Functional defect at the rice choline monooxygenase locus from an unusual post-transcriptional processing is associated with the sequence elements of short-direct repeats

Glycine betaine (GB), a quaternary ammonium solute, plays a crucial role in developing osmotic tolerance. Rice contains a choline monooxygenase (CMO) and two betaine aldehyde dehydrogenase homologues that are required for GB synthesis, but usually no GB is accumulated in rice (Oryza sativa). To elucidate the molecular processes that underlie the GB deficiency in rice, an experiment involving rice and spinach (Spinacia oleracea) was conducted to analyze the products transcribed from CMO genes. Reverse transcription- polymerase chain reaction (RT-PCR) was used to obtain CMO transcripts and a sequencing approach was employed to analyze the structural composition of various CMO transcripts. The results showed that most rice CMO transcripts were processed incorrectly, retaining introns or deleted of coding sequences; the unusual deletion events occurred at sequence elements of the short-direct repeats. In conclusion, the production of incorrect CMO transcripts results in a deficiency of the full-length CMO protein and probably reduces GB accumulation considerably in rice plants. Sequence comparison results also implied that the unusual deletion-site selection might be mediated by the short-direct repeats in response to stress conditions.