New Insights into the Biology of Cytokinin Degradation

Abstract

A survey of recent results is presented concerning the role of cytokinin degradation in plants, which is catalyzed by cytokinin oxidase/dehydrogenase (CKX) enzymes. An overview of Arabidopsis CKX gene expression suggests that their differential regulation by biotic and abiotic factors contributes significantly to functional specification. Here, we show using reporter gene and semiquantitative RT-PCR analyses regulation of individual CKX genes by cytokinin, auxin, ABA, and phosphate starvation. Partially overlapping expression domains of CKX genes and cytokinin-synthesizing IPT genes in meristematic tissues and endo-reduplicating cells lend support for a locally restricted function of cytokinin. On the other hand, their expression in vascular tissue suggests a function in controlling transported cytokinin. Recent studies led to a model for the biochemical reaction mechanism of CKX-mediated catalysis, which was refined on the basis of the three-dimensional enzyme structure. Last but not least, the developmental functions of CKX enzymes are addressed. The recent identification of the rice OSCKX2 gene as an important novel breeding tool is highlighted. Together the results corroborate the relevance of metabolic control in determining cytokinin activity.

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

“…Furthermore, we did not observe an additional decrease in CK levels in CK-deficient plants under stress conditions (see Supplemental Data Set 2 online). In addition, because the expression of CKX genes is regulated by CKs (Werner et al, 2006), a reduction in CK content would subsequently cause downregulation of CKX genes. To confirm this hypothesis, we examined the expression of the CKX genes in the CK-deficient plants.…”

“…Formation of tZ by the hydroxylation of iP requires cytochrome P450 monooxygenases (CYP735A1 and CYP735A2) (Takei et al, 2004). Irreversible CK degradation is catalyzed by CKX, which selectively cleaves unsaturated isoprenoid side chains, resulting in the formation of adenine/adenosine and the corresponding side chain aldehyde (Sakakibara, 2006;Werner et al, 2006). Therefore, CKX enzymes are important players in regulating CK concentrations and thereby influence plant growth and development.…”

“…It has been postulated that CK and ABA exert antagonistic activities during a number of growth and physiological processes, including plant adaptation to environmental stresses (Zdunek and Lips, 2001;Chang et al, 2003;Chow and McCourt, 2004;Werner et al, 2006). When plants are exposed to stress, the accumulation of ABA helps plants to avoid stress by several different mechanisms.…”

Cytokinin signaling regulates cambial development in poplar

“…These trees expressed a cytokinin catabolic gene from Arabidopsis cytokinin oxidase 2, under the promoter of a birch cytokinin receptor, BpCRE1. CKX family enzymes irreversibly degrade active cytokinin species (29), and they have been successfully expressed under the systemic 35S promoter to reduce the general cytokinin levels in several plant species (13,14,30,31). The BpCRE1 promoter was chosen for its high cambial expression (Fig.…”

Cytokinin signaling regulates pavement cell morphogenesis in Arabidopsis

“…The structural diversity of natural cytokinins and modifications of the adenine moiety confer specificity of the cytokinin receptor interaction [14]. Cytokinin homeostasis is achieved through regulated conjugation or irreversible degradation [15], and cytokinin oxidase/dehydrogenase (CKX) catalyzes the irreversible degradation by cleavage of the side chain [14]. In Arabidopsis, three Arabidopsis thaliana histidine kinases serve as cytokinin receptors: AHK2, AHK3, and AHK4/CRE1/WOL [16][17][18].…”