Salt stress is one of the environmental threats
that have devastating impacts on plant distribution, growth
and production. Different plants are believed to have salt
tolerance mechanisms that occur at the cellular level. One
facet of the cellular mechanisms of adaptation to salinity
stress is to accumulate either inorganic and/or organic
solutes. Glycinebetaine (GB), as well as other organic
solutes, has been referred to as compatible solutes, for the
reason that they are innocent with essential biochemical
reactