The Accumulation of Glycine Betaine Is Dependent on Choline Monooxygenase (OsCMO), Not on Phosphoethanolamine N-Methyltransferase (OsPEAMT1), in Rice (Oryza sativa L. ssp. japonica)

Glycine betaine (GB) is an important osmoprotectant, which improves plant tolerance to various abiotic stresses. In higher plants, GB is synthesized through two-step oxidations of choline, catalyzed by choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively. Choline, the precursor of GB, is synthesized by phosphoethanolamine N-methyltransferase (PEAMT). Rice is known as a typical non-GB-accumulated species. However, the underlying mechanism related to GB accumulation remains elusive. Here, we determined whether the endogenous accumulation of choline is sufficient to GB biosynthesis in rice and whether the rice CMO protein has the function of oxidizing choline to generate betaine aldehyde. The results showed that overexpression of the rice PEAMT1 gene (OsPEAMT1) resulted in increased levels of choline, while GB content remained unchanged in the transgenic rice plants overexpressing OsPEAMT1. However, the intracellular GB level and the tolerance to salt stress of the transgenic lines overexpressing OsCMO were significantly enhanced. Immunoblotting analysis demonstrated that abundant functional OsCMO proteins with correct size were detected in OsCMO-overexpressing transgenic rice plants, but rarely accumulated in the wild type. Collectively, these results implicated that the endogenous accumulation level of choline is not the major factor leading to non-GB accumulation in rice. Instead, the defective expression of OsCMO resulted in non-GB accumulation.