A gnotobiotic bioassay for studying interactions between potatoes and plant growth-promoting rhizobacteria

A gnotobiotic bioassay, using potato plantlets derived from single-node explants and grown in test tubes containing potato nodal cutting medium (PNCM), was found to be highly useful for investigations of direct growth promotion by a nonfluorescent Pseudomonas sp. strain PsJN. Strain PsJN was used to optimize and evaluate this bioassay for purposes of screening other rhizosphere bacteria and identification of Tn5 mutants of strain PsJN deficient in growth-promoting properties. The selection of potato cultivar used in this bioassay was critical, as growth promotion of potatoes by strain PsJN was cultivar specific. Inoculated plantlets of cultivars Norchip, Kennebec, Shepody, and Chaleur showed, in root dry weight, a five-to eight-fold increase, two-to three-fold increase, no response, and a decrease of 50%, respectively. Haulm dry weight followed similar trends but was not as consistent an indicator of growth promotion. Bioassay results were not altered to any extent by minor changes in PNCM composition or by slight changes in temperature and light conditions. A rapid method for preparation of bacterial suspensions and inoculation of explants was developed. Inoculation of three explants taken from 6-week-old stock plantlets of cv. Kennebec for each Tn5 transconjugate of strain PsJN (total of 1500 transconjugates) enabled the elimination of 93% of those isolates that retained growth-promoting activity. The remaining 7% of isolates were retested and seven were confirmed to have lost growth-promoting ability. Bacteria from different genera were also screened with this bioassay. None of these bacteria increased the growth of potato plantlets, but several inhibited root and haulm growth.