Biology Reference
In-Depth Information
17.3 Photodynamic Herbicides: Concept
and Phenomenology
Tetrapyrrole-dependent photodynamic herbicides (TDPH) are compounds that
force green plants to accumulate undesirable amounts of metabolic intermediates
of the chlorophyll (Chl) and heme metabolic pathways, namely tetrapyrroles
(Duke and Rebeiz 1994 ;Rebeizetal. 1984b , 1987 , 1988b , 1991 , 1994 ;Rebeiz
1991 ; Reddy and Rebeiz 1994 ). In the light the accumulated tetrapyrroles photo-
sensitize the formation of singlet oxygen that kills the treated plants by oxidation
of their cellular membranes. Tetrapyrrole-dependent photodynamic herbicides
usually consist of a 5-carbon amino acid, ALA, the precursor of all tetrapyrroles
in plant and animal cells, and one of several chemicals referred to as modulators.
Delta-aminolevulinic acid and the modulators act in concert. The amino acid
serves as a building block of tetrapyrrole accumulation, while the modulator
alters quantitatively and qualitatively the pattern of tetrapyrrole accumulation
(Amindari et al. 1995 ). The tetrapyrrole-dependent connotation is meant to
differentiate between this class of photodynamic herbicides and other light
activated herbicides such as paraquat, that are not dependent on tetrapyrrole
metabolism for herbicidal activity. During the past several years, the scope of
TDPH research has expanded considerably, as some established herbicides and a
plethora of new compounds that act via the TDPH phenomenon have been
discovered. However commercialization of ALA-dependent photodynamic
herbicides has not yet been achieved. The rest of this chapter will therefore
attempt to lay down the foundations for such an undertaking by future researchers
in academia and industry.
17.4 Photodynamic Effects of Metabolic
Tetrapyrroles on Isolated Chloroplasts
While delta-aminolevulinic acid (ALA)-dependent photodynamic destruction of
insect and animal tissues is mainly photosensitized by protoporphyrin IX (Proto),
additional Mg-containing tetrapyrroles are involved in the photodynamic destruction
of plant tissues. To gain better understanding of the destructive photodynamic effects
of these plant tetrapyrroles, the effects of divinyl (DV) Proto, DV Mg-Proto and its
monomethyl ester and DV and monovinyl (MV) protochlorophyllides (Pchlides) on
isolated chloroplasts was compared. Incubation of isolated cucumber chloroplasts
with tetrapyrroles, in the light, exhibited various effects on the pigments and pigment-
protein complexes of the plastids. These effects are described below. The state of
pigment- protein complexes was monitored by analysis of pigment content and by
spectrofluorometry of isolated chloroplasts at 77 K (Amindari et al. 1995 ).
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