Effects of plant-beneficial fungi on plant growth and herbivore resistance under contrasting fertilizer conditions

Aims Plant-beneficial fungi play an important role in enhancing plant growth and protecting plants from biotic and abiotic stresses. However, context-dependency of such effects and differences among fungi often lead to inconsistent results that hamper their widespread use. Here, we investigated the effect of plant-beneficial fungi on plant growth and herbivore resistance, and how effects are mediated by fertilization. Methods Sweet pepper (Capsicum annuum L.) plants were root-inoculated with the plant-beneficial fungi Beauveria bassiana ARSEF 3097 and Trichoderma harzianum T22 and grown in a low-nutrient potting mix, with or without additional nutrients. Plant growth and herbivore resistance against the southern green stink bug (Nezara viridula L.) were compared between fungal treatments and fertilization levels by measuring several growth traits and quantifying feeding damage and plant defense-related gene expression. Results Fertilization significantly increased plant growth, but at the same time made plants more susceptible to herbivory. Irrespective of fertilization, T. harzianum stimulated growth and reduced feeding damage (number of leaf punctures), while B. bassiana only enhanced growth. For both strains, fungal inoculation generally increased the expression of marker genes involved in salicylic acid- and jasmonic acid-dependent defense responses upon herbivory, but this was less pronounced for salicylic acid-dependent defense signaling under fertilization. Conclusions We conclude that fungal inoculation improved plant growth and generally elicited a stronger defense response to stink bug feeding. Accordingly, plant damage was reduced by T. harzianum. Overall these results show that plant-beneficial fungi have the potential to promote plant growth and reduce feeding damage, irrespective of fertilization.