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Spatial Colonization Patterns and Interaction of Bacteria on Inoculated Sugar Beet Seed. R. Fukui, Division of Entomology and Plant and Soil Microbiology, University of California, Berkeley 94720, Present address: Department of Plant Pathology, University of Hawaii at Manoa, Honolulu 96822; E. I. Poinar(2), P. H. Bauer(3), M. N. Schroth(4), M. Hendson(5), X. -L. Wang(6), and J. G. Hancock(7). (2)(4)(5)(6)(7)Division of Entomology and Plant and Soil Microbiology, University of California, Berkeley 94720; (3)Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523. Phytopathology 84:1338-1345. Accepted for publication 18 August 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1338.

Development and spatial distribution of microcolonies of Pseudomonas spp. and Bacillus subtilis GB03 inoculated singly and in combination on sugar beet (Beta vulgaris) seed were observed with a scanning electron microscope (SEM). SEM examination of seed directly after inoculation with Pseudomonas strain 33-2 or ML5 at population densities of approximately 104 cfu per seed revealed a random distribution of individual cells. By 24 h, when population densities had reached the stationary phase (approximately 106 cfu per seed), microcolonies had developed in a random pattern over the seed surface. However, even at these populations, only 1040% of the seed surface was colonized. Most microcolonies developed as separate entities on the indented surface of cells of the perianth and the operculum. The colonization patterns at 48 h were similar to those at 24 h, except that the colonies were larger. Since the number of cfu measured by dilution plating (detectable population) was similar at both time periods, it was assumed that many cells were dead or dormant in the larger microcolonies. The spatial colonization patterns were entirely different, depending on the density of the initial inoculum. The entire seed surface was covered when sufficient inoculum was applied to attain a detectable population size of approximately 107 cfu per seed. Yet, even when the detectable population size increased to 107 cfu per seed following growth from an initial inoculum density of 104 cfu per seed, only 4050% of the seed surface was colonized. This indicates the need for differentiating among live, dormant, and dead cells. The spatial colonization pattern of strain GB03 differed greatly from Pseudomonas strains. At temperatures favoring its growth, microcolonies of GB03 were located primarily near the basal pore of the seed, whether inoculated singly or coinoculated with Pseudomonas putida 33-2. In coinoculations, few microcolonies of 33-2 developed near the basal pore. However, this localized interaction could not be detected by dilution plating of bacteria from the spermosphere, demonstrating the difficulty of determining microbial interactions without visual examination. The overall conclusion from the study was that the spatial distribution patterns of developing and established microbial colonies of pseudomonads are such that little direct interaction occurs in the spermosphere unless massive amounts of inocula are present, and that availability of nutrients is the limiting factor in population size. An interaction was detected with GB03 only at 37 C.