Piperine, Reserpine and ß-Sitosterol Attenuate Stem Rot (Sclerotium rolfsii Sacc.) of Groundnut by Inducing the Secretion of defense Enzymes and Phenolic Acids.

Groundnut stem rot caused by Sclerotium rolfsii is a major constraint as it affects the productivity. Although managing this disease using synthetic fungicides is a more feasible method, environmental pollution and side effects caused by them demand some safe fungicides. Seven phytochemicals piperine, quercetin, reserpine, atropine sulfate, ß-sitosterol, ethyl protocatechuate and salicylic acid were initially tested against S. rolfsii under in vitro methods. All the compounds exhibited significant effects on mycelial inhibition (except atropine sulfate), sclerotial development, ooze formation, maturity, sclerotial number and germination of S. rolfsii. The more active compounds, piperine, reserpine and ß-sitosterol were then evaluated under glasshouse condition by adopting various application methods (seed treatment, foliar application and micro-injection at 2000 µg/mL) on groundnut plants challenged with and without S. rolfsii. All the treatments effectively reduced the plant mortality when tested every 15 days of infection with S. rolfsii. However, the magnitude of reduction varied among the treatments, with the mortality ranging between 9.37 % and 29.68 % compared to the control (40.68 %). The piperine-microinjected plants recorded minimum mortality (3.12 %). The defense enzymes (PAL and PPO) and key end products such as phenolics (total and individual) were determined in the leaf samples collected after 24, 48 and 72 h of infection with S. rolfsii to understand the systemic resistance induction effect. An increase in PAL and PPO activity was observed in all the samples. While microinjection of ß-sitosterol caused a maximum PAL induction, piperine caused a maximum PPO activity. Further, samples of piperine treated group showed higher induction of phenolic acids (86.46 µg g-1 micro-injection) compared to ß-sitosterol and reserpine groups in all the methods. When the samples were analysed (HPLC) for individual phenolic acids, maximum accumulation of various acids was observed in the samples collected after 48 h. Tannic and gallic acids were found to be accumulated in higher quantities in most of the samples. The maximum accumulation of phenolic acids was found in micro-injected samples. These results verified the reduction of mortality through the induction of defensive chemicals by the action of phytochemicals. Thus, the study recommends the use of these natural molecules for the integrated management of stem rot of groundnut after necessary field trials.

Isolation and characterization of antimicrobial cyclic dipeptides from Pseudomonas fluorescens and their efficacy on sorghum grain mold fungi.

This study was aimed at isolation and characterization of natural antifungal compounds for grain mold, a key parasitic fungal disease of sorghum. Pseudomonas fluorescens strain isolated from rhizosphere of groundnut crop was selected as a source. Its biocontrolling ability was assessed by testing some biochemical attributes such as phosphate-solubilization, and HCN, NH3 , indole-3-acetic acid, and siderophore production. The strain showed positive result for all except indole-3-acetic acid, revealing its suitability for a further study. The antibiotic-sensitivity pattern of the strain against 43 antibiotics was also established, which showed resistance to 15 antibiotics. The efficacy of P. fluorescens strain against grain mold was identified by dual culture technique. Hundred percent inhibition was found against Fusarium moniliforme, an important causative agent of this disease. The strain was fermented for secondary metabolites and extracted with AcOEt. Chromatographic separation of the extract yielded four known compounds, cyclo(L-Pro-L-Phe) (1), cyclo(trans-4-hydroxy-L-Pro-L-Leu) (2), cyclo(trans-4-hydroxy-L-Pro-L-Phe) (3), and cyclo(Gly-L-Pro) (4), which were characterized by spectral analysis and optical rotation. The crude extract, a mixture of 2 and 3, and isolated 1 were proved to be significantly effective against grain mold fungi. This is the first report on production of these cyclic dipeptides by P. fluorescens and their antagonistic properties.