The interaction between fungal endopolygalacturonases and a plant cell wall PGIP (PolyGalacturonase-Inhibiting Protein) in plant-pathogen recognition is being investigated. This protein-protein interaction has been shown to favour the formation of oligogalacturonides able to elicit plant defense responses. A single mutation in the endopolygalacturonase gene of Fusarium moniliforme abolishes the hydrolytic activity but does not affect the elicitor activity of the enzyme and its ability to interact with PGIP. Accumulation of pgip mRNA in different race-cultivar interactions (either compatible or incompatible) between Colletotrichum lindemuthianum and Phaseolus vulgaris has been followed by Northern blot and in situ hybridisation analyses. Rapid accumulation of pgip mRNA correlates with the appearance of the hypersensitive response in incompatible interactions, while a more delayed increase, coincident with the onset of lesion formation, occurs in compatible interactions. PGIP exhibits a modular structure: its amino acid sequence can be divided into a set of 10.5 leucine-rich tandemly repeated units (LRR=leucinerich repeats), each derived from modifications of a 24-amino acid peptide. A LRR structure has been observed in several proteins implicated in protein-protein interactions and in the extracellular domain of a cloned Arabidopsis receptor-like protein kinase (RLK5); a LRR structure has also been observed in the products of several resistance genes recently cloned. A plasma membrane-associated high molecular weight protein cross-reacting with an antibody prepared agaist PGIP is being purified in our laboratory. We suggest that PGIP may belong to a class of receptor complexes specialized for defense against microbes. © 1996 Elsevier B.V. All rights reserved.

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