INCOMPLETE DOMINANCE OF TOMATO CF GENES FOR RESISTANCE TO CLADOSPORIUM-FULVUM

Different tomato Cf resistance genes confer distinct abilities to rest rict Cladosporium fulvum infections. Measurements of fungal growth rev ealed that their relative efficiencies decreased in the order Cf-2, Cf -5, Cf-9, Cf-4, Cf-11, Cf-3. Plants homozygous for a given Cf gene wer e more effective in containing infections than when heterozygous. Cf h omozygotes also responded to a two-fold lower concentration of race-sp ecific elicitor (IF) than heterozygotes. The effectiveness of heterozy gotes was even further reduced if produced by crosses to Lycopersicon pennellii instead of L. esculentum. Incompatibility usually occurred i n the mesophyll layers and involved the gradual arrest of hyphal growt h, frequent and nondichotomous hyphal branching, and a failure to form straight runner hyphae. Contained hyphae were often swollen and disto rted, but those observed at the margin of the larger infections or whe n hyphae had been arrested within 1-2 days of entry into a substomatal cavity appeared normal. Localized host responses triggered by incompa tibility included guard cell death (only in the Cf-2 containing line), enlargement of lower mesophyll cells, deposition of phenolic extracel lular material on cell walls and later some cell death at the center o f infections. Compatibility involved rapid colonization of the lower m esophyll apoplast by straight runner hyphae, the accumulation of highl y branched mycelium in close proximity to vascular tissue in the lesio n center, and finally the death of mesophyll cells directly below the sporulating conidiophores. The implications of the incomplete dominanc e of Cf genes, and the mechanisms by which they restrict fungal growth , are discussed.