Plant growth-promoting rhizobacteria (PGPR) enhance plant development by many mechanisms. Indirect growth effects result from PGPR activities that displace soilborne pathogens and thereby reduce disease. Direct effects include improved nutrition, reduced disease due to activation of host defenses, and bacterial production of phytohormones. An understanding of the mode of action is essential for exploitation of PGPR for field use. For instance, bacteria that act as biological control agents can only be of benefit at locations where disease occurs. PGPR that stimulate plant growth directly will likely have more universal uses and greater impacts. Thus, we have been developing model systems for identifying PGPR with such traits. In this presentation, the effects of bacterization of tissue culture-grown plants, plug transplants, and seed with a growth-promoting Pseudomonas sp. (PsJN) will be described. Potential uses for this and other PGPR will also be identified. The talk will consider the advantages and limitations of: a) screening methods used for selection of PGPR, b) model systems available for studying the mechanisms of action, and c) why transplants offer an ideal delivery system for rhizobacteria. Results from field trials with PGPR with different modes of action will be presented and their future role in agriculture considered.