Polar transport of the plant hormone auxin controls many aspects of plant growth and development. A number of synthetic compounds have been shown to block the process of auxin transport by inhibition of the auxin efflux carrier complex. These synthetic auxin transport inhibitors may act by mimicking endogenous molecules. Flavonoids, a class of secondary plant metabolic compounds, have been suggested to be auxin transport inhibitors based on their in vitro activity. The hypothesis that flavonoids regulate auxin transport in vivo was tested in Arabidopsis by comparing wild-type (WT) and transparent testa (tt4) plants with a mutation in the gene encoding the first enzyme in flavonoid biosynthesis, chalcone synthase. In a comparison between tt4 and WT plants, phenotypic differences were observed, including three times as many secondary inflorescence stems, reduced plant height, decreased stem diameter, and increased secondary root development. Growth of WT Arabidopsis plants on naringenin, a biosynthetic precursor to those flavonoids with auxin transport inhibitor activity in vitro, leads to a reduction in root growth and gravitropism, similar to the effects of synthetic auxin transport inhibitors. Analyses of auxin transport in the inflorescence and hypocotyl of independent tt4 alleles indicate that auxin transport is elevated in plants with a tt4 mutation. In hypocotyls of tt4, this elevated transport is reversed when flavonoids are synthesized by growth of plants on the flavonoid precursor, naringenin. These results are consistent with a role for flavonoids as endogenous regulators of auxin transport.

The plant hormone cytokinin regulates numerous growth and developmental processes. A signal transduction pathway for cytokinin has been elucidated that is similar to bacterial twocomponent phosphorelays. In Arabidopsis, this pathway is comprised of receptors that are similar to sensor histidine kinases, histidine-containing phosphotransfer proteins, and response regulators (ARRs). There are two classes of response regulators, the type-A ARRs, which act as negative regulators of cytokinin responses, and the type-B ARRs, which are transcription factors that play a positive role in mediating cytokinin-regulated gene expression. Here we show that several closely related members of the Arabidopsis AP2 gene family of unknown function are transcriptionally up-regulated by cytokinin through this pathway, and we have designated these AP2 genes CYTOKININ RESPONSE FACTORS (CRFs). In addition to their transcriptional regulation by cytokinin, the CRF proteins rapidly accumulate in the nucleus in response to cytokinin, and this relocalization depends on the histidine kinases and the downstream histidine-containing phosphotransfer proteins, but is independent of the ARRs. Analysis of loss-of-function mutations reveals that the CRFs function redundantly to regulate the development of embryos, cotyledons, and leaves. Furthermore, the CRFs mediate a large fraction of the transcriptional response to cytokinin, affecting a set of cytokinin-responsive genes that largely overlaps with type-B ARR targets. These results indicate that the CRF proteins function in tandem with the type-B ARRs to mediate the initial cytokinin response. Thus, the evolutionarily ancient two-component system that is used by cytokinin branches to incorporate a unique family of plant-specific transcription factors.cell signaling ͉ plant hormones

Auxin transport has been reported to occur in two distinct polarities, acropetally and basipetally, in two different root tissues. The goals of this study were to determine whether both polarities of indole-3-acetic acid (IAA) transport occur in roots of Arabidopsis and to determine which polarity controls the gravity response. Global application of the auxin transport inhibitor naphthylphthalamic acid (NPA) to roots blocked the gravity response, root waving, and root elongation. Immediately after the application of NPA, the root gravity response was completely blocked, as measured by an automated video digitizer. Basipetal

The phytohormone cytokinin is an important regulator of plant growth and development; however, relatively few genes that mediate cytokinin responses have been identified. Genome-wide analyses of Arabidopsis seedlings using the approximately 8,300-element Affymetrix Arabidopsis GeneChips (Affymetrix, Santa Clara, CA) to examine cytokinin-responsive genes were conducted, revealing at least 30 genes whose steady-state level of mRNA was elevated and at least 40 that were down-regulated at multiple time points after application of cytokinin. The cytokinin up-regulated genes include the type-A Arabidopsis response regulators (ARRs), which had been shown previously to be cytokinin primary response genes, cytokinin oxidase, which encodes an enzyme that degrades cytokinins, and several transcription factors. Cytokinin down-regulated genes include several peroxidases and kinases and an E3 ubiquitin ligase. We identified a common sequence motif enriched in the upstream regions of the most consistently cytokinin up-regulated genes. This motif is highly similar to the optimal DNA-binding sites for ARR1/ARR2, type-B ARRs that have been implicated in the transcriptional elevation of the type-A ARRs. Additionally, genome-wide analyses of cytokinin receptor mutants (wol/cre1) revealed large-scale changes in gene expression, including down-regulation of the type-A ARRs and several meristem and cell cycle genes, such as CycD3. Mutations in CRE1 reduced but did not eliminate the effect of cytokinin on gene expression for a subset of cytokinin-responsive genes and had little or no effect on others, suggesting functional redundancy among the cytokinin receptors.Cytokinins are a group of adenine derivatives that affect multiple aspects of plant growth and development, including cell division, vascular development, sink/source relationships, apical dominance, and leaf senescence (Binns, 1994; Mok, 1994, 2001). A pathway for cytokinin biosynthesis and metabolism is emerging from molecular and biochemical studies. This has been highlighted by the recent cloning of several genes encoding enzymes involved in cytokinin biosynthesis or metabolism, including ipt (Kakimoto, 2001; Takei et al., 2001), which catalyzes the first committed step in cytokinin biosynthesis; cytokinin oxidase (Houba-Hérin et al., 1999;Morris et al., 1999), which cleaves the N 6 side chain from cytokinins; and several enzymes that catalyze the conjugation of sugar moieties to cytokinins (Martin et al., 1999a(Martin et al., , 1999b(Martin et al., , 2001. A model for cytokinin perception and signal transduction has emerged that is similar to prokaryotic two-component response pathways (Haberer and Kieber, 2001; Hutchison and Kieber, 2002;Lohrmann and Harter, 2002). A family of genes that are similar to bacterial two-component response regulators, the type-A Arabidopsis response regulators (ARRs), was identified as cytokinin primary response genes (Brandstatter and Kieber, 1998;Sakakibara et al., 1998;Taniguchi et al., 1998; D'Agostino et al., 2000). The cytokinin re...

Plants have evolved elaborate mechanisms for sensing and responding to sub-optimal environmental conditions. Abiotic stresses caused by these conditions trigger a wide range of local and long-distance signals which must be co-ordinated and integrated into whole-plant processes, such as development, in order for the plant to respond properly and survive. Several hormones function as key regulators of stress tolerance, connecting local stimuli to systemic responses. Cytokinin is a hormone well known for its role in numerous aspects of growth and development, although abundant evidence also indicates that cytokinin functions in stress responses as well. At present, a full understanding of the effects of cytokinin on plant resistance to stress is lacking, possibly as a result of the complex interactions between cytokinin and stress signalling. Current knowledge of the physiological relationship between cytokinin and abiotic stress, based on measurements of cytokinin levels under stress conditions and the effects of cytokinin treatment on stress tolerance, has been examined here. A pattern of transcriptional regulation of stress-related genes by cytokinin in different plant species has also been identified. In addition, research regarding the role of specific cytokinin signalling components in a variety of stress responses is presented. We discuss what this body of research collectively implies with regard to cross-talk between cytokinin and abiotic stress tolerance.