Agriculture Reference
In-Depth Information
Fig. 1.1
An illustration of the rhizosphere.
A
amoeba-consuming bacteria,
BL
energy-limited
bacteria,
BU
non-energy-limited bacteria,
RC
root-derived carbon,
SR
sloughed root hair cells,
F
fungal hyphae,
N
nematode worm (adapted from http://en.wikipedia.org./wiki/rhizosphere)
the competence of bacteria varies among different rhizospheres/rhizoplane
(Gamalero et al.
2003
) which has been described to be linked to root exudation
(Lugtenberg et al.
2001
). For instance, carbon fixed by plant photosynthesis is
known to be partly translocated into the root zone and released as root exudates
(Bais et al.
2006
). Moreover, various carbohydrates, amino acids, organic acids and
other compounds, which provide a source of nutrients for root-associated bacteria,
are released in the rhizosphere (Walker et al.
2003
). Such exudates act as
chemoattractants towards which the bacterial population moves and in effect
allow them to colonize and multiply in both the rhizosphere and the rhizoplane
(Lugtenberg and Kamilova
2009
). Plant exudates thus provide a rich source of
energy and nutrients for the bacteria in rhizosphere, resulting in more microbial
populations in the region than outside the region (Haas and Defago
2005
). The
colonization of plant rhizosphere by
Bacillus subtilis
sp. and
Pseudomonas
sp. has
been well studied (Trivedi et al.
2005
; Steenhoudt and Vanderleyden
2000
).
Rhizobacteria may depend on other microbes for nutrient sources as one microbe
may convert plant exudates into a form that can be used by another microbe. Thus,
rhizosphere has appeared as a versatile and dynamic ecological environment of
intense plant-microbe interactions (Mayak et al.
2004
) harnessing essential micro-
and macronutrients affecting plant growth, although the process of root coloni-
zation is under the influence of various parameters such as bacterial traits, root
exudates and several other biotic and abiotic factors (Benizri et al.
2002
). Broadly,
chemotaxis is generally considered to play an important role for successful
rhizosphere/rhizoplane
colonization
(Andrews
and Harris
2000
; Walsh
