Type B Response Regulators of Arabidopsis Play Key Roles in Cytokinin Signaling and Plant Development

Abstract

The type B Arabidopsis Response Regulators (ARRs) of Arabidopsis thaliana are transcription factors that act as positive regulators in the two-component cytokinin signaling pathway. We employed a mutant-based approach to perform a detailed characterization of the roles of ARR1, ARR10, and ARR12 in plant growth and development. The most pronounced phenotype was found in the arr1-3 arr10-5 arr12-1 triple loss-of-function mutant, which showed almost complete insensitivity to high levels of exogenously applied cytokinins. The triple mutant exhibited reduced stature due to decreased cell division in the shoot, enhanced seed size, increased sensitivity to light, altered chlorophyll and anthocyanin concentrations, and an aborted primary root with protoxylem but no metaxylem. Microarray analysis revealed that expression of the majority of cytokinin-regulated genes requires the function of ARR1, ARR10, and ARR12. Characterization of double mutants revealed differing contributions of the type B ARRs to mutant phenotypes. Our results support a model in which cytokinin regulates a wide array of downstream responses through the action of a multistep phosphorelay that culminates in transcriptional regulation by ARR1, ARR10, and ARR12.

The Cytokinin Type-B Response Regulator PtRR13 Is a Negative Regulator of Adventitious Root Development inPopulus

“…Similar phenotypes have been observed in cytokinin-deficient tobacco plants, where overexpression of the cytokinin-degrading enzyme cytokinin oxidase results in abundant adventitious roots (Werner et al, 2001(Werner et al, , 2003. The Arabidopsis triple mutant arr1 arr10 arr12 lacks the function of three type-B RRs, shows almost complete insensitivity to exogenously applied cytokinin, and spontaneously produces adventitious roots in hypocotyls (Argyros et al, 2008). While these genetic studies imply a potential role of cytokinin in adventitious rooting, they do not illuminate when or where these effects occur.…”

“…While phosphorylated type-B RRs act as transcriptional activators of cytokinin-regulated genes, the type-A RRs down-regulate cytokinin signaling by competing with the type-B RRs for phosphoryl groups (for review, see To and Kieber, 2008). Genetic analyses to define the function of RRs in Arabidopsis (Arabidopsis thaliana) suggest functional redundancy among family members, since phenotypes are not obvious unless mutant lines contain null alleles from several loci Argyros et al, 2008;Ishida et al, 2008). Because of their potential functions as transcriptional activators, type-B RRs are thought to represent key elements orchestrating transcriptome changes triggered by cytokinin.…”

Sugar-Hormone Cross-Talk in Anthocyanin Biosynthesis

“…2A and 2B). The type-B ARRs are redundant positive regulators of CK signaling and are the immediate upstream activators of type-A ARR gene expression (Argyros et al, 2008;Ishida et al, 2008). Of 11 known type-B ARRs in Arabidopsis, the subfamily 1 contains 7 ARRs that are associated with CK signaling (Mason et al, 2005).…”

The Cytokinin Receptors of Arabidopsis Are Located Mainly to the Endoplasmic Reticulum

“…The cytoplasmic part of all three receptors interact redundantly with His phosphotransfer proteins (AHP), which transmit the signal to the nucleus (Hwang and Sheen, 2001;Punwani et al, 2010), where type B response regulators, a class of Myb transcription factors, are activated. These mediate the majority, if not all, of the transcriptional responses to cytokinin (Argyros et al, 2008;Heyl et al, 2008;Ishida et al, 2008).…”