The role of ACC deaminase in plant growth promotion

Abstract

We have proposed that plant growth-promoting bacteria, that possess the enzyme ACC (1-aminocyclopropane-1-carboxylic acid) deaminase, can reduce the amount of ethylene produced by the plant and promote root elongation. Ethylene is required by many plants for seed germination but high levels of ethylene produced by fast growing roots can inhibit root elongation. In this study, levels of ACC, the immediate precursor of ethylene, were measured in canola seedlings, and in canola seed exudates and extracts. We show that ACC is exuded by the seeds and that, in the presence of the plant growth-promoting bacterium, Enterobacter cloacae CAL3, ACC levels in both exudates and extracts are lowered.

Enterobacter cloacae CAL3, the plant growth-promoting bacterium used in this study, produces low levels of ACC deaminase activity and is capable of synthesizing high levels of IAA (indole acetic acid). An attempt was made to isolate the ACC deaminase gene from this bacterium. Four clones, based on their growth on minimal medium containing ACC as the sole source of nitrogen, were selected from the Enterobacter cloacae CAL3 clone bank for DNA sequence analysis. None of the derived amino acid sequences of the DNA inserts were homologous with known ACC deaminase genes but were homologous with genes encoding enzymes or functional proteins that might enhance the growth of the bacterium or the uptake of ACC.