Effect of low pO2 and host plant on colonization of roots by Pseudomonas putida PH6 (L1019)

Abstract

Effective use of genetically altered microorganisms to enhance growth requires a full understanding of the factors governing their colonization potential, as well as survival in soil. A low oxygen concentration is known to increase leakage of substrates from roots, therefore affecting the rhizosphere bacterial population. In this study, I have evaluated the effects of low oxygen partial pressure on "total" numbers of bacteria, numbers of Pseudomonas putida PH6(L1019) associated with maize roots, and the amount of soluble carbon exuded or stored in roots. Low pO2 experiments were conducted by exposing roots of maize plants (grown in Weswood silt loam) to low pO2 in solution (hydroponic experiments), or by growing maize plants in fritted clay, and inoculating with L1019. Root and solution (from hydroponics only) samples were collected at selected time intervals, for enumeration of bacteria and determinations of total organic carbon (TOC) and hexose carbon analysis. In the hydroponic experiments, roots tended to accumulate 1.3-fold more carbon, and "total" bacterial numbers were 1.8 to 2.4-fold higher in the low pO2 treatments than under aerated conditions. However, numbers of L1019 were 1.2 to 5.4-fold higher under aerated conditions, suggesting that substrate availability was not the only factor affecting establishment of L1019. However, when similar experiments were conducted using fritted clay or with individual plants in smaller volumes of nutrient solution, results showed that low P02 conditions favored establishment of L1019, and this was related to greater carbon availability. Rhizosphere survival experiments were conducted using maize, sorghum, oat, cotton, and soybean plants grown in Weswood soil and inoculated with L1019. L1019 successfully colonized and persisted (up to 49 days) in the rhizosphere of these plants at densities of log 4.65.6 cfu/gram root dry weight. L1019 increased plant growth parameters of C3 but not of C4 plants. Selection of pseudomonads using apomictic hosts was done by growing apomictic grasses in diverse soils and identifying the isolates using fatty acid profiles. Different microbial biotypes tended to be isolated from the rhizosphere of different host plants when planted in the same soils, suggesting that the host plant was an important factor influencing the microflora present in the rhizosphere.