The phytohormones auxin and cytokinin interact to regulate many plant growth and developmental processes. Elements involved in the biosynthesis, inactivation, transport, perception, and signaling of these hormones have been elucidated, revealing the variety of mechanisms by which signal output from these pathways can be regulated. Recent studies shed light on how these hormones interact with each other to promote and maintain plant growth and development. In this review, we focus on the interaction of auxin and cytokinin in several developmental contexts, including its role in regulating apical meristems, the patterning of the root, the development of the gynoecium and female gametophyte, and organogenesis and phyllotaxy in the shoot.

Type-B Arabidopsis thaliana response regulators (ARRs) are transcription factors that function in the final step of two-component signaling systems. To characterize their role in plant growth and development, we isolated T-DNA insertions within six of the genes (ARR1, ARR2, ARR10, ARR11, ARR12, and ARR18) from the largest subfamily of type-B ARRs and also constructed various double and triple combinations of these mutations. Higher order mutants revealed progressively decreased sensitivity to cytokinin, including effects on root elongation, lateral root formation, callus induction and greening, and induction of cytokinin primary response genes. The triple mutant arr1,10,12 showed almost complete insensitivity to cytokinin under many of the assay conditions used. By contrast, no significant change in the sensitivity to ethylene was found among the mutants examined. These results indicate that there is functional overlap among the type-B ARRs and that they act as positive regulators of cytokinin signal transduction.

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.

• Background The phytohormone ethylene is a key regulator of plant growth and development. Components of the pathway for ethylene signal transduction were identified by genetic approaches in Arabidopsis and have now been shown to function in agronomically important plants as well. • Scope This review focuses on recent advances in our knowledge on ethylene signal transduction, in particular on recently proposed components of the pathway, on the interaction between the pathway components and on the roles of transcriptional and post-transcriptional regulation in ethylene signalling. • Conclusions Data indicate that the site of ethylene perception is at the endoplasmic reticulum and point to the importance of protein complexes in mediating the initial steps in ethylene signal transduction. The expression level of pathway components is regulated by both transcriptional and post-transcriptional mechanisms, degradation of the transcription factor EIN3 being a primary means by which the sensitivity of plants to ethylene is regulated. EIN3 also represents a control point for cross-talk with other signalling pathways, as exemplified by the effects of glucose upon its expression level. Amplification of the initial ethylene signal is likely to play a significant role in signal transduction and several mechanisms exist by which this may occur based on properties of known pathway components. Signal output from the pathway is mediated in part by carefully orchestrated changes in gene expression, the breadth of these changes now becoming clear through expression analysis using microarrays.