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

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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.
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