Plant basal resistance - Universiteit Utrecht

Chapter 1
between defence signalling pathways. For instance, suppression of the SA-dependent
pathway by mycorrhizal fungi results in a potentiation of JA-dependent defences (Pozo and
Azcón-Aguilar, 2007), while trans-generational priming of the SA response in Arabidopsis
coincides with a repression of JA response (Luna et al., 2012) Interestingly augmented levels
of JA have been associated with primed callose deposition in grapevine against Plasmopara
(Hamiduzzaman et al., 2005). Similarly priming of papillae formation was observed in the
roots of mycorrhiza-infected tomatoes with Phytophthora (Cordier et al., 1998). These
observations point to a mechanism by which suppression of the SA-response results in a
beneficial side effect: systemic priming of JA-dependent defences and callose deposition.
reduced scavenging capacity of reactive oxygen species. Several recent studies
have pointed to an important role of reactive oxygen species (ROS) in priming of defence.
Thiamine (vitamin B 1 ) induces resistance against Pseudomonas syringae pv. tomato
DC3000 (PstDC3000), which is associated with hydrogen peroxide (H 2 O 2 )-dependent
priming of defence genes and callose deposition (Ahn et al., 2007). Vitamin B 2 (riboflavin)
induces a phenotypically similar resistance response that is associated with priming of
ROS production, callose deposition and SA-inducible genes (Zhang et al., 2009). The plant
secondary metabolite quercetin has also been demonstrated to induce SA- and NPR1-
dependent resistance against PstDC3000, which is associated with augmented deposition
of ROS, callose, PR1 and PAL gene transcripts (Jia et al., 2010). A recent study by Mukherjee
et al. (2010) provided a plausible mechanism for ROS-dependent regulation of priming. The
authors performed a phenotypic analysis of different alleles of the ascorbic acid deficient
mutant vtc1 and demonstrated that the enhanced disease resistance of this mutant is
based on priming of pathogen-induced accumulation of ROS, SA and NPR1 gene transcripts
(Mukherjee et al., 2010). The authors suggested that the reduced ROS scavenging capacity
of vtc1 causes constitutive priming of pathogen-induced H 2 O 2 , thereby causing augmented
SA accumulation and enhanced defence induction.
Costs & benefits of priming
The full development of an inducible defence response requires energy and, therefore,
involves costs on growth and reproduction. Apart from allocation costs, costs can also arise
from toxicity of the defence to the plant’s own metabolism, or when the defence response
affects the plant’s interaction with beneficial organisms (Heil, 2002). It is commonly accepted
that plants only express inducible defences if the benefits (i.e. protection against the
attackers) outweigh the associated costs (Heil, 2002; Walters and Boyle, 2005). Van Hulten
et al. (2006) conducted a laboratory study to compare the costs and benefits of defence
priming versus direct induction of defence in Arabidopsis. By using low doses of BABA to
induce priming and high doses of either BABA or BTH to induce defence expression directly,
it was found that priming is associated with relatively minor costs on plant growth and seed
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