Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 356

17.4.2 Effects of Exogenous Tetrapyrroles
on Isolated Chloroplasts
Only one of the five exogenous tetrapyrroles failed to trigger chloroplast destruction
in the light, namely divinyl (DV) Mg-Protoporphyrin IX (Mg-Proto). Esterification of
DV Mg-Proto to yield DV Mg-Proto monomethyl ester (Mpe) rendered this tetrapyr-
role extremely destructive. While overall destructive effects were manifested by Chl
a and b disappearance and the appearance of Chl degradation products, such as
chlorophyllide a ,and b and pheophytin and pheophorbide a , more specific effects on
the pigment-protein complexes became evident from in organello 77 K fluorescence
spectroscopy. DV Proto, an early intermediate in Chl a biosynthesis, affected the
photosystem (PS) II antenna Chl a pigment-protein complexes, but had no effect on
the PS I antenna complex and the Chl a / b light harvesting antenna complex (LHCII).
On the other hand DVMpe and DV Pchlide a , destroyed completely all the thylakoid
pigment-protein complexes. As for DV-Pchlide a , it exhibited its strongest effect on
the disorganization of the PS I antenna LHCI-730 complex. Altogether these results
indicate that individual tetrapyrroles have distinct and different disruptive effects on
the structure of thylakoid membranes in the light. Specific effects appear to be related
to the position of particular tetrapyrrole in the Chl a biosynthetic chain and its
electrostatic properties (Amindari et al. 1995 )
17.4.2.1 Effect of Exogenous DV Proto on Photodynamic Damage
in Isolated Cucumber Chloroplasts
As described in Chaps. 5 and 7 , DV Proto is the precursor of DV Mg-Proto. In its
native state DV Proto is loosely bound to the plastid membranes (Smith and Rebeiz
1979 ). It is formed by oxidation of DV protoporphyrinogen IX (Protogen) by
protoporphyrinogen oxidase (Protox). DV Protogen is the hexahydro reduction
product of DV Proto. It is a highly mobile metabolite. It moves readily from one
cellular compartment to another where it is rapidly converted to DV Proto by Protox.
Indeed, diphenyl ethers belong to a family of potent herbicides that act via Protox
inhibition in the chloroplast. Protogen that can no longer be converted to Proto in the
chloroplast, diffuses out of the chloroplast to various subcellular compartments.
There, it causes considerable photodynamic damage after conversion to DV Proto
by Protoxes, that are resistant to inhibition by diphenyl ethers. A large number of
photodynamic herbicide modulators also result in the accumulation of DV Proto in
the chloroplast, when plants are treated with modulators and ALA (Rebeiz
et al. 1988b , 1990 , 1991 , 1994 ).
After 2 h of incubation of isolated chloroplasts with Proto in the light, about
90 % of the added Proto disappeared (Rebeiz et al. 1984a , b ). Proto photosensitiza-
tion exerted negligible effects, however, on other pigments. Except for a very
modest increase in Chlide a and b content, Proto had essentially no effects on
other pigment pools a (Amindari et al. 1995 ).