14C Tracer Evidence for Synthesis of Choline and Betaine via Phosphoryl Base Intermediates in Salinized Sugarbeet Leaves

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Abstract
Like other chenopods, sugarbeets (B. vulgaris L. cv. Great Western D-2) accumulate glycine betaine when salinized; this may be an adaptive response to stress. The pathway of betaine synthesis in leaves of salinized (150-200 mM NaCl) sugarbeet plants was investigated by supplying [14C]formate, phosphoryl[14C]monomethylethanolamine .**GRAPHIC**. MME) or phosphoryl[14C]choline .**GRAPHIC**. choline) to leaf discs and following 14C incorporation into prospective intermediates. The 14C kinetic data were used to develop a computer model of the betaine pathway. When [14C]formate was fed, .**GRAPHIC**. MME, phosphoryldimethylethanolamine .**GRAPHIC**. DME) and .**GRAPHIC**. choline were the most prominent methylated products at short labeling times, after which 14C appeared in free choline and in betaine. Phosphatidylcholine labeled more slowly than .**GRAPHIC**. choline, choline and betaine, and behaved as a minor end product. Very little 14C entered the free methylethanolamines. When .**GRAPHIC**. MME was supplied, a small amount was hydrolyzed to the free base but the major fate was conversion to .**GRAPHIC**. DME, .**GRAPHIC**. choline, free choline and betaine; label also accumulated slowly in phosphatidylcholine. Label from supplied .**GRAPHIC**. choline entered choline and betaine rapidly, while phosphatidylcholine labeled only slowly and to a small extent. These results are consistent with the pathway .**GRAPHIC**. MME .fwdarw. .**GRAPHIC**. DME .fwdarw. .**GRAPHIC**. choline .fwdarw. choline .fwdarw. .fwdarw. betaine, with a minor side branch leading from .**GRAPHIC**. choline into phosphatidylcholine. This contrasts markedly with the pathway of stress-induced choline and betaine synthesis in barley, in which phosphatidylcholine apparently acts as an intermediate (Hitz, et. al 1981) and with choline biogenesis in mammalian liver and microorganisms. Computer modeling of the experimental data pointed strongly to regulation at the .**GRAPHIC**. choline .fwdarw. choline step and also indicated that the rate of .**GRAPHIC**. choline synthesis is subject to feedback inhibition by .**GRAPHIC**. choline.