# Hormonal Interactions Underlying Rootstock-Induced Vigor Control in Horticultural Crops
## 4 Effect of Multiple Plant Hormones in Rootstock-Induced Dwaring  
### Auxin (IAA)  
Auxins are a class of phytohormones that control various processes of plant growth and development [48]. Auxins are primarily recognized for their potential to promote embryogenesis, tissue patterning, differential growth, and cell elongation [49; 50]. Indole-3-acetic acid (IAA) is predominantly produced in meristematic tissues of young leaves and transported to the root sections by phloem and cambium [51]. Additionally, IAA application can stimulate plant height and shoot length [52]. IAA content in the scion portion improved after bridging and replacing interstock bark with vigorous interstock, and the scion part became a standard-level tree [53; 54]. The polar IAA inhibitor \(N\)-1-naphthylphthalamic acid restricted IAA transport in the stem of vigorous rootstocks, contributing to a substantial reduction in shoot growth and root zeatin levels [12]. Similarly, [PERSON] et al. [55] found that 'Fuji' apple trees grown onto taller 'MM111' rootstock had greater IAA levels than those grafted onto size-controlling 'M.9' rootstock. In another study, it was reported that dwarfism is caused by a decline in indole-acetic acid (IAA) content and by the associated changes in the IAA:ABA (abscisic acid) content ratio in the crabapple plants in both growing conditions [1; 56].  
Additionally, dwarfing rootstocks had noticeably lower expression levels of the _MdYUCCA10a_ gene in their roots and leaves than invigorating rootstocks. Therefore, the best-supported argument is that dwarfing rootstocks acquire less IAA from the scion part, consequently, limit root growth and root-synthesized cytokinin transported to the aerial part again, which in turn reduces plant growth [57; 58]. In this way, the dwarfing 'M.9' rootstock had lower amounts of labeled IAA in cambial sap than the more vigorous 'M.26' and 'MM.106' rootstocks [59]. Moreover, the 'M.9' dwarfing rootstock transported the labeled IAA downward at a slower rate than the 'MM.11' vigorous rootstock [60]. In another study, [PERSON] et al. [34] found that the vigorous 'Canton lemon' (_Citrus limonia_ Osbeck) rootstock had a maximum level of IAA in roots. However, the roots of the dwarfing 'Fragrant orange' rootstock had the lowest levels. Previous research also suggests that the lowered transport ability of IAA in dwarfing rootstock is linked with the imbalanced distribution of IAA efflux carrier proteins [61; 62; 63]. It has been demonstrated that different _PIN_ gene expression levels may affect IAA levels [64].  
Furthermore, IAA is transported from the scion part to the root portion through polar auxin transport (PAT), regulated by the PIN auxin efflux carriers [65; 66; 67]. On the other hand, [PERSON] et al. [68] revealed that tobacco plants overexpressing apple _MdPIN1b_ enhanced polar IAA transport and reduced _MdPIN1b_ expression in the M9 interstem, which may cause apple dwarfism. The inflorescence axis of the mutant (_pinl-1_) exhibits a distinctive structure and lowered IAA polar transport activity [69; 70]. Modifications in the _Arabidopsis_ gene _PIN3_ significantly influence stem and lateral root development [71]. A CT repeat deletion in the promoter of the _PcPIN-L_ gene is associated with decreased promoter activity in dwarf-type pears, which may impair polar IAA transport and result in a dwarf phenotype [43].