Corolla of the Large-flowered Petunia (Petunia hybrida Vilm.) Cultivars Exhibit Low Endogenous Cytokinin Concentration through Enhanced Expression of the Genes Encoding Cytokinin Oxidases

Abstract

We previously reported that cytokinin (CK) application enlarged the corolla of petunia (Petunia hybrida Vilm.). In this paper, the effect of the large-flowered trait of petunia caused by the Grandiflora (G) allele to CK biosynthesis was investigated. Concentrations of endogenous CK nucleobases (i.e., N6-(Δ2-isopentenyl)adenine (iP) and trans-Zeatin (tZ)), CK nucleosides (iP riboside and tZ riboside), and a CK glucoside (iP-7-glucoside) in the developing corolla were markedly lower in large-flowered cultivars with a Gg genotype than small- and medium-flowered cultivars with a gg genotype. We isolated the cDNAs encoding CK biosynthesis enzymes expressed in the young corolla, i.e., cDNAs of an adenosine phosphate-isopentenyltransferase gene (SHO), a CK riboside 5'-monophosphate phosphoribohydrolase gene (PhLOG), and two CK oxidase/dehydrogenase genes (PhCKX1 and PhCKX2). Expression levels of SHO and PhLOG in the developing corolla were not markedly different among small-, medium-, and large-flowered cultivars. However, expression levels of PhCKX1 and PhCKX2, which catalyze CK degradation through removal of the isoprenoid side chain, were much higher in large-flowered cultivars than in small- and medium-flowered cultivars. Therefore, low CK concentrations in large-flowered cultivars are very probably induced by the promoted expressions of PhCKX1 and PhCKX2, which may partially repress corolla enlargement.