This study demonstrates that the application of the PGPB strain, Kosakonia radicincitans enhances a plant's resistance against phloem-feeding and chewing insects in Arabidopsis thaliana. The plant growth-promoting bacterial strain K. radicincitans DSM 16656 applied to A. thaliana reduced the number of phloem-feeding insects of both the specialist Brevicoryne brassicae and the generalist Myzus persicae. While weight gain of the generalist chewing insect Spodoptera exigua was reduced by 30% on A. thaliana plants treated with K. radicincitans, growth of the specialist caterpillar Pieris brassicae was not affected when compared with caterpillars from control plants. Since generalist and specialist chewing insects responded differentially to PGPB application, the implication of signaling pathways in PGPB mediated changes in plant defense was studied using two signaling pathway mutants impaired in their salicylic acid (npr1-1 mutant) or jasmonic acid (coi1-1 mutant) pathway. We found that the jasmonic acid pathway is relevant for upregulation of aliphatic glucosinolates and suppression of the chewing generalist S. exigua larval growth. Chewing from generalist P. brassicae increased glucosinolate content in A. thaliana leaves mediated via both signaling pathways. However, only in the npr1-1 mutant, which contains the highest aliphatic glucosinolate content, the P. brassicae induced further enrichment of glucosinolates, resulting in a reduction of larval growth. Effects of K. radicincitans on plant resistance could not be explained by changes in glucosinolate contents or composition. Our results demonstrate the distinct role played by K. radicincitans in suppressing insect performance in A. thaliana.
Keywords: Defense signaling pathways; Glucosinolates; Kosakonia spp.; Pieris brassicae; Spodoptera exigua; npr1-1 and coi1-1 mutants.