Overexpression of Serotonin N-Acetyltransferase Cloned from the Green Algae Chara braunii Confers Peroxidizing Herbicide Tolerance Through the Induction of Protoporphyrinogen Oxidase 1 in Rice.

Serotonin N-acetyltransferase (SNAT) is pivotal to melatonin biosynthesis, catalyzing either serotonin into N-acetylserotonin or 5-methoxytryptamine (5-MT) into melatonin. Many SNAT genes have been cloned from a wide range of organisms, including animals and plants, but none has been reported in the model streptophyte green alga Chara braunii. Here, we found one archaeal CbSNAT homologue in the C. braunii genome, which showed 28% and 27% amino acid homology with human Naa50 and rice SNAT3, respectively. The CbSNAT gene, encoding a 172-amino acid protein, was expressed and purified in Escherichia coli. The recombinant CbSNAT protein exhibited SNAT enzyme activity toward serotonin (Km = 293 μM) and 5-MT (Km = 28 μM), and was located in the cytoplasm of tobacco cells, similar to other archaeal SNAT homolog proteins. To assess whether CbSNAT was functionally coupled to melatonin biosynthesis, CbSNAT was overexpressed in rice. Transgenic CbSNAT-overexpressing rice seedlings showed enhanced melatonin synthesis and resistance to both cadmium and the herbicide butafenacil. The cadmium resistance was due to the increased expression of chaperone genes such as BIP3, BIP4, and BIP5, whereas the butafenacil resistance resulted from increased expression of protoporphyrinogen oxidase 1, a target gene of butafenacil, via the transcriptional regulatory effects of melatonin. The discovery of a CbSNAT gene in C. braunii opens a new avenue for its use as a genetic resource in the development of crops exhibiting stress resistance to toxic compounds found in modern agriculture.