CHARACTERISTICS OF INSERTIONAL MUTANTS OF PSEUDOMONAS-SYRINGAE WITH REDUCED EPIPHYTIC FITNESS

Random Tn5 mutagenesis was used to identify genes ir Pseudomonas syrin gae which contribute to epiphytic fitness. Mutants were selected on th e basis of deficiencies in epiphytic growth or survival on plants rath er than deficiencies in predetermined phenotypes exhibited in culture. A sample freezing procedure was used to measure the population sizes of 5,300 mutants of P. syringae exposed to alternating wet and dry con ditions on bean leaves in growth chambers. Eighty-two mutants exhibite d reduced population sizes. Of these mutants, over half exhibited a re duced ability to survive the stresses associated with dry leaves, whil e others grew more slowly or attained reduced stationary-phase populat ion sizes on leaves. While some epiphytic fitness mutants were altered in phenotypes that could be measured in culture, many mutants were no t altered in any in vitro phenotype examined. Only three of the epiphy tic fitness mutants were auxotrophs, and none had catabolic deficienci es for any of 31 organic compounds tested. Other mutants that exhibite d reductions in one or more of the following were identified: motility , osmotolerance, desiccation tolerance, growth rate in batch culture, and extracellular polysaccharide production. All of the mutants retain ed the abilities to produce disease symptoms on the compatible host pl ant, bean, to incite a hypersensitive response on the non-host plant, tobacco, and to produce a fluorescent pyoverdine siderophore.