in the root. Tissue printing analysis of the enzyme expression showed that the enzyme is predominantly expressed after pathogen inoculation and is localised in the epidermal and vascular regions. Temporal analysis of transcript accumulation showed that in resistant seedlings PPO mRNAs was expressed earlier and more abundantly than in susceptible seedlings. Our studies demonstrate, for the first time, that PPO is actively involved in plant defence and can be used as a marker of resistance to downy mildew infection in pearl millet.

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Abstract. Biopriming pearl millet seeds with Pseudomonas fluorescens isolates resulted in improved growth of the plants and also induction of resistance against downy mildew disease caused by the fungus Sclerospora graminicola. Although all of the test isolates reduced disease severity and promoted growth both under greenhouse and field conditions, high variability was observed in their efficiency. Treatment due to Pseudomonas fluorescens resulted in enhancement of: germination, seedling vigour, plant height, leaf area, tillering capacity, seed weight (measured for 1000 seeds) and yield. The time required for flowering was advanced by 5 days. Isolate UOM SAR 14 registered higher levels of vegetative and reproductive growth; most importantly there was a 22% increase in grain yield. The isolates also effectively induced resistances against downy mildew disease both under greenhouse and field conditions. The isolates offered protection ranging from 20 to 75%. However, the level of disease control was less than that achieved by the systemic fungicide Apron. UOM SAR 14 offered 71 and 75% and remained consistent under all test conditions. Further studies showed that the resistance induced was systemic, required a minimum of 3 days to build up, and was sustained throughout the plant's life.

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