Plant basal resistance - Universiteit Utrecht

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Summary
but repressed transcription of genes controlling BX biosynthesis downstream of BX1. This
repression was also obtained after treatment with the BX precursor indole and DIMBOA,
but not with HDMBOA-glucoside. Hence, apoplastic DIMBOA acts as a negative regulator
of its own biosynthesis genes. To further examine the regulatory role of apoplastic DIMBOA
in plant defence, BX-deficient bx1 mutant lines were examined for callose deposition upon
treatment with the fungus- and insect-derived PAMP chitosan. This experiment revealed
that bx1 mutant lines deposited considerably less chitosan-induced callose than did Bx1
wild-type lines. Moreover, apoplastic leaf infiltration with DIMBOA, but not HDMBOA-
glucoside, mimicked chitosan-induced callose. Together, these results strongly suggest that
apoplastic DIMBOA not only functions as a biocidal defence compound, but also act as a
within-plant signal to control PAMP-induce callose deposition.
BXs have also been implicated in basal defences belowground, where they can
exert allelochemical or antimicrobial activities. Chapter 4 of this thesis describes a study
into the impact of BXs on the interaction between maize and Pseudomonas putida
KT2440, a competitive coloniser of the maize rhizosphere with plant-beneficial traits.
Chromatographic analyses revealed that DIMBOA is the dominant BX compound in root
exudates of maize during the early developmental stages of the plant. In vitro analyses of
DIMBOA stability indicated that DIMBOA tolerance of P. putida KT2440 bacteria is based on
metabolism-dependent breakdown of DIMBOA. Transcriptome analysis of DIMBOA-exposed
P. putida confirmed increased transcription of genes controlling benzoate catabolism. This
transcriptome analysis also revealed DIMBOA-inducible expression of genes that is involved
in bacterial motility. Subsequent chemotaxis assays verified motility of P. putida towards
DIMBOA. Moreover, colonisation essays with GREEN FLUORESCENT PROTEIN (GFP)-
expressing P. putida showed that DIMBOA-producing roots of Bx1 wild-type lines attract
significantly higher numbers of P. putida cells than roots of DIMBOA-deficient bx1 mutant
lines. In combination with Chapter 3, the results described in Chapter 4 demonstrate a central
signalling role for DIMBOA during the regulation of basal plant defence. Aboveground,
DIMBOA acts as a defence regulatory signal of maize basal resistance, while belowground
this compound acts as a semiochemical for recruitment of beneficial rhizobacteria during
the relatively young and vulnerable growth stages of maize.
Preliminary results presented in the general Discussion (Chapter 5) indicate that
root colonization by Pseudomonas putida primes aboveground basal defences against
herbivores, thereby further highlighting the central and multifaceted function of DIMBOA
in maize basal resistance.