Nonsense-mediated decay of mutant waxy mRNA in rice.

Two rice (Oryza sativa) waxy mutations of the Japonica background were shown to contain approximately 20% of the fully spliced mRNA relative to the wild type. Sequencing analysis of the entire waxy genes of the two mutants revealed the presence of premature translation termination codons in exon 2 and exon 7. These results indicated that the lower accumulation of fully spliced RNA in the mutants was caused by nonsense-mediated decay (NMD), which is an RNA surveillance system universally found in eukaryotes. It is interesting that levels of RNA retaining intron 1 were not changed by premature nonsense codons, suggesting that splicing may be linked with NMD in plants, as previously found in mammalian cells. Measurements of the half-lives of waxy RNAs in transfected rice protoplasts indicated that the half-life of waxy RNA with a premature nonsense codon was 3.3 times shorter than that without a premature nonsense codon. Because the wild-type waxy transcripts, which are derived from the Wx(b) gene predominantly distributed among Japonica rice, have been shown to be less efficiently spliced and their alternative splicing has been documented, we examined whether these splicing properties influenced the efficiency of NMD. However, no effects were observed. These results established that NMD occurs in rice waxy RNA containing a premature nonsense codon.