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Chemotaxis and motility of Rhizobium trifolii TA1

Catlow, Helga Y. (1988) Chemotaxis and motility of Rhizobium trifolii TA1. PhD thesis, Murdoch University.

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The work in this thesis set out to examine the importance of motility and chemotaxis to the behaviour of Rhizobium trifolii TA1 in soil. Mutants defective in chemotaxis or motility were therefore generated by Tn5- mutagenesis (Beringer et a1., 1978) of Rhizobium trifolii TA1.

Attempts to produce totally non-chemotactic mutants of R. trifolii were unsuccessful, though several partially non-chemotactic mutants were isolated. These mutants lacked chemotaxis to a wide range of single carbon sources, but were responsive to yeast extract in every case. They were therefore considered unsuitable for investigating the role of chemotaxis in nodulation and migration of root nodule bacteria in soil, since the possibility remained that plant exudates or soil substrates could still elicit chemotactic responses.

Non-motile mutants were obtained using transposon Tn-5 mutagenesis. These mutants were indistinguishable from the wild type in growth rates in both laboratory media and in the clover rhizosphere. They also produced as many nodules on clover plants (Trifolium subterraneum cv Mount Barker) as the wild type and were as effective as the parent at nitrogen fixation.

When motile and non-motile (flagellate or nonflagellate) strains were inoculated simultaneously onto clover plants the non-motile strain was found in only about 15% of the resultant nodules and the motile strain in 85%. This implies that the non-motile mutants are at some disadvantage in competition with the motile parent.

The vertical movement of root nodule bacteria was examined in vertical columns of steam-treated sand. The non-motile strain did not move away from the inoculation site whereas the motile one did. Other experiments to determine the nodulation pattern of motile and non-motile organisms, on the tap roots of clover in steamed soil, showed that the motile strain could produce nodules further down the clover tap root than the non-motile strains. Similar experiments examining the lateral movement of both motile and non-motile strains from a central inoculated clover plant again showed that the motile strain had a definite advantage over the non-motile strain in its ability to move through steamed soil to produce nodules on clover roots.

When these experiments were carried out in untreated sand, quite different results were obtained. In experiments using soil columns to examine the vertical movement of root nodule bacteria, neither the motile or non-motile strains moved significantly from the inoculation point. Similarly, in experiments to examine lateral movement of root nodule bacteria, in non-treated soil, little movement of either motile or non-motile strain was observed.

These experiments showed that in steam-treated soil, motile strains, but not non-motile strains, have the capacity to move considerable distances. However, this movement potential may not be achieved under natural conditions in soil.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Dilworth, Michael and Glenn, Andrew
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