plant surface microbiology.pdf

342
M. Zakaria Solaiman and Lynette K. Abbott
arbuscular mycorrhizas. An essential step in this process has been the generation,
screening and analysis of mycorrhizal mutants (Marsh and Schultze
2001).
Plant mutants are valuable tools in unravelling complex events that occur
during cell and tissue differentiation in plants that show impaired formation
of arbuscular mycorrhizas (Peterson and Guinel 2000). Since the first description
of myc – mutants (Duc et al. 1989), there has been increasing interest in
using them to address questions related to various key steps in the colonisation
process (Senoo et al. 2000; Wyss et al. 1990). As these fungi are obligate
symbionts, it has been difficult to study the interaction between the symbionts
during colonisation. The interaction in early colonisation phases of
Allium porrum L. (leek) roots by the AM fungus Glomus versiforme have been
described (Garriock et al. 1989) and reviewed (Giovanetti et al. 1994). Mutants
of pea (Pisum sativum L.) and faba bean (Vicia faba L.) were not colonised by
AM fungi (Duc et al. 1989). These mycorrhizal (myc – ) mutants were also
unable to form functional root nodules (nod – ). The myc – mutants have
aborted infections (Gianinazzi-Pearson et al. 1991). In contrast, nod – mutants
of soybean were colonised by Glomus mossae to the same extent as wild-type
(nod + ) soybean plants (Wess et al. 1990). The myc – mutants should be
screened against different AM fungi in a range of soils to see whether resistance
is horizontal or if some fungi can overcome resistance as in the case of
certain nod – plants in the presence of different Rhizobium populations (Lie
and Timmermans 1983). Recently, it has been shown that some AM fungi can
colonise the mutant of tomato, rmc, demonstrating that the fungi can overcome
resistance to successful colonisation (Gao et al. 2001). This new tool
would help exploration of genetic variability in AM fungi. It would also open
the possibility of controlling plant – fungus specificity in the presence of communities
of AM fungi in field soils.
Mycorrhizal mutants were screened from the model plant Lotus japonicus
(Senoo et al. 2000) after inoculation with Glomus sp. R-10. These mutants were
characterized and categorized into mcbep (mycorrhizal colonisation blocked
at epidermis) and mcbex (mycorrhizal colonisation blocked at exodermis)
based on the detailed assessment of colonisation and microscopic observation
(Senoo et al. 2000). Isolation and cloning of the gene will facilitate understanding
of its function, and it could be used to probe a range of hosts to
determine its distribution and expression. It is essential to expand the collection
of mutants in order to build up a comprehensive description of the molecular
genetic basis of successful mycorrhization.