Evolution and roles of cytokinin genes in angiosperms 2: Do ancient s play housekeeping roles while non-ancient s play regulatory roles?

Cytokinin oxidase/dehydrogenase (CKX) is a key enzyme responsible for the degradation of endogenous cytokinins. However, the origins and roles of genes in angiosperm evolution remain unclear. Based on comprehensive bioinformatic and transgenic plant analyses, we demonstrate that the s of land plants most likely originated from an ancient chlamydial endosymbiont during primary endosymbiosis. We refer to the s retaining evolutionarily ancient characteristics as "ancient s" and those that have expanded and functionally diverged in angiosperms as "non-ancient s". We show that the expression of some non-ancient s is rapidly inducible within 15 min upon the dehydration of , while the ancient () is not drought responsive. Tobacco plants overexpressing a non-ancient display improved oxidative and drought tolerance and root growth. Previous mutant studies have shown that non-ancient s regulate organ development, particularly that of flowers. Furthermore, ancient CKXs preferentially degrade -zeatin (Z)-type cytokinins, while non-ancient CKXs preferentially target -(Δ-isopentenyl) adenines (iPs) and -zeatins (Zs). Based on the results of this work, an accompanying study (Wang et al. 10.1038/s41438-019-0211-x) and previous studies, we hypothesize that non-ancient CKXs and their preferred substrates of iP/Z-type cytokinins regulate angiosperm organ development and environmental stress responses, while ancient CKXs and their preferred substrates of Zs play a housekeeping role, which echoes the conclusions and hypothesis described in the accompanying report (Wang, X. et al. Evolution and roles of cytokinin genes in angiosperms 1: Doancient IPTs play housekeeping while non-ancient IPTs play regulatory roles? , (2020). 10.1038/s41438-019-0211-x).