Biosynthesis, Translocation, and Accumulation of Betaine in Sugar Beet and Its Progenitors in Relation to Salinity

Abstract

Like other halophytic chenopods, sugar beet (Beta vulgaris L.) can accumulate high betaine levels in shoots and roots. N,N,N-trimethylglycine impedes sucrose crystallization and so lowers beet quality. The objective of this research was to examine the genetic variability and physiological significance of betaine accumulation in sugar beet and its relatives. Three cultivated genotypes of B. vulgaris and two genotypes of the wild progenitor B. maritima L. were grown with and without gradual salinization (final NaCl concentration = 150 millimolar). At 6 weeks old, all five genotypes had moderately high betaine levels in shoots and roots when unsalinized (averages for all genotypes: shoots = 108 micromoles per gram dry weight; roots = 99 micromoles per gram dry weight). Salinization raised betaine levels of shoots and roots 2- to 3-fold, but did not greatly depress shoot or root growth. The genotype WB-167—an annual B. maritima type—always had approximately 40% lower betaine levels in roots than the other four genotypes, although the betaine levels in the shoots were not atypically low.The site and pathway of betaine synthesis were investigated in young, salinized sugar beet plants by: (a) supplying 1 micromole [14C]ethanolamine to young leaf blades or to the taproot sink of intact plants; (b) supplying tracer [14C]formate to discs of leaf, hypocotyl, and taproot tissues in darkness. Conversion of both 14C precursors to betaine was active only in leaf tissue. Very little 14C appeared in the phospholipid phosphatidylcholine before betaine was heavily labeled; this was in marked contrast to the labeling patterns in salinized barley. Phosphorylcholine was a prominent early 14C metabolite of both [14C]ethanolamine and [14C]formate in all tissues of sugar beet. Betaine translocation was examined in young plants of sugar beet and WB-167 by applying tracer [methyl-14C]betaine to a young expanded leaf and determining the distribution of 14C after 3 days. In all cases, extensive 14C translocation to young leaves and taproot sink occurred; neither in the fed leaf nor in sink organs were any 14C metabolites of betaine detected.