A rice chloroplast transit peptide sequence does not alter the cytoplasmic localization of sheep serotonin N ‐acetyltransferase expressed in transgenic rice plants

Ectopic overexpression of melatonin biosynthetic genes of animal origin has been used to generate melatonin‐rich transgenic plants to examine the functional roles of melatonin in plants. However, the subcellular localization of these proteins expressed in the transgenic plants remains unknown. We studied the localization of sheep ( Ovis aries ) serotonin N ‐acetyltransferase (Oa SNAT ) and a translational fusion of a rice SNAT transit peptide to Oa SNAT ( TS :Oa SNAT ) in plants. Laser confocal microscopy analysis revealed that both Oa SNAT and TS :Oa SNAT proteins were localized to the cytoplasm even with the addition of the transit sequence to Oa SNAT . Transgenic rice plants overexpressing the TS :Oa SNAT fusion transgene exhibited high SNAT enzyme activity relative to untransformed wild‐type plants, but lower activity than transgenic rice plants expressing the wild‐type Oa SNAT gene. Melatonin levels in both types of transgenic rice plant corresponded well with SNAT enzyme activity levels. The TS :Oa SNAT transgenic lines exhibited increased seminal root growth relative to wild‐type plants, but less than in the Oa SNAT transgenic lines, confirming that melatonin promotes root growth. Seed‐specific Oa SNAT expression under the control of a rice prolamin promoter did not confer high levels of melatonin production in transgenic rice seeds compared with seeds from transgenic plants expressing Oa SNAT under the control of the constitutive maize ubiquitin promoter.