Cytokinin signaling: two-components and more

Cytokinins modulate auxin-induced organogenesis in plants via regulation of the auxin efflux

Postembryonic de novo organogenesis represents an important competence evolved in plants that allows their physiological and developmental adaptation to changing environmental conditions. The phytohormones auxin and cytokinin (CK) are important regulators of the developmental fate of pluripotent plant cells. However, the molecular nature of their interaction(s) in control of plant organogenesis is largely unknown. Here, we show that CK modulates auxin-induced organogenesis (AIO) via regulation of the efflux-dependent intercellular auxin distribution. We used the hypocotyl explants-based in vitro system to study the mechanism underlying de novo organogenesis. We show that auxin, but not CK, is capable of triggering organogenesis in hypocotyl explants. The AIO is accompanied by endogenous CK production and tissue-specific activation of CK signaling. CK affects differential auxin distribution, and the CK-mediated modulation of organogenesis is simulated by inhibition of polar auxin transport. CK reduces auxin efflux from cultured tobacco cells and regulates expression of auxin efflux carriers from the PIN family in hypocotyl explants. Moreover, endogenous CK levels influence PIN transcription and are necessary to maintain intercellular auxin distribution in planta. Based on these findings, we propose a model in which auxin acts as a trigger of the organogenic processes, whose output is modulated by the endogenously produced CKs. We propose that an important mechanism of this CK action is its effect on auxin distribution via regulation of expression of auxin efflux carriers.

Regulation of ACS protein stability by cytokinin and brassinosteroid

A major question in plant biology is how phytohormone pathways interact. Here, we explore the mechanism by which cytokinins and brassinosteroids affect ethylene biosynthesis. Ethylene biosynthesis is regulated in response to a wide variety of endogenous and exogenous signals, including the levels of other phytohormones. Cytokinins act by increasing the stability of a subset of ACC synthases, which catalyze the generally rate-limiting step in ethylene biosynthesis. The induction of ethylene by cytokinin requires the canonical cytokinin two-component response pathway, including histidine kinases, histidine phosphotransfer proteins and response regulators. The cytokinin-induced myc-ACS5 stabilization occurs rapidly (<60 min), consistent with a primary output of this two-component signaling pathway. We examined the mechanism by which another phytohormone, brassinosteroid, elevates ethylene biosynthesis in etiolated seedlings. Similar to cytokinin, brassinosteroid acts post-transcriptionally by increasing the stability of ACS5 protein, and its effects on ACS5 were additive with those of cytokinin. These data suggest that ACS is regulated by phytohormones through regulatory inputs that probably act together to continuously adjust ethylene biosynthesis in various tissues and in response to various environmental conditions.

Cytokinin signaling during root development

The cytokinin class of phytohormones regulates division and differentiation of plant cells. They are perceived and signaled by a phosphorelay mechanism similar to those observed in prokaryotes. Research into the components of phosphorelay had previously been marred by genetic redundancy. However, recent studies have addressed this with the creation of high-order mutants. In addition, several new elements regulating cytokinin signaling have been identified. This has uncovered many roles in diverse developmental and physiological processes. In this review, we look at these processes specifically in the context of root development. We focus on the formation and maintenance of the root apical meristem, primary and secondary vascular development, lateral root emergence and development, and root nodulation. We believe that the root is an ideal organ with which to investigate cytokinin signaling in a wider context.

Comprehensive transcriptome analysis of phytohormone biosynthesis and signaling genes in microspore/pollen and tapetum of rice

To investigate the involvement of phytohormones during rice microspore/pollen (MS/POL) development, endogenous levels of IAA, gibberellins (GAs), cytokinins (CKs) and abscisic acid (ABA) in the mature anther were analyzed. We also analyzed the global expression profiles of genes related to seven phytohormones, namely auxin, GAs, CKs, brassinosteroids, ethylene, ABA and jasmonic acids, in MS/POL and tapetum (TAP) using a 44K microarray combined with a laser microdissection technique (LM-array analysis). IAA and GA(4) accumulated in a much higher amount in the mature anther compared with the other tissues, while CKs and ABA did not. LM-array analysis revealed that sets of genes required for IAA and GA synthesis were coordinately expressed during the later stages of MS/POL development, suggesting that these genes are responsible for the massive accumulation of IAA and GA(4) in the mature anther. In contrast, genes for GA signaling were preferentially expressed during the early developmental stages of MS/POL and throughout TAP development, while their expression was down-regulated at the later stages of MS/POL development. In the case of auxin signaling genes, such mirror-imaged expression observed in GA synthesis and signaling genes was not observed. IAA receptor genes were mostly expressed during the late stages of MS/POL development, and various sets of AUX/IAA and ARF genes were expressed during the different stages of MS/POL or TAP development. Such cell type-specific expression profiles of phytohormone biosynthesis and signaling genes demonstrate the validity and importance of analyzing the expression of phytohormone-related genes in individual cell types independently of other cells/tissues.

Multiple roles for cytokinin receptors and cross-talk of signaling pathways

Cytokinin receptors (CRs) are hybrid-type histidine kinases, membrane proteins with a cytokinin-binding extracellular domain. CRs initiate and propagate cytokinin signaling by means of phosphorylation and phosphotransfer to downstream proteins. In legumes, some members of the CR multigenic family are essential for nodulation. In two recent works, we investigated the involvement of two new CRs, MsHK1 from Medicago sativa, and LaHK1 from Lupinus albus, in nodule morphogenesis, senescence and stress response. LaHK1 expression increased during the first stages of lupin nodule development, while MsHK1 expression was localized in the meristem and the invasion zone of alfalfa nodules pointing to a role for CRs in nodule cell proliferation and differentiation. Both CRs were also induced during nodule senescence. MsHK1 expression increased under osmotic stress and both genes were induced following dark stress, indicating that CRs are also likely to play a significant role in the response to stress. We propose multiple roles for CRs which, when analyzed jointly with recent results from other authors, suggest coordinated cross-talk of different signaling systems.

Plant hormones and signaling: common themes and new developments

About 200 plant biologists convened in Keystone, Colorado, for the "Plant Hormones and Signaling" symposium, which was organized by Joanne Chory, Joe Ecker, and Mark Estelle. The meeting was run concurrently with the "Plant Innate Immunity" symposium organized by Jonathan Jones and Jane Glazebrook. In this report, we summarize the progress in plant hormones and signaling.