Paenibacillus polymyxa improves glutathione reductase activity and reduces Fusarium verticillioides incidence in maize grains

Fusarium verticillioides (Hypocreales: Nectriaceae) infects maize, causing stalk rot disease and producing toxins that accumulate in grains. The antifungal activity of seven strains of Paenibacillus polymyxa (Bacillales: Paenibacillaceae) against F. verticillioides was evaluated in vitro using the microbiological growth media potato dextrose agar, malt extract agar, and crushed maize grains. The strain LIS03, which demonstrated the strongest antagonistic activity, was selected for further investigation based on its ability to reduce the severity of stalk rot disease and induce plant-defense enzymes in maize. In the greenhouse, seed inoculation and plant spraying (V4 phenological stage) with LIS03 prior to F. verticillioides injection in the stem were evaluated regarding the activity of phenylalanine ammonia-lyase (PAL) and glutathione reductase (GR) in maize leaves. PAL activity significantly increased in plants inoculated with LIS03 at three and nine days post-inoculation (dpi). In contrast, plants simultaneously inoculated with LIS03 and the pathogen showed an increase of 82% in GR activity 12 dpi. Higher GR activity was correlated with reduced stalk rot severity at 6 dpi (r = − 0.53). In the field, the combination of seed inoculation followed by plant spraying, as well as plant spraying alone, resulted in a 33% reduction in disease severity. Only seed inoculation led to an 18.7% decrease. Although the grains did not show symptoms of fungal diseases in the field experiment, a culture-based blotter test revealed a high incidence of F. verticillioides. The blotter test also showed that grains from silks sprayed with LIS03 presented a reduction of 16% and 25% in the incidence of F. verticillioides in naturally infected and artificially inoculated plants, respectively. These results with P. polymyxa provide a promising avenue for developing biological control strategies for controlling maize diseases caused by F. verticillioides.