Plant basal resistance - Universiteit Utrecht

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General discussion
There are different ways by which rhizobacteria can protect plants. These include
competition for nutrients (Kamilova et al., 2005), siderophore-mediated competition for iron
(Schippers et al., 1987), signal interference (Lin et al., 2003), antibiosis (Haas and Keel, 2003)
and ISR (Zamioudis and Pieterse, 2011). Interestingly, transcriptome analysis of DIMBOA-
exposed P. putida revealed enhanced expression of the phzF gene (Chapter 4, Table I),
which encodes an enzyme in the biosynthesis of the broad-spectrum antibiotic phenazine
(Blankenfeldt et al., 2004). These results suggest that DIMBOA exudation from cereal roots
has the potential to boost phenazine production by natural Pseudomonas bacteria in the
rhizosphere, which has been linked to suppression of take-all disease (Thomashow and
Weller, 1988). Moreover, preliminary evidence suggests that root colonization by P. putida
KT2440 primes emission of wounding-inducible volatiles aboveground (Figure 3). These
volatiles have well-known functions in the recruitment of natural enemies of herbivores
(Turlings and Ton, 2006), and some can also serve as long-distance signals to prime defence
in neighbouring plants (Heil and Ton, 2008). The study described in Chapter 4 demonstrates
that BXs can act as belowground signals to recruit and select for plant-beneficial P. putida
bacteria. This discovery is exploitable for plant breeder and biotech companies to sustainably
manage pests and diseases by selecting for cereal crops that exude increased amounts
of DIMBOA from their roots. Furthermore, it was recently reported that mycorrhization
increases DIMBOA levels in maize roots (Song et al., 2011). It is known that mycorrhization
causes major changes in the rhizobacterial community (Linderman, 1988). Considering
the results in Chapter 4, DIMBOA may be a driving factor behind this plant-beneficial
mycorrhizoshere effect.
Classical breeding strategies can be used to select varieties with an enhanced
capacity to exudate DIMBOA in the roots. If these breeding schemes can be combined with
the selection for increased BX content aboveground, these varieties will not only be able to
better in recruiting a disease-suppressing rhizosphere, but they will also be more resistant
to pests and diseases aboveground. Less targeted approaches can be used to identify similar
traits in non-cereal crops. With the next-generation sequencing technologies becoming
more cost-efficient, it will become possible to carry out large-scale QTL analyses in order
to associate crop performance in terms of growth and stress resistance with metagenomic
profiles in the rhizosphere (Poland et al., 2011; Bisseling et al., 2009). Such an approach will
lead to the identification of novel plant genes that promote the establishment of a disease-
suppressing and growth-promoting rhizosphere.
AKNOWLEDGEMENTS
We thank Georg Jander and Adewale Martins Adio from the Boyce Thompson Institute
for Plant Research (Cornell University; New Yourk, United States of America) for analysing