Movement of Pseudomonas aureofaciens from the rhizosphere to aerial plant tissue

Following inoculation onto seeds, the rhizobacterium Pseudomonas aureofaciens Ps3732RNL11 (L11), which contains the constitutively expressed lacZ and lacY genes from Escherichia coli, was recovered from the interior of aerial tissues of all 16 monocot and dicot plants tested, and the exterior of aerial surfaces of 15. In more detailed studies with corn, wheat, and broccoli, both Ps3732RNL11 and its nonengineered parent strain PS3732RN (RN) rapidly established large populations on all root systems and smaller densities within the aerial tissues, all of which persisted at stable levels throughout 12- to 23-day test periods. There were no differences in the behavior of L11 and RN on any of the three plant species. L11 invaded the aeriel tissues of corn in at least two distinct ways. First, it moved into the interior of leaves following inoculation of guttation drops, suggesting that the bacteria may contaminate the developing shoot prior to its emergence from the soil and then invade through natural openings. However, when this route was blocked by inoculating the roots after shoot emergence in either soil or hydroponic systems, the bacteria still invaded the aerial tissues within 24 h, suggesting direct vascular transport from the roots. Such bacterial movement is an important consideration in future field releases of both native and genetically modified rhizobacteria.