Jones B, Gunnerås SA, Petersson SV, Tarkowski P, Graham N, May S, Dolezal K, Sandberg G, Ljung K.
Cytokinin regulation of auxin synthesis in Arabidopsis involves a homeostatic feedback loop regulated via auxin and cytokinin signal transduction
The Plant Cell: 2010 22:2956-2969

Abstract
Together, auxin and cytokinin regulate many of the processesthat are critical to plant growth, development, and environmentalresponsiveness. We have previously shown that exogenous auxinregulates cytokinin biosynthesis in Arabidopsis thaliana. Inthis work, we show that, conversely, the application or inducedectopic biosynthesis of cytokinin leads to a rapid increasein auxin biosynthesis in young, developing root and shoot tissues.We also show that reducing endogenous cytokinin levels, eitherthrough the induction of CYTOKININ OXIDASE expression or themutation of one or more of the cytokinin biosynthetic ISOPENTENYLTRANSFERASEgenes leads to a reduction in auxin biosynthesis. Cytokininmodifies the abundance of transcripts for several putative auxinbiosynthetic genes, suggesting a direct induction of auxin biosynthesisby cytokinin. Our data indicate that cytokinin is essential,not only to maintain basal levels of auxin biosynthesis in developingroot and shoot tissues but also for the dynamic regulation ofauxin biosynthesis in response to changing developmental orenvironmental conditions. In combination with our previous work,the data suggest that a homeostatic feedback regulatory loopinvolving both auxin and cytokinin signaling acts to maintainappropriate auxin and cytokinin concentrations in developingroot and shoot tissues.

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