CULTIVATED AND WILD RICES DO NOT ACCUMULATE GLYCINEBETAINE DUE TO DEFICIENCIES IN 2 BIOSYNTHETIC STEPS

Many cereals, grasses and other plants accumulate glycinebetaine in response to drought or salinity. Glycinebetaine can act as a nontoxic or protective cytoplasmic osmolyte, and is therefore a biochemical component of drought- and salt-stress resistance. It is synthesized by a two-step pathway: choline --> betaine aldehyde --> glycinebetaine. There are contradictory literature reports as to whether cultivars of rice (Oryza sativa L.) accumulate glycinebetaine. To resolve this conflict, we used sensitive and specific fast atom bombardment mass spectrometry methods to analyze 35 diverse rice cultivars, together with accessions of nine wild rices. No glycinebetaine accumulators were found; levels of glycinebetaine were in all cases < 1 mumol g-1 dry wt., and no other betaines were detected. We then applied fast atom bombardment mass spectrometry to determine why rice, unlike other cereals, does not accumulate glycinebetaine. Rice leaves supplied with deuterium-labeled betaine aldehyde did not convert it to glycinebetaine, and they lacked an endogenous betaine aldehyde pool. Together, these results indicate that rice lacks both the steps in the glycinebetaine biosynthesis pathway. Levels of the precursor choline, however, were comparable to those found in betaine-accumulating cereals. Rice cultivars are therefore rational candidates for the introduction of heterologous genes for glycinebetaine biosynthesis, with the aim of improving resistance to drought and salinity.