1.10.11
cAMP SIGNALLING IN FUNGAL PATHOGENESIS: DIVERGENT PATHWAYS REGULATE GROWTH AND PATHOGENESIS IN THE RICE BLAST FUNGUS

K ADACHI and JE HAMER

Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA

Background and objectives
cAMP plays a central role as a signalling molecule in fungal growth, morphogenesis and pathogenesis. In Magnaporthe grisea, causal agent of rice blast disease, increase of internal cAMP concentration seems to trigger the formation of infection structures (appressoria). However, cAMP-dependent protein kinase catalytic subunit gene (CPKA) disruption mutants still make appressoria, although appressorium function for penetration is deficient [1, 2]. To understand more details of the cAMP signalling pathway in appressorium formation, we cloned and disrupted a gene encoding adenylate cyclase (MAC1), a key enzyme producing cAMP from ATP in the cAMP signalling pathway, and analysed Mac1- suppressor mutants in detail.

Results and conclusions
mac1 knockout mutants show pleiotropic effects on growth, conidiation, sexual development and appressorium formation. Defects in appressorium formation are overcome by the addition of exogenous cAMP. By-pass suppressor mutants of Mac1- phenotype, designated sum, are isolated very frequently on a variety of culture media. Protein kinase A (PKA) assays and molecular cloning show that one suppressor mutation (sum1-99) alters a conserved amino acid in the cAMP binding domain A of the regulatory subunit gene of PKA (SUM1), while other suppressor mutations act independently of PKA activity. Although suppressor mutations restore growth and enhance appressorium formation, they do not restore full pathogenicity. PKA assays demonstrate that the catalytic subunit gene, CPKA, encodes the major detectable PKA activity in M. grisea. However cpkA null mutants have none of the growth defects associated with adenylate cyclase mutations. These results suggest a model where both saprophytic and pathogenic growth of M. grisea are regulated by adenylate cyclase, but that different effectors of cAMP mediate downstream effects specific for either cell morphogenesis or pathogenesis.

References
1. Mitchell TK, Dean RA, 1995. Plant Cell 7, 1869-1878.
2. Xu J-R, Urban M, Sweigard JA, Hamer JE, 1997. Molecular Plant-Microbe Interactions 10, 187-194.