NITROGEN LIMITATION INDUCES EXPRESSION OF THE AVIRULENCE GENE AVR9 INTHE TOMATO PATHOGEN CLADOSPORIUM-FULVUM

The avirulence gene avr9 of the fungal tomato pathogen Cladosporium fu lvum encodes a race-specific peptide elicitor that induces the hyperse nsitive response in tomato plants carrying the complementary resistanc e gene Cf9. The avr9 gene is not expressed under optimal growth condit ions in vitro, but is highly expressed when the fungus grows inside th e tomato leaf. In this paper we present evidence for the induction of avr9 gene expression in C. fulvum grown in vitro under conditions of n itrogen limitation. Only growth medium with very low amounts of nitrog en (nitrate, ammonium, glutamate or glutamine) induced the expression of avr9. Limitation of other macronutrients or the addition of plant f actors did not induce the expression of avr9. The induced expression o f avr9 is possibly mediated by a positive-acting nitrogen regulatory p rotein, homologous to the Neurospora crassa NIT2 protein, which induce s the expression of many genes under conditions of nitrogen limitation . The avr9 promoter contains several putative NIT2 binding sites. The expression of avr9 during the infection process was explored cytologic ally using transformants of C. fulvum carrying an avr9 promoter-beta-g lucuronidase reporter gene fusion. The possibility that expression of avr9 in C. fulvum growing in planta is caused by nitrogen limitation i n the apoplast of the tomato leaf is discussed.