The role of DMSP in oxidative stress protection in Spartina alterniflora Loisel

DMSP is present in S. alterniflora tissues at high intracellular concentrations, yet its physiological functions remain unclear. It has been suggested that DMSP and its degradation products may function as antioxidants. We carried out a variety of field and laboratory experiments investigating the role of DMSP in oxidative stress protection in S. alterniflora. Our data indicate that the concentrations of DMSP and its oxidation product DMSO are highest in leaf tissue followed by stem and root tissue, while the ratio DMSO/DMSP in root tissue was more than twice that in leaf or stem tissue. This suggests a higher rate of DMSP oxidation in roots, perhaps due to stress associated with the anoxic sulfidic conditions in the S. alterniflora root zone. This idea is supported by our finding that leaf DMSO levels are positively related to porewater salinity and sulfide. In addition, we found that the ratio DMSO/DMSP is higher in tissues presumably experiencing increased oxidative stress such as; yellowing S. alterniflora leaves and senescing segments of individual leaves compared to healthy tissue, leaves treated with the oxidizing herbicides paraquat and DCMU, and isolated S. alterniflora protoplasts treated with paraquat. Interestingly, we did not see significant differences in the concentrations of DMSP and DMSO, or the ratio DMSO/DMSP in isolated S. alterniflora chloroplasts. Nor did we see an increase in DMSP synthesis and accumulation in response to oxidative stress at any of the levels investigated (whole leaf, whole cell, and chloroplast). These findings suggest that S. alterniflora may not synthesize DMSP solely as an antioxidant but that DMSP may serve a constitutive role in oxidative stress protection. Lending strong support to this contention is our observation that treatment with paraquat led to significantly greater decline in maximum quantum yield of PSII in the non-DMSP synthesizing S. patens, and S. cynosuroides than in S. alterniflora; and, perhaps most significantly, less tissue necrosis, chlorophyll bleaching, and membrane-lipid peroxidation in paraquat treated leaf discs of the non-DMSP producing grass P. commutatum with the inclusion of exogenous DMSP in the treatment medium.