Cytokinin: secret agent of symbiosis.

The symbiotic interaction between Rhizobium bacteria and legumes leads to the induction of a new root organ: the nitrogen-fixing nodule. Recent findings have uncovered that cytokinin is instrumental in this developmental process, but they also suggest a broader role for cytokinin in mediating rhizobial infection. In this opinion article, we propose that cytokinin is the key differentiation signal for nodule organogenesis. Furthermore, we discuss a model in which cytokinin might also influence bacterial infection by controlling the expression of NIN (Nodule Inception) and other transcriptional regulators through mechanisms operating both locally and systemically.