J. Collemare (1), H. U. Böhnert (1),
M. Pianfetti, I. Fudal (1), D. Tharreau (2), M.-H. Lebrun (1)
(1) UMR2847 CNRS-Bayer
Cropscience, Physiologie des plantes et des champignons lors de l’infection,
Bayer Cropscience, 14-20 rue P Baizet, 69263 Lyon Cedex 09
(2) CIRAD - UMR BGPI,
Campus International de Baillarguet, TA41 / K, 34398 Montpellier Cedex 05
Recognition of the blast fungus Magnaporthe grisea by resistant rice
cultivars frequently involves interaction between a fungal avirulence gene and
a specific rice resistance gene, as observed for the avirulence gene ACE1 that controls the production of a
signal specifically recognized by rice cultivars carrying the resistance gene Pi33. ACE1 encodes for a polyketide synthase fused to a non-ribosomal
peptide synthetase, an enzyme involved in the biosynthesis of a microbial
secondary metabolite. ACE1 is
specifically expressed in mature appressoria during penetration of the fungus
into host plant leaves. The protein Ace1 is only detected in the cytoplasm of
appressoria and does not seems to move into infectious hyphae produced inside
host plant tissues. Deletion analysis of ACE1
promoter has led to the identification of a 58-bp region required for its
appressorium-specific transcription. This region contains putative binding
sites for a Ste12 transcription
factor and a fungal binuclear zinc finger transcription factors. Site-directed
mutagenesis of these putative binding sites will be used to assess their role
in the control of ACE1 expression. Ace1-ks0, a non-functional ACE1 allele obtained by site-directed
mutagenesis of an essential amino acid of the polyketide synthase KS domain, is
unable to confer avirulence. This result suggests that the avirulence signal
recognized by Pi33 resistant rice is
not the Ace1 protein, but is likely the secondary metabolite synthesized by
Ace1. In order to characterize this metabolite, we have performed a metabolic
profiling of M. grisea appressoria by
LC-MS-MS, using appressoria from either virulent or avirulent isolates
differentiated on onion epidermis. Fungal metabolites were detected but none
was specific of avirulent isolate. Close to ACE1,
we identified 15 genes predicted to encode enzymes involved in secondary
metabolism, including two enoyl-reductases and a binuclear zinc-finger
transcription factor. All these 15 genes are located within a region of 70-kb
and display the same penetration-specific expression pattern as ACE1, defining a cluster of co-expressed
genes. The inactivation of these genes in an avirulent isolate is underway to
evaluate their role in the biosynthesis of the avirulence signal recognized by Pi33 resistant rice cultivars.