Environmental Engineering Reference
In-Depth Information
Senescent mes16 mutants exhibit a strong UV-excitable fluorescence, which
is due to accumulation of FCCs. This derives, at least in part, from the fact that
FCC isomerization to the respective NCC in the presence of an intact C132-
carboxymethylester is slower than with a free carboxylic acid group (Christ et al.
2012
; Oberhuber et al.
2008
). The most likely reason is differences in the vacuolar
pH, which determine the rate of FCC-to-NCC isomerization. Therefore, whether
a plant can accumulate FCCs or NCCs might depend on the presence/absence of
O13
4
-demethylation and/or on the vacuolar pH (Christ et al.
2012
). Accumulation
of 'hypermodified' FCCs (
h
FCCs) in ripening bananas (
Musa acuminata
,
Cavendish cultivar) can indicate a new role of Chl catabolites. Moreover,
h
FCCs
are a group of unprecedented FCC-esters, and their accumulation in the peels of
ripening bananas is rationalized by the corresponding deactivation of the natural,
acid-induced (FCC-to-NCC) isomerization (Moser et al.
2008
). Such isomerization
occurs rapidly in weakly acidic solution (at pH 4.9) and at ambient temperature
in aqueous solution. It also occurs in the vacuoles of senescent leaves, in senes-
cent leaves of banana plants and of the peace lily (
Spathiphyllum wallisii
) (Matile
et al.
1988
; Matile
1997
; Oberhuber et al.
2003
; Moser et al.
2009
; Banala et al.
2010
; Kräutler et al.
2010
). The
h
FCCs are esterified at the C17-propionic acid side
chain, but they are not isomerized to NCCs in some senescing leaves and in ripen-
ing banana fruits (Moser et al.
2009
; Banala et al.
2010
; Kräutler et al.
2010
).
The conversion of FCCs to NCCs in vacuole is partly due to either Fenton-type
or photo-Fenton type reactions that can generate the HO
•
, a strong oxidizing agent.
This issue is supported by the observation of hydroxylated NCC products or of prod-
ucts with OH-containing other functional groups in place of CH
3
(R
1
or R
3
positions)
(Moser et al.
2009
; Hörtensteiner and Kräutler
2011
; Müller et al.
2007
; Pruzinská et al.
2005
; Christ et al.
2012
; Kräutler et al.
1991
; Mühlecker and Kräutler
1996
; Oberhuber
et al.
2003
; Kräutler et al.
1992
; Curty and Engel
1996
; Berghold et al.
2004
; Berghold
et al.
2006
). Further evidence is the occurrence of the reactions under acidic conditions
(pH 4.9), which is vital for obtaining sufficiently high efficiency of Fenton or photo-
Fenton reactions. Note that Fenton reaction occurs in an aqueous solution of H
2
O
2
and
ferrous or ferric salts, which can produce HO
•
(see also '
Photoinduced Generation
of Hydroxyl Radical in Natural Waters
”) (Fenton
1894
; Barb et al.
1951
; Zepp et al.
1992
; Kwan and Voelker
2002
). The efficiency of the Fenton reaction is highest at pH
3, whilst the photo-Fenton process takes place in the presence of light. The occurrence
of various salts, minerals, proteins, FCCs, water and so on in vacuole may favor such
type of reactions. The reduction of the rate of formation of hydroperoxides of linoleic
acid (induced by H
2
O
2
) in the presence of NCC may also support the occurrence of
such reactions in vacuole (Moser et al.
2009
; Müller et al.
2007
). High production rates
of H
2
O
2
in vacuole can be due either to light-sensitive FCCs or from the complexes
of FCCs with metal ions present in vacuole. Upon irradiation, such compounds yield
electrons (e
−
) that can subsequently produce superoxide radical anions (O
2
•
−
), H
2
O
2
,
and finally HO
•
from H
2
O
2
. The latter process can take place by either direct photo-
dissociation (H
2
O
2
+
h
υ
→
HO
•
) or upon Fenton and photo-Fenton reactions. Such
processes are discussed in detail in other chapters (see chapters
“
Photoinduced and
