Plant growth promotion by Bacillus megaterium involves cytokinin signaling

Accumulating evidence indicates that plant growth promoting rhizobacteria (PGPR) influence plant growth and development by the production of phytohormones such as auxins, gibberellins, and cytokinins. Little is known on the genetic basis and signal transduction components that mediate the beneficial effects of PGPRs in plants. We recently reported the identification of a Bacillus megaterium strain that promoted growth of A. thaliana and P. vulgaris seedlings. In this addendum, the role of cytokinin signaling in mediating the plant responses to bacterial inoculation was investigated using A. thaliana mutants lacking one, two or three of the putative cytokinin receptors CRE1, AHK2 and AHK3, and RPN12 a gene involved in cytokinin signaling. We show that plant growth promotion by B. megaterium is reduced in AHK2-2 single and double mutant combinations and in RPN12. Furthermore, the triple cytokinin-receptor CRE1-12/AHK2-2/AHK3-3 knockout was insensitive to inoculation in terms of growth promotion and root developmental responses. Our results indicate that cytokinin receptors play a complimentary role in plant growth promotion by B. megaterium.