Induced resistance for plant disease control - Disease Control in Crops - page 82

dynamics of SA, JA and ET signalling following attack by pathogens and insect pests.
When global gene expression profi les were compared, the workers found considerable
overlap in the changes induced by pathogens and insects. Thus, all of the different patho-
gen and insect attackers stimulated JA biosynthesis, although most of the changes in
JA-responsive gene expression were attacker specifi c (De Vos et al. , 2005). The authors
suggest that although SA, JA and ET play a primary role in orchestrating plant defence,
the fi nal defence response is shaped by other regulatory mechanisms, for example cross-
talk between different pathways. A recent study indicates that herbivorous nymphs of the
silverleaf whitefl y ( Bemisia tabaci ) may activate the SA signalling pathway as a decoy
strategy to sabotage JA-dependent defences and so enhance insect performance (Zarate
et al ., 2007).

4.4.3 Trade-offs with mutualistic symbioses

Since induced resistance is a broad-spectrum resistance against microorganisms, it is
likely to exert an impact on plant interactions with mutualistic symbionts, including
mycorrhizal fungi and nitrogen fi xing Rhizobia and Bradyrhizobia bacteria (Figure 4.2).
Perhaps surprisingly, this area of research has received little attention to date. However,
some studies of the legume Rhizobium symbiosis have shown that application of SA to
the rooting substrate exerted a negative effect on nodule formation and/or functioning
(Martínez-Abarca et al., 1998; Ramanujam et al., 1998; Lian et al., 2000), while treat-
ment of Vicia faba plants with ASM led to a reduction in the number and size of nod-
ules compared with untreated controls (Heil, 2001). Although some studies have reported
negative effects on colonisation of tobacco roots by the arbuscular mycorrhizal fungus
Glomus mosseae in plants constitutively expressing -1,3-glucanase (Vierheilig et al.,
1994; Glandorf et al., 1997), other workers found no effect of ASM treatment on mycor-
rhizal infection of barley roots (Sonnemann et al., 2002).
Interestingly, there are a number of reports indicating an effect of mycorrhizal infec-
tion on plant defence against pathogens. For example, colonisation of tomato roots
by G. mosseae induced cell defence responses and localised and systemic resistance
to Phytophthora parasitica , while infection of barley roots with G. mosseae induced
systemic resistance to the take-all fungus Gaeumannomyces graminis f. sp. tritici
(Cordier et al., 1998; Khaosaad et al., 2007). Mycorrhizal infection and colonisation has
also been shown to modify the effectiveness of induced resistance. Thus, Sonnemann
et al. (2005) found that at low and medium levels of colonisation of barley roots by
Glomus etunicatum , ASM had either no effect or decreased foliar infection by powdery
mildew, while high levels of mycorrhizal colonisation increased mildew infection.

4.5 Future prospects

Induced resistance offers the prospect of broad-spectrum disease control and yet, it
remains on the fringes of mainstream crop protection. As indicated above, this is due in
part to the variability in effi cacy whenever activators are used. Such variability should
not be surprising, since induced resistance is a host response and as such, will be infl u-
enced by genotype and environment. At present, our understanding of this area of induced
resistance is woefully inadequate. For fi eld crops, it is also likely that plants will already