Functional Expression of Cf9 and Avr9 Genes in Brassica napus Induces Enhanced Resistance to Leptosphaeria maculans

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Abstract
The tomato Cf9 resistance gene induces an Avr9-dependent hypersensitive response (HR) in tomato and transgenic Solanaceae spp. We studied whether the Cf9 gene product responded functionally to the corresponding Avr9 gene product when introduced in a heterologous plant species. We successfully expressed the Cf9 gene under control of its own promoter and the Avr9 or Avr9R8K genes under control of the p35S1 promoter in transgenic oilseed rape. We demonstrated that the transgenic oilseed rape plants produced the Avr9 elicitor with the same specific necrosis-inducing activity as reported for Cladosporium fulvum. An Avr9-dependent HR was induced in Cf9 oilseed rape upon injection of intercellular fluid containing Avr9. We showed Avr9-specific induction of PR1, PR2, and Cxc750 defense genes in oilseed rape expressing Cf9. Cf9 × Avr9 oilseed rape did not result in seedling death of the F1 progeny, independent of the promoters used to express the genes. The F1 (Cf9 × Avr9) plants, however, were quantitatively more resistant to Leptosphaeria maculans. Phytopathological analyses revealed that disease development of L. maculans was delayed when the pathogen was applied on an Avr9-mediated HR site. We demonstrate that the Cf9 and Avr9 gene can be functionally expressed in a heterologous plant species and that the two components confer an increase in disease resistance.

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