Poly-β-hydroxybutyrate metabolism in Azospirillum brasilense and the ecological role of PHB in the rhizosphere

Azospirillum brasilense is a rhizosphere microorganism which has potential use for promoting plant growth in economically important crops. Its ability to survive the adverse conditions imposed by nutrient starvation and competition in the rhizosphere is of great importance. A. brasilense accumulates up to 70% of its cell dry weight with poly-β-hydroxybutyrate (PHB). In the presence of stress factors such as ultraviolet radiation, desiccation and osmotic stress, PHB-rich cells survived better than PHB-poor cells. Polymer-rich cells of Azospirillum fixed N₂ in the absence of exogenous carbon and combined nitrogen. The enzymes of the PHB cycle in both the synthesis and degradation processes as well as during starvation were more active in PHB-rich cells. After 24 h of starvation there was a peak of activity of D(-)β-hydroxybutyrate dehydrogenase, β-ketothiolase and thiophorase due to PHB degradation. Additionally, acetoacetyl-CoA reductase dropped to a minimum level because PHB could not be synthesized. The possible utilization of PHB as a sole carbon and energy source by A. brasilense and other bacteria during establishment, proliferation and survival in the rhizosphere will be discussed.