IRON STRESS AND PYOVERDIN PRODUCTION BY A FLUORESCENT PSEUDOMONAD IN THE RHIZOSPHERE OF WHITE LUPINE (LUPINUS-ALBUS L.) AND BARLEY (HORDEUM-VULGARE L.)

Induction of high-affinity iron transport during root colonization by Pseudomonas fluorescens Pf-5 (pvd-inaZ) was examined in lupine and bar ley growing in microcosms, P. fluorescens Pf-5 (pvd-inaZ) contains a p lasmid carrying pvd-inaZ; thus, in this strain, ice nucleation activit y is regulated by pyoverdin production, Lupine or barley plants were g rown for 18 or 8 days, respectively, in soil amended with 2% calcium c arbonate and inoculated with P. fluorescens Pf-5 (pvd-inaZ) at a densi ty of 4 x 10(8) CFU g (dry weight) of soil(-1). A filter paper blottin g technique was used to sample cells from the rhizosphere in different root zones, and then the cells were resuspended for enumeration and m easurement of ice nucleation activity, The population density of P. fl uorescens Pf-5 (pvd-inaZ) in the rhizosphere decreased by one order of magnitude in both lupine and barley over time, The ice nucleation act ivity ranged from -3.4 to -3.0 log ice nuclei CFU-1 for lupine and -3. 0 to -2.8 log ice nuclei CFU-1 for barley, was similar in all root zon es, and did not change over time, An in vitro experiment was conducted to determine the relationship between ice nucleation activity and pyo verdin production in P. fluorescens Pf-5 (pvd-inaZ), An ice nucleation activity of approximately -3.0 log ice nuclei CFU-1 was measured in t he in vitro experiment at 25 to 50 mu M FeCl3. By using the regression between ice nucleation activity and pyoverdin production determined i n vitro and assuming a P. fluorescens Pf-5 (pvd-inaZ) population densi ty of 10(8) CFU g of root(-1), the maximum possible pyoverdin accumula tion by P. fluorescens Pf-5 (pvd-inaZ) in the rhizosphere was estimate d to be 0.5 and 0.8 nmol g of root(-1) for lupine and barley, respecti vely, The low ice nucleation activity measured in the rhizosphere sugg ests that nutritional competition for iron in the rhizosphere may not be a major factor influencing root colonization by P. fluorescens Pf-5 (pvd-inaZ).