EFFECT OF ROOT EXUDATES AND BACTERIAL METABOLIC-ACTIVITY ON CONJUGAL GENE-TRANSFER IN THE RHIZOSPHERE OF A MARSH PLANT

Genetic exchange between bacteria in natural habitats is believed to b e strongly influenced by availability of growth substrates and growth rate. To test this hypothesis, conjugal transfer of an RP4 derivative between Pseudomonas fluorescens and Serratia sp. was studied in a simp le sand/plant microcosm and was related to availability of root exudat es and bacterial metabolic activity. The presence of the plant (Echino chlora crusgalli) greatly stimulated transfer. Average transfer ratios (T/D.R) were 8.9 x 10(-11) in the rhizosphere and 5.5 x 10(-15) in sa nd unaffected by the plant root. The bacteria used root exudates as gr owth substrate and depending on cell density, the metabolic activity ( [H-3]leucine uptake) of the cells was higher in the rhizosphere than i n the sand (around 1.0 x 10(-2) and 0.5 x 10(-2) fmol leucine CFU-1 h( -1), respectively). Thus, an apparent correlation between metabolic ac tivity and transfer was observed. Additional experiments, however, sug gested that there was no causal relationship between the two. When inc ubated in a sand microcosm containing root exudates, the metabolic act ivity of the bacteria increased while conjugal transfer ratios remaine d constant. Hence, contrary to the accepted view, root exudates and me tabolic activity did not appear to be responsible for the stimulation of conjugal transfer in the rhizosphere. (C) 1998 Federation of Europe an Microbiological Societies. Published by Elsevier Science B.V.