Transcripts of two ent-copalyl diphosphate synthase genes differentially localize in rice plants according to their distinct biological roles

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Abstract

Gibberellins (GAs) are diterpenoid phytohormones that regulate various aspects of plant growth. Tetracyclic hydrocarbon ent-kaurene is a biosynthetic intermediate of GAs, and is converted from geranylgeranyl diphosphate, a common precursor of diterpenoids, via ent-copalyl diphosphate (ent-CDP) through successive cyclization reactions catalysed by two distinct diterpene synthases, ent-CDP synthase and ent-kaurene synthase. Rice (Oryza sativa L.) has two ent-CDP synthase genes, OsCPS1 and OsCPS2. It has been thought that OsCPS1 participates in GA biosynthesis, while OsCPS2 participates in the biosynthesis of phytoalexins, phytocassanes A–E, and oryzalexins A–F. It has been shown previously that loss-of-function OsCPS1 mutants display a severe dwarf phenotype caused by GA deficiency despite possessing another ent-CDP synthase gene, OsCPS2. Here, experiments were performed to account for the non-redundant biological function of OsCPS1 and OsCPS2. Quantitative reverse transcription–PCR (qRT–PCR) analysis showed that OsCPS2 transcript levels were drastically lower than those of OsCPS1 in the basal parts, including the meristem of the second-leaf sheaths of rice seedlings. qRT–PCR results using tissue samples prepared by laser microdissection suggested that OsCPS1 transcripts mainly localized in vascular bundle tissues, similar to Arabidopsis CPS, which is responsible for GA biosynthesis, whereas OsCPS2 transcripts mainly localized in epidermal cells that address environmental stressors such as pathogen attack. Furthermore, the OsCPS2 transgene under regulation of the OsCPS1 promoter complemented the dwarf phenotype of an OsCPS1 mutant, oscps1-1. The results indicate that transcripts of the two ent-CDP synthase genes differentially localize in rice plants according to their distinct biological roles, OsCPS1 for growth and OsCPS2 for defence.

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