Pairwise inoculation of plant growth-promoting rhizobacteria unveils predominant strain-specific effects in root growth and hormone-responsive promoter activity in Arabidopsis thaliana.

Plant growth-promoting rhizobacteria are promising tools for agricultural management. We previously identified Achromobacter sp. 5B1, Bacillus sp., Micrococcus luteus, and Pseudomonas brassicae strains that influence the growth of Arabidopsis seedlings in highly contrasting manners. Here, pairwise combinations of these four bacterial isolates were tested to identify single or additive traits influenced by cocultivation with Arabidopsis seedlings in vitro. Root architectural modifications, such as primary root growth and lateral root formation, as well as root and shoot biomass production were correlated with expression of gene promoters involved in phytohormone responses using auxin, cytokinin and jasmonic acid-inducible DR5:GUS, ARR5:GUS, and JAZ1:GUS constructs, respectively. Noteworthy, the presence of P. brassicae in all consortia led to the exhaustion of the root meristem that coincided with high expression of jasmonic acid-inducible JAZ1:GUS in primary root tips, whereas Achromobacter sp. 5B1 induced cytokinin-signaling in roots. These effects were prevalent over other bacteria when grown together. Additive effects in expression of JAZ1:GUS were observed in primary and lateral roots of P. brassicae - Achromobacter sp. 5B1 consortium indicating elicitation of the jasmonic acid response. Transfer of plants to soil with and without pairwise inoculum showed that early interaction with Bacillus sp. - Achromobacter sp. 5B1 promoted the transition to flowering and increased fruit and seed production. These results advance our understanding of how mixed consortia of plant growth-promoting rhizobacteria affect plant growth and reveal dominant effects of particular strains in mixed populations.