G. ANDRADE2, ROBERT LINDERMAN1 & GABOR BETHLENFALVAY1
1.U.S. Department of Agriculture, Agricultural Research Service, Horticultural Crops Research Laboratory, 3420 NW Orchard Avenue, Corvallis, OR 97330, USA. 2Universidade Estadual de Londrina, CCB, Depto. Microbiologia, cx. Postal 6001, Londrina, PR, BRAZIL 86051-970.
Roots and mycorrhizal fungi may not associate with soil bacteria randomly, but rather in a hierarchical structure of mutual preferences. Elucidation of such structures would facilitate the management of the soil biota to enhance the stability of the plant-soil system. We conducted an experiment utilizing two isolates of soil bacteria to determine their persistence in distinct mycorrhizal regions of the root zone. Split-root sorghum (Sorghum bicolor L.) plants were grown in four-compartment containers, constructed so that the soils in individual compartments held either. Roots colonized by the arbuscular-mycorrhizal (AM) fungus Glomus. mosseae (M), 2. nonAM roots only (R), 3. hyphae of G. mosseae (H), or 4. no mycorrhizal structures (S). The soils were inoculated with antibiotic-resistant (rifampicin, rif; streptomycin, sm) strains of the soil bacteria, Alcaligenes eutrophus (rif r 50) or Arthrobacter globiformis (smr 250), or were left uninoculated as control . A. eutrophus had been isolated from a specific source (hyphosphere soil of G. mosseae), and A. globiformis from nonspecific mycorrhizosphere soil. After 10 weeks of growth, the presence of A. eutrophus was barely detectable (<10 cfu g-1 dry soil) in nonAM (R and S) soils, but persisted well (104 cfu g-1 dry soil) in AM (H and M) soils. Numbers of A. globiformis were more evenly distributed in all soils, but were highest in the presence of AM roots (M soil): A. eutrophus decreased hyphal length in H soil, while A. globiformis stimulated root length in M soil. The results indicate that the hyphosphere-specific A. eutrophus depended on the presence of G. mosseae as its AM-fungal host, but that the nonspecific A. globiformis did not. The mycorrhizal status of soils may selectively influence persistence of bacterial inoculants as well as affecting the numbers of other native bacteria.
Key words: Glomus mosseae, Pseudomonas, hyphosphere, mycorrhiza, mycorrhizosphere, rhizobacteria, rhizosphere.