The pollen-stigma interaction in the grasses. 4. An interpretation of the self-incompatibility response

Features of the self-incompatibility system in the grasses are reviewed, and related to characteristics of the stigma and pollen tube growth. From a consideration of the existing data, a general hypothesis is advanced for the self-incompatibility reaction, based upon the following propositions: (A) the pistil-side incompatibility factors are proteins (probably glycoproteins) with lectin-like properties present in the stigma surface secretions and in the transmitting tracts; (B) their binding specificities are such that they are complementary to sugar sequences or arrays displayed by wall carbohydrate in the growth zone of incompatible pollen tubes, but not complementary to those presented by the compatible tubes; (C) binding at the tip of an incompatible tube leads to a disruption of apical growth by preventing the dissociation of the polysaccharide content of the wall precursor bodies and interfering with the extension of polysaccharide microfibrils in the sub-apical zone. The implications of the hypothesis are briefly discussed, and also its relevance to other gametophytic self-incompatibility systems.