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 GR 12–2 lacd 68 (deficient in the activity of the enzyme 1-aminocyclopropane-1-carboxylate deaminase) alone and in conjunction with either an inhibitor of ethylene biosynthesis, L-α-(aminoethoxyvinyl)-glycine (AVG), or the chemical ethylene 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 GR 12–2 and/or AVG but not with the mutant strain, in comparison with a MgSO 4 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 GR 12–2 is a consequence of inhibition of ethylene production within the developing seedling.