Plant basal resistance - Universiteit Utrecht

Chapter 5
defence against different organisms has been well documented, the mechanisms underlying
these effects are poorly explored. The objective of the study presented in Chapter 3 was
to examine role of BXs in defence against aphids and fungi along with the underlying
mechanisms. To this end, we have undertaken a detailed analysis of BX pathway regulation
and BX localisation in response to different biotic stresses. The analysis critically depended
on the construction of maize mutant lines that are impaired in the first dedicated step of
the benzoxazinoid pathways: the conversion of indole-3-glycerol phosphate into indole. In
maize, this reaction is catalysed by Indole glycerol phosphate lyase (IGL) or Benzoxazineless1
(BX1). The Igl gene is inducible by herbivory, wounding, insect elicitors or jasmonates (Frey
et al., 2000; Frey et al., 2004; Ton et al., 2007; Erb et al., 2009), and is responsible for indole
emission. The Bx1 gene is under developmental control and mediates indole production
as a substrate for BX biosynthesis. To determine the role of BXs in defence against aphids
and fungi, we determined basal resistance of wild-type and mutant bx1 plants in the igl
mutant background, thereby preventing BX production from aphid- or fungus-induced IGL.
These bx1 igl double mutant lines were derived from two independent reciprocal crosses
between a Mutator (Mu)-induced mutant in the Igl gene, and the original bx1bx1 mutant
(Hamilton, 1964). Since both parental plants have different genetic backgrounds, their
progeny lines in the F3, F4 and F5 generation are still segregating for genetically different
alleles from the parental plants. However, these segregation patterns can be expected to
largely differ between the two independent progeny lines. Hence, defence phenotypes that
are consistently expressed in both bx1 igl progeny lines are unlikely due to segregating genes
from the parental backgrounds, but are rather caused by their inability to produce BXs.
Moreover, apoplastic infiltration with the BX compound DIMBOA (2,4-dihydroxy-7-methoxy-
2H-1,4-benzoxazin-3(4H)-one) stimulated callose deposition (Chapter 3; Figure 8). Together
with the defence phenotypes of the BX-deficient bx1 igl lines, these results validate the
conclusion that apoplastic DIMBOA stimulates callose deposition in maize.
One major objective of this study was to establish whether there is a causal link
between aphid resistance and the presence of an intact BX1 gene. Our results clearly
demonstrate that this is the case; aphid survival rate and weight gain were higher on bx1
igl plant than Bx1 igl plants from both crosses (Figure 1; Chapter 3). Our results compliment
numerous studies which report correlations between aphid performance and BX
concentration in plants or in their feeding substrates. For example, rate of intrinsic weight
increase of the grain aphid Sitobion avenae, the cherry-oat aphid Rhopalosiphum padi and
mean relative growth rates of greenbug Schizaphis graminum and S.avenae were negatively
correlated with BX levels in wheat seedlings(Leszczynski and Dixon, 1990; Thackray et
al., 1990; Givovich and Niemeyer, 1994). In another study, 20 Hungarian wheat varieties
differing in BX levels were tested for infestation rating by R.padi under field conditions
and an inverse relationship was reported between infestation rate and BX concentration
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