In planta functions of the Arabidopsis cytokinin receptor family

Abstract

Since their discovery as cell-division factors in plant tissue culture about five decades ago, cytokinins have been hypothesized to play a central role in the regulation of cell division and differentiation in plants. To test this hypothesis in planta, we isolated Arabidopsis plants lacking one, two, or three of the genes encoding a subfamily of histidine kinases (CRE1, AHK2, and AHK3) that function as cytokinin receptors. Seeds were obtained for homozygous plants containing mutations in all seven genotypes, namely single, double, and triple mutants, and the responses of germinated seedlings in various cytokinin assays were compared. Both redundant and specific functions for the three different cytokinin receptors were observed. Plants carrying mutations in all three genes did not show cytokinin responses, including inhibition of root elongation, inhibition of root formation, cell proliferation in and greening of calli, and induction of cytokinin primary-response genes. The triple mutants were small and infertile, with a reduction in meristem size and activity, yet they possessed basic organs: roots, stems, and leaves. These results confirm that cytokinins are a pivotal class of plant growth regulators but provide no evidence that cytokinins are required for the processes of gametogenesis and embryogenesis.S ince the discovery of kinetin in 1956 as a degradation product of DNA that promotes cell division in plants (1), a considerable amount of biochemical, physiological, and, most recently, genetic research has focused on elucidating the diverse roles that cytokinins play in plant growth and development. Perturbations of cytokinin levels in plants via over-expression of bacterial cytokinin synthesis genes (2-4), recovery of mutant plants with a higher-than-normal cytokinin content (5), and characterization of loss-of-function mutants of the cytokinin receptor CYTOKININ RESPONSE 1 (CRE1) (6-9) have implicated cytokinins in a wide variety of processes, including cell division, organ formation and regeneration, senescence, apical dominance, vascular development, response to pathogens, and nutrient mobility. These numerous roles for cytokinins, coupled with the failure of mutant screens to yield plants with nondetectable cytokinin levels, led to the longstanding belief that cytokinins are essential for plant growth and development.Plants respond to cytokinin through a multistep phosphorelay system, consisting of sensor histidine kinase (HK) proteins, histidine phosphotransfer (HPt) proteins, and effector response regulator (RR) proteins. Over-expression and loss-of-function analyses of particular HK, HPt, and RR proteins in Arabidopsis (8-13), combined with transient expression assays in protoplasts (14), have led to a model for cytokinin signaling (for a review, see refs. 15 and 16), beginning with perception of cytokinins by HK proteins.The Arabidopsis genome encodes six nonethylene receptor HKs: CRE1͞WOL͞AHK4, AHK2, AHK3, AtHK1, CKI1, and CKI2͞AHK5. Among them, CRE1, Arabidopsis HK2 (AHK2), and Arabidopsis HK3 (A...

Arabidopsis ABCG14 protein controls the acropetal translocation of root-synthesized cytokinins

“…4a,b). This phenotype is also consistent with perturbation of cytokinin status, as increase in cytokinin levels normally inhibit root growth and reduce the size of root meristems [22][23][24] . Thus, we speculated that disrupting AtABCG14 may impair root meristem activity.…”

The ArabidopsisGRF-INTERACTING FACTORGene Family Performs an Overlapping Function in Determining Organ Size as Well as Multiple Developmental Properties

“…Therefore, it is conceivable that the small size of leaf primordia in the gif mutants may be, in part, attributable to the small size of the SAM cell pool, probably because the SAM tissues do not have sufficient numbers of founder cells. This notion is in line with the fact that all of the cytokinin-signaling mutants and transgenic plants with reduced cytokinin activities have both the smaller SAM cell pool and leaf organs with fewer cells compared with the wild type (Werner et al, 2001(Werner et al, , 2003Higuchi et al, 2004;Nishimura et al, 2004;Miyawaki et al, 2006). However, precise data on the causal relationship between altered numbers of the SAM cells and leaf cells are scarce in Arabidopsis.…”

Sugar-Hormone Cross-Talk in Anthocyanin Biosynthesis

“…CK signaling, which involves a phosphorelay mechanism similar to the bacterial two-component system (Ferreira and Kieber, 2005), is mediated by sensor His kinases (AHKs), Hiscontaining phosphotransfer proteins (AHPs), and type-B and -A response regulators (ARRs) (Hwang and Sheen, 2001). AHKs are positive and functionally overlapping regulators of CK signaling (Higuchi et al, 2004;Nishimura et al, 2004;Riefler et al, 2006). Several Arabidopsis CK signaling pathway mutants have been used to investigate the role of CK in the regulation of anthocyanin accumulation.…”