Seed inoculation with entomopathogenic fungi (EPF) causes plant-mediated effects against arthropod herbivores, but the responses vary among EPF isolates. We used a wheat model system with three isolates representing Beauveria bassiana and Metarhizium spp. causing either negative or positive effects against the aphid Rhopalosiphum padi. Activities of six carbohydrate enzymes increased in plants showing biomass build-up after EPF inoculations. However, only aldolase activity showed positive correlation with R. padi numbers. Plants inoculated with M. robertsii hosted fewest aphids and showed increased activity of superoxide dismutase, implying a defense strategy of resistance towards herbivores. In M. brunneum-inoculated plants, hosting most R. padi, activities of catalase and glutathione reductase were increased suggesting enhanced detoxification responses towards aphids. However, M. brunneum simultaneously increased plant growth indicating that this isolate may cause the plant to tolerate herbivory. EPF seed inoculants may therefore mediate either tolerance or resistance towards biotic stress in plants in an isolate-dependent manner.

AbstractInsect herbivores, such as aphids, are common on plants, yet how they interact with plant microbiomes remains largely unknown. For instance, for the widespread bacterial epiphyte and potential aphid pathogen Pseudomonas syringae, aphids could impact bacterial populations by serving as secondary hosts or by altering the epiphytic habitat through feeding and/or waste secretion. Here, we examined whether the pea aphid, Acyrthosiphon pisum, could influence epiphytic populations of P. syringae. First, we quantified epiphytic growth ability without aphids and virulence to aphids across 21 diverse P. syringae strains. For eight strains that varied in these traits we then assessed the influence of aphid presence on epiphytic bacterial growth. In some cases P. syringae benefited significantly from the presence of aphids, with up to 3.8 times more cell doublings. This benefit was not correlated with strain traits but rather with initial population densities; smaller bacterial populations received relatively more benefit from aphids, and larger populations received less benefit. Honeydew, the sugary waste product of aphids, in the absence of aphids was sufficient to increase P. syringae density on leaves. We conclude that aphid honeydew can sometimes increase P. syringae epiphytic growth but that the bacteria may not benefit from using aphids as hosts.

BACKGROUND: Inoculation of bermudagrass with rhizobacterial biostimulants can increase plant growth and influence relationships with grass-feeding herbivores. Tunneling and root-feeding behaviors of tawny mole crickets cause severe damage to grass in pastures, golf courses, and lawns. The goal of this study was to determine if inoculation of bermudagrass by a rhizobacteria blend (plant growth-promoting rhizobacteria, PGPR) increases the tolerance of hybrid bermudagrass to tawny mole crickets in captivity and under field conditions.
RESULTS: Bermudagrass in arenas treated with a rhizobacteria blend then infested with tawny mole crickets produced significantly greater root lengths compared to bermudagrass that was either fertilized and infested with mole crickets, or bermudagrass without mole crickets. Bermudagrass treated with either the rhizobacteria blend or fertilizer produce similar top growth, and both treatments in the presence of mole crickets produced greater top growth than bermudagrass without mole crickets. Bermudagrass field plots infested naturally with mole crickets and treated twice with the rhizobacteria blend, or the rhizobacteria blend mixed with bifenthrin produced similar damage ratings as plots treated twice with bifenthrin. The rhizobacteria blend mixed with bifenthrin provided more consistent reductions in the surface activity of mole crickets relative to non-treated plots.
CONCLUSIONS: Enhanced growth of bermudagrass from fertilizer or rhizobacteria increased tolerance of bermudagrass to damage from tawny mole crickets. Application of PGPR or a PGPR-bifenthrin mixture to turfgrass damaged by mole crickets provided comparable reductions to a short residual, synthetic pyrethroid insecticide. Rhizobacterial products have potential utility for IPM of root herbivores. © 2019 Society of Chemical Industry.

Herbivore microbial associates can affect diverse interactions between plants and insect herbivores. Some insect symbionts enable herbivores to expand host plant range or to facilitate host plant use by modifying plant physiology. However, little attention has been paid to the role of herbivore-associated microbes in manipulating plant defenses. We have recently shown that Colorado potato beetle secrete the symbiotic bacteria to suppress plant defenses. The bacteria in oral secretions from the beetle hijack defense signaling pathways of host plants and the suppression of induced plant defenses benefits the beetle\'s performance. While the defense suppression by the beetle-associated bacteria has been investigated in local damaged leaves, little is known about the effects of the symbiotic bacteria on the manipulation of plant defenses in systemic undamaged leaves. Here, we demonstrate that the symbiotic bacteria suppress plant defenses in both local and systemic tissues when plants are attacked by antibiotic-untreated larvae.