Recognition in flowering plants: A comparison of the Brassica self‐incompatibility system and plant pathogen interactions

Abstract

summaryMorphological, genetic, and biochemical data on the sporophytic self‐incompatibility (SI) system of Brassica spp. are evaluated and compared with those from host‐pathogen (HP) interactions which conform to the gene‐for‐gene hypothesis and are characterized by the production of phytoalexins.SI and HP systems have many features in common, reflecting similarities in the processes of discrimination, particularly with respect to the response to an incompatible recognition event. Thus, both recognition reactions involve cell surface components which are likely to be glycoproteins in at least one of the partners. Both incompatible responses are active processes requiring enzyme synthesis, are biostatic rather than biocidal with respect to pollen and pathogen, and are accompanied by the production of low molecular weight phytoalexin‐like compounds. Recent evidence also suggests that incompatible HP interactions may involve recognition of ‘self’ cell wall components. These similarities are distinguished from those, such as the production of cuticle and cell wall degrading enzymes, which may result only from the common requirement of pollen and pathogen to penetrate through the plant surface. The significance of the most obvious differences between the two systems, such as the evidence that SI may be mediated through the supply of water lo the pollen grain and the absence, in SI interactions, of cell necrosis or ‘cross protection’ reactions is also explored. It is suggested that some of the observed differences may result from ‘pre‐programming’ of the SI response.