In my previous post (The smell of the ocean - dimethylsulfide:
https://steemit.com/steemstem/@davidrhodes124/the-smell-of-the-ocean-dimethylsulfide), I discussed the pathway of synthesis of dimethylsulfoniopropionate (DMSP) in marine algae and phytoplankton.

![Dimethylsulfoniopropionate_Structural_Formula_V1.svg.png](https://steemitimages.com/DQmaRvKKhkSdZ2gE8kVdmQKiSPuGG6zQE6B6md6yHc3RggK/Dimethylsulfoniopropionate_Structural_Formula_V1.svg.png)
Dimethylsulfoniopropionate - Wikipedia (https://en.wikipedia.org/wiki/Dimethylsulfoniopropionate)

Surprisingly, this pathway of synthesis is not the same in higher plants (angiosperms). In contrast to marine algae, the synthesis of 3-dimethylsulfoniopropionate (DMSP) in the salt marsh grass and higher plant (angiosperm), *Spartina alterniflora*, a member of the Poaceae, proceeds via the pathway methionine --->  S-methylmethionine (SMM) ---> dimethylsulfoniopropylamine (DA) ---> dimethylsulfoniopropionaldehyde (DMSP-ald) ---> dimethylsulfoniopropionate (DMSP): 

This was verified by computer modeling of *in vivo* radioisotope tracer data:

Kocsis, M.G., Nolte, K.D., Rhodes, D., Shen, T.L., Gage, D.A., Hanson, A.D. 1998. Dimethylsulfoniopropionate biosynthesis in *Spartina alterniflora* Loisel.: evidence that S-methylmethionine and dimethylsulfoniopropylamine are intermediates. Plant Physiol. 117 (1): 273-281 (1998)
http://www.plantphysiol.org/content/117/1/273.long

This implicated 4 enzyme steps involved in this biosynthesis pathway. Because the first and last steps of the pathway are catalyzed by enzymes that are widely distributed in angiosperms, it was postulated that *Spartina alterniflora* needed only to evolve two novel enzymes to complete this pathway (i.e. enzymes catalyzing the steps S-methylmethionine (SMM) ---> dimethylsulfoniopropylamine (DA), and dimethylsulfoniopropylamine (DA) ---> dimethylsulfoniopropionaldehyde (DMSP-ald)). These enzymes were subsequently found:

Kocsis, M.G., Hanson, A.D. Biochemical evidence for two novel enzymes in the biosynthesis of 3-dimethylsulfoniopropionate in *Spartina alterniflora*. Plant Physiol. 123 (3): 1153-1161 (2000)
http://www.plantphysiol.org/content/123/3/1153.long

More recently, it has been established that DMSP is not synthesized by all *Spartina* species, but only in the hexaploid species:

Rousseau, H., Rousseau-Gueutin, M., Dauvergne, X., Boutte, J., Simon, G., Marnet, N., Bouchereau, A., Guiheneuf, S., Bazureau, J.P., Morice, J., Ravanel, S., Cabello-Hurtado, F., Ainouche, A., Salmon, A., Wendel, J.F., Ainouche, M.L. Evolution of DMSP (dimethylsulfoniopropionate) biosynthesis pathway: Origin and phylogenetic distribution in polyploid *Spartina* (Poaceae, Chloridoideae). Mol. Phylogenet. Evol. 114: 401-414 (2017)
https://hal.archives-ouvertes.fr/hal-01579439

"We found that the hexaploid lineage containing *S. alterniflora*, *S. foliosa* and *S. maritima* and their derived hybrids and allopolyploids are all able to produce DMSP, in contrast to species in the tetraploid clade. Thus, examination of DMSP synthesis in a phylogenetic context implicated a single origin of this physiological innovation, which occurred in the ancestor of the hexaploid *Spartina* lineage, 3-6MYA. Candidate genes specific to the *Spartina* DMSP biosynthesis pathway were also retrieved from *Spartina* transcriptomes, and provide a framework for future investigations to decipher the molecular mechanisms involved in this plant phenotypic novelty that has major ecological impacts in saltmarsh ecosystems." (Rousseau et al., 2017).

DMSP is not found in all angiosperms. In fact there are relatively few species that are known to accumulate this compound:

Otte, M.L., Wilson, G., Morris, J.T., Moran, B.M. Dimethylsulphoniopropionate (DMSP) and related compounds in higher plants. J. Exp. Bot. 55(404):1919-1925 (2004) 
https://academic.oup.com/jxb/article/55/404/1919/772378

*Wollastonia biflora* (a member of the Asteraceae) is another species known to accumulate DMSP. While this species produces DMSP from SMM, and DMSP-aldehyde is an intermediate (as in *Spartina alterniflora*), surprisingly, DMSP-amine is **NOT** an intermediate:

Hanson, A.D., Rivoal, J., Paquet, L., Gage, D.A. Biosynthesis of 3-dimethylsulfoniopropionate in *Wollastonia biflora* (L.) DC. Evidence that S-methylmethionine is an intermediate. Plant Physiol. 105 (1): 103-110 (1994)
http://www.plantphysiol.org/content/105/1/103.long

James, F,, Paquet, L., Sparace, S.A., Gage, D.A., Hanson, A.D. Evidence implicating dimethylsulfoniopropionaldehyde as an intermediate in dimethylsulfoniopropionate biosynthesis. Plant Physiol. 108 (4):1439-1448 (1995)
http://www.plantphysiol.org/content/108/4/1439.long

It is postulated that *Wollastonia biflora* has an unusual enzyme that converts SMM to DMSP-aldehyde:

Rhodes, D., Gage, D.A., Cooper, A., Hanson, A.D. S-Methylmethionine conversion to dimethylsulfoniopropionate: evidence for an unusual transamination reaction. Plant Physiol. 115 (4): 1541-1548 (1997)
http://www.plantphysiol.org/content/115/4/1541.long

SMM's structure is shown below:

![220px-Methylmethionine.png](https://steemitimages.com/DQmWpfKunx8nELyBijKmTUGtMgWT4KfkWmGpVRdecrsGyGt/220px-Methylmethionine.png)
S-Methylmethionine - Wikipedia (https://en.wikipedia.org/wiki/S-Methylmethionine)

SMM is found in most higher plants: 

Ranocha, P., McNeil, S.D., Ziemak, M.J., Li, C., Tarczynski, M.C., Hanson, A.D. The S-methylmethionine cycle in angiosperms: ubiquity, antiquity and activity. Plant J. 25 (5): 575-584 (2001)
http://onlinelibrary.wiley.com/doi/10.1046/j.1365-313x.2001.00988.x/abstract

It is present at relatively high levels in asparagus spears, and sweet corn ears. When heated under alkaline conditions, SMM degrades to the sulfur gas, dimethylsulfide (DMS). This gives the characteristic odor of cooked sweet corn:

Wang A.D., Swiader, J.M, Juvik, J.A. Nitrogen and sulfur fertilization influences
aromatic flavor components in *shrunken2* sweet corn kernels. J. AMER. Soc. HORT. SCI. 120 (5): 771-777 (1995)
http://journal.ashspublications.org/content/120/5/771.abstract