OSMOPROTECTIVE COMPOUNDS IN THE PLUMBAGINACEAE - A NATURAL EXPERIMENTIN METABOLIC ENGINEERING OF STRESS TOLERANCE

In common with other zwitterionic quaternary ammonium compounds (QACs) , glycine betaine acts as an osmoprotectant in plants, bacteria, and a nimals, with its accumulation in the cytoplasm reducing adverse effect s of salinity and drought. For this reason, the glycine betaine biosyn thesis pathway has become a target for genetic engineering of stress t olerance in crop plants. Besides glycine betaine, several other QAC os moprotectants have been reported to accumulate among flowering plants, although little is known about their distribution, evolution, or adap tive value. We show here that various taxa of the highly stress-tolera nt family Plumbaginaceae have evolved four QACs, which supplement or r eplace glycine betaine-namely, choline O-sulfate and the betaines of b eta-alanine, proline, and hydroxyproline. Evidence from bacterial bioa ssays demonstrates that these QACs function no better than glycine bet aine as osmoprotectants. However, the distribution of QACs among diver se members of the Plumbaginaceae adapted to different types of habitat indicates that different QACs could have selective advantages in part icular stress environments. Specifically, choline O-sulfate can functi on in sulfate detoxification as well as in osmoprotection, beta-alanin e betaine may be superior to glycine betaine in hypoxic saline conditi ons, and proline-derived betaines may be beneficial in chronically dry environments. We conclude that the evolution of osmoprotectant divers ity within the Plumbaginaceae suggests additional possibilities to exp lore in the metabolic engineering of stress tolerance in crops.