Compatible osmolyte
AtProT2 can be induced by water stress, and AtProT2 and
LeProT1 transport GABA as well as other stress-related compounds,
such as proline and glycine betaine36–38. These findings indicate
that GABA might have a role as a compatible osmolyte42. All
three compounds are zwitterionic at neutral pH, are highly soluble
in water, can accumulate to low mM concentrations, and apparently
contribute no toxic effects to the cell. At high concentrations
(25–200 mM), GABA stabilizes and protects isolated thylakoids
against freezing damage in the presence of salt, exceeding the cryoprotective
properties of proline43. In addition, GABA possesses in
vitro hydroxyl-radical-scavenging activity, exceeding that of proline
and glycine betaine at the same concentrations (16 mM)44.
GABA might be synthesized from g-aminobutyraldehyde (a product
of the polyamine catabolic pathway45) by the chloroplast-localized
betaine aldehyde dehydrogenase, which is involved in glycinebetaine synthesis46, but the relative fluxes via polyamines versus
glutamate decarboxylation are unknown. Whether GABA has a
specific role (i.e. osmolyte or osmoprotectant) under water stress,
or is metabolized (e.g. to support the production of known
osmolytes, such as proline) is unknown.