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Biosynthetic Heterogeneity of MV Chlorophyll
b
in DDV-LDV-LDDV Plant Species
Six different pools of MV Chl
b
are formed from MV Chlide
b
, MV Chl
a
and MV
Pchlide
b
. The biosynthesis of these pools is discussed below.
Formation of MV Chl b from MV Chlide a and MV Chl a via Routes
2, and 5 and from MV Chlide b via Route 4 in Etiolated
DDV-LDV-LDDV Plants Subjected to Illumination
We have repeatedly observed that in DDV-LDV-LDDV plant tissues such as
etiolated cucumber cotyledons subjected to a brief light illumination then returned
to darkness, MV Chl
b
accumulation as monitored by sensitive fluorescence
techniques (Rebeiz
2002
) is observed after 15-30 min of dark-incubation. By
using less sensitive spectrophotometric techniques, it is observed that etiolated
cucumber cotyledons subjected to continuous illumination, start accumulating
measurable amounts of MV Chl
b
after 2 h of illumination (Rebeiz
1967
).
We propose that the biosynthesis of this MV Chl
b
can originate in routes 2, 4
and 5, as described below.
In route 2, MV Chl
b
would be formed from MV Chl
a
which is formed in turn
from MV Pchlide
a
and MV Chlide
a
(Fig.
13.9
).
In route 5, MV Chl
b
would be formed from MV Chl
a
which is formed in turn
from DV Chlide
a
, and MV Chlide
a
(Fig.
13.9
). In both cases the conversion of MV
Chl
a
to MV Chl
b
is substantiated by favorable precursor-product relationships
between these two tetrapyrroles in DDV-LDV-LDDV plants (Rebeiz et al.
1999
).
In route 4, MV Chl
b
would be formed by esterification of nascent MV Chlide
b
which is formed in turn from DV Chlide
a
and MV Chlide
a
(Fig.
13.9
). Esterifi-
cation of MV Chlide
b
with GG followed by stepwise hydrogenation of MV Chl
b-GG is strongly suggested by the detection of MV Chlide
b
-GG, Chlide
b
-DHGG,
MV Chlide
b
-THGG, and MV Chl
b
-phytol in greening etiolated cucumber cotyledons
(Shio and Sasa
1983
).
Since during the initial phases of greening of etiolated tissues, most of the Chl
consists of MV Chl, located in PSI and PSII (Akoyunoglou
1978
; Akoyunoglou
et al.
1981
; Alberte et al.
1972
), we propose that the MV Chl
b
formed via routes
2, 4, and 5 is destined to PSI and/or PSII inner antenna Chl-protein complexes. It is
not certain at this stage whether the MV Chl
b
formed via routes 2, and 5 during the
very early phases of greening is convertible to MV Chl
a
or not as has been reported
for later stages of greening (Ohtsuka et al.
1996
).
Biosynthesis of MV Chl b from MV Chl a via Route 0 in Greening DDV-LDV-LDDV
Plants During the Light Phases of the Photoperiod
The operation of this biosynthetic route depends on the Detection of DV Mg-Proto
reductase in DDV-LDV-LDDV plant species (Fig.
13.10
).
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