A Subset of Cytokinin Two-component Signaling System Plays a Role in Cold Temperature Stress Response in Arabidopsis

Abstract

A multistep two-component signaling system is established as a key element of cytokinin signaling in Arabidopsis. Here, we provide evidence for a function of the two-component signaling system in cold stress response in Arabidopsis. Cold significantly induced the expression of a subset of A-type ARR genes and of GUS in Pro ARR7 :GUS transgenic Arabidopsis. AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. Cold neither significantly induced AHK2 and AHK3 expression nor altered the cytokinin contents of wild type within the 4 h during which the A-type ARR genes exhibited peak expression in response to cold, indicating that cold might induce ARR expression via the AHK2 and AHK3 proteins without alterations in cytokinin levels. The ahk2 ahk3 and ahk3 ahk4 mutants exhibited enhanced freezing tolerance compared with wild type. These ahk double mutants acclimated as efficiently to cold as did wild type. The overexpression of the cold-inducible ARR7 in Arabidopsis resulted in a hypersensitivity response to freezing temperatures under coldacclimated conditions. The expression of C-repeat/dehydration-responsive element target genes was not affected by ARR7 overexpression as well as in ahk double mutants. By contrast, the arr7 mutants showed increased freezing tolerance. The ahk2 ahk3 and arr7 mutants showed hypersensitive response to abscisic acid (ABA) for germination, whereas ARR7 overexpression lines exhibited insensitive response to ABA. These results suggest that AHK2 and AHK3 and the cold-inducible A-type ARRs play a negative regulatory role in cold stress signaling via inhibition of ABA response, occurring independently of the cold acclimation pathway.Cytokinins are plant hormones that regulate a variety of developmental and physiological processes, including cell division, cell proliferation, root and leaf differentiation, chloroplast biogenesis, and the inhibition of leaf senescence (1). Arabidopsis cytokinin signaling utilizes a multistep phospho-relay composed of a sensor kinase, a histidine phosphotransfer protein, and a response regulator similar to the TCS 2 of bacterial and yeast cells (2). A hybrid-type histidine kinase referred to as CYTOKININ INDEPENDENT1 (CKI1) is essential for megagametogenesis (3). CYTOKININ RESPONSE1 (CRE1)/ WOODEN LEG1 (WOL1)/ARABIDOPSIS HISTIDINE KINASE4 (AHK4) were shown to bind directly to a variety of natural and synthetic cytokinins in vitro with high specificity as well as in a yeast system and thus to be a primary receptor for cytokinins (4 -8). The experiments conducted using a heterologous phospho-relay system demonstrated that AHK2 and AHK3 are also cytokinin receptors. The primary functions of these Arabidopsis histidine kinase (AHK) genes involve the triggering of cell division and the maintenance of the meristematic competence of cells to prevent subsequent differentiation (9, 10). Partially redundant functions of cytokinin receptors have also been revealed in shoot growth, root development, leaf sen...

Analysis of Cytokinin Mutants and Regulation of Cytokinin Metabolic Genes Reveals Important Regulatory Roles of Cytokinins in Drought, Salt and Abscisic Acid Responses, and Abscisic Acid Biosynthesis

“…We observed that both the stresstolerant CK-deficient and CK receptor His kinase mutant plants have enhanced root growth but retarded shoot growth ( Figures 1 and 2) (Werner et al, 2003;Higuchi et al, 2004;Nishimura et al, 2004;Miyawaki et al, 2006;Riefler et al, 2006;Tran et al, 2007b;Jeon et al, 2010). Root enhancement is one of the key traits that is correlated with drought resistance (Sharp et al, 2004).…”

“…Strong lines of evidence have demonstrated that various members of CK signaling, including CK receptor AHKs and a number of ARRs, such as ARR5 and ARR7, function as negative regulators in osmotic and cold stress responses (Tran et al, 2007b;Wohlbach et al, 2008;Jeon et al, 2010). Furthermore, all of the ahk2, ahk3, and ahk4 single mutants and the ahk2 3 double mutant showed an ABA-hypersensitive phenotype (Tran et al, 2007b;Jeon et al, 2010). Taken together, we suggest a mechanism in which plants adapt to adverse conditions by triggering an efficient regulation of CKs and CK signaling.…”

“…Interestingly, it has been reported that CKresponsive His kinase mutants displayed hypersensitivity to exogenous ABA (Tran et al, 2007b;Jeon et al, 2010). Therefore, we determined whether CK-deficient plants exhibit an altered ABA response.…”

“…Functional analysis of ahk2, ahk3, and ahk4 single, double, and triple mutants suggests that AHK2, AHK3, and AHK4/CRE1 function as CK receptor AHKs and act as positive regulators in CK signaling and plant growth (Higuchi et al, 2004;Nishimura et al, 2004;Riefler et al, 2006). Increasing evidence indicates that CK signaling is involved in response to environmental stimuli and ABA signaling (Tran et al, 2007b;Jeon et al, 2010). In Arabidopsis, alterations of external osmolarity induce changes in the transcript levels of CK-responsive AHK2, AHK3, and AHK4, suggesting a functional importance of these AHKs for the efficient sensing of environmental signals.…”

Arabidopsis AHP2, AHP3, and AHP5 histidine phosphotransfer proteins function as redundant negative regulators of drought stress response

“…Within the CK signaling cascade, all three CK receptor Arabidopsis HKs-AHK2, AHK3, and AHK4/cytokinin response 1 (CRE1)-are known to be involved in the response to cold, drought, and salt stresses (10,11). Analyses of ahk2, ahk3, and ahk4 single, double, and triple mutants suggest that AHK2, AHK3, and AHK4 function as negative regulators in abscisic acid (ABA), salt, and drought signaling pathways (10,12).…”