SALINITY PROMOTES ACCUMULATION OF 3-DIMETHYLSULFONIOPROPIONATE AND ITS PRECURSOR S-METHYLMETHIONINE IN CHLOROPLASTS

Wollastonia biflora (L.) DC, plants accumulate the osmoprotectant 3-di methylsulfoniopropionate (DMSP), particularly when salinized. DMSP is known to be synthesized in the chloroplast from S-methylmethionine (SM M) imported from the cytosol, but the sizes of the chloroplastic and e xtrachloroplastic pools of these compounds are unknown. We therefore d etermined DMSP and SMM in mesophyll protoplasts and chloroplasts. Sali nization with 30% (v/v) artificial seawater increased protoplast DMSP levels from 4.6 to 6.0 mu mol mg(-1) chlorophyll (Chl), and chloroplas t levels from 0.9 to 1.9 mu mol mg(-1) Chl. The latter are minimum val ues because intact chloroplasts leaked DMSP during isolation. Correcti ng for this leakage, it was estimated that in vivo about one-half of t he DMSP is chloroplastic and that stromal DMSP concentrations in contr ol and salinized plants are about 60 and 130 mM, respectively. Such co ncentrations would contribute significantly to chloroplast osmoregulat ion and could protect photosynthetic processes from stress injury. SMM levels were measured using a novel mass-spectrometric method. About 4 0% of the SMM was located in the chloroplast in unsalinized W. biflora plants, as was about 80% in salinized plants; the chloroplastic pool in both cases was approximately 0.1 mu mol mg(-1) Chl. In contrast, gr eater than or equal to 85% of the SMM was extrachloroplastic in pea (P isum sativum L.) and spinach (Spinacia oleracea L.), which lack DMSP. DMSP synthesis may be associated with enhanced accumulation of SMM in the chloroplast.