Strigolactone promotes cytokinin degradation through transcriptional activation of CYTOKININ OXIDASE/DEHYDROGENASE 9 in rice

Jingbo Duan, Hong Yu, Kun Yuan, Zhigang Liao, Xiangbing Meng, Yanhui Jing, Guifu Liu, Jinfang Chu, and Jiayang Li

PNAS July 9, 2019 116 (28) 14319-14324

Significance

Strigolactone plays a vital role in plant growth and development, but its response genes remain to be identified. In this study, we found that cytokinin content is markedly increased in the strigolactone signaling mutant d53, and that OsCKX9, which encodes a cytokinin oxidase to catalyze the degradation of cytokinin, functions as a primary strigolactone-responsive gene to regulate rice tillering, plant height, and panicle size, likely via a secondary response gene, OsRR5, which encodes a cytokinin-inducible rice type-A response regulator, demonstrating that strigolactone regulates rice shoot architecture through enhanced cytokinin catabolism by modulating OsCKX9 expression.

Abstract

Strigolactones (SLs), a group of terpenoid lactones derived from carotenoids, are plant hormones that control numerous aspects of plant development. Although the framework of SL signaling that the repressor DWARF 53 (D53) could be SL-dependently degraded via the SL receptor D14 and F-box protein D3 has been established, the downstream response genes to SLs remain to be elucidated. Here we show that the cytokinin (CK) content is dramatically increased in shoot bases of the rice SL signaling mutant d53. By examining transcript levels of all the CK metabolism-related genes after treatment with SL analog GR24, we identified CYTOKININ OXIDASE/DEHYDROGENASE 9 (OsCKX9) as a primary response gene significantly up-regulated within 1 h of treatment in the wild type but not in d53. We also found that OsCKX9 functions as a cytosolic and nuclear dual-localized CK catabolic enzyme, and that the overexpression of OsCKX9 suppresses the browning of d53calli. Both the CRISPR/Cas9-generated OsCKX9 mutants and OsCKX9-overexpressing transgenic plants showed significant increases in tiller number and decreases in plant height and panicle size, suggesting that the homeostasis of OsCKX9 plays a critical role in regulating rice shoot architecture. Moreover, we identified the CK-inducible rice type-A response regulator OsRR5 as the secondary SL-responsive gene, whose expression is significantly repressed after 4 h of GR24 treatment in the wild type but not in osckx9. These findings reveal a comprehensive plant hormone cross-talk in which SL can induce the expression of OsCKX9 to down-regulate CK content, which in turn triggers the response of downstream genes.

See https://www.pnas.org/content/116/28/14319

Figure 1: Expression levels of CK metabolism genes on treatment with rac-GR24, 6-BA, tZ, or 2iP. (A) Fold change in CK metabolism gene expression levels after 1 h treatment with 5 μM rac-GR24 compared with mock. **P < 0.01, Student’s t test. (B) Expressions of eight OsCKXs on treatment with 5 μM 6-BA. Results are presented relative to mock. Values are mean ± SD, n = 3. **P < 0.01 compared with mock, Student’s t test; ns, no significant difference. (C) Expression levels of eight OsCKXs on treatment with 5 μM tZ or 2iP. Results are presented relative to mock. Values are mean ± SD, n = 3. **P < 0.01 compared with mock, Student’s t test; ns, no significant difference.