Differential accumulation of mRNAs encoding extracellular and intracellular PR proteins in tomato induced by virulent and avirulent races of Cladosporium fulvum.

Tomato leaves infected by the fungal pathogen Cladosporium fulvum contain several types of intracellular and extracellular pathogenesis-related (PR) proteins. Previously, we reported the purification and serological characterization of five extracellular PR proteins: P2, P4, P6, a chitinase and a beta-1,3-glucanase [22, 23]. Here we describe the purification of a basic intracellular 33 kDa beta-1,3-glucanase and the isolation and characterization of cDNA clones encoding the two extracellular P14 isomers P4 and P6, the extracellular acidic beta-1,3-glucanase and a basic 35 kDa beta-1,3-glucanase, different from the purified 33 kDa protein. Southern blot analysis demonstrated that tomato PR proteins are not encoded by large gene families, as is the case in tobacco. The number of genes corresponding to each protein was estimated to vary between one and three. A northern blot analysis indicated that the mRNAs for the extracellular PR proteins (P4, P6 and acidic beta-1,3-glucanase) accumulate to similar levels in compatible and incompatible tomato-C. fulvum interactions, although the maximum level of expression is reached much faster in the incompatible interaction. On the other hand, the mRNA for the basic 35 kDa beta-1,3-glucanase is induced rapidly to high levels in both interactions, but declines in time to background levels only in the incompatible interaction. The relevance of this difference in relation to plant defence is discussed.