Cytokinin Oxidase Gene Expression in Maize Is Localized to the Vasculature, and Is Induced by Cytokinins, Abscisic Acid, and Abiotic Stress

Abstract

Cytokinins are hormones that play an essential role in plant growth and development. The irreversible degradation of cytokinins, catalyzed by cytokinin oxidase, is an important mechanism by which plants modulate their cytokinin levels. Cytokinin oxidase has been well characterized biochemically, but its regulation at the molecular level is not well understood. We isolated a cytokinin oxidase open reading frame from maize (Zea mays), called Ckx1, and we used it as a probe in northern and in situ hybridization experiments. We found that the gene is expressed in a developmental manner in the kernel, which correlates with cytokinin levels and cytokinin oxidase activity. In situ hybridization with Ckx1 and transgenic expression of a transcriptional fusion of the Ckx1 promoter to the Escherichia coli beta-glucuronidase reporter gene revealed that the gene is expressed in the vascular bundles of kernels, seedling roots, and coleoptiles. We show that Ckx1 gene expression is inducible in various organs by synthetic and natural cytokinins. Ckx1 is also induced by abscisic acid, which may control cytokinin oxidase expression in the kernel under abiotic stress. We hypothesize that under non-stress conditions, cytokinin oxidase in maize plays a role in controlling growth and development via regulation of cytokinin levels transiting in the xylem. In addition, we suggest that under environmental stress conditions, cytokinin oxidase gene induction by abscisic acid results in aberrant degradation of cytokinins therefore impairing normal development.

SPINDLY inhibits class I TCP proteolysis to promote sensitivity to cytokinin

“…The reduced levels of active CKs might be the result of a negative feedback regulation, initiated by increased CK signaling (Kieber and Schaller, 2014). High CK activity induces the expression of CYTOKININ OXIDASE/DEHYDROGENASE genes for CK degradation (Brugière et al, 2003;Werner et al, 2003Werner et al, , 2006 and rapidly converts free active CKs to inactive derivatives (Singh et al, 1988;Moffatt et al, 1991;Yonekura-Sakakibara et al, 2004). Indeed, we found higher levels of inactive CKs in TCP14-overexpressing flowers.…”

Arabidopsis CYP735A1 and CYP735A2 Encode Cytokinin Hydroxylases That Catalyze the Biosynthesis of trans-Zeatin

“…5 and 6). Genes for cytokinin oxidase/dehydrogenase (CKX), a CK degradation enzyme, are also known to be up-regulated by CKs (38). These regulation patterns suggest that the genes mentioned are involved as antagonists in the regulation of the cellular CK level and of the balance between iP-and tZ-type CKs that interact with auxin.…”

Localized High Expression of Type-A Response Regulator and Cytokinin Oxidase/dehydrogenase Genes in Relation to Forchlorfenuron-induced Changes in Flower Morphology in Torenia fournieri Lind.

“…The expression of type-A RRs is rapidly and strongly induced in response to cytokinin, and thus, the expression level reflects the extent of the cytokinin signal (Brandstatter and Kieber, 1998;D'Agostino et al, 2000;Nishijima et al, 2011b;Taniguchi et al, 1998). In addition, CKX is involved in negative feedback regulation of cytokinin biosynthesis, because CKXs expression is induced by cytokinin (Brugière et al, 2003;Kiba et al, 2005;Nishijima et al, 2011a). Thus, the expression of type-A RRs and CKXs may be used as indices of cytokinin signaling.…”