AHP4 attenuates cytokinin signalling in the phloem companion cells to control root growth under osmotic stress

Cytokinins regulate diverse aspects of plant development. They are perceived by membrane-localised receptors that transmit a signal to nuclear-localised response regulators via the Histidine Phosphotransfer proteins (HPts). These HPts can be divided into two classes: authentic HPts (AHPs), which positively regulate cytokinin signalling, and pseudo HPts (PHPs), which inhibit it. Whilst four of the five Arabidopsis AHPs form a well-conserved monophyletic clade, AHP4 evolved separately and is more closely related to the monocot PHPs than to the dicot AHPs. AHP4's role in cytokinin signalling has been ambiguous; in some cases, it has been shown to act as a positive regulator, whilst in others, it acts negatively. Here, we propose that AHP4 act like a dimmer switch to dampen cytokinin output. Under optimal growth conditions, AHP4 is expressed at a low level in the phloem companion cells and has little effect on either cytokinin signalling or plant development. However, AHP4 expression is induced under osmotic stress, and under this condition, we show a novel role for AHP4 in reducing cytokinin signalling in the phloem and maintaining primary root growth. We propose that AHP4 fine-tunes responses to the osmotic environment by attenuating cytokinin signalling.