Expression Profiling of Cytokinin Action in Arabidopsis  

Abstract

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...

Transcription Analysis of Arabidopsis Membrane Transporters and Hormone Pathways during Developmental and Induced Leaf Senescence

“…Of two phosphorelay mediators and 11 B-type ARRs on the array, only AtARR1 seems to be weakly (2.6-to 3.4-fold) up-regulated. Several A-type ARRs are rapidly induced by CK (Rashotte et al, 2003;Brenner et al, 2005;Kiba et al, 2005). Consistent with a depletion of CK during senescence progression, four of those are down-regulated in NS, DIS, and DET (Fig.…”

“…Down-regulation of these genes also was observed by Buchanan-Wollaston et al (2005). A remarkable exception is AtARR16, which is strongly up-regulated upon CK treatment (D'Agostino et al, 2000;Rashotte et al, 2003;Kiba et al, 2005). This gene is also up-regulated during NS and DIS (but not DET; Fig.…”

GeBP and GeBP-Like Proteins Are Noncanonical Leucine-Zipper Transcription Factors That Regulate Cytokinin Response in Arabidopsis

“…A, In vitro growth of wild types and single, double, and triple mutants on MS in the absence (2K) or presence (1K) of kinetin (10 mg mL 21 ). The triple mutant was distinguishable from wild type on lower concentrations (2-5 mg mL 21 ) of kinetin and was observed on trans-zeatin (1 mg mL 21 ) as previously described (Higuchi et al, 2004;To et al, 2004;Hutchison et al, 2006 Kiba et al, 2002;Rashotte et al, 2003). Analyses of type-A overexpressing or high-order loss-of-function mutants indicate that type-A ARRs are redundant negative regulators of cytokinin signaling (Taniguchi et al, 1998;D'Agostino et al, 2000;Hwang and Sheen, 2001;To et al, 2004).…”

“…Type-A ARRs are considered to be primary cytokinin response genes that act as redundant negative regulators of cytokinin signaling and their transcription is rapidly elevated in response to exogenous cytokinin (D'Agostino et al, 2000;Kiba et al, 2002;Rashotte et al, 2003). Three lines of evidence indicate that the GeBP/ GPL genes are involved in cytokinin responses through type-A ARR gene regulation.…”

“…This result suggests there is a nonadditive interaction between GeBP/ GPL and cytokinin-dependent regulation of type-A ARRs. One hypothesis is that the GeBP/GPL genes interfere with the possible direct induction of type-A by type-B ARRs (Sakai et al, 2000;Rashotte et al, 2003). Third, 35S:VP16:GPL2 plants have reduced ARR transcript levels and display increased cytokinin sensitivity.…”