Nodulation in legumes is part of a rhizobacterial symbiosis allowing nitrogen fixation within the roots of the plant. Nodule development is induced by rhizobial signalling that interacts with phytohormonal regulation in the plant. In the roots, nodule organogenesis and the progression of infection are regulated by cytokinin and auxin, but the mechanism of this regulation is poorly understood.
In a recent article published in Annals of Botany, lead author Elena A. Dolgikh and colleagues attempted to uncover the functions of cytokinin and auxin signalling during late symbiosis development. The group studied mutants defective in the production of certain transcription factors involved in the late stages of nodule development, analysing alterations in the distribution of the two hormones.
The researchers found that when genes involved in late-stage nodule development were impaired, the distribution of cytokinin looked very different. In wild-type nodules, cytokinin was present through the meristem, infection zone, and within the apical region of the nitrogen fixation zone, while auxin occurred in the meristem and peripheral tissues. In a mutant (sym33) impaired in bacterial accommodation and nodule differentiation, cytokinin was largely confined to the meristem. In a second mutant (sym40) defective in a key transcription factor, EFD, required for functional nodule formation, cytokinins were detected in the infection zone only.
Overall, these findings suggest that cytokinins may have a role in controlling both bacterial accommodation and tissue differentiation related to nodule formation. “Importantly, cytokinins are involved in regulating the root system architecture,” the authors write. “In the parental root meristem, cytokinin signalling ensures the balance between cell proliferation and differentiation. Cytokinins control the cell differentiation rate by negatively regulating the positive effect of auxin on cell proliferation.” This hints at a common mechanism behind the developmental regulation of both roots and nodules. The authors note, however, that “the exact role of cytokinins during the late stages of nodulation associated with plant cell and bacteroid differentiation remains to be elucidated.”