Fig. 5: A working model for the actions of auxin-enriched rootstock during grafting.

At the morphological and developmental levels, elevated auxin levels in rootstock tissues can exert a number of beneficial effects on rootstock and scion tissues. These effects on rootstock tissue include enhanced root initiation and inhibited lateral bud release, whereas the effects on scion tissue include more vigorous growth at early stages after grafting. At the biochemical and molecular levels, higher levels of auxin in rootstock tissue result in elevated auxin concentrations in scion tissues, which is likely due to the upward transport or passive diffusion of auxin from the rootstock to scion tissues. Higher auxin levels in rootstock tissues can enhance the expression of genes involved in callus formation and vascular tissue development/reconnection in both rootstock and scion tissues. Additionally, elevated auxin levels induce more ethylene production, and ethylene plays a positive role in callus formation and vascular tissue development/reconnection. Together, these effects lead to higher grafting success. The genes showing enhanced expression in rootstock tissue containing elevated auxin levels include Wound induced protein 1 (WIND1)³⁶, Lateral Organ Boundaries Domain (LBD) 16, LBD17, LBD18³⁷, Histone H4⁸, cyclin genes²⁵, such as CYCA1-1, CYCA2-2, CYCB2-3, CYCB2-4 and CYCD3-1, the NAC domain-containing proteins NAC096 and NAC071⁷, high cambial activity 2 (HCA2)⁸, expansin-A15, and expansin-like B1