Foreword / Preface / Introduction to Photobiology, Photosynthesis and Photmorphogenesis / Part A: Photosynthesis/ Section I: Basic Principles and Primary Processes / Section II: Photosynthetic Apparatus / Section III: Photophosphorylation and CO2 Metabolism/ Part B: Stress Response: Section IV: Light Stress/ Section V: Other Stress Factors / Part C: Gene Regulation and Photomorphogenesis: Section VI: Chloroplast Molecular Biology and Regulation / Section VII: Photoreceptors and Functions/ Section VIII: Photoresponses and Molecular Mechanism.

Concepts in photobiology : photosynthesis and photomorphogenesis

We have investigated the formation of porphyrin intermediates by isolated barley (Hordeum vulgare) plastids incubated for 40 min with the porphyrin precursor 5-aminolevulinate and in the presence and absence of a diphenylether herbicide that blocks protoporphyrinogen oxidase, the enzyme in chlorophyll and heme synthesis that oxidizes protoporphyrinogen IX to protoporphyrin IX. In the absence of herbicide, about 50% of the protoporphyrin IX formed was found in the extraplastidic medium, which was separated from intact plastids by centrifugation at the end of the incubation period. In contrast, uroporphyrinogen, an earlier intermediate, and magnesium protoporphyrin IX, a later intermediate, were located mainly within the plastid. When the incubation was carried out in the presence of a herbicide that inhibits protoporphyrinogen oxidase, protoporphyrin IX formation by the plastids was completely abolished, but large amounts of protoporphyrinogen accumulated in the extraplastidic medium. To detect extraplastidic protoporphyrinogen, it was necessary to first oxidize it to protoporphyrin IX with the use of a herbicide-resistant protoporphyrinogen oxidase enzyme present in Escherichia coli membranes. Protoporphyrinogen is not detected by some commonly used methods for porphyrin analysis unless it is first oxidized to protoporphyrin IX. Protoporphyrin IX and protoporphyrinogen found outside the plastid did not arise from plastid lysis, because the percentage of plastid lysis, measured with a stromal marker enzyme, was far less than the percentage of these porphyrins in the extraplastidic fraction. These findings suggest that of the tetrapyrrolic intermediates synthesized by the plastids, protoporphyrinogen and protoporphyrin IX, are the most likely to be exported from the plastid to the cytoplasm. These results help explain the extraplastidic accumulation of protoporphyrin IX in plants treated with photobleaching herbicides. In addition, these findings suggest that plastids may export protoporphyrinogen or protoporphyrin IX for mitochondrial heme synthesis.

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Porphyrin Accumulation and Export by Isolated Barley (Hordeum vulgare) Plastids (Effect of Diphenyl Ether Herbicides).

With a leaf disc assay, 11 species were tested for effects of the herbicide acifluorfen on porphyrin accumulation in darkness and subsequent electrolyte leakage and photobleaching of chlorophyll after exposure to light. Protoporphyrin IX (Proto IX) was the only porphyrin that was substantially increased by the herbicide in any of the species. However, there was a wide range in the amount of Proto IX accumulation caused by 0.1 millimolar acifluorfen between species. Within species, there was a reduced effect of the herbicide in older tissues. Therefore, direct quantitative comparisons between species are difficult. Nevertheless, when data from different species and from tissues of different age within a species were plotted, there was a curvilinear relationship between the amount of Proto IX caused to accumulate during 20 hours of darkness and the amount of electrolyte leakage or chlorophyll photobleaching caused after 6 and 24 hours of light, respectively, following the dark period. Herbicidal damage plateaued at about 10 nanomoles of Proto IX per gram of fresh weight. Little difference was found between in vitro acifluorfen inhibition of protoporphyrinogen oxidase (Protox) of plastid preparations of mustard, cucumber, and morning glory, three species with large differences in their susceptibility at the tissue level. Mustard, a highly tolerant species, produced little Proto IX in response to the herbicide, despite having a highly susceptible Protox. Acifluorfen blocked carbon flow from δ-aminolevulinic acid to protochlorophyllide in mustard, indicating that it inhibits Protox in vivo. Increasing δ-aminolevulinic acid concentrations (33-333 micromolar) supplied to mustard with 0.1 millimolar acifluorfen increased Proto IX accumulation and herbicidal activity, demonstrating that mustard sensitivity to Proto IX was similar to other species. Differential susceptibility to acifluorfen of the species examined in this study appears to be due in large part to differences in Proto IX accumulation in response to the herbicide. In some cases, differences in Proto IX accumulation appear to be due to differences in activity of the porphyrin pathway.

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Physiological basis for differential sensitivities of plant species to protoporphyrinogen oxidase-inhibiting herbicides.

Tetrapyrrole-dependent photodynamic herbicides (TDPH)* (Rebeiz et al., 1984) are compounds which force green plants to accumulate undesirable amounts of metabolic intermediates of the chlorophyll (Chl) and heme metabolic 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, 6-aminolevulinic acid (ALA), the precursor of all tetrapyrroles in plant and animals cells, and one of several chemicals referred to as modulators. 6-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. The tetrapyrroledependent connotation is meant to differentiate between this class of photodynamic herbicides and other light activated herbicides such as paraquat which are not dependent on tetrapyrrole metabolism for herbicidal activity. During the past 6 years, the scope of TDPH research has expanded considerably, as some established herbicides and a plethora of new compounds which act via the TDPH phenomenon have been discovered. In this review an attempt is made to describe this new field of research and to summarize the progress achieved since the field was last reviewed (Rebeiz et al., 1987, 1988a).

Tetrapyrrole-dependent photodynamic herbicides

Quantitative structure-activity relationships of protoporphyrinogen oxidase-inhibiting diphenyl ether herbicides

It is shown that continuous illumination is mandatory for the induction of tetrapyrrole accumulation in acifluorfen-sodium-treated plants and for the photosensitization of tetrapyrrole-dependent photodynamic damage. At low concentrations of acifluorfen-sodium (up to 20 μM), photoporphyrin IX appears to be the major light-induced tetrapyrrole that accumulates in the treated plants. At higher concentrations of acifluorfen-sodium, monovinyl chlorophyllide a accumulates in addition to protoporphyrin IX. In the light, the development of photodynamic injury appears to be directly related to the accumulation of the light-induced tetrapyrroles. For example when acifluorfen-sodium-treated plants are returned to darkness, or are treated with tetrapyrrole biosynthesis inhibitors, tetrapyrrole accumulation and photodynamic injury come to a halt. In-vivo and in-organello studies failed, however, to support the commonly held hypothesis that the induction of tetrapyrrole accumulation in the light, in acifluorfen-sodium-treated plants, is only dependent on the inhibition of protoporphyrinogen oxidase. Indeed, when plastids capable of very high rates of tetrapyrrole biosynthesis and accumulation were incubated with δ-aminolevulinic acid and acifluorfen-sodium, either in darkness or in the light, a severe inhibition of protoporphyrin IX and total terapyrrole formation was observed. Althoung these results are compatible with the inhibition of tetrapyrrole formation by acifluorfensodium at the level of protoporphyrinogen oxidase, they indicate that, in addition to that inhibition, other cuellular processes are probably involved in the light-dependent accumulation of protoporphyrin IX in acifluorfensodium-treated plants.

Ujjanagouda B. Nandihalli

Papers

Photodynamic herbicides VIII. Mandatory requirement of light for the induction of protoporphyrin IX accumulation in acifluorfen-treated cucumber†

The primary mode of action of acifluorfen-Na in intact seedlings is not via tetrapyrrole accumulation during the first dark period following treatment