5.3 hrp Genes and Their Function

This chapter briefly discusses the hrp genes and their functions. The presence of hrp genes appears to be the characteristic of phytopathogenic Erwinia, Pseudomonas, Xanthornonas, and Rulstoniu spp. These Gram-negative bacteria cause diseases in virtually all crop plants, although individual strains vary considerably in their host specificity. The most highly specific strains are found in Pseudomonas syringae and Xanthornonas campestris, two species that are subdivided into pathovars and then races based on their specificity for plant species and then cultivars. The hrp genes carried by these bacteria are so named, because mutations in them can abolish two important plant-related bacterial phenotypes: hypersensitive response (HR) elicitation in non-hosts and pathogenicity in hosts. The HR is a rapid localized active death of plant cells in contact with potentially pathogenic bacteria and is associated with the failure of these bacteria to continue multiplying or to produce disease symptoms. hrp genes direct the production and secretion of protein elicitors of the HR. The HR is a highly variable biological phenomenon, and even plants with the same genotype can vary in their response. Factors reducing responsiveness are low-light intensity, wilting, and previous infection by pathogens that trigger systemic-acquired resistance. The chapter also determines the functions of hrp genes in regulation.