Picolinic acid spray stimulates the antioxidative metabolism and minimizes impairments on photosynthesis on wheat leaves infected by Pyricularia oryzae

Abstract

Fungal pathogens produce toxins that are important for their pathogenesis and/or aggressiveness towards their hosts. Picolinic acid (PA), a non-host selective toxin, causes lesions on rice leaves resembling those originated from Pyricularia oryzae infection. Considering that non-host selective toxins can be useful for plant diseases control, this study investigated whether the foliar spray with PA on wheat (Triticum aestivum L.) plants, in a non-phytotoxic concentration, could increase their resistance to blast, stimulate the anti-oxidative metabolism, and minimize alterations in photosynthesis. The PA spray at concentrations greater than 0.1 mg ml^-1 caused foliar lesions, compromised the photosynthesis and was linked with greater accumulation of hydrogen peroxide (H₂ O₂ ) and superoxide anion radical (O₂ ^•- ). Fungal mycelial growth, conidia production and germination decreased by PA at 0.3 mg ml^-1 . Blast severity was significantly reduced by 59 and 23%, respectively, at 72 and 96 h after inoculation for plants sprayed with PA (0.1 mg ml^-1 ) at 24 h before fungal inoculation compared to non-sprayed plants. Reduction on blast symptoms was linked with increases on ascorbate peroxidase (EC 1.11.1.11), catalase (EC 1.11.1.6), glutathione peroxidase (EC 1.11.1.9), glutathione reductase (EC 1.8.1.7), glutathione-S-transferase (EC 2.5.1.18), peroxidase (EC 1.11.1.7), and superoxide dismutase (EC 1.15.1.1) activities, lower H₂ O₂ and O₂ ^•- accumulation, reduced malondialdehyde production as well as less impairments to the photosynthetic apparatus. A more efficient antioxidative metabolism that rapidly scavenges the reactive oxygen species generated during P. oryzae infection, without dramatically decreasing the photosynthetic performance, was a remarkable effect obtained with PA spray.