ROOT ELONGATION IN VARIOUS AGRONOMIC CROPS BY THE PLANT GROWTH PROMOTING RHIZOBACTERIUM PSEUDOMONAS PUTIDA GR12–2

Abstract

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.