The Role of Recognition in Plant Disease

Abstract

As noted by Sequeira (110,111), the term recognition has become very popular in the literature of host-pathogen interactions over the last ten years. Despite recent facile usage, its meaning is not always clear to me. Recognition is defined by Clarke & Knox (16) as the initial in cell-cell communication that elicits a defined biochemical, physiological, or morphological response. This requires the not-so-easy task of defining initial event and communica­ tion with precise chemical or physical mechanisms. For pathologists, Se­ queira (110) has provided a more restricted meaning: An early specific that triggers a rapid, overt response by the host, either facilitating or impeding further growth of the pathogen. The word specific in this definition is the key to both the current popularity of the term and the recent appearance of models of host-pathogen interactions predicated on recognition phenomena. It is trite to repeat, but any chemical or physical theory of disease reaction (resistance or susceptibility) must account for the specificity of that reaction (19). In molecular terms, it also should be consistent with the mode of inheritance of the reaction (18). In recent years, the latter requirement has entailed, for many workers, allegiance with Flor's gene-for-gene concept (43), as further amplified by the considerations of Person (102), Day (27), Ellingboe (40, 41), and Sidhu (114) . The genetic data, although complete for only a relative small number of plant diseases (19), is generally interpreted to mean that the product of a host resistance gene interacts