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Article|01 Mar 2020|OPEN
Evolution and roles of cytokinin genes in angiosperms 2: Do ancient CKXs play housekeeping roles while non-ancient CKXs play regulatory roles?
Xiaojing Wang1, Jing Ding1,, Shanshan Lin1, Decai Liu1, Tingting Gu1, Han Wu1, Robert N. Trigiano2, Richard McAvoy3, Jinling Huang4,5 & Yi Li1,3,
1State Key Laboratory of Crop Genetics and Germplasm Enhancement and College of Horticulture, Nanjing Agricultural University, Nanjing, P. R. China
2Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, TN 37996-4560, USA
3Department of Plant Science and Landscape Architecture, University of Connecticut, Storrs, CT 06269, USA
4State Key Laboratory of Crop Stress Adaptation and Improvement, Key Laboratory of Plant Stress Biology, School of Life Sciences, Henan University, Kaifeng, China
5Department of Biology, East Carolina University, Greenville, NC 27858, USA

Horticulture Research 7,
Article number: 29 (2020)
doi: 10.1038/hortres.2020.29
Views: 514

Received: 27 Sep 2019
Revised: 27 Dec 2019
Accepted: 04 Jan 2020
Published online: 01 Mar 2020

Abstract

Cytokinin oxidase/dehydrogenase (CKX) is a key enzyme responsible for the degradation of endogenous cytokinins. However, the origins and roles of CKX genes in angiosperm evolution remain unclear. Based on comprehensive bioinformatic and transgenic plant analyses, we demonstrate that the CKXs of land plants most likely originated from an ancient chlamydial endosymbiont during primary endosymbiosis. We refer to the CKXs retaining evolutionarily ancient characteristics as “ancient CKXs” and those that have expanded and functionally diverged in angiosperms as “non-ancient CKXs”. We show that the expression of some non-ancient CKXs is rapidly inducible within 15 min upon the dehydration of Arabidopsis, while the ancient CKX (AtCKX7) is not drought responsive. Tobacco plants overexpressing a non-ancient CKX display improved oxidative and drought tolerance and root growth. Previous mutant studies have shown that non-ancient CKXs regulate organ development, particularly that of flowers. Furthermore, ancient CKXs preferentially degrade cis-zeatin (cZ)-type cytokinins, while non-ancient CKXs preferentially target N6-(Δ2-isopentenyl) adenines (iPs) and trans-zeatins (tZs). Based on the results of this work, an accompanying study (Wang et al. https://doi.org/10.1038/s41438-019-0211-x) and previous studies, we hypothesize that non-ancient CKXs and their preferred substrates of iP/tZ-type cytokinins regulate angiosperm organ development and environmental stress responses, while ancient CKXs and their preferred substrates of cZs 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? Hortic Res 7, (2020). https://doi.org/10.1038/s41438-019-0211-x).