# Hormonal Interactions Underlying Rootstock-Induced Vigor Control in Horticultural Crops
## 4 Effect of Multiple Plant Hormones in Rootstock-Induced Dwaring
### Cytokinin (CK)  
Cytokinins (CKs) are synthesized in the root tips and transported to the shoot part, where they control the cell cycle and various developmental processes [74, 107]. Furthermore, CKs also play a critical role in modulating plant vigor when dwarfing rootstocks are employed. According to [PERSON] [108], the more vigorous rootstock MM106 had more Cks than dwarfing rootstocks (M.9 and M27). Depletion of solutes (e.g., CKs) in xylem sap due to graft union may also reduce vigor [60, 109]. [PERSON] and [PERSON] [110] found similar findings that grapevine scions grafted on the vigor-controlling rootstock had minimum levels of CKs passing from roots to shoots compared to those grafted onto the rootstock of more vigorous ones. Moreover, CK-like activity in branch sap is greater in vigorous 'Volkamer lemon' rootstock compared with trees grafted onto the least vigorous 'Troyer' (_Poncirus trifoliata_\(\times\)_Citrus sinensis_) rootstock [111]. [PERSON] et al. [103] reported that the amount of CKs in shoot sap varies depending on the source of the rootstock; the more vigorous (MM106) rootstock has more CKs than dwarfing (M9 and M27) rootstocks.  
Similarly, [PERSON] et al. [112] reported a positive correlation between tree vigor and the transport rate of CK in grafted and un-grafted peach trees. Furthermore, Isopentenyl transferases are essential enzymes for cytokinin synthesis [113]. Gene expression of _IPT_ genes directly affects endogenous CK levels [114, 115, 116]. IPT fusion gene expression has been shown to improve cytokinin levels in transgenic plants [117]. The zeatin-riboside (ZR) synthetase _IPT5b_ gene mutants in _Arabidopsis_ have low endogenous active forms and severe dwarfism [118]. In apples, reduced _IPT3_ expression led to decreased cytokinin production in the roots of dwarfing rootstocks. As a result, the scion part received insufficient amounts of cytokinin, which lowered their IAA concentration and permanently reduced tree vigor and vegetative growth [64]. Moreover, lower expression of _IPT5b_ gene in apples resulted in poor zeatin biosynthesis and dwarfing [13]. They hypothesized that the dwarfing effect in 'Fuiji'/M.9 (scion/rootstock combination) was due to the weak production ability of CK in the roots of M.9 rootstock. However, dwarfing in 'Fuiji'/M.9/Baleng Crab (scion/interstem/rootstock) graft combination was mediated by decreased expression of _MMPIN8_ gene in the bark of M.9 interstem, which lowered basipetal transport of IAA and root IAA supply, thereby inhibiting root zeatin biosynthesis. In addition, inadequate zeatin synthesis in the roots also caused a mild zeatin shortage in the shoots, restricting the shoots' vegetative growth. In a recent study, [PERSON] et al. [90] reported that greater expression levels of _MMPTS_ gene in vigorous rootstock boost zeatin amounts to encourage cell division and internode length. On the other hand, lower expression levels of _MMPTS_ gene in dwarfing rootstock deliver defective zeatin, which in turn causes a reduction in internode length. This could be explained by the inadequate cytokinin synthesis capacity triggered by poorly developed roots [119, 120]. As a result, one of the underlying mechanisms that contribute to the dwarf tree phenotype may be a reduced capacity for cytokinin synthesis and transport.