Burkholderia phytofirmans-induced shoot and root growth promotion is associated with endogenous changes in plant growth hormone levels

Plant growth promoting bacteria (PGPB) are capable of significantly altering the growth phenotype of inoculated plants. Changes in growth phenotype are often attributed to the ability of PGPB to assimilate minerals and/or increase mineral uptake, leading to increased plant root growth. However, many PGPB are also capable of either synthesizing plant hormones, such as auxins (mainly indole-3-acetic acid or IAA), gibberellins (GAs) and cytokinins (CKs) or affecting plant hormone biosynthesis (homeostasis) in planta. Burkholderia phytofirmans strain PsJN is a PGPB capable of inducing biomass growth of several plant species, including potatoes (Solanum tuberosum L.). In this paper we examined the effect of PsJN inoculation of two potato cultivars with similar root growth, but different shoot growth patterns (faster-growing Kennebec and slower-growing Yukon gold) to asses the bacteria's impact on growth and plant hormone homeostasis. Both cultivars showed similar and massive root growth increases after inoculation and this was associated with a twofold to threefold increase in IAA and CK (trans-zeatin or tZ) levels, expressed on a per plant basis. However, PsJN inoculation resulted in a different shoot growth response, which appeared to depend on the inherent growth characteristics of each cultivar. That is, the slower-growing Yukon gold plants matched the growth rate of faster-growing Kennebec plants 20 days after inoculation and this was associated with higher GA(1) levels and lower tZ levels. It is thus concluded that B. phytofirmans strain PsJN-induced plant phenotypic changes are associated with, and likely dependent on, changes in biosynthesis of plant growth hormones.