Regulation of Photosynthetic Light Energy Capture
43
Perhaps the strongest evidence for a causal involvement of the xanthophyll cycle in mediating energy dissipation in the pigment bed (NRD) comes
from studies in which V to Z de-epxidation was blocked by application
of dithiothreitol (DTI). DTT is a powerful inhibitor of violaxanthin deepoxidase (Yamamoto and Kamite 1972). Application of a DTT solution to
leaves through the cut petioles effectively prevented Z formation and caused
a strong suppression of NPQ and an increased degree of reaction center
closure (Bilger et al. 1989; Bilger and Bjorkman 1990, 1991; DemmigAdams et al. 1990). Such blocking of Z formation also resulted in an
apparently increased susceptibility to photoinhibition during longer-term
exposure to high PFD (Winter and Koniger 1989; Adams et al. 1990; Bilger
and Bjorkman 1990). However, DTI treatments had no detectible shortterm effects on photosynthetic O2 or CO2 exchange, nor on the efficiency of
energy conversion in PS II. Moreover, subsequent studies have shown that
application of DTI to cotton leaves in which high levels of zeaxanthin
are already present does not suppress the development of NPQ nor of
the putative conformational change as determined by absorbance changes
(Bilger and Bjorkman, unpubl. data). Similar results were obtained with
isolated lettuce chloroplasts by Gilmore and Yamamoto (1992). No significant NPQ development was obtained upon exposure to conditions that
caused the proton gradient to rise if Z formation was prevented by DTI and
the preparations initially did not contain any Z. However, full NPQ development occurred when DTT was applied to preparations in which high
levels of Z had been preformed. The above studies indicate that the main
effect of DTT on the development of NPQ was through its prevention
of Z formation alone both in leaves and isolated chloroplasts. Moreover,
according to Gilmore and Yamamoto (pers. comm.), at any given lumen
proton concentration NPQ was found to be linearly dependent on the Z
concentration or perhaps even more closely on the Z + A concentration, in
both lettuce and pea chloroplasts. Conversely, at any given Z or Z + A
concentration, NPQ was linearly dependent on the lumen proton concentration. Hence, NPQ was directly proportional to the product of [H+] and
[Z] or [Z + A].
As was discussed above, in the presence of excess light the lumen proton
concentration rises, causing a high activity of violaxanthin deepoxidase and
subsequent depoxidation of V to A and Z. A unifying hypothesis is that
the resulting substitution of Z (and perhaps also A) for V in thylakoid
membranes, together with the increased proton concentration, causes a conformational change which alters the association among the pigment molecules. As a result of this change, NRD is favored over fluorescence and
energy transfer to the reaction centers, thus causing an increase in NPQ.
Horton et al. (1991) proposed that protonation of the lumen surface causes
an aggregation of the major light harvesting pigment-protein complex of PS
II and that this aggregation is amplified by Z. While it still needs to be
established whether Z is involved directly or indirectly in NRD, this is
43
Perhaps the strongest evidence for a causal involvement of the xanthophyll cycle in mediating energy dissipation in the pigment bed (NRD) comes
from studies in which V to Z de-epxidation was blocked by application
of dithiothreitol (DTI). DTT is a powerful inhibitor of violaxanthin deepoxidase (Yamamoto and Kamite 1972). Application of a DTT solution to
leaves through the cut petioles effectively prevented Z formation and caused
a strong suppression of NPQ and an increased degree of reaction center
closure (Bilger et al. 1989; Bilger and Bjorkman 1990, 1991; DemmigAdams et al. 1990). Such blocking of Z formation also resulted in an
apparently increased susceptibility to photoinhibition during longer-term
exposure to high PFD (Winter and Koniger 1989; Adams et al. 1990; Bilger
and Bjorkman 1990). However, DTI treatments had no detectible shortterm effects on photosynthetic O2 or CO2 exchange, nor on the efficiency of
energy conversion in PS II. Moreover, subsequent studies have shown that
application of DTI to cotton leaves in which high levels of zeaxanthin
are already present does not suppress the development of NPQ nor of
the putative conformational change as determined by absorbance changes
(Bilger and Bjorkman, unpubl. data). Similar results were obtained with
isolated lettuce chloroplasts by Gilmore and Yamamoto (1992). No significant NPQ development was obtained upon exposure to conditions that
caused the proton gradient to rise if Z formation was prevented by DTI and
the preparations initially did not contain any Z. However, full NPQ development occurred when DTT was applied to preparations in which high
levels of Z had been preformed. The above studies indicate that the main
effect of DTT on the development of NPQ was through its prevention
of Z formation alone both in leaves and isolated chloroplasts. Moreover,
according to Gilmore and Yamamoto (pers. comm.), at any given lumen
proton concentration NPQ was found to be linearly dependent on the Z
concentration or perhaps even more closely on the Z + A concentration, in
both lettuce and pea chloroplasts. Conversely, at any given Z or Z + A
concentration, NPQ was linearly dependent on the lumen proton concentration. Hence, NPQ was directly proportional to the product of [H+] and
[Z] or [Z + A].
As was discussed above, in the presence of excess light the lumen proton
concentration rises, causing a high activity of violaxanthin deepoxidase and
subsequent depoxidation of V to A and Z. A unifying hypothesis is that
the resulting substitution of Z (and perhaps also A) for V in thylakoid
membranes, together with the increased proton concentration, causes a conformational change which alters the association among the pigment molecules. As a result of this change, NRD is favored over fluorescence and
energy transfer to the reaction centers, thus causing an increase in NPQ.
Horton et al. (1991) proposed that protonation of the lumen surface causes
an aggregation of the major light harvesting pigment-protein complex of PS
II and that this aggregation is amplified by Z. While it still needs to be
established whether Z is involved directly or indirectly in NRD, this is