CYTOKININ OXIDASE/DEHYDROGENASE4 Integrates Cytokinin and Auxin Signaling to Control Rice Crown Root Formation.

Shaopei Gao,Yuqi Feng,Baodong Cai,Wei Wang,Chengcai Chu,Fan Xu,Xiaohong Sun,Jinfang Chu,Jun Fang

doi:10.1104/pp.114.238584

Abstract

Crown roots constitute the majority of the rice (Oryza sativa) root system and play an important role in rice growth and development. However, the molecular mechanism of crown root formation in rice is not well understood. Here, we characterized a rice dominant mutant, root enhancer1 (ren1-D), which was observed to exhibit a more robust root system, increased crown root number, and reduced plant height. Molecular and genetic analyses revealed that these phenotypes are caused by the activation of a cytokinin oxidase/dehydrogenase (CKX) family gene, OsCKX4. Subcellular localization demonstrated that OsCKX4 is a cytosolic isoform of CKX. OsCKX4 is predominantly expressed in leaf blades and roots. It is the dominant CKX, preferentially expressed in the shoot base where crown root primordia are produced, underlining its role in root initiation. OsCKX4 is induced by exogenous auxin and cytokinin in the roots. Furthermore, one-hybrid assays revealed that OsCKX4 is a direct binding target of both the auxin response factor OsARF25 and the cytokinin response regulators OsRR2 and OsRR3. Overexpression and RNA interference of OsCKX4 confirmed that OsCKX4 plays a positive role in crown root formation. Moreover, expression analysis revealed a significant alteration in the expression of auxin-related genes in the ren1-D mutants, indicating that the OsCKX4 mediates crown root development by integrating the interaction between cytokinin and auxin. Transgenic plants harboring OsCKX4 under the control of the root-specific promoter RCc3 displayed enhanced root development without affecting their shoot parts, suggesting that this strategy could be a powerful tool in rice root engineering.

Highlights

Crown roots constitute the majority of the rice (Oryza sativa) root system and play an important role in rice growth and development
The rice WUSCHEL-related homeobox gene WOX11 is expressed in the early crown root primordia, and its expression is maintained in the cell division zone of the root meristem and induced by both auxin and cytokinin (Zhao et al, 2009)
Examination of a number of cross sections of the coleoptilar node of the wild type and ren1-D mutants revealed that the crown root primordia were present in ren1-D mutants at 2 d after germination (DAG) but not in the wild type (Fig. 1B)

Summary

Introduction

Crown roots constitute the majority of the rice (Oryza sativa) root system and play an important role in rice growth and development. Expression analysis revealed a significant alteration in the expression of auxin-related genes in the ren1-D mutants, indicating that the OsCKX4 mediates crown root development by integrating the interaction between cytokinin and auxin. The connection between auxin signaling and cytokinin-responsive type A response regulators has been documented in rice roots, where auxin induces OsRR1 via the APETALA2/ETHYLENE RESPONSIVE FACTOR transcription factor CROWN ROOTLESS5 (CRL5) to allow the initiation of crown root primordia in rice seedlings (Kitomi et al, 2011). The rice WUSCHEL-related homeobox gene WOX11 is expressed in the early crown root primordia, and its expression is maintained in the cell division zone of the root meristem and induced by both auxin and cytokinin (Zhao et al, 2009). Plants overexpressing WOX11 show overproduction of crown roots (Zhao et al, 2009)

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