Glycinebetaine:
There is a lot of cold tolerance research presently focusing on this particular substance, which is an important osmolyte in crop plants such as barley, sugar beet and spinach. Some of the research is cited in the review linked above:
http://jxb.oxfordjournals.org/content/63/4/1593.long
Drought, salt, and temperature stress-induced metabolic rearrangements and regulatory networks Julia Krasensky and Claudia Jonak
Here are some links to research on glycinebetaine:
Glycinebetaine regulates tolerance of both drought and cold tolerance:
http://www.ncbi.nlm.nih.gov/pubmed/20946588
whether endogenous or externally applied/supplied. overview:
http://www.ncbi.nlm.nih.gov/pubmed/18703379
Genetic engineering of glycinebetaine synthesis in tomato protects seeds, plants, and flowers from chilling damage.
http://www.ncbi.nlm.nih.gov/pubmed/15500464
GMO research found GB expression in chloroplasts was more effective than in cytosol:
http://www.ncbi.nlm.nih.gov/pubmed/17617827
http://www.ncbi.nlm.nih.gov/pubmed/15500464
A full text on GB engineering in crops:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3329348/
Natural sources of glycinebetaine: seaweed
http://www.ncbi.nlm.nih.gov/pubmed/19363684 betaines in seaweed
http://www.ncbi.nlm.nih.gov/pubmed/10526988 nematode pest control associated with betaines incl gb in seaweed
http://www.ncbi.nlm.nih.gov/pubmed/19363684 lipophylic fraction and whole extract increased frost tolerance and activated genes
"In field-grown tomato subjected to salt or high temperature stress, fruit yields increased up to 39% following exogenous glycine betaine application at mid-flowering. In a previous study by Makela et al. (16) osmotic shock to plant foliage occurred when rates of application were too high, and this was more pronounced in plants that were not natural glycine betaine accumulators. "
http://www.plantmanagementnetwork.or.../2003/betaine/
Proline is an osmolyte that accumulates naturally in tomato, and high levels are found in pollen. The proline transporter in tomato pollen was also found to have a high affinity for glycine betaine (even higher than for proline itself). So tomato plants can readily take up gb and use it as an osmolyte, although not naturally produced/ accumulated in tomato stress response.
http://www.plantcell.org/content/11/3/377.full