Host-selective toxins and disease specificity: perspectives and progress.

The high degree of specificity that many pathogens have with their plant hosts is well-documented from field observations and genetic analysis. Hundreds of disease-specificity genes have been described in plants (resistance genes) and in pathogens (pathogenicity genes). Significant progress has been made in the genetics of specificity, in Mendelian and, more recently, molecular terms. However, a major challenge confronting molecular plant pathologists today is to understand the biochemical and physiological mechanisms controlled by specificity genes. Mendelian genetics is limited in the information it can supply about mechanisms, and although molecular genetics supplies detailed information about a gene, including its pattern of expression and the size and amino acid sequence of the product, there have been many cases, both in plant/microbe interactions and in other fields of biology, in which having a cloned gene in hand has not led to insights into the function of that gene.