EFFECT OF HOST PLANT GENOTYPE AND GROWTH-RATE ON AGROBACTERIUM-TUMEFACIENS-MEDIATED GALL FORMATION IN PINUS-RADIATA

Effect of host genotype on Agrobacterium tumefaciens-mediated gall formation in Pinus radiata was examined using clonal tissue culture shoots and rooted plantlets. Tissue culture shoots were inoculated in vitro with A. tumefaciens strain 542 or C2/74, both known to be highly infective in pines. Rooted plantlets of the identical genotypes used in vitro and seedlings from the same seedlot were established in the greenhouse and inoculated with strain 542. Gall formation frequency of tissue culture shoots and rooted plantlets was genotype dependent. Rooted plantlets and seedlings of the same age did not differ in susceptibility to A. tumefaciens strain 542. In vitro gall formation frequency did not predict greenhouse gall formation frequency. Higher gall formation frequencies were found in rooted plantlets in the greenhouse as compared to tissue culture shoots of the same clones. Gall formation occurred earlier and over a shorter period with in vitro as compared to greenhouse inoculation. Galls on seedlings and tissue culture shoots were spherical and succulent, whereas rooted plantlets had less localized and woodier galls. The correlation between mean growth rate at the time of inoculation and gall formation frequency was significant for rooted plantlets. When only the more rapidly growing rooted plantlets within each clone (top 50%) were considered, gall formation frequency was higher than for the more slowly growing individuals within the same clones, and the effect of genotype was obscured. Overall, 58% of gall tissue had detectable opine levels. An inverse relationship existed between the woodiness of gall tissue and the amount of opine detected. Our data are consistent with the idea that a major component of the host-pathogen interaction is the genetic and environmental control of host plant cell division at the time of inoculation.