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Growth of Genetically Altered Pseudomonas solanacearum in Soil and Rhizosphere

Dep. of Agronomy, 3111 Plant Sciences, Univ. of Georgia, Athens, GA 30602

Joseph W. Williamson

USDA-ARS, Russell Research Center, Athens, GA 30613

Mark A. Schell

Dep. of Microbiology, Univ. of Georgia

*Corresponding author.

ABSTRACT

The extent to which a bacterium can grow in soil and in the rhizosphere is influenced by the presence or absence of various genes. The effect of genetic alteration of the egl gene, encoding a ß-1,4-endoglucanase, on the growth of Pseudomonas solanacearum E.F. Smith (the causal agent of bacterial wilt) in soil and in the rhizosphere of tomato (Lycopersicon esculentum Mill.), common purslane (Portulaca oleracea L.), and pearl millet (Pennisetum glaucum (L.) R. Br.) was determined. Endoglucanase production by P. solanacearum was either inactivated by transposon (Tn5) insertion mutagenesis or enhanced twofold by increasing the egl copy number with a recombinant plasmid (pHE3). In a test of the stability of the genetic alterations in two different soils, the transposon in the chromosome was detectable for >120 d, but the recombinant plasmid was apparently lost after 14 d in one soil and after 120 d in the other. In comparison to the wild-type strain, the generation times of the strains with the transposon and the recombinant plasmid increased 16 and 25%, respectively, in one soil solution and 35 and 53% in the other soil solution; this correlated well with the ability of the strains to grow in the same freshly moistened soils. Under conditions of soil inoculation, the genetically altered strains did not wilt tomato seedings significantly faster or slower than the wild-type strain. Although all of the P. solanacearum strains, including the wild type, grew in the rhizosphere of gnotobiotically grown tomato plants, none of the strains grew in the rhizosphere of tomato, common purslane, or pearl millet under nonsterile conditions. The data suggest that genetic alterations slow the growth of P. solanacearum in freshly moistened soil, and that endoglucanase is of minor importance to the growth of P. solanacearum under these conditions.

Received for publication November 9, 1989.