Biology Reference
In-Depth Information
In higher plants, the end product of the Chl
a
biosynthetic heterogeneity is
invariably MV Chl
a
and
b
, with the only known exception of a lethal maize mutant
(Bazzaz
1981
) which forms only DV Chl
a
and
b
. However, in the prochlorophyte
picoplankton of the subtropical waters of the North Atlantic as well as in the
picoplankton of the euphotic zone of the world tropical and temperate oceans,
and the Mediterranean sea, DV Chl
a
and
b
are the predominant Chl species
(Chisholm et al.
1988
,
1992
; Goerike and Repeta
1992
; Shioi and Sasa
1983
;
Veldhuis and Kraay
1990
). It has been proposed that in green plants, the multiplic-
ity of Chl
a
biosynthetic routes, produces different pools of MV Chl
a
, complexed
to different pigment-proteins at specific sites of the photosynthetic membranes
(Rebeiz et al.
1983
,
1994
,
2003
,
2005
).
The DV and MV Chl
a
biosynthetic routes are linked at the level of DV Mg-Proto
(Kim and Rebeiz
1996
), DV Mpe (Kolossov and Rebeiz
2010
), DV Pchlide
a
(Tripathy and Rebeiz
1988
), DV Chlide
a
(Parham and Rebeiz
1992
,
1995
), and
DV Chl
a
(Adra and Rebeiz
1998
) by [4-vinyl] reductase(s) that convert the 4-vinyl
group at position 4 to ethyl, thus converting a DV tetrapyrrole to a MV tetrapyrrole.
It is relevant to point out that Whyte and Griffiths (
1993
) have interpreted the
accumulation of DV and MV Pchlide
a
in term of a dual pathway with a single vinyl
reductase of broad specificity. In their scheme the major route converts DV Pchlide
a
to MV Chl
a
via DV Chlide
a
, and MV Chlide
a
. The minor route consists of the
formation of MV Chlide
a
, and by inference of MV Chl
a
via DV Pchlide
a
, which
is converted to MV Pchlide
a
by the non-specific vinyl reductase. This hypothesis is
not compatible with the following observations: (a) It has been demonstrated that
during DV and MV Pchlide
a
biosynthesis, only a fraction of the MV Pchlide
a
pool
can be formed by reduction of DV Pchlide
a
(Tripathy and Rebeiz
1988
), and (b) In
Rhodobacter capsulatus
in which the bchJ gene which codes for DV Pchlide
a
reductase (4VpideR) has been deleted, in addition to the accumulation of DV
Pchlide
a
, accumulation of MV Mg-Proto, its monoester (precursors of Pchlide
a
),
and MV Pchlide
a
have been observed (Suzuki and Bauer
1995
). This in turn
indicates that at least one separate [4-vinyl] reductase exists which acts prior to DV
Pchlide
a
and DV Chlide
a
vinyl reduction. This enzyme would be responsible for
the accumulation of MV Mg protoporphyrins in plants (Belanger and Rebeiz
1982
),
as well as for MV Pchlide
a
formation in the absence of 4VpideR. Finally Signifi-
cant evidence indicates the existence of multiple vinyl-reductases in green plants
(Kolossov and Rebeiz
2010
).
Very recently the Chl
a
biosynthetic heterogeneity has been extended to the level
of Pchlide
a
photoreduction, by the discovery of a second Pchlide
a
oxidoreductase
(POR), in addition to the conventional POR of etiolated tissues (Armstrong
et al.
1995
; Holtorf et al.
1995
). One, POR-A, is the conventional photoenzyme,
which occurs in etiolated tissues but disappears during greening. The second,
POR-B is present throughout angiosperm development. Armstrong et al. (
1995
)
have suggested that POR-A performs a specialized function restricted to the initial
stages of greening, while POR-B is involved in maintaining Chl levels throughout
angiosperm development.
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