Altered regulation of porphyrin biosynthesis and protective responses to acifluorfen-induced photodynamic stress in transgenic rice expressing Bradyrhizobium japonicum Fe-chelatase

Abstract

We examined the molecular regulation of porphyrin biosynthesis and protective responses in transgenic rice (Oryza sativa) expressing Bradyrhizobium japonicum Fe-chelatase (BjFeCh) after treatment with acifluorfen (AF). During the photodynamic stress imposed by AF, transcript levels of BjFeCh in transgenic plants increased greatly; moreover, transcript levels of OsFeCh2 remained almost constant, whereas in wild type (WT) plants they were considerably down-regulated. In the heme branch, transgenic plants exhibited greater levels of OsFC and HO transcripts than WT plants in the untreated stems as well as in the AF-treated leaves and stems. Both WT and transgenic plants treated with AF substantially decreased transcript levels for all the genes in the chlorophyll branch, with less decline in transgenic plants. After AF treatment, ascorbate (Asc) content and the redox Asc state greatly decreased in leaves of WT plants; however, in transgenic plants both parameters remained constant in leaves and the Asc redox state increased by 20% in stems. In response to AF, the leaves of WT plants greatly up-regulated CatA, CatB, and GST compared to those of transgenic plants, whereas, in the stems, transgenic plants showed higher levels of CatA, CatC, APXb, BCH, and VDE. Photochemical quenching, q_P, was considerably dropped by 31% and 18% in WT and transgenic plants, respectively in response to AF, whereas non-radiative energy dissipation through non-photochemical quenching increased by 77% and 38% in WT and transgenic plants, respectively. Transgenic plants treated with AF exhibited higher transcript levels of nucleus-encoded photosynthetic genes, Lhcb1 and Lhcb6, as well as levels of Lhcb6 protein compared to those of WT plants. Our study demonstrates that expression of BjFeCh in transgenic plants influences not only the regulation of porphyrin biosynthesis through maintaining higher levels of gene expression in the heme branch, but also the Asc redox function during photodynamic stress caused by AF.

Keywords: Acifluorfen; Ascorbate redox state; Photodynamic stress; Photosystem; Porphyrin biosynthetic genes; ROS-scavenging genes.