REVIEW https://doi.org/10.48130/FruRes-2021-0003 Fruit Research 2021, 1: 3 An insight into dwarfing mechanism: contribution of scion-rootstock interactions toward fruit crop improvement Faisal Hayat1, Shahid Iqbal1, Daouda Coulibaly1, Muhammad Khuram Razzaq2, Muhammad Azher Nawaz3, Weibing Jiang1, Ting Shi1, and Zhihong Gao1* 1 Laboratory of Fruit Tree Biotechnology, College of Horticulture, Nanjing Agricultural University, Nanjing 210095, China 2 Soybean Research Institute, National Centre for Soybean Improvement, Nanjing Agricultural University, Nanjing 210095, China 3 University College of Agriculture, University of Sargodha, Sargodha 40100, Pakistan These authors contributed equally: Faisal Hayat, Shahid Iqbal * Corresponding author, E-mail: [email protected] Abstract Grafting has been commonly practiced for many centuries in the cultivation of horticultural crops. The use of dwarfing rootstocks has enabled a high-density plantation to produce maximum yield. Rootstock regulates scion phenotype, including precocity, fruit size, yield, quality characteristics, and tolerance to various environmental stresses. This review summarizes the existing information on the influence of rootstocks on scion growth and dwarfing mechanisms induced by multiple factors, including hormone signaling, photosynthesis, mineral transport, water relations, anatomical characteristics, and genetic markers. It has been shown that the complex interactions between scion and rootstock can regulate plant development and its structure. This information will provide interesting insights for future research related to rootstock-mediated dwarfing mechanisms and accelerate the breeding progress of dwarfing rootstocks. Citation: Hayat F, Iqbal S, Coulibaly D, Razzaq MK, Nawaz MA, et al. 2021. An insight into dwarfing mechanism: contribution of scion-rootstock interactions toward fruit crop improvement. Fruit Research 1: 3 https://doi.org/10.48130/FruRes-2021-0003 Introduction scion growth[11]. Several studies have reported the diversity and Grafting is an ancient plant propagation technique that domestication of rootstocks and their influence on plant combines the aerial portion (scion) with another segment vigour, hormonal communication between scion and [1,2] (rootstock) to form a new plant . Modern fruit cultivation rootstock, and various stresses[1,12−16]. However, due to the systems are mainly based on scion varieties, grafted with complex interaction of rootstock-induced scion vigour, these rootstock to control plant-specific characteristics, such as fruit studies have not fully elucidated the possible dwarfing [3,4] size, early fruiting, and to reduce disease susceptibility . mechanism. This review summarizes the existing information Rootstocks provide grafted plants with more favorable of rootstock influence on scion growth, nutrient uptake, tolerance against multiple environmental stresses[5]. The hormonal communication, carbohydrate distribution, and the selection of a suitable rootstock is one of the major decisions molecular mechanism. in establishing a tree garden and achieving excellent tree performance in different ecological regions. Rootstock controls scion vigour Rootstocks have been used in fruit plants to improve nutrient absorption and transportation by increasing fruit Rootstocks have a substantial effect on the physiological production, taste and quality, and tolerance against several features of grafted plants and other aspects of growth and environmental stresses[6,7]. Dwarfing rootstock is an impor- development, among which the reduction of scion growth is tant germplasm resource for high-density cultivations to one of the most interesting phenomena (Fig. 1). Previous ensure maximum yield and mechanical harvesting[8]. In studies have demonstrated that dwarfing rootstock and recent years, dwarfing rootstocks have become popular due interstock contribute to restricting plant height, crown size, to reduced vegetative growth of scion varieties, increased and tree volume. However, the plants grafted on vigourous production, and reduced labour costs[9]. It is speculated that rootstocks have more nutritional properties but lower scion and rootstock are the essential components in fruit yield[17−20]. Gjamovski and Kiprijanovski[21] documented that production that interact with each other to release mineral 'Granny Smith' apple trees, grafted with taller rootstock nutrients, hormones and carbohydrates[10]. The hypothesis (Supporter 4), have higher vegetative growth than size- behind rootstock-induced size control may be triggered by controlling rootstocks (Panjam 1, M.9 T337 and Mark 9). Zhou several factors such as water, nutrients, and particularly et al.