ANALYSIS OF EXPRESSION OF A PHENAZINE BIOSYNTHESIS LOCUS OF PSEUDOMONAS-AUREOFACIENS PGS12 ON SEEDS WITH A MUTANT CARRYING A PHENAZINE BIOSYNTHESIS LOCUS ICE NUCLEATION REPORTER GENE FUSION

A derivative of Pseudomonas aureofaciens PGS12 expressing a promoterle ss ice nucleation gene under the control of a phenazine biosynthesis l ocus was used to study the expression of a phenazine antibiotic locus (Phz) during bacterial seed colonization. Seeds of various plants were inoculated with wild-type PGS12 and a PGS12 ice nucleation-active phz :inaZ marker exchange derivative and planted in soil, and the expressi on of the reporter gene was monitored at different intervals for 48 h during seed germination. phz gene expression was first detected 12 h a fter planting, and the expression increased during the next 36-h perio d. Significant differences in expression of bacterial populations on d ifferent seeds were measured at 48 h. The highest expression level was recorded for wheat seeds (one ice nucleus per 4,000 cells), and the l owest expression level was recorded for cotton seeds (one ice nucleus per 12,000,000 cells). These values indicate that a small proportion o f bacteria in a seed population expressed phenazine biosynthesis. Repo rter gene expression levels and populations on individual seeds in a s ample were lognormally distributed. There was greater variability in r eporter gene expression than in population size among individual seeds in a sample. Expression on sugar beet and radish seeds was not affect ed by different inoculum levels or soil matric potentials of -10 and - 40 J/kg; only small differences in expression on wheat and sugar beet seeds were detected when the seeds were planted in various soils. It i s suggested that the nutrient level in seed exudates is the primary re ason for the differences observed among seeds. The lognormal distribut ion of phenazine expression on seeds and the timing and difference in expression of phenazine biosynthesis on seeds have implications for th e potential efficacy of biocontrol microorganisms against plant pathog ens.