3-Dimethylsulfoniopionate Biosynthesis and use by Flowering Plants

Summary

3-Dimethylsulfoniopropionate (DMSP) is known to be an effective osmoprotectant. It is accumulated to high levels (5-50 μmol g 1 fresh weight) by several diverse flowering plants, including intertidal Spartina species and sugarcane (Poaceae), and the coastal plant Wollastonia biflora (Asteraceae). Many other species may have traces of DMSP (0.01-0.5 mol g-1 fresh weight). The biosynthetic pathway has been investigated in W. biflora leaves. In vivo isotope labeling data are consistent with the following sequence of steps:

$$\begin{gathered} C{H_3}SC{H_2}C{H_2}CH\left( {N{H_2}} \right)COOH \to {\left( {C{H_3}} \right)_2}{S^ + }C{H_2}C{H_2}CH\left( {N{H_2}} \right)COOH \to {\left( {C{H_3}} \right)_2}{S^ + }C{H_2}C{H_2}CHO \to {\left( {C{H_3}} \right)_2}{S^ + }C{H_2}C{H_2}COOH \hfill \\ Methionine S - Methylmethionine DMSP - aldehyde DMSP \hfill \\ \end{gathered} $$

It is not clear whether S-methylmethionine is converted to DMSP-aldehyde directly or via an intermediate. The methyltransferase catalyzing the conversion of methionine to S-methylmethionine has been purified and shown to be a tetramer of 115-kD subunits; the other biosynthetic enzymes are not yet known. In vivo radiotracer studies of DMSP catabolism in W. biflora leaves indicate that demethylation to methylthiopropionate and breakdown to dimethyl sulfide are very slow (1 % of the DMSP pool per day). In W. biflora leaves, both S-methylmethionine and DMSP-aldehyde are degraded to dimethyl sulfide far more rapidly than DMSP, which suggests that part of the dimethyl sulfide emitted by DMSP-producing plants might come from these compounds. The efficacy of DMSP as an osmoprotectant coupled with its simple biosynthetic pathway, metabolic stability and lack of nitrogen make DMSP a rational candidate for the genetic engineering of osmotic stress resistance in crop plants.