The impact of Pseudomonas fluorescens strains on aspen growth during phosphorus and nitrogen limitation
Symbiotic relationships influence survival and productivity in forest ecosystems and consequently affect ecosystem carbon- nutrient cycling. In addition to fungal associations, some bacteria can also play a major role in nutrient uptake and plant protection from pathogens. Many strains of Pseudomonas fluorescens are characterized as mycorrhizal helper bacteria (MHB) based on their ability to promote mycorrhizal formation and enhance plant productivity in low nutrient soils. To better understand the role of these MHB-plant associations, we examined the impact of several P. fluorescens strains (WH6, SBW-25, Pf0-1, and PF-5) on the in vitro growth of aspen seedlings at control versus low concentrations of nitrogen (4mM versus1mM) and phosphorous (1.5mM versus 20µM). Control and nutrient limited seedlings were grown with and without the MHB on vertical petri-plates in controlled environmental conditions. While growth rate and morphology were assessed as primary indicators of plant health, number of leaves, plant shoot height, root length and root structure such as branching pattern and rootlets, were also evaluated weekly.
Nutrient limitation was observed to decrease the number of leaves, but increase root length and branching structure. The number of seedling rootlets decreased significantly during phosphorus stress, but increased during nitrogen stress. The effects of MHB colonization on plant growth and root morphology were found to be strain dependent. Root length, for example increased in control plants colonized with PF-5 and Pf0-1, while plants colonized with SBW-25 exhibited enhanced root branching relative to the controls. An increase in the number of leaves was observed in control seedlings colonized with all bacterial strains. Similar strain dependent effects were observed under nutrient limiting conditions. Under phosphorus stress, root length increased significantly in aspen colonized by PF-5, Pf0-1 and SBW-25, while the number of leaves was higher in plants colonized with WH6 relative to the control seedlings. More complex root branching was observed for seedlings under phosphorus and nitrogen stress in comparison to the control seedlings when plants were colonized by Pf0-1 and SBW-25. These preliminary results demonstrate some of the profound effects of MHB on plant growth and morphology. Further understanding of nutrient limitation effects is expected to increase our insight into community structure and carbon cycling in terrestrial ecosystems.