AVAILABILITY OF IRON TO PSEUDOMONAS-FLUORESCENS IN RHIZOSPHERE AND BULK SOIL EVALUATED WITH AN ICE NUCLEATION REPORTER GENE

The biological availability of iron in the rhizosphere was assessed by evaluating ice nucleation activity (INA) expressed in situ by Pseudom onas fluorescens Pf-5 containing a transcriptional fusion (pvd-inaZ) o f an iron-regulated promoter to an ice nucleation reporter gene (inaZ) , Pf-5 containing pvd-inaZ expresses INA that is inversely related to the iron availability of a growth medium (J. E. Loper and S. E. Lindow , Appl. Environ. Microbiol, 60:1934-1941, 1994), INA expressed by rhiz osphere populations of Pf-5 containing pvd-inaZ was at a maximum withi n 12 to 24 h following inoculation of the bacterium onto bean roots an d typically decreased gradually during the following 4 days, Iron avai lability in the soil, which was altered by the addition of chelators, influenced INA expressed by rhizosphere populations of Pf-5 containing pvd-inaZ, In soil adjusted to a pH of 7.0 or 8.0 by adding Ca(OH)(2), rhizosphere populations of Pf-5 containing pvd-inaZ expressed greater INA, indicating lower iron availability, than they did in the nonamen ded soil at a pH of 5.4, Similarly, rhizosphere populations of Pf-5 co ntaining pvd-inaZ expressed less INA in an agricultural soil of pH 5.4 than in other agricultural soils ranging in pH from 6.4 to 7.7, These results conform to the predictions of chemical models stating that pH is a major factor influencing iron availability in soil solutions, Th e results of this study indicate that P. fluorescens Pf-5 encountered an iron-limited environment immediately after it was inoculated onto b ean roots planted in agricultural field soils. One to two days after t he bacterium was inoculated onto root surfaces, however, iron became m ore available to rhizosphere populations of Pf-5, We speculate that ir on acquisition systems of plants and other rhizosphere organisms may p rovide available sources of iron to established rhizosphere population s of P. fluorescens.