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Plant mechanisms contributing to acid impairment of nodulation of Medicago murex and Medicago sativa by Sinorhizobium medicae

Cheng, Yvonne (2003) Plant mechanisms contributing to acid impairment of nodulation of Medicago murex and Medicago sativa by Sinorhizobium medicae. PhD thesis, Murdoch University.

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      Abstract

      The widespread sowing of the perennial forage legume Medicago sativa to lower groundwater tables in south-western Australia is limited as many soils targeted for its use are too acidic (pHCaCl2 < 5) for consistent nodulation with Sinorhizobium spp. The annual medic M. murex grows and nodulates well in these acidic soils, but it cannot fill the niche of M. sativa in lowering groundwater tables. The differential ability of M. murex and M. sativa to nodulate in acid soils provided the opportunity to compare the nodulation responses between the two species and to identify the mechanisms contributing to the poor nodulation of M. sativa in soil of low pH.

      An initial glasshouse experiment compared the nodulation of M. murex cv. Zodiac and M. sativa cv. Aquarius with S. medicae strains WSM419 and CC169. Subsequent glasshouse and laboratory experiments used only the more acid-tolerant S. medicae strain WSM419. In the glasshouse in soil of pHCaCl2 4.3, the uppermost nodule on both M. murex and M. sativa formed at 4-5 cm below the hypocotyl, but the nodules on M. sativa formed almost 4 weeks later than those on M. murex. The difference in nodulation response between M. murex and M. sativa was related to numbers of S. medicae in the rhizosphere. After 24 d growth in soil of pHCaCl2 4.3, there were 100-fold higher numbers of S. medicae WSM419 associated with the roots of M. murex than M. sativa. This difference in rhizobial numbers was not due to differences in root growth as there were similar rates of root elongation in M. murex and M. sativa, or differences in the root products released as root exudates of M. murex and M. sativa produced at low pH had no significant effect on the growth of S. medicae.

      Using a 'root mat' approach on soil disks of pHCaCl2 4.49, M. sativa acidified its rhizosphere by approximately 0.2 0.4 pH-units within 4 d, while M. murex did not acidify its rhizosphere. Rates of H+ release were higher from M. sativa than from M. murex. Using videodensitometry with agarose of pH 4.5, ature parts of the tap-root of both pecies exuded OH- ions, but this was pproximately 2-times higher in M. murex than in M. sativa. Consequently, oung parts of the M. sativa rhizosphere were more acidic than hat of . murex. The higher rate of acidification by the oots of M. sativa made its hizosphere less favourable for the survival and growth of S. medicae. oot hair development was initially similar for both M. murex and . sativa. However by 7 d after sowing in soil of pHCaCl2 .3, he density of root airs on M. murex increased to 37 root hairs mm-1 root, while the density of root airs on M. sativa decreased to 20 root hairs mm-1 root. Due to higher oot hair density, the roots of M. murex provided a greater surface area for the attachment and colonisation of S. medicae compared to the roots of M. sativa.

      Indeed, confocal laser scanning microscopy at 7 d after sowing showed there were larger populations of a green fluorescent protein-marked transconjugant of S. medicae WSM419 colonised at 4-5 cm below the hypocotyl on the root of M. murex (3.28 pixel intensity units) compared to M. sativa (1.78 pixel intensity units). The smaller population of S. medicae colonised on the M. sativa root resulted in the observed delay in nodule development in M. sativa compared to M. murex.

      Two plant mechanisms contributed to the greater numbers of S. medicae in the M. murex rhizosphere compared to M. sativa rhizosphere when plants were grown in an acidic soil: (1) roots of M. murex had a higher density of root hairs, and thus provided a larger root surface area for the growth and colonisation of S. medicae than M. sativa, and (2) roots of M. murex acidified the rhizosphere less, and thus provided more favourable conditions for the growth and colonisation of S. medicae than the rhizosphere of M. sativa. Models explaining the different nodulation responses between M. murex and M. sativa in soil of pHCaCl2 4.3 and 7.0 are presented.

      Publication Type: Thesis (PhD)
      Murdoch Affiliation: School of Biological Sciences and Biotechnology
      Supervisor: O'Hara, Graham
      URI: http://researchrepository.murdoch.edu.au/id/eprint/655
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