Agriculture Reference
In-Depth Information
The fungicidal effect of foliar-applied S
0
has to be distinguished strictly from the
health promoting effect of soil-applied sulphate-S, but the mode of action shows marked
parallels to different plant metabolites putatively connected with sulphur-induced
resistance (SIR).
11.2
Sulphur-induced resistance - agronomic,
physiological and molecular aspects
In greenhouse and fi eld experimentation, soil-applied S fertilisation in the form of sulphate
reduced the disease index for various host/pathogen relationships and in 1997 Schnug
coined the term Sulphur-Induced Resistance (SIR) to describe the complex biological
phenomenon behind these observations (Schnug, 1997). The term SIR denotes the
reinforcement of the natural resistance of plants against fungal pathogens through trigger-
ing the stimulation of metabolic processes involving sulphur by targeted sulphate-based
and soil-applied fertiliser strategies. It is important to note that SIR is one constituent of
the complex phenomenon of induced resistance (IR) (see Chapter 4). In contrast to the
induction of resistance by application of elicitors such as potassium phosphate (Mitchell &
Walters, 2004; Walters
et al.
, 2005; Chapter 4), the magnitude and effi ciency of SIR
seems to be regulated by the external, plant available S reserves and plant-inherent S
pools and fl uxes (Haneklaus
et al.
, 2007a).
11.2.1
Sulphur metabolism and plant disease
For triggering SIR on the farm it is necessary to control the dynamic interactions between
plant, environment and pathogen, and this requires knowledge of the physiological and
molecular background.
Jost
et al.
(2005) investigated the reaction of 2487 selected genes after applying
methyl jasmonate to mimic signalling of host-pathogen interactions in
Arabidopsis
thaliana
. Their studies revealed that genes, which are related to S metabolism were more
strongly upregulated than stress-related genes. Sasaki-Sekimoto
et al.
(2005) showed that
jasmonates increased gene expression of the ascorbate and glutathione metabolic path-
ways and biosynthesis of indole glucosinolates. Hell & Kruse (2006) summarised current
knowledge at the molecular level with respect to the link of different S metabolites to
different phases of pathogenesis: signal perception, signal transduction and expression
of resistance, and their possible role in defence. Plants with a lower level of reduced
glutathione for instance proved to be more susceptible to biotic and abiotic stresses (Ball
et al.
, 2004). Changes in the ratio of oxidised (GSSG) to reduced glutathione (GSH) seem
to be involved in the activation of regulatory proteins and are required for salicylic acid
and abscisic acid signalling (Hell & Kruse, 2006).
In general, it is diffi cult to assign a change in plant metabolism to a specifi c stress
factor, as usually a variety of abiotic and biotic stress affects the plant at the same time.
In addition, cross-talk in defence signalling may yield distinct, but also antagonistic
response pathways, often initiated simultaneously in response to the specifi c stress. These
processes result in accumulation, degradation and consumption of primary and secondary
metabolites. Metabolites with antifungal potential include prohibitins and phytoanticipins,
which contribute to constitutive resistance, and phytoalexins, for example, which are
