Perturbed porphyrin biosynthesis contributes to differential herbicidal symptoms in photodynamically stressed rice (Oryza sativa) treated with 5-aminolevulinic acid and oxyfluorfen.

This paper focuses on the molecular mechanism of deregulated porphyrin biosynthesis in rice plants under photodynamic stress imposed by an exogenous supply of 5-aminolevulinic acid (ALA) and oxyfluorfen (OF). Plants treated with 5 mM ALA or 50 µM OF exhibited differential herbicidal symptoms as characterized by white and brown necrosis, respectively, with substantial increases in cellular leakage and malondialdehyde production. Protoporphyrin IX accumulated to higher levels after 1 day of ALA and OF treatment, whereas it decreased to the control level after 2 days of ALA treatment. Plants responded to OF by greatly decreasing the levels of Mg-protoporphyrin IX (MgProto IX), MgProto IX methyl ester, and protochlorophyllide to levels lower than control, whereas their levels drastically increased 1 day after ALA treatment and then disappeared 2 days after the treatment. Enzyme activity and transcript levels of HEMA1, GSA and ALAD for ALA synthesis greatly decreased in ALA- and OF-treated plants. Transcript levels of PPO1, CHLH, CHLI, and PORB genes involving Mg-porphyrin synthesis continuously decreased in ALA- and OF-treated plants, with greater decreases in ALA-treated plants. By contrast, up-regulation of FC2 and HO2 genes in Fe-porphyrin branch was noticeable in ALA and OF-treated plants 1 day and 2 days after the treatments, respectively. Decreased transcript levels of nuclear-encoded genes Lhcb1, Lhcb6, and RbcS were accompanied by disappearance of MgProto IX in ALA- and OF-treated plants after 2 days of the treatments. Under photodynamic stress imposed by ALA and OF, tight control of porphyrin biosynthesis prevents accumulation of toxic metabolic intermediates not only by down-regulation of their biosynthesis but also by photodynamic degradation. The up-regulation of FC2 and HO2 also appears to compensate for the photodynamic stress-induced damage.