Involvement of plant stress hormones in Burkholderia phytofirmans-induced shoot and root growth promotion

Abstract

Plant growth promoting bacteria (PGPB) enhance plant growth by often influencing plant hormone homeostasis. Inoculation of potato nodal explants with a known PGPB B. phytofirmans (strain PsJN) significantly enhanced shoot and root growth under gnotobiotic conditions. There was a proportionally higher increase in root than shoot biomass. The increases in shoot and root growth were assessed for association with endogenous levels of plant stress hormones, salicylic acid (SA), abscisic acid (ABA) and jasmonic acid (JA) using stable isotope dilution technique coupled with liquid chromatography–mass spectrometry. Inoculation of potato plants with PsJN caused about a 1.5-fold increase in shoot endogenous SA levels. However, shoot ABA levels were not affected, whereas shoot JA levels were not detectable. For roots, the PsJN inoculation caused almost a fourfold increase in endogenous SA levels, whereas increases in ABA and JA levels were about 1.5-fold. To test if the massive increases in SA levels following PsJN inoculation were directly related to observed shoot (about 1.5-fold) and especially root (almost threefold) growth increases, a range of exogenous SA concentrations were applied to control and PsJN-inoculated plants. Applied SA similarly inhibited root growth of control and PsJN-inoculated plants, whereas a 10 µM concentration inhibited shoot growth of PsJN-inoculated, but not control plants. It is thus concluded that while PGPB can modify in planta biosynthesis of several plant stress hormones, it is unlikely that changes in endogenous levels of these hormones are directly related to the observed plant growth increases.