Cell wall integrity and high osmolarity glycerol pathways are required for adaptation of Alternaria brassicicola to cell wall stress caused by brassicaceous indolic phytoalexins

Camalexin, the characteristic phytoalexin of Arabidopsis thaliana, inhibits growth of the fungal necrotroph Alternaria brassicicola. This plant metabolite probably exerts its antifungal toxicity by causing cell membrane damage. Here we observed that activation of a cellular response to this damage requires cell wall integrity (CWI) and the high osmolarity glycerol (HOG) pathways. Camalexin was found to activate both AbHog1 and AbSlt2 MAP kinases, and activation of the latter was abrogated in a AbHog1 deficient strain. Mutant strains lacking functional MAP kinases showed hypersensitivity to camalexin and brassinin, a structurally related phytoalexin produced by several cultivated Brassica species. Enhanced susceptibility to the membrane permeabilization activity of camalexin was observed for MAP kinase deficient mutants. These results suggest that the two signalling pathways have a pivotal role in regulating a cellular compensatory response to preserve cell integrity during exposure to camalexin. AbHog1 and AbSlt2 deficient mutants had reduced virulence on host plants that may, at least for the latter mutants, partially result from their inability to cope with defence metabolites such as indolic phytoalexins. This constitutes the first evidence that a phytoalexin activates fungal MAP kinases and that outputs of activated cascades contribute to protecting the fungus against antimicrobial plant metabolites.     

Sequence of a Plastocyanin cDNA From Wheat and the Use of the Gene Product to Determine Serologically Tissue Degradation after Infection with Pseudocercosporella herpotrichoidesa

Pseudocercosporella herpotrkhoides, the causal agent of the eyespot disease of wheat, induces lesions at the stetn bases, in degraded tissue a specific protein (Pc) cannot more be detected whereas it is abundantly produced in the green tissue showing no disease symptotns. In plant satnples including the stem bases, the amount of protein Pc is therefore correlated with the degree of tissue degradation. A serological test (ELiSA) was developed for the quantitative determination of the protein. ELISA readings can be used as estimates of disease severity. By sequence analysis, Pc was identified as plastocyanin. Based on the nucleic acid sequence, wheat pre-plastocyanin consists of a transit peptide of 61 amino acids and a tnature peptide of 97 residues. Comparison of the nucleotide sequences of plastocyanin of wheat and barley showed homology of 93.7%.     

NaCl and wounding induced changes in NAD reductase in hypocotyls and root tips ofPhaseolus vulgaris L.

The effect of 300 mM NaCl and wounding on the nicotinamide adenine dinucleotide (NAD) kinase, revealed by the guanosine triphosphate-dependent NAD reductase activity, was studied in two differently resistant bean cultivars using densitometric analysis of electrophoretic gels. In the presence of NaCl the total activity of NAD reductase was increased, in hypocotyls and root tips of resistant cultivar. The contribution of each of NAD reductase isoforms to the total activity was not significantly different between cultivars. Conversely, after wounding the hypocotyl, an increase could be observed in both cultivars and differences were demonstrated in the contribution of the different isoforms.