30Understanding how root systems modulate shoot system phenotypes is a fundamental question in 31 plant biology and will be useful in developing resilient agricultural crops. Grafting is a common 32 horticultural practice that joins the roots (rootstock) of one plant to the shoot (scion) of another, 33 providing an excellent method for investigating how these two organ systems affect each other. 34 65 66Grapevine (Vitis L. spp.) is an excellent model for examining rootstock-scion interactions due to 67 the ease of cloning, available genomic resources, ability to grow across diverse environments, 68 3 and high economic value. Widespread grafting of grapevine began in the late 19 th century after 69 the European wine industry was devastated by the spread of phylloxera (Daktulosphaira 70 vitifoliae Fitch), an aphid-like insect introduced from North America. While many North 71 American Vitis species can withstand phylloxera infestations, roots of the European wine grape 72 Vitis vinifera L. cannot tolerate phylloxera attacks, which lead to a rapid decline in vigour and 73 often death 3 . However, V. vinifera vines with susceptible roots can be grafted to phylloxera-74 tolerant North American Vitis rootstocks, thus circumventing phylloxera sensitivity. Worldwide 75 more than 80% of all vineyards grow vines grafted onto rootstocks composed of American Vitis 76 species or hybrids 3 .
78Although initial grapevine grafting was driven by the need for phylloxera tolerance, additional 79 benefits exist. For example, certain Vitis rootstocks provide resistance to additional pests and 80 pathogens such as nematodes 4 . Rootstocks can also be used to increase tolerance to abiotic 81 stresses including drought 5,6 , salinity 7 , and calcareous soils 8 . Lastly, grafting can modify mineral 82 nutrition 9 , scion vigor 10 , rate of ripening 11 , and fruit phenolic compounds 12 . Thus, grafting is a 83 valuable tool for improving grapevine fruit quality and response to stress.
85Grapevine producers rely on experimental trials to identify elite rootstocks that will best fit their 86 specific growing conditions. Most commonly used grapevine rootstocks are hybrid derivatives of 87 two or three phylloxera-tolerant native North American species, Vitis riparia and Vitis rupestris, 88 which root easily from dormant cuttings, and Vitis cinerea var. helleri (Vitis berlandieri), which 89 is adapted to chalky soils 13 . Interestingly, despite the global diversity of soils, climates and grape 90 varieties, only a handful of rootstock cultivars derived from these three species are in widespread 91 use 3 .
93The result of over a century of grafting grapevines is a wealth of information characterizing 94 graft-transmissible traits. In some cases, the biological mechanisms underlying beneficial effects 95 are now understood. For example, salt (NaCl) tolerant rootstocks can exclude sodium (Na + ) from 96 the shoot, due to VisHKT1;1, a gene which can could serve as a valuable genetic marker for 97 rootstock breeding 14 . However, for many oth...