Agriculture Reference
In-Depth Information
inducing plant resistance to pathogen infection (Kloepper, 1993). Two bacterial strains
(
Burkholderia cepacia
RAL3 &
Pseudomonas fl uorescens
64-3) have been identifi ed that
reduced disease of white spruce seedlings caused by
Fusarium
spp
.,
and
Pythium
spp., in
a commercial nursery and increased the survival of out-planted bare root white spruce in
a re-forestation site (Reddy
et al.
, 1997). The treatments were applied by soaking either
seeds or the seedling roots in bacterial suspension before planting.
Enebak & Carey (2004) reported on the effects of seed treatment with
Bacillus
pumilus
strains on seedling growth and on fusiform rust infection (
Cronartium quercuum
f. sp.
fusiforme
) in bare root nurseries in Alabama and in Georgia, USA. In Georgia,
seed treatment with
B. pumilus
strains T4 and SE34 promoted seedling growth whilst
treatment with T4 signifi cantly reduced infection by the rust fungus
and resulted in the
development of fewer galls. However, in Alabama there was no treatment effect. Such site
specifi city mirrors the fi ndings reported for effects of PGPR on growth promotion and
highlights the importance of understanding the impact of biotic and abiotic soil factors on
PGPR-induced resistance.
4.3
Costs associated with induced resistance
Plants respond almost immediately to an inducing agent but, nevertheless, require a lag
period of a few days to activate its full complement of inducible defences. Therefore,
unlike constitutive resistance, where defences are always present, with induced resis-
tance, the plant remains vulnerable to attack for a short period. The question therefore
remains, why do plants have inducible defences? The continued and widespread exis-
tence of induced resistance suggests a selective advantage over constitutive resistance or
that inducible defences can be more selectively targeted at an attacker than previously
thought. One possible explanation for this selective advantage lies with fi tness costs,
where resistant plants would have decreased reproductive success (e.g. seed production)
than non-resistant plants under conditions where there was no pathogen pressure. In
other words, in addition to their positive effects, expression of defence genes also has
negative effects (Heil & Baldwin, 2002). In fact, the various theories of plant defence
all assume the existence of such costs. For example, the optimal defence theory focuses
on the spatial and temporal distribution of the limited resources plants can invest in
resistance (McKey, 1974, 1979; Rhoades, 1979), while the growth-differentiation bal-
ance hypothesis looks at the actual costs of resistance and highlights the compromise
between growth and differentiation in growing plants attempting to defend themselves
against attackers (Herms & Mattson, 1992). Costs include allocation costs arising from
diversion of metabolites and energy from growth and other processes towards defence,
as well as the negative effects (trade-offs) of the resistance on symbiotic associations
and effects on resistance to insects (Gomez
et al.,
2007; Walters & Heil, 2007; see also
Section 4.4).
4.3.1
Allocation costs
Although there is much evidence that induced resistance to insects incurs costs
(e.g. Zavala
et al.,
2004), the situation with respect to pathogens is less clear (Walters &
Heil, 2007). In some early work, Smedegaard-Petersen & Stolen (1981) demonstrated a
