LOCAL CYTOKININ (CK) BIOSYNTHESIS AND ZEATIN-TYPE CKs ARE CONTROLLING ROSE AXILLARY BUD GROWTH

Axillary bud growth is promoted by cytokinins (CKs). However, it has not been demonstrated unequivocally that root-derived CKs trigger release of axillary buds. The role of endogenous CKs in rose root-shoot relations was studied; in particular, the involvement of CKs in axillary bud growth. Variation in root formation and axillary bud growth was induced by two indol 3-butyric acid (IBA) pre-treatments in two cutting sizes. At six morphological developmental stages around the onset of axillary bud growth concentrations of CKs [zeatin (Z), isopentenyl adenine (iP), zeatin riboside (ZR), dihydro ZR (DHZR), isopentenyl adenosine (iPA), Z O-glucoside (ZOG), ZR O-glucoside (ZROG), ZR 5'-monophosphate (ZRMP) and iPAMP] were determined by LC-ESP-MS/MS. Early onset of axillary bud growth occurred in long cuttings pre-treated at low IBA, whereas early root formation was found in long cuttings pretreated at high IBA. In axillary bud tissue, the concentration of iPA and iPAMP was strongly positively correlated, and both were negatively correlated with Z. Furthermore Z, ZR, ZRMP, ZOG, and ZROG were strongly positively correlated with coefficient of correlations above 0.66. IBA pre-treatment influenced the concentration of CKs in axillary bud tissue more than cutting size. The most abundant CK was iPAMP with concentrations up to 90 pmol g^-1 fresh weight (FW). ZROG, ZR, and iPA were found in concentrations up to 29 pmol g^-1 FW, and ZRMP, ZOG, Z and iP peaked below 11 pmol g^-1 FW. An early large increase in iPAMP showed rapid CK biosynthesis in rootless cuttings, suggesting that green parts and the axillary bud can synthesize CKs. At onset of axillary bud growth an increase of Z, ZR, ZRMP, ZOG and ZROG was largely coincident with a decrease in iPAMP, iPA, iP and DHZR. The results strongly indicate a positive role of CKs in axillary bud growth, and that the axillary buds and/or their surroundings are able to synthesize CKs. The results also indicate that axillary bud growth in rose is controlled through changes in endogenous levels of CKs, and further that Z, ZR and ZRMP are the active CKs.