Differences in Agrobacterium infections in silver birch and Scots pine

Summary The aim of this study was to investigate the differences in infections caused by Agrobacterium tumefaciens in a conifer, Scots pine ( Pinus sylvestris ), and in a non‐host deciduous species, silver birch ( Betula pendula ). All the Agrobacterium tumefaciens strains tested caused crown‐gall formation in both tree species, but the infection rates varied remarkably. In Scots pine, the development of galls was rare, and slower than in silver birch. Inoculation into the base of the stem were the most successful in gall induction. Silver‐birch galls were large, often surrounding the whole stem, in contrast to Scots pine galls, which were characterized by their small size and neck‐like connection with the host plant. In silver birch, no other morphological changes could be seen. In Scots pine, abnormal phenotypes with proliferating short shoots above the galls were observed during the second and third growing season. The results indicate that, of the two non‐host tree species, the deciduous one, silver birch, is more susceptible to an A. tumefaciens infection than the conifer, Scots pine. The matrix for A. tumefaciens infection in silver birch differs from that in Scots pine, since the terpene compounds of Scots pine seem either to kill the agrobacteria or to suppress their growth. The differences between the species could be partly caused by their difference in sensitivity to phytohormones. These features reflect evolutionary incompatibility between A. tumefaciens and a gymnosperm.