Agriculture Reference
In-Depth Information
7% reduction in grain yield in barley plants inoculated with powdery mildew, com-
pared with uninoculated control plants. These workers suggested that the reduction in
grain yield was the result of the greatly increased rates of dark respiration, required to
provide rapid resistance to mildew infection (Smedegaard-Petersen & Stolen, 1981).
This contrasts with later work reporting yield increases associated with induced resis-
tance to powdery mildew infection in barley (Steiner
et al.,
1988; Oerke
et al.,
1989)
and the lack of any effect on yield in barley induced by application of yeast-derived
elicitors (Reglinski
et al.,
1994). However, because these studies were conducted in
the presence of pathogen challenge, they cannot be used to quantify the costs asso-
ciated with induced resistance. Subsequent work by Heil
et al.
(2000) showed that
ASM, applied to wheat in the absence of pathogen pressure reduced plant growth and
yield and provided a clear indication that use of ASM incurred allocation costs. In
fact, similar results have been reported in other crop plants, including sunfl ower (Prats
et al.,
2002), tobacco (Csinos
et al.,
2001), caulifl ower (Ziadi
et al.,
2001), straw-
berry (Hukkanen
et al.,
2007), melon (Buzi
et al.,
2004) and cowpea (Latunde-Dada &
Lucas, 2001). Further evidence that SAR is costly comes from studies on
Arabidopsis
mutants that overexpress SAR; such transformants usually have stunted growth and
reduced seed yields (Bowling
et al.,
1994; Greenberg
et al.,
2000; Jirage
et al.,
2001;
Mauch
et al.,
2001).
SAR might indeed be costly, but whether such costs are incurred will depend on
environmental factors, both abiotic and biotic, and very likely will also be genotype-
dependent, although this has received little attention to date. In wheat and
Arabidopsis,
whether or not costs were incurred following induction of resistance depended on nitrogen
supply (Heil
et al.,
2000; Dietrich
et al.,
2005), while successful induction of resistance
by ASM in barley depended on whether the plants were mycorrhizal or not (Sonnemann
et al.,
2005). The importance of the rhizosphere community is becoming increasingly
evident. For example, the root colonising basidiomycete (
Piriformospora indica
) has
been shown to induce resistance in barley to powdery mildew and to root rot caused by
Fusarium culmorum
(Waller
et al.,
2005). In addition, there was a signifi cant increase in
grain yield thus challenging the notion that an enhanced resistance status imposes a cost.
Furthermore, a recent report by Nair
et al.
(2007) suggests that the presence of certain
rhizobacteria may be able to compensate for some of the costs associated with induced
resistance. In their study, growth of the leafy vegetable, amaranthus, was retarded when
treated with ASM but not when plants were treated with a combination of the ASM and
the rhizobacterial strain
Pseudomonas fl uorescens
PN026R. It was proposed that growth
retardation effect of ASM was compensated for by the growth promoting properties of
the rhizobacteria.
As for the mechanisms by which SAR-associated costs are incurred, a number of work-
ers have reported negative effects of resistance induction on plant carbon and nitrogen
metabolism (e.g. Logemann
et al.,
1995; Ramanujam
et al.,
1998). Such reports have
been confi rmed by gene array studies, which have generally found that genes involved in
photosynthesis and growth are downregulated during the expression of induced resistance
(Scheideler
et al.,
2002; Heidel
et al.,
2004). It seems likely therefore that a switch from
housekeeping to pathogen defence metabolism may be a prerequisite for the full com-
mitment of a plant to transcriptional activation of resistance pathways (Logemann
et al.,
1995).
