Synopsis - Chlorophyll Biosynthesis and Technological Applications - page 56

2.10.4 Discovery of the Greening Group Affiliation of Plants

With the belief in a uniform, single-branched Chl biosynthetic pathway (Granick
1950 ), there was no need to even contemplate the existence of plants greening
differently from one another, depending on their taxonomical affiliation. However,
soon after the discovery of the DV-MV Chl biosynthetic heterogeneity, and after a
brief survey of the plant kingdom, it became apparent that plants used different MV
or DV Chl biosynthetic routes to make Chl (Abd-El-Mageed et al. 1997 ; Carey and
Rebeiz 1985 ; Carey et al. 1985 ; Ioannides et al. 1994 ). The greening group affilia-
tion of plants will be discussed at length in Chap. 14 .

2.10.5 Discovery of Photodynamic Herbicides

In 1982, after having researched the chemistry and biochemistry of the greening
process for 18 years, it was felt that enough was known about this important biological
phenomenon to translate it into biotechnological developments. In looking for a
handle on the problem we opted for the development of photodynamic herbicides.
That decision was prompted by two considerations: (a) the size and importance of the
herbicide industry, and (b) the interesting photosensitizing properties of tetrapyrroles,
which came to our attention.
Tetrapyrrole-dependent photodynamic herbicides (TDPH) consist of compounds
that force green plants to accumulate undesirable amounts of metabolic inter-
mediates of the Chl and heme biosynthetic pathways, namely tetrapyrroles. In the
light the accumulated tetrapyrroles photosensitize the formation of singlet oxygen
which kills the treated plants by oxidation of their cellular membranes. Tetrapyrrole-
dependent photodynamic herbicides usually consist of a 5-carbon amino acid,
δ

-aminolevulinic acid (ALA), the precursor of all tetrapyrroles in plant and animal
cells, and one of several chemicals referred to as modulators. -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 (Rebeiz et al. 1988b ). In the light, tetrapyr-
roles are excited to the singlet state. It is believed that the excited, singlet tetrapyrroles
can readily be converted to the triplet state via intersystem crossing. Since, in the
ground state, oxygen exists in the triplet state, triplet-triplet energy transfer can readily
take place between the excited triplet tetrapyrroles and the ground state triplet oxygen.
As a consequence of this energy transfer, oxygen is excited to the singlet state. Being a
very powerful oxidant, singlet oxygen oxidizes the unsaturated fatty acids of the
lipoprotein membranes which are converted to hydroperoxides. The latter in turn
produce free radicals which attack the unsaturated membrane lipoproteins thus
setting in motion a greatly damaging free-radical chain reaction. The reaction
stops when most of the cell membranes have been destroyed, causing plant death
(Rebeiz et al. 1984 ).

δ