Plant basal resistance - Universiteit Utrecht

119
General discussion
or exert antimicrobial activities (Niemeyer, 2009). The study described in Chapter 4 describes
the belowground impact of BXs in maize-rhizobacteria interactions.
The plant root system not only provides support and intake of nutrient and water,
but is also a source of secondary metabolites. Plant roots produce and store a blend of
chemicals/metabolites, and can release them in the form of root exudates into their
immediate vicinity: the rhizosphere. The presence of chemicals in root exudates has a well-
documented role in manipulating and dictating the plant’s relationship with pathogenic
and symbiotic microbes in the rhizosphere. A well know class of chemicals that are present
in root exudates are the isoflavones. These compounds have been identified from root
exudates of soybean, which attract nitrogen fixing bacteria Bradyrhizobium japonicum and a
pathogen Phytopthora sojae towards roots (Bais et al., 2006). Similarly L-malic acid has been
reported to be released by Arabidopsis thaliana in their root exudates, which selectively
attracts the plant-beneficial rhizobacterial strain Bacillus subitilis FB17 (Rudrappa et al.,
2008). In support of this, the results presented in Chapter 4 provides evidence that maize
roots release DIMBOA, which stimulates chemotaxis-related gene expression and in vitro
chemotaxis in P. putida KT2440 (Chapter 4; Figures 3 & 4). However, in order for rhizobacteria
to establish a plant-beneficial interaction, they not only need to locate their host, but they
also need to be capable of tolerating toxic chemicals that are present in the exudates. The
ability to tolerate and detoxify these chemicals will provide a competitive advantage to these
microbes over other strains that are incapable of tolerating these toxic chemicals in the
rhizosphere. Indeed, P. putida KT2440 were tolerant to DIMBOA in comparison to other soil-
borne bacteria and its exposure to DIMBOA elicited gene expression that is associated with
benzoate breakdown (Chapter 4; Table I). Moreover, in vitro analysis of DIMBOA stability
indicated that DIMBOA tolerance of KT2440 bacteria is based on metabolism-dependent
breakdown of DIMBOA and 6-methoxy-benzoxazolin-2-one (MBOA), a product of DIMBOA
degradation (Chapter 4; Figure 2). Chapter 4 finally revealed that DIMBOA-exuding roots of
Bx1 igl plants allowed higher levels of P. putida KT2440 colonized than did DIMBOA-deficient
bx1 igl plants. It is, therefore, likely that the increased bacterial colonization of BX-producing
roots is the additive result of positive chemotaxis and tolerance to DIMBOA.
Root colonization of Arabidopsis by P. putida KT2440 elicits an induced systemic
resistance response against P. syringae pv. tomato DC3000 (Matilla et al., 2010). Moreover,
preliminary data from our laboratory have revealed that colonization of maize roots by
KT2440 bacteria primes emission of wound-inducible volatiles (Figure 3). Hence, P. putida
KT2440 is a rhizobacterial strain with plant-beneficial characteristics. Together with the
data presented in Chapter 4, these results suggest that BXs fulfil a signalling role in the
rhizosphere to recruit and select for plant-beneficial bacteria. Interestingly, however, a
recent study by Robert et al., (2012) revealed that these BX signals can also be used by the
specialised root herbivore (Diabrotica virgifera) to locate their hosts. Hence, BXs are potent