Manipulation of beneficial microorganisms in crop rhizospheres - Microbial Ecology in Sustainable Agroecosystems - page 27

certain functions and two-way signaling between plants and microbes. One of the better-
understood plant-microbe communications is between rhizobia and legumes at the begin-
ning of the symbiotic N 2 fixation process and other stages of symbiosis and involves the
exchange of flavonoids and chitin-based compounds.
Another example is fungal elicitors detected by plants such as chitin fragments that
cause the plant to have defense reactions to fungal pathogens and to produce phytoalexins
(Ebel and Mithöfer, 1998). Most often, these phytoalexins are flavonoids and isoflavonoids,
such as genistein.
Protein elicitins are unique to the genus Phytophthora and are encoded by a large multi-
gene family divided into at least eight different classes based on their sequence homology
and protein motif diversity (Jiang, Tyler, and Govers, 2006; Jiang, Tyler, Whisson, et al.,
2006). Class I elicitins became of interest when it was shown that these proteins induced
a plant hypersensitive response (HR) when infiltrated on tobacco leaves (Ponchet et al.,
1999). Consequently, further research was done to establish whether elicitin genes are
avirulence genes that determine the host range (Kamoun, 2001; Ponchet et al., 1999). As a
result, studies of elicitins have largely focused on elicitor activity on plants (e.g., Baillieul
et al., 2003), characterizing protein structure (e.g., Pantoea and Zoogloea ) (Lascombe et al.,
2004), and discovering new elicitins secreted by Phytophthora spp. (e.g., Churngchow and
Rattarasarn, 2000).
Yousef et al. (2009) showed that sterols trigger the expression of elicitin genes in the
plant pathogen Phytophthora sojae , and that elicitin secretion provides a protein sterol carrier
in the soil/rhizosphere environment. The latter is important as it shows how this eukary-
ote organism can acquire essential sterols from a plant that it does not produce itself.
Plant growth-promoting (PGP) microorganisms will produce compounds and cause
systemic acquired resistance (SAR), which causes a range of defense mechanisms (Metraux
et al., 2001). For instance, SAR involves signal transduction, generation of phytoalexins,
oxidative stress protection, and lignification (Reymond and Farmer, 1998).
At the same time, there is evidence that plants send signals to microorganisms, which
in turn result in stimulation of microorganisms that can have synergistic and positive
effects on plants. However, the relative importance of plant signaling versus the increase in
nutrients and energy sources provided by plant roots has on promoting beneficial micro-
organisms is not known. Given that some rhizosphere bacteria are reported to stimulate
plant growth through production of plant hormones, it may be that ethylene, salicylic acid,
and jasmonic acid are active here as well.
In-depth research is needed to identify the range of signaling compounds produced by
rhizosphere microorganisms and by plant roots and the associated response mechanisms.
For example, riboflavin and lumichrome, both known to stimulate plant root growth, have
been shown to activate the LasR QS receptor, thus acting as QS agonists (J. Robinson,
personal communication, 2011). This finding raises the possibility that vitamins and their
derivatives might play important roles as QS mimic compounds as well as precursors to
essential cofactors. Additional research is needed to exploit the use of activator induc-
ers on beneficial microbial activator production that could further increase plant growth.
An improved understanding of this aspect of plant-microbe relationships will allow for
manipulation of these compounds to enhance plant growth, increasing crop yield and
allowing for efficient and less environmentally damaging production of plant biomass for
food and feed production. In summary, microbial and plant signaling is not well under-
stood, and in general, this type of effect remains to be investigated and is strongly needed
to fully exploit the potential of beneficial microorganism for crop production. If this break-
through could be made, it offers the possibility of “programming” plant roots to release