The Pto gene in tomato confers resistance to races of Pseudomonas syringae pv. tomato that carry the avirulence gene avrPto. A yeast artificial chromosome clone that spans the Pto region was identified and used to probe a leaf complementary DNA (cDNA) library. A cDNA clone was isolated that represents a gene family, at least six members of which genetically cosegregate with Pto. When susceptible tomato plants were transformed with a cDNA from this family, they were resistant to the pathogen. Analysis of the amino acid sequence revealed similarity to serine-threonine protein kinases, suggesting a role for Pto in a signal transduction pathway.

Tomato plants resistant to the fungal pathogen, Fusarium oxysporum f. sp. lycopersici, race 2, were obtained using in vitro selection against fusaric acid, a non-specific toxin, as well as non-challenged cells. Protoplasts were isolated from cotyledonary tissue of tomato cv. 'UC-82', which is susceptible to Fusarium race 2. Protoplasts were challenged with the toxin, and the resistant calli were further subjected to the toxin. Plants regenerated from toxin-resistant calli were screened for resistance to the pathogen by using the Fusarium slurry inoculation technique. Seeds were collected from the surviving individuals, germinated and rescreened for resistance to the pathogen. Data obtained from this test showed a ratio of three resistant to one susceptible among R1 progenies. Further analysis of the R2 progenies confirmed that the fusarium-resistant plants were either homozygous or heterozygous dominant for the gene conferring the resistance. Similar results were recorded for tomato plants regenerated from cells that received no selection pressure. The nature of this single dominant gene-type of resistance is under investigation.

A protocol for Agrobacterium-mediated transformation with mannose selection was developed for cotyledon petiole, hypocotyl and leaf explants of tomato (Lycopersicon esculentum L. Mill). More than 400 transgenic plants from three tomato varieties were selected with 1% mannose in combination with 0.1-0.5% glucose. Average transformation frequencies ranged from 2.0 to 15.5% depending on the construct, genotype and type of tissue used for transformation. The highest transformation rate was obtained for hypocotyl explants from tomato variety SG048. The ploidy levels of 264 independent transgenic events and 233 non-transgenic plants regenerated from tissue culture were assessed by flow cytometry. The incidence of polyploids within the total population of transgenic plants varied from 10 to 78% and was not significantly different from the non-transgenic population. The greatest variation in the proportion of polyploids was observed in plants derived from different explant types, both in transgenic and non-transgenic regenerants, across three studied genotypes. Transgenic and non-transgenic plants regenerated from leaves included the highest number of normal diploid plants (82-100%), followed by cotyledon petiole-derived plants (63-78%). Transgenic plants produced from hypocotyls contained 22-58% diploids depending on the genotype used in transformation. Results described in this study demonstrate that, although transformation frequencies for leaf tissue are still lower under current protocols, the high percentage of diploids obtained make leaf tissue an attractive transformation target.