The Large-flowered Petunia (Petunia hybrida Vilm.) Genotype Promotes Expressions of Type-A Response Regulator and Cytokinin Receptor Genes like Cytokinin Response

Expressions of the genes involved in the cytokinin (CK) phosphorelay signaling circuitry in petunia (Petunia hybrida Vilm.) were investigated with regard to the large-flowered trait induced by the semidominant Grandiflora (G) gene. We isolated three cDNAs encoding type-A response regulators (RRs, PhRR1-3) and one cDNA encoding CK receptor histidine protein kinase (PhHK). Expression levels of PhRR1-3 and PhHK in the developing corolla were markedly higher in large-flowered cultivars with the Gg genotype than in smalland medium-flowered cultivars with the gg genotype. Higher expression levels of PhRR1-3 and PhHK in the Gg than in the gg genotypes was also consistent in BC4 plants in which the genetic background except for G allele was mostly uniform. These results suggest that Gg genotype induces a high expression level of PhRR1-3 and PhHK. These changes in expression caused by the G allele were the same as those caused by the CK response. Since type-A RRs repress CK signaling, the G-allele-induced high expression levels of PhRR1-3 presumably have partial inhibitory effects on G-alleleinduced corolla enlargement.