Agriculture Reference
In-Depth Information
strong reduction in chloroplastic and cytosolic Cu/Zn SODs by Cd was reported
and to a lesser extent for Fe-SOD, while Mn-SOD was affected only by the highest
Cd concentration tested. This showed that Mn-SOD was the isozyme more resistant
to Cd (Sandalio et al.
2001
). In pea leaf peroxisomes, the Mn-SOD activity did not
change in response to Cd treatment (Romero-Puertas et al.
1999
). On the contrary,
increases in total SOD activity were detected following the application of Cd in
Pisum sativum
(Dalurzo et al.
1997
),
Solanum tuberosum
(Stroinski and Kozlowska
1997
),
Hordeum vulgare
(Guo et al.
2004
),
Arabidopsis thaliana
(Skorzynska-Polit
et al.
2003
/
2004
),
Oryza sativa
(Hsu and Kao
2004
),
Triticum aestivum
(Khan et al.
2007
),
Zea mays
(Krantev et al.
2008
),
Brassica juncea
(Mobin and Khan
2007
),
Vigna mungo
(Singh et al.
2008
),
Cicer arietinum
(Hasan et al.
2008
) and hyperac-
cumulator plants of the genus
Alyssum
(Schickler and Caspi
1999
). SOD activity
remained unaltered in
Helianthus annuus
(Gallego et al.
1996b
,
1999
) and declined
in
Amaranthus lividus
(Bhattacharjee
1998
),
Phragmites australis
(Iannelli et al.
2002
),
Capsicum annuum
plants (Leon et al. 2002),
Glycine max
(Noriega et al.
2007
) under Cd stress. In clonal, hydroponically grown poplar plants (
Populus
×
Canescens
, a hybrid of
Populus termula
×
Populus alba
) (Schutzendubel et al.
2002
) and
Arabidopsis lividus
(Bhattacharjee
1998
) exposed to Cd resulted in in-
hibition of SOD activity. Romero-Puertas et al. (
2004
) studied the involvement of
H
2
O
2
and O
2
·ˉ in the signaling events that lead to the variation of the transcript
levels of Cu/Zn-SOD in
Pisum sativum
plants under Cd stress.
Monodehydroascorbate reductase (EC 1.6.5.4) is a flavin adenin dinucleotide
(FAD) enzyme that is present as chloroplastic and cytosolic isozymes which share
similar properties. MDHAR exhibits a high specificity for monodehydro ascorbate
(MDHA) as the electron acceptor, preferring NADH rather than NADPH as the
electron donor. Asada (
1999
) studied the multi-step reduction of FAD in detail. The
first step is the reduction of the enzyme-FAD to form a charge transfer complex.
The reduced enzyme donates electrons successively to MDHA, producing two mol-
ecules of ascorbate via a semiquinone form [E-FAD-NADP(P)
+
]. The dispropor-
tionation by photoreduced ferrodoxin (redFd) in the thylakoids is of great impor-
tance. Since redFd (reduced ferrodoxin) can reduce MDHA more effectively than
NADP
+
; MDHAR cannot participate in the reduction of MDHA in the thylakoidal
scavenging system. Therefore, MDHAR would function at a site where NAD(P)
H is available, but redFd is not. MDHAR is also located in peroxisomes and mito-
chondria accompanying APX. MDHAR could remove and scavenge H
2
O
2
in these
organelles similar to that in chloroplasts, which has escaped from breakdown by
peroxisomal CAT (del Rio et al.
2002
). Schutzendubel et al. (
2001
) have noted
enhanced MDHAR activity in Cd-exposed
Pinus sylvestris
and a declined MDHAR
activity in Cd-exposed poplar hybrids (
Populus
×
canescens
).
The tripeptide (γ-GluCysGly) glutathione (GSH) plays a central role in several
physiological processes, including regulation of sulfate transport, signal transduc-
tion, conjugation of metabolites, detoxification of xenobiotics (Xiang et al.
2001
)
and the expression of stress-responsive genes (Mullineaux and Rausch
2005
). The
reduced form of glutathione, GSH, is an abundant compound in plant tissue that
exists interchangeably with the oxidized form, GSSG. GSH is abundant (3-10 mM)
