The evolution of cytokinin signaling and its role in development before Angiosperms

Cytokinin signaling by way of a multistep two-component signaling pathway has been highly examined over the last 20 years with a lot of attention to how its signaling components regulate essential growth and developmental processes from seedlings to senescence. Most studies have focused on Angiosperm plants, particularly Arabidopsis, with very little attention across the rest of the plant kingdom, making it difficult to know if cytokinin-regulated developmental processes are have remained evolutionarily conserved or distinct among different plant groups. Here the prevalence of cytokinin signaling components throughout plants, algae, and other organisms are examined. Particular examination is paid to how minimal sets of these base signaling components are related to development in Bryophytes, a primary group of study outside of Angiosperms. General comparisons of the role of signaling components between these distant groups suggest some evolutionary conservation of function has been maintained across plants as a whole. This review also highlights the need to investigate how cytokinin signaling components, recently identified in every plant group, contribute to development; this will give higher resolution to our understanding of cytokinin roles in development across plant taxa. Also revealed is a strong need to further pursue how cytokinin signaling components recently identified in every plant group contribute to development in order to further dissect these connections.