Root elongation in various agronomic crops by the plant growth promoting rhizobacterium Pseudomonas putida GR12-2

Seeds of canola, lettuce, tomato, barley, wheat, and oats were inoculated with either the wild-type plant growth promoting rhizobacterium (PGPR), Pseudomonas putida GR12-2, or the mutant P. putida GR12-2/acd68 (defi cient in the activity of the enzyme 1-aminocyclopropane-1-carboxylate deaminase) alone and in conjunction with either an inhibitor of ethylene biosynthesis, L-alpha-(aminoethoxyvinyl)-glycine (AVG), or the chemical ethyl ene generator, (2-chloroethyl) phosphonic acid (ethophon). For the different treatments, variations in root length under gnotobiotic conditions were compared. Canola, lettuce, tomato, and wheat responded to all of the treatments in a similar manner: The root lengths increased when seeds were treated with P. putida GR12-2 and/or AVG but not with the mutant strain, in comparison with a MgSO4 control treatment, while the ethophon treatment inhibited root elongation. With barley and oat, none of the treatments had any effect on root lengths; however, when the ethophon concentration was increased, root elongation of these two plants was also inhibited. These observations are consistent with a model in which promotion of root growth by P. putida GR12-2 is a consequence of inhibition of ethylene production within the developing seedling.