Agriculture Reference
In-Depth Information
11.2.4
Changes in S metabolism during pathogenesis
For linking mineral nutrition with disease, different approaches have been used:
determination of the infl uence of fertilisation on disease incidence and/or severity;
comparative analysis of mineral concentrations in healthy or resistant tissues and infected
or susceptible tissues; analysis of conditions controlling the availability of the nutrient
and disease (Huber & Haneklaus, 2007). In general, it can be expected that the greatest
benefi t to the plant is provided when nutrient supply is in the suffi ciency range, but the
response to a pathogen may well be different when going from defi ciency to suffi ciency,
or suffi ciency to excess (Huber & Haneklaus, 2007).
Raj & Srivastava (1977) showed that total S content in infected tissues of
Brassica
juncea
plants was inversely correlated with the pathogenicity of different isolates. While
the S content was 50% and 26% lower in plants infected with highly and medium virulent
isolates, respectively, a two-fold increase in S level was determined in plants infected with
weakly virulent isolates (Raj & Srivastava, 1977). The authors suggested metabolism of
host S by pathogens of high and medium virulence, which is, however, limited so that
higher plant S concentrations may exert toxic effects on the pathogen. In contrast, weakly
virulent isolates lacked the ability to utilise plant S. Earlier experiments by Yarwood &
Jacobson (1955) showed similar results.
Glutathione is supposedly a systemic messenger in the hypersensitive response as GSH
is rapidly accumulated after fungal attack (Edwards
et al.
, 1991; Foyer & Rennenberg,
2000; Gullner & Kömives, 2001). Participation in anti-oxidative defence reactions is the
primary role of GSH in plant tissues, either by direct reactions with reactive oxygen
species or through the ascorbate-GSH cycle (Winterbourn & Metodiewa, 1999; Foyer
& Rennenberg, 2000). In general, a rapid accumulation of GSH occurs after infection
(Vanacker
et al.
, 2000). A positive relationship between GSH content and protection
against fungal diseases exists (Gullner & Kömives, 2001), but speed of GSH accumula-
tion might be equally important (Vanacker
et al.
, 2000). Field experiments conducted
by Salac
et al.
(2003) revealed that infections of oilseed rape by the black leg fungus
(
Leptosphaeria maculans
) led to increased synthesis of GSH, whereby initiation of GSH
synthesis appeared to be dependent on some threshold infection. Furthermore, GSH con-
centration and accumulation rate might not only depend on the availability of its precur-
sor cysteine, but a suffi cient sulphate pool in the soil.
Long-term fi eld experiments in Northern Germany and Scotland have shown that
an infection by
P. brassicae
may yield both signifi cant increases and decreases of the
cysteine and GSH contents in oilseed rape leaf discs (Salac
et al.
, 2005). The authors
assumed a connection to the different inoculum pressures in both countries; when
inoculum pressure was extraordinarily high, a decline in cysteine and GSH occurred. It is
not known whether such a decrease is a transient phenomenon and related to the consump-
tion of these metabolites during defence processes, or else a shift to catabolic processes
when infection severity is extremely high (Salac
et al.
, 2005). Kuzniak & Sklodowska
(2005) observed such a decrease of the GSH content after successful establishment of the
pathogen, together with a decline in ascorbate and associated enzyme activities.
The fi eld trials of Burandt
et al.
(2001) and Bloem
et al.
(2004) suggested a relationship
between the activity of H
2
S releasing enzymes, S status of the crop and infection with
fungal diseases. Fungal infection yielded a higher potential for H
2
S release, since
