ACC Deaminase is Central to the Functioning of Plant Growth Promoting Rhizobacteria

Abstract

In addition to the more well known mechanisms that are used by plant growth promoting rhizobacteria (PGPR), these organisms contain the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which has no known function in bacteria, and use this enzyme to lower plant ethylene levels resulting in an increase in root length. Mutants of one PGPR strain that lack ACC deaminase activity were no longer able to promote the elongation of canola seedling roots. All bacterial isolates that are able to grow on ACC as a sole nitrogen source contain ACC deaminase and promote root elongation. The effect of ACC deaminase-containing PGPR on canola seedlings is identical to the effect of the ethylene inhibitor AVG. Treatment of canola seeds with ACC deaminase-containing PGPR results in a lowering of ACC levels in both roots and shoots. When an ACC deaminase gene was expressed in soil pseudomonads that did not contain this enzyme activity and did not stimulate canola root elongation, the transformants gained both ACC deaminase activity and canola root elongation activity. These data are explained in terms of a model in which a PGPR binds to canola seed coats and, during seed imbibition, the bacterium sequesters and then hydrolyzes ACC from the seed, thereby lowering the level of ethylene that can form during early plant development.