Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 10, 1998
Publication Date: N/A
Interpretive Summary: Plant roots are colonized by soil fungi that improve plant growth through an increased uptake of mineral nutrients from the soil. They also improve soil stability through the transport of organic nutrients from the plant to the soil, where soil organisms help the plant roots and their fungal associates to form stable soil aggregates. Some plants prefer certain fungi, and some of these fungi may prefer to associate with certain bacteria. We have introduced bacteria that were taken from different soil environments into soils colonized by one root-soil fungus, and showed that the bacteria preferred to grow in soil that was similar to the soil from which they originated. The introduction of these bacteria also affected other bacteria native to the soil. These results show that bacterial communities occur and function in soils not in a random, haphazard manner, but are probably organized into structures of mutual preference with each other and with other soil organisms, especially roots and root-soil fungi. This is an important consideration when bacteria are used to control pathogens or to promote plant growth. These processes are important for the management of agricultural systems, because only an understanding of the relationships between soil minerals and soil organisms can guarantee that beneficial organisms will dominate under benign soil conditions.
Technical Abstract: Mycorrhizal fungi may form hierarchial structures of mutual preference with soil bacteria. Elucidation of such structures would facilitate managing of the soil biota to enhance the stability of the plant-soil system. We utilized two isolates of soil bacteria to determine their responses to distinct mycorrhizal regions of the root zone. Sorghum plants were grown in nfour-part containers, where soils in individual compartments held either roots colonized by arbuscular-mycorrhizal (AM) fungus Glomus mosseae (M), roots only (R), hyphae of the AM fungus (H), or no mycorrhizal structures (S). The soils were inoculated with antibiotic-resistant (rifampicin, rif; streptomycin, sm) strains of one of two soil bacteria, Alcaligenes eutrophus (rif**r50) and Arthrobacter globiformis (sm**r250), or left uninoculated. A. eutrophus had been isolated from hyphosphere soils of G. mosseae, and A. globiformis from nonspecific mycorrhizosphere soil. After 10 weeks, A. eutrophus was barely detectable in nonAM soils, but persisted well in AM soils. Numbers of A. globiformis were more evenly distributed in all soils, but were highest in the presence of AM roots (M soil). Bacterial effects on root and AM-hyphal development varied: A. eutrophus decreased hyphal length in H soil, while A. globiformis stimulated root length in M soil. The two bacterial inoculants did not affect numbers of the soil bacterium Pseudomonas fluorescens in H, R, and M soils, but the AM status of the soils did: the numbers of P. fluorescens increased in the order M>R>H>S. The hyphosphere-specific A. eutrophus depended on the presence of G. mosseae as its host, but the non-specific A. globifomis did not. The mycorrhizal status of soils may selectively influence the persistence of bacterial inoculants and affect other native bacteria.