Abstract
Summary S23142 and Acifluorfen-ethyl (AFE) inhibit protoporphyrinogen oxidase (Protox) and induce accumulation of protoporphyrin IX (Proto IX) which is a strong phytotoxic photosensitizer. A S23142-resistant cell line, YZI-1 S, of photomixotrophically cultured tobacco was selected and its resistance mechanism was characterized. While growth rates of wild-type and YZI-1 S cells were similar in the absence of the herbicide, S23142 concentrations that reduced the chlorophyll contents by 50 % were 2 and 250 nM for wild-type and YZI-1 S cell lines, respectively. The YZI-1 S cells also exhibited resistance for other types of Protox inhibiting herbicides (acifluorfenethyl, acif luorf en, bif enox, oxadiazon, chlomethoxynil, nitrof en and chloronitrofen), but were sensitive to atrazine and DCMU, which inhibit photosynthetic electron transport. YZI-1S cells did not accumulate Proto IX, even at 100nM S23142 in which the wild-type cells accumulated large amounts of Proto IX. Protox isolated from YZI-1S cells showed a 2-fold higher activity than that of wild-type cells and also exhibited a 20-fold increase in tolerance to S23142. On the other hand, treatment with 1 mM 6-Aminolevulinic acid (ALA), a tetrapyrrole precursor, induced photobleaching by accumulation of Proto IX in both YZI-1 S and wild-type cells under high light irradiation. From these results, we conclude that the resistance of YZI-1S cells to S23142 is due mainly to the increase of Protox activity.
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