6 Wood Discoloration

5.2 Bacteria 111
Gram staining divides Gram-positive and negative species according to their
wall structure and the occurrence of pepitoglycan or lipids. With regard to
oxygen, aerobes grow only in the presence of oxygen and anaerobes in its
absence. The latter behavior may be either facultative or obligate. Many bacteria
are motile, using either flagella, axial filaments, gliding, or changes in
buoyancy. In some genera (Bacillus, Clostridium), the mother cell develops
an endospore, which is rather resistant against heat, radiation and chemicals
(Schlegel 1992).
Actinobacteria are bacteria, which often live in the soil. They play important
roles in plant decomposition, humus formation, and degradation (Filip
et al. 1998) and are found on timber in soil contact. Some form branching filaments,
which resemble the fungal mycelium (in former times: Actinomycetes),
whereby the cell diameter of about 1µm is however smaller than that of most
fungal hyphae. Some actinobacteria (e.g., Streptomyces) developaplentyof
air-borne spores.
There are various interactions between bacteria and plants, like increase of
soil fertility by nutrient release, nitrogen fixation (Azotobacter), root symbioses
(Rhizobium), decomposition and humification, and parasitism as causal agents
of diseases.
The pathogenic bacteria of woody plants belong to the genera Agrobacterium,
Erwinia, Pseudomonas, andXanthomonas (Nienhaus and Kiewnick
1998). Bacteria cause the fire blight [Erwinia amylovora (Burrill) Winslow
et al.] of many species of the rose family (Tattar 1978), canker of poplar [Xanthomonas
populi ssp. populi (Ridé) Ridé & Ridé], willow (X. populi ssp. salicis
de Kam) and ash (Pseudomonas syringae ssp. savastanoi pv. fraxini Janse)
(Butin 1995). Agrobacterium species infect the roots of a wide range of dicotyledonous
plants and some gymnosperms causing crown gall and hairy
root diseases.
Since the late 1970s, Agrobacterium-mediated gene transfer is an important
tool in genetic transformation of forest trees. During the disease process,
a DNA segment of the bacterium (T-DNA) is integrated into the host plant
genome. The T-DNA originates from a 200-kb plasmid (Ti plasmid) and foreign
genes can be inserted into this DNA for transfer into the plant (Palli
and Retnakaran 1998; Häggman and Aronen 1996), e.g., for gene-manipulated
poplars and white spruce (Séguin et al. 1998). Kajita et al. (2004) transferred the
gene for the enzyme feruloyl-CoA hydratase/lyase, which is involved in lignin
(hydroxycinnamates) metabolism, from a bacterium into aspen by Agrobacterium
tumefaciens (E.F. Smith & Townsend) Conn in view of producing trees
with novel characteristics.
Rickettsia and Rickettsia-like organisms (RLOs) (Proteobacteria) (100–
800 nm) are obligate intracellular, Gram-negative bacteria with reduced metabolic
activity. They cannot be cultured in nutrient medium. RLOs in plants are
transferred by arthropodes, particularly cicadas, and multiply in the vector
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