Agriculture Reference
In-Depth Information
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100
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Figure 11.3
Release of H
2
S after infection of grapes with
Uncinula necator
(adapted from Bloem
et al.
,
2007 with permission from Wiley-Blackwell).
L-cysteine-desulphhydrase (LCD) activity increased following fungal infection. In a
greenhouse experiment with grapes that had been suffi ciently supplied with S, Bloem
et al.
(2007) determined H
2
S peaks of 140 and 120 pg g
−1
min
−1
seven days after inoculation
without and with additional S supply (Figure 11.3). Although the peak value was higher
when no additional S was supplied, the increase in H
2
S emissions in the previous three
days was about 20% higher when S was added. At later stages, H
2
S emissions were below
the minimum amount of total H
2
S release when no S was added (255 pg), while after-
wards, H
2
S emissions remained at a level of about 50 pg g
−1
min
−1
(Figure 11.3).
A steep and fast increase during the initial phase of pathogenesis was not only determined
for H
2
S, but also cysteine, GSH and phytoalexins (see above). Although the sequence, mag-
nitude and effi cacy of individual S metabolites involved in the activation and strengthening
of plant defences by S fertilisation are not yet known, these could be released in a chain
reaction triggered by the pathogen and mediated by the S status of the plant (Figure 11.4).
It seems possible that infection triggers the activation of all effective resistance
mechanisms of the host. The accumulation of salicylic acid, which initiates and maintains
systemic acquired resistance (Parker, 2000), is linked to plant S metabolism as synthe-
sis of salicylic acid requires coenzyme A (CoASH) in the
-oxidation pathway (Ryals
et al.
, 1996); cysteine is one of the precursors of CoASH synthesis (Luckner, 1990). In the
course of accumulation of cysteine it is possible that the release of H
2
S is a by-product to
keep the level below the phytotoxicity threshold, or a targeted stimulation of LCD activity
may result in an accordingly higher H
2
S emission that requires elevated cysteine concen-
trations. A storage experiment with broccoli revealed, however, that enzyme activity was
not limiting the release of H
2
S, while higher substrate availability coincided with higher
emission rates (Derbali & Makhlouf, 1998). Additionally, the free cysteine pool is the
precursor for all of the relevant S-containing metabolites putatively involved in SIR.
It was shown previously that an increasing S supply to plants was associated with a
higher concentration of cysteine, glutathione, glucosinolates and H
2
S, so that these plants
β
