Mechanisms of symptom production by foliar bacterial pathogens

Most foliar bacterial pathogens are pathovars ofPseudomonas syringae andXanthomonas campestris. Many of them live during the greater part of their life-cycle on the upper parts of the plant. In order to survive and develop in this environment, the pathogens have elaborated an array of mechanisms which enable them to penetrate the plant foliage, produce a substantial endophytic population, and use the host tissue as a nutrient source. These activities result in the formation of different categories of symptoms,e.g. necrotic and chlorotic lesions, wilting, gall formation, leaf abscission and inhibition of plant growth. The operating mechanisms of foliar phytopathogenic bacteria can be divided into four major groups: (i) production of toxins (mostly non-specific), mainly byP. syringae pathovars; (ii) excessive production of plant growth hormones; (iii) enzymatic hydrolyzing activity; and (iv) inhibition of seedling growth by unknown mechanism(s). More than one pathogenic mechanism may operate in a given leaf disease or by a single pathogen. Despite the large amount of literature describing visual, biochemical and biophysical aspects of any leaf disease, basic knowledge is fragmental and comprehensive understanding of the mode of action of any disease is lacking. It is proposed that in order to close up these gaps in our knowledge other approaches, such as molecular biology technology and more intensive use of both bacterial and plant mutants, should be incorporated in pathological studies of foliar bacterial diseases.