Methods for stimulating plant growth and/or promoting plant health using free enzymes or recombinant microorganisms that overexpress enzymes are provided. Plant seeds coated with free enzymes or recombinant microorganisms that overexpress enzymes are also provided. Compositions comprising a fertilizer and an enzyme or a recombinant microorganism that overexpresses an enzyme are provided. Modified enzymes having ACC deaminase activity, recombinant microorganisms expressing the modified enzymes, plant seeds treated with the modified enzymes or recombinant microorganisms, and methods for stimulating plant growth and/or promoting plant health using the modified enzymes or recombinant microorganisms are also provided.