plant surface microbiology.pdf

1 The State of the Art 7
responses of plants to colonization by communities of arbuscular mycorrhizal
fungi comprising species with different capacities to take up phosphorus from
soil, transport it along hyphae and transfer it to the plant.
When microbial communities are established in association with roots,
they may be affected by changes in rooting patterns and exudates (Chap. 20).
Introduction of plant growth promoting rhizobacteria (PGPRs) into the
soil/plant/microbial environment can influence organisms already present
(e.g., pathogenic and mycorrhizal fungi) in addition to the roots themselves.
Techniques for microbial community fingerprinting are being adapted for
assessment of PGPRs, in addition to in situ methods such as confocal laser
scanning microscopy, to understand root – microbial associations from the
perspective of communities of organisms that perform different, and sometimes
contrasting, functions.
Nutrients introduced into the rhizosphere from plants and decaying
organic matter can influence physiological responses of microorganisms and
their interactions with plants. Gene regulation in some ectomycorrhizal fungi
has been shown to be altered in nutrient-limiting environments and this
could have consequences for nutrient uptake and transfer to plants. For example,
regulation of gene expression associated with some sugars has been
shown to depend on the concentration of specific carbohydrates in the
medium with threshold responses identified (Chap. 21). Expression of ammonium
transporter genes can be stimulated for some fungi grown under nitrogen-limiting
conditions and this could have important consequences for plant
establishment in nitrogen-limiting natural ecosystems. Different patterns of
gene regulation have been identified for the ectomycorrhizal fungus Amanita
muscaria in relation to carbon and nitrogen nutrition. Some genes are regulated
by both nitrogen and carbon nutrition, while others by either nitrogen
or carbon (Chap. 21). Recent advances in the adaptation of molecular techniques
to studies of plant and fungal biochemistry have contributed to understanding
nitrogen metabolism in plants and microorganisms (Chap. 22). For
some time, studies of nitrogen assimilation by ectomycorrhizal fungi have
investigated nitrate and nitrite uptake kinetics, ammonium transport and
amino acid transport. Techniques such as immunogold and 14 C labelling can
now be combined with gene cloning to clarify physiological processes
involved in nitrogen assimilation in ectomycorrhizal fungi to highlight their
differences from saprophytic and pathogenic fungi.
Section E deals with the sophisticated and novel techniques to formulate
critical experiments and their design in order to retrieve excellent and reliable
results. Background information for the selection of beneficial properties of
Pseudomonas and Bacillus strains from the rhizospheric antagonistic to phytopathogenetic
community requires elaboration, evaluation and bioassay
(Chap. 23). After the selection of strains, these can be marked with a reporter
gene and used to study cellular and molecular interactions between one or
more beneficial microbes. These strains can also serve as a tool to study the