Evolution and roles of cytokinin genes in angiosperms 1: Do ancient play housekeeping while non-ancient play regulatory roles?

Isopentenyltransferase (IPT) genes, including those encoding s and s, control the rate-limiting steps of the biosynthesis of -(Δ-isopentenyl)adenine (iP)-type and -zeatin (Z)-type cytokinins and -zeatin (Z)-type cytokinins, respectively. However, the evolution and roles of these s in angiosperms are not well understood. Here, we report comprehensive analyses of the origins, evolution, expression patterns, and possible roles of and s in angiosperms. We found that Class I and II s likely coexisted in the last common ancestor of eukaryotes, while s likely originated from a Class II before the divergence of angiosperms. s are conservatively retained as 2-3 copies, but s exhibit considerable expansion and diversification. Additionally, s are constitutively expressed throughout the plant, whereas the expression of -s is tissue-specific and rapidly downregulated by abiotic stresses. Furthermore, previous studies and our present study indicate that -s and their products, iPs/Zs, may regulate responses to environmental stresses and organ development in angiosperms. We therefore hypothesize that s and the associated Zs play a housekeeping role, whereas s and the associated iP/Z-type cytokinins play regulatory roles in organ development and stress responses in angiosperms, which echoes the conclusions and hypothesis presented in the accompanying study by Wang, X. et al Evolution and roles of cytokinin genes in angiosperms 2: Do ancient CKXs play housekeeping roles while non-ancient CKXs play regulatory roles? 10.1038/s41438-020-0246-z.