The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), whose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in Arabidopsis thaliana. The pathogeninducible genes PR-1, PR-2, and PR-5 require SA signaling for activation, whereas the plant defensin gene PDF1.2, along with a PR-3 and PR-4 gene, are induced by pathogens via an SA-independent and JA-dependent pathway. An Arabidopsis mutant, coi1, that is affected in the JA-response pathway shows enhanced susceptibility to infection by the fungal pathogens Alternaria brassicicola and Botrytis cinerea but not to Peronospora parasitica, and vice versa for two Arabidopsis genotypes (npr1 and NahG) with a defect in their SA response. Resistance to P. parasitica was boosted by external application of the SA-mimicking compound 2,6-dichloroisonicotinic acid [Delaney, T., et al. (1994) Science 266, 1247-1250] but not by methyl jasmonate (MeJA), whereas treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resistance to Alternaria brassicicola. The protective effect of MeJA against A. brassicicola was the result of an endogenous defense response activated in planta and not a direct effect of MeJA on the pathogen, as no protection to A. brassicicola was observed in the coi1 mutant treated with MeJA. These data point to the existence of at least two separate hormone-dependent defense pathways in Arabidopsis that contribute to resistance against distinct microbial pathogens.

The avirulence gene Avr4 conditions avirulence of the biotrophic fungus Cladosporium fulvum on tomato genotypes carrying resistance gene Cf-4 (MM-Cf4). Strains of the fungus that circumvent Cf-4-specific resistance show various single point mutations in the coding region of the Avr4 gene. Similar to expression of the Avr4 gene, expression of the various virulent avr4 alleles is specifically induced during pathogenesis. Polyclonal antibodies raised against the AVR4 elicitor, however, did not detect AVR4 isoforms in MM-Cf4 plants infected by the different virulent strains, indicating that these isoforms are unstable. To analyze whether the AVR4 isoforms still possess specific elicitor activity, the avr4 alleles were expressed in MM-Cf4 plants by using the potato virus X (PVX)-based expression system. Inoculation with PVX::Avr4 resulted in the development of spreading lesions, eventually leading to plant death, whereas the various PVX::avr4 derivatives induced symptoms ranging from severe necrosis to no lesions at all. We conclude that instability of the AVR4 isoforms that are produced by virulent strains is a crucial factor in circumvention of Cf-4-mediated resistance.

The avirulence gene Avr4 conditions avirulence of the biotrophic fungus Cladosporium fulvum on tomato genotypes carrying resistance gene Cf-4 (MM-Cf4). Strains of the fungus that circumvent Cf-4-specific resistance show various single point mutations in the coding region of the Avr4 gene. Similar to expression of the Avr4 gene, expression of the various virulent avr4 alleles is specifically induced during pathogenesis. Polyclonal antibodies raised against the AVR4 elicitor, however, did not detect AVR4 isoforms in MM-Cf4 plants infected by the different virulent strains, indicating that these isoforms are unstable. To analyze whether the AVR4 isoforms still possess specific elicitor activity, the avr4 alleles were expressed in MM-Cf4 plants by using the potato virus X (PVX)-based expression system. Inoculation with PVX::Avr4 resulted in the development of spreading lesions, eventually leading to plant death, whereas the various PVX::avr4 derivatives induced symptoms ranging from severe necrosis to no lesions at all. We conclude that instability of the AVR4 isoforms that are produced by virulent strains is a crucial factor in circumvention of Cf-4-mediated resistance.

Chickpea (Cicer arietinum L.) cell-suspension cultures were used to isolate one beta-1,3-glucanase (EC 3.2.1.29) and two chitinases (EC 3.2.1.14). The beta-1,3-glucanase (M(r) = 36 kDa) and one of the chitinases (M(r) = 32 kDa) belong to class I hydrolases with basic isoelectric points (10.5 and 8.5, respectively) and were located intracellularly. The basic chitinase (BC) was also found in the culture medium. The second chitinase (M(r) = 28 kDa), with an acidic isoelectric point of 5.7, showed homology to N-terminal sequences of class III chitinases and represented the main protein accumulating in the culture medium. Polyclonal antibodies raised against the basic beta-1,3-glucanase (BG) and the acidic chitinase (AC) were shown to be monospecific. The anti-AC antiserum failed to recognize the BC on immune blots, confirming the structural diversity between class I and class III chitinases. Neither chitinase exhibited lysozyme activity. All hydrolases were endo in action on appropriate substrates. The BC inhibited the hyphal growth of several test fungi, whereas the AC failed to show any inhibitory activity. Expression of BG activity appeared to be regulated by auxin in the cell culture and in the intact plant. In contrast, the expression of neither chitinase was apparently influenced by auxin, indicating a differential hormonal regulation of beta-1,3-glucanase and chitinase activities in chickpea. After elicitation of cell cultures or infection of chickpea plants with Ascochyta rabiei, both system were found to have hydrolase patterns which were qualitatively and quantitatively comparable.(ABSTRACT TRUNCATED AT 250 WORDS)