[22] stated that 'Red Fuji' apple scion grafted on size- hormones, which pass through the graft union to influence controlling rootstock M.9 had the lowest vegetative growth, www.maxapress.com/frures © The Author(s) www.maxapress.com Rootstock mediated scion vigour Lower stomata and Higher stomata and photosynthesis increased photosynthesis decreased scion growth enhanced shoot growth Lower mineral uptake Higher mineral uptake Lower leaf water Higher leaf water potential reduces shoot potential enhances shoot growth and induced growth and increase mild water stress transpiration Small xylem vessels and Large xylem vessels and lower conductance higher conductance restricts water flow facilitates water flow Lower ZR Higher Weak root system and GA3 Vigorous root system cytokinin biosynthesis Lower values of root Higher values of root biosynthesis morphological traits morphological traits Fig. 1 Schematic diagram of rootstock control scion vigour. canopy width, and trunk cross-sectional area (TCSAs), cultivar 'Flavorcrest' and found significant variations in stem compared with vigorous rootstocks. water potential (ΨSTEM) and stem extension growth. Solari et Sitarek and Bartosiewicz[23] studied that trees grafted on al.[31] and Tombesi et al.[32] explained the relationship M46 and P. divaricata had substantially higher values of trunk between dwarfing rootstocks and hydraulic conductance. cross-sectional area. In comparison, those grafted on They found that dwarfing rootstocks limit the water supply to Wangenheim Prune showed lower values and dwarfing the aerial portion. characteristics. Several studies have shown that apple scions Lowered hydraulic conductivity of size-controlling root- grafted onto dwarfing rootstocks have shorter shoot lengths, stocks is probably due to the greater hydraulic resistance and lower trunk cross-sectional area (TCSA), and weak growth limited water absorption capacity of size-controlling root- characteristics[21,24,25]. Tworkoski and Fazio[23] also studied the stocks. In pear, Chen et al.[33] studied the anatomical structure effect of various apple scion cultivars grafted onto rootstocks of leaves and stem of vigorous and dwarf type pears to clarify of Geneva and Malling series. They observed that plants the mechanism of a short tree structure. They found that the grafted with G.41 and G.935 rootstocks had the maximum dwarf type had a smaller vessel diameter and a greater stem shoot growth and stem diameter compared with other cross-sectional area than the vigorous type. In citrus, Saeed et rootstocks. al.[34] explored the relationships among growth vigour and Moreover, the application of interstocks with greater anatomical characteristics of leaves, stems and roots. They dwarfing effects had lower net photosynthesis rate and root- found that vigorous rootstocks, rough lemon (C. jambhiri), shoot ratio but improved yield efficiency and fruit quality of have larger xylem vessel elements in the stems and roots with Red Fuji apple trees[26]. In another study, Hayat et al.[27] lower phloem percentage (%), when compared with dwarfing reported that apple scion grown onto dwarfing M.9 rootstock rootstocks- flying dragon (P. trifoliata). Zoric et al.[35] noticed had the smallest plant height and reduced scion diameter, that dwarfing rootstocks had lower theoretical hydraulic and the lowest hydraulic conductance (Kleaf). In contrast, conductance (kh) and a smaller vessel lumen size, especially plants grown onto more vigorous rootstocks M.26, Chistock- when compared with vigorous rootstocks. Martínez-Alcántara 1, and Baleng rootstocks had longer shoot length, greater et al.[36] also demonstrated lower leaf water potential and scion diameter, and hydraulic conductance. decreased hydraulic conductance during times of high evapo- rative demand, leading to reduced stomatal conductance in Water restrictions and anatomical characteristics plants grafted onto size-controlling rootstocks. In apple, Zhou et al.[37] studied the effect of different vigour controlling root- Root hydraulic conductivity can influence shoot growth by stock combinations (vigorous, semi-dwarfing and dwarfing modifying water supply to the aerial part; however, its role in characteristics) grafted with 'Red Fuji' scion cultivar. They rootstock-induced vigour is still controversial[28]. The concluded that reduction in scion growth of dwarfing physiological mechanisms of the size reduction of rootstock rootstocks (M.9 and B.9) was related to lower palisade/spongy or interstock are triggered by restricting water flow to the parenchyma ratios and hydraulic conductance stomatal scion[29]. In another study, Basile et al.[30] reported that water density compared with more vigorous rootstocks. Therefore, status might significantly affect the rootstock-induced it is concluded from the above literature (Table