IDENTIFICATION AND DS-TAGGED ISOLATION OF GENES AT THE CF-4 LOCUS OF TOMATO INVOLVED IN DISEASE RESISTANCE TO CLADOSPORIUM FULVUM RACE 5
FLW TAKKEN, D SCHIPPER, HJJ NIJKAMP and J HILLE
Department of Genetics, IMBW, BioCentrum Amsterdam, Vrije Universiteit, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands
Background and objectives
Results and conclusions
To isolate the gene involved in AVR4 recognition, a second transposon-tagging population was generated. A transposon-tagging population of plants expressing both the Cf-4 and Avr4 genes was screened for survivors. The surviving mutants, no longer showing a systemic HR, were expected to have a disrupted Cf-4 gene . 1500 plants were screened and 22 survivors were found, of which two showed necrosis on their secondary leaves, later leading to leaf shed. These last two plants were shown to have a Ds tag in Cf-4A. The 20 other survivors showed a disruption of the Cf-4 gene which is closely homologous (>95%) to the Cf-4A gene present at the same locus. Six of the surviving plants also had, in addition to the disruption of Cf-4, a disruption of the Cf-4A gene. An inoculation assay of the survivors with race 5 of C. fulvum revealed that only plants having a disruption of both genes (Cf-4 and Cf-4A) become susceptible to infection.
Both identified genes are members of a gene family and show high sequence homology to the C. fulvum resistance genes Cf-9 and Cf-2. The genes are predicted to encode extracellular transmembrane proteins containing a divided domain of 22/25 leucine-rich repeats.
We propose a model in which the Cf-4 locus contains two active resistance genes (Cf-4 and Cf-4A) that both recognise different elicitors, AVR4 and AVR4A, respectively. In the first tagging experiment, susceptible plants were isolated as having only a tag in Cf-4A; this indicates 'cross-talk' between these two genes upon insertion of a transposon. This hypothesis is supported by the isolation of the two necrotic survivors obtained from the second tagging population having only a Ds tag in Cf-4A.