Deciphering the molecular mechanisms of rootstock induced dwarfing in cherries (Prunus spp.)

Rootstock induced dwarfing has been one of the major breakthroughs in orchard management in the twentieth century. The majority of the cherry rootstocks have been produced in the last 30 years. Nevertheless, breeding of new rootstocks has proven challenging, due to the lack of understanding of the dwarfing phenomenon. This project explores the phenotypic and genetic differences between cherry graft combinations that exhibit varying degrees of vigor. Growth data have indicated a consistent cessation of shoot growth across rootstocks of varying vigor and across growing seasons. The initial rate of shoot elongation was the same for all graft combinations tested, but dwarfing graft combinations showed faster cessation of shoot growth. The same pattern was observed for the number of nodes added during the elongation of the shoot. The average metamer length though was not affected between grafts, indicating that cessation of growth is due to reduced cell growth and expansion at the apical meristem. This was further confirmed by the absence of significant difference in the size or number of cells within the metamer, between grafts. Complementary DNA Amplified Fragment Length Polymorphism of shoot and graft union samples revealed a high degree of co-regulation in gene expression between the dwarfing 'Bing'/Gi5 and semi-vigorous 'Bing'/Gi6 graft combinations. Few genes showed differential expression between the two graft combinations. Forty-three of those genes were differentially expressed in the shoot samples and 56 in the graft union samples. The differentially expressed genes had a variety of functions with the most interesting being a group of genes previously involved in brassinosteroid signaling. The analysis of gene expression also revealed the presence of the Cherry Virus A in the dwarfing combination 'Bing'/Gi5. Screening of rootstocks that confer different degrees of vigor did not show any correlation between the presence of the virus and the vigor of the rootstock. Also, the absence of the virus from some rootstocks is circumstantial rather than due to resistance, which was shown by screening different scions grafted on the same rootstock variety. The current study provides an initial cataloging of genes that may be involved in the process of rootstock-induced dwarfing. No certain conclusion can be drawn from these results and further study will be necessary to identify which of these genes contribute significantly to this phenomenon.