Cloning and salt-induced, ABA-independent expression of choline mono-oxygenase in Atriplex prostrata

Certain plants accumulate glycinebetaine, a type of osmoprotectant, in response to salinity. Glycinebetaine is synthesized in these plants via the two-step oxidation of choline, and the first step is catalysed by choline mono-oxygenase (CMO; EC 1.14.15.7). Cloned by RT-PCR and 3'-RACE, the cDNA of Atriplex prostrata CMO (ApCMO) is 1669 bp in length and encodes a full-length protein of 438 amino acids. The deduced amino acid sequence of ApCMO revealed a Rieske-type [2Fe-2S] cluster motif and a mononuclear non-heme Fe binding motif, and shares 82.9% identity and 87.2% similarity with the deduced amino acid sequence of spinach CMO. Accumulation of CMO transcript and glycinebetaine both increased in response to NaCl treatment. Without salt treatment, CMO mRNA was detected in stems and 5-day-old seedlings, but not in leaves, roots and older seedlings. With salt treatment, CMO mRNA accumulated dramatically in stems, leaves and roots, with the most abundant accumulation occurring in young stems. Although abscisic acid may initiate global physiological reactions in response to osmotic stress, it did not induce the expression of CMO in A. prostrata. In summary, salt-induction of CMO mRNA in A. prostrata is more substantial than that reported in spinach and sugar beet, and the plant may serve as a useful model to study regulation of glycinebetaine synthesis by environmental stress.