The tomato Cf‐9 gene confers resistance to races of the leaf mould fungus Cladosporium fulvum that carry the Avr9 avirulence gene. Cf‐9 was isolated by transposon tagging using a modified maize Dissociation (Ds) element. This generated
an allelic series of Ds‐induced mutations of Cf‐9, of which two were found to confer novel phenotypes in a screen for mutants affecting wild‐type Cf‐9 function in trans. Genetic and molecular analysis of these mutants suggested semidominant,
Avr9‐dependent, negative‐interfering mutations involving Ds insertions in a defined subregion of Cf‐9. Interference was associated with expression of the 5′‐end of Cf‐9 upstream of the Ds insertions in these mutants,
suggesting that truncated Cf‐9 proteins were the likely cause of interference. Transgenic tomato lines harbouring Cf‐9 constructs with premature stop codons in positions similar to the Ds insertions also showed interference, indicating that the presence of Ds
was not required for interference to occur. Interestingly, interference in these transgenic lines was completely dominant and was associated with a pronounced developmental phenotype that was dependent on co‐expression of Cf‐9, Avr9 and a truncated Cf‐9
transgene. However, interference with a weakly autoactive Hcr9 gene was Avr9‐independent and did not cause a developmental phenotype, suggesting that localized restoration of Cf‐9/Avr9‐dependent cell death was responsible for the developmental phenotype.
The restricted region in which truncation of Cf‐9 results in dominant‐negative interference suggests that leucine‐rich repeats (LRR) 16–19 of Cf‐9 may mediate dimerization of Cf‐9 and LRRs 20–23 may mediate interactions with downstream partner
proteins required for Cf‐9 signalling, or vice versa.
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plant disease resistance;
Document Type: Research Article
Plant Cell Biology Group, Research School of Biological Sciences, The Australian National University, Canberra ACT 0200, Australia, and
Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich, Norfolk NR4 7UH, UK
May 1, 2006