Abstract
The hormones auxin and cytokinin are essential for plant growth and development. Because of the central importance of root and shoot apical meristems in plant growth, auxin/cytokinin interactions have been predominantly analyzed in relation to apical meristem formation and function. In contrast, the auxin/cytokinin interactions during organ growth have remained largely unexplored. Here, we show that a specific interaction between auxin and cytokinin operates in both the root and the shoot where it serves as an additional determinant of plant development. We found that auxin at low concentrations limits the action of cytokinin. An increase in cytokinin level counteracts this inhibitory effect and leads to an inhibition of auxin signaling. At higher concentrations of both hormones, these antagonistic interactions between cytokinin and auxin are absent. Thus, our results reveal a bidirectional and asymmetrical interaction of auxin and cytokinin beyond the bounds of apical meristems. The relation is bidirectional in that both hormones exert inhibitory effects on each other's signaling mechanisms. However, this relation is also asymmetrical because under controlled growth conditions, auxin present in nontreated plants suppresses cytokinin signaling, whereas the reverse is not the case.
Highlights
The Arabidopsis cytokinin response pathway is a two-component signaling mechanism that starts with a family of Arabidopsis histidine kinases receptors that self-phosphorylate after binding cytokinin
The cytokinin response promoting type-B Arabidopsis Response Regulator proteins (ARRs) are transcription factors expressed in their latent forms and are activated by phosphorylation to induce the expression of primary cytokinin response genes (Argyros et al, 2008; Ishida, Yamashino, Yokoyama, & Mizuno, 2008)
As the cytokinin induction of ARR5p:GUS was enhanced in auxin-resistant mutants (Supporting Information Figure S3) and the auxin induction of DR5p:GUS was not altered in the cytokinin- resistant seedlings, we concluded that the cytokinin at levels present in nontreated plants does not impact auxin signaling and that the signaling relation between auxin and cytokinin is unidirectional
Summary
The Arabidopsis cytokinin response pathway is a two-component signaling mechanism that starts with a family of Arabidopsis histidine kinases receptors that self-phosphorylate after binding cytokinin. It is well documented that plant development is to a large extent controlled by interactions between auxin and cytokinin (Schaller, Bishopp, & Kieber, 2015) The existence of this hormonal crosstalk implies that, in addition to negative feedback mechanisms regulating the strength and duration of individual hormonal responses, there are regulatory mechanisms that calibrate the intensity of a hormone response based on spatial and temporal characteristics of the hormonal interaction. These interactions are of particular importance during the development of shoot and root apical meristems. Higher concentrations of both hormones convert antagonistic interactions into additive signaling
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