Abstract
Response regulator 9 (RR9) is a typical member of type-A RRs whose transcription is rapidly increased by exogenous cytokinin (CTK). CTK stimulates the outgrowth of axillary buds in apples (Malus domestica). Previous studies showed that the decreased bud outgrowth activity of type-A rr mutants is paradoxical given that type-A RRs act as negative feedback regulators of CTK signaling in Arabidopsis. However, no systematic study of RR9 has been performed in apples. In this study, the genomic sequences of MdRR9, located on chromosome 14, were obtained. Sequencing identified MdRR9 as having an open reading frame (ORF) of 648 bp. Expression pattern showed MdRR9 has the highest expression level in the stem. MdRR9 was up-regulated earlier in response to 6-benzylaminopurine (6-BA, a synthetic type of CTK) than decapitation treatment. The subcellular localization of MdRR9 was in the nucleus. Ectopic expression of MdRR9 in tomatoes indicates that MdRR9 plays a positive role in branch control. A yeast double-hybrid assay showed that MdRR9 may interact with MADS-box transcription factor 22-like (MdAGL9), which gives us a new perspective to study the regulation of the bud outgrowth mechanism of RR9 in the future.
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