Effect of soft rot erwinias on micropropagated potato plantlets and greenhouse plants and characterization of a factor inhibiting root elongation
Creators: name: Lan, Xuesong
Publisher: name: University of British Columbia
Year: 2009

description: Symptoms induced by soft rot erwinias were observed in micropropagated potato plantlets of cultivar Russet Burbank, Teton, and Urgenta grown from nodal cuttings inoculated with Erwinia carotovora subsp. atroseptica strain 31 (Eca 31) in preliminary experiments. The development of short roots, stunting, and yellowing in inoculated plantlets was evident after 25—30 days of growth in a tissue culture growth chamber. Two strains from each of E. c. subsp. atroseptica and carotovora, and E. chrysanthemi were tested individually for their effect on the growth of micropropagated potato plantlets by a medium—inoculation method. All strains tested significantly inhibited root growth of micropropagated plantlets of cv. Kennebec. Most strains, except for E. c. subsp. atroseptica strain 196 and E. c. subsp. carotovora strain 21 caused stunting. Aerial root formation was caused by all soft rot erwinia strains. Root tip browning, stem tip blackening, leaf curling and lesions which appeared as black necrotic dots also occurred, although the frequencies were low within each experiment. Root growth inhibition, stem length reduction and severity of yellowing increased as the inoculum concentration increased. Root elongation was dramatically inhibited by Eca 31 at inoculum concentrations between 0.2—20 colony forming units (cfu)/mL in tissue culture growth medium. All four cultivars Kennebec, Russet Burbank, Red Pontiac, and Red Lasoda, responded similarly to the low bacterial concentration with respect to stem length, but differently to the high inoculum concentration. The roots of cv. Red Lasoda were more susceptible to Eca 31 at 0.2 cfu/mL, but were more resistant at 20 cfu/mL than those of the three other cultivars. The concentration of bacterial antigens at stem ends of the four cultivars increased directly with an increase in inoculum concentration as indicated by the indirect enzyme—linked immunosorbent assay (ELISA) . Electron microscopy revealed that a large population of Eca 31 cells covered the epidermal surface of the root, and that a large number of bacteria was present in the intercellular space of root epidermal cells and in the vascular bundles of roots. The inhibition of root elongation and reduction in root dry weight occurred in potato plants grown from seed pieces in soil inoculated with E. c. subsp. atroseptica after 6 days of growth at 18°C in a greenhouse. The roots were inhibited by up to 50% at an inoculum concentration of 1.1 x 10⁷ cfu/g soil. Stunting occurred, but no other foliage symptoms were observed. The root inhibitory factor was shown to be present in bacterial supernatant of 5-day old nutrient broth cultures of Eca 31. The inhibition of root elongation was directly correlated with supernatant concentration in a “Petri dish bioassay”. The factor responsible for root inhibition was relatively resistant to heat, of high molecular weight (&gt; 12000—14000), and precipitable by ammonium sulphate at 70% saturation.