Plant grafting: new mechanisms, evolutionary implications

Abstract

Grafting, an old plant propagation practice, is still widely used with fruit trees and in recent decades also with vegetables. Taxonomic proximity is a general prerequisite for successful graft-take and long-term survival of the grafted, composite plant. However, the mechanisms underlying interspecific graft incompatibility are as yet insufficiently understood. Hormonal signals, auxin in particular, are believed to play an important role in the wound healing and vascular regeneration within the graft union zone. Incomplete and convoluted vascular connections impede the vital upward and downward whole plant transfer routes. Long-distance protein, mRNA and small RNA graft-transmissible signals currently emerge as novel mechanisms which regulate nutritional and developmental root/top relations and may play a pivotal role in grafting physiology. Grafting also has significant pathogenic projections. On one hand, stock to scion mechanical contact enables the spread of diseases, even without a complete graft union. But, on the other hand, grafting onto resistant rootstocks serves as a principal tool in the management of fruit tree plagues and vegetable soil-borne diseases. The ‘graft hybrid’ historic controversy has not yet been resolved. Recent evidence suggests that epigenetic modification of DNA-methylation patterns may account for certain graft-transformation phenomena. Root grafting is a wide spread natural phenomenon; both intraspecific and interspecific root grafts have been recorded. Root grafts have an evolutionary role in the survival of storm-hit forest stands as well as in the spread of devastating diseases. A more fundamental evolutionary role is hinted by recent findings that demonstrate plastid and nuclear genome transfer between distinct Nicotiana species in the graft union zone, within a tissue culture system. This has led to the formation of alloploid cells that, under laboratory conditions, gave rise to a novel, alloploid Nicotiana species, indicating that natural grafts may play a role in plant speciation, under certain circumstances.

Graft incompatibility in Prunus spp. preceded by SPAD index reduction

“…Kester and Hansen (1964) reported that almond cultivars and selections grafted on 'Marianna 2624' exhibited different behaviors, from lack of symptoms up to full incompatibility, six years after field planting, and Grasselly (1983) reported that some 'Myrobalan' clones (P. cerasifera) showed graft compatibility with various peach cultivars. Even though early detection of graft incompatibility has been reported (RODRIGUES et al, 2001;ERREA;BORRUEY, 2004;ZARROUK et al, 2006b;ZARROUK et al, 2010), the methods usually have limitations and evaluation cycles in the field for some rootstock combinations may require several years before any assumptions on compatibility and yield efficiency can be made, especially when genotypes are taxonomically distant (GOLDSCHMIDT, 2014). Thus, graft incompatibility is one of the major hurdles for rootstock improvement and analysis (CIOBOTARI et al, 2010).…”

“…A common form of incompatibility is "translocated" incompatibility, in which symptoms occur during plant development, and include early termination of scion and root growth, shriveling and early leaf drop, reduced carbohydrate translocation between scion, and rootstock leaf chlorosis evolving to leaf reddening. Thus, the appropriate choice of scion/ rootstock combination is key to achieving graft compatibility (MOSSE, 1962;HARTMANN et al, 2002;GOLDSCHMIDT, 2014).…”

Anatomy of Rootstocks and Scions in Four Pine Species

“…A decisive factor in the failure of grafts is incompatibility, which refers to the lack of union between stock and scion, which affects the growth and productivity of the plants, and may even lead to their death [15][16][17].…”

Exogenous auxin improves root morphology and restores growth of grafted cucumber seedlings

“…Hormonal signals, auxin in particular, are believed to play an important role in the wound healing and vascular regeneration within the graft union zone (Pina, Errea 2005;Goldschmidt 2014), as well as profoundly influence root morphology, increasing lateral root production and inducing adventitious roots (Aloni et al 2006;Overvoorde et al 2010;Woodward, Bartel 2005). The present data as well confirm the influence of exogenous auxin application on adventitious root formation and root architecture in the rootstocks of root pruned splice grafted (RPSG) cucumber seedlings.…”