Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The interaction of potato explants (Solanum tuberosum ssp. tuberosum) and a rhizobacterium which was identified as a nonfluorescent Pseudomonas sp. (Ps JN) was studied under in vitro conditions. The culture medium used to propagate potato plantlets was modified such that bacterial growth was only obtained in association with plant tissues. In this medium, plantlets arising from Ps JN-inoculated nodal cuttings exhibited enhanced growth and important developmental modifications. In repeated experiments root and haulm dry weights were consistently increased (30-260%), and were used as a measure of growth stimulation. The elicited effects were unique to strain Ps JN and did not occur upon inoculation with other beneficial or nonstimulatory rhizobacteria. Bacterization also enhanced leaf hair formation (55-110%), secondary root branching (30-110%), and total plant lignin content (43%). Excised leaves from in vitro bacterized plants had more functional stomata and controlled water loss more efficiently than controls. Upon direct transplantation to the field a higher survival of bacterized plants was observed (30-64%). Treatment of potato seed pieces with strain Ps JN statistically improved plant growth in the greenhouse (35-57%), and final yield in three of four field experiments (43, 23 and 17%). Positive plant growth responses were correlated with the occurrence of high numbers of strain Ps JN in the endo and exorhizosphere of potato plants. The results produced from greenhouse and field experiments demonstrated that the developed in vitro assay gives a true measure of the growth promoting ability of strain PsJN (Pgp phenotype).;This assay was used to screen Tn5 insertion mutants of strain Ps JN for their Pgp phenotype. Seven Pgp{dollar}\sp-{dollar} mutants were identified. Pgp activity was fully regained "in vitro" and in the greenhouse upon complementation with the cosmid pMF 738 from the Ps JN wild type DNA library. Subcloning, and analyses for restoration of the Pgp phenotype defined four complementation groups. All Tn5 insertions occurred in a 14.7 kb Bam HI chromosomal DNA fragment. Mobilization of the cosmid pMF 738 to E. coli, or subclone pMF 1A (containing the 14.7 kb DNA fragment) to Pseudomonas putida resulted in the expression of the Pgp phenotype upon plant inoculation, but not in Serratia marcescens or Pseudomonas fluorescens.



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