Infection by Pythium aphanidermatum increases production of phenolics in hydroponically grown peppers and predisposes healthy plants to root rot

Abstract

The type and concentration of phenolics found in the cytosol (free), cell wall (bound), and nutrient solution of hydroponic pepper (Capsicum annuum L.) inoculated with Pythium aphanidermatum (Edson) Fitzp. were compared with non-inoculated controls. The quantities and types of free phenolics found in the roots and nutrient solution were greater in inoculated than in control plants. With regard to bound phenolics, while there was no difference between the types of phenolics in inoculated and control roots, quantities were greater in inoculated roots. High Pressure Liquid Chromatography analyses revealed two compounds in root extracts, and one in nutrient solution extracts that were contributors to root discolouration caused by Pythium aphanidermatum colonization. The compounds were subsequently isolated and identified as 4-hydroxybenzoic acid and 3-methoxy-4-hydroxybenzoic acid (vanillic acid) by their High Pressure Liquid Chromatography retention times, UV signature and Nuclear Magnetic Resonance spectroscopic comparisons with pure standards. Addition of various concentrations of 4-hydroxybenzoic acid or vanillic acid to the nutrient solution predisposed healthy plants to infection by zoospores of Pythium aphanidermatum when compared to plants treated with only Pythium aphanidermatum. 4-hydroxybenzoic acid and vanillic acid had little or no effect on colony growth of Pythium aphanidermatum in vitro, but respectively increased and decreased numbers of sporangia produced. These results are the first demonstration of host predisposition to infection by Pythium aphanidermatum caused by phenolic compounds that accumulate in inoculated roots and leach into the nutrient system of hydroponic systems.