Auxin and cytokinin antagonistically regulate Arabidopsis root distal stem cell fate through opposing pairs of HD-Zip II transcription factors

The antagonistic action of auxin (IAA) and cytokinin (CK) regulates several aspects of plant development including stem cell specification and maintenance. In the Arabidopsis thaliana root, the fate of distal stem cells (DSCs) is mainly regulated by IAA, while the function of CK is still ill-defined. Here, by investigating the role of the HD-Zip II transcription factor family in distal tissues, we uncover CK as a proliferative input for DSCs, acting in opposition to IAA. We show that opposing pairs of HD-Zip II proteins regulate DSCs fate by integrating IAA and CK signals. The δ module, composed of HAT3 and ATHB4, is activated by CK and promotes DSCs division. Conversely, the γ module, constituted by HAT1 and ATHB2, acts downstream of IAA to repress CK biosynthesis and DSCs division. The mutual inhibition between δ and γ modules downstream of opposite hormone inputs sustains the precise regulation of DSCs homeostasis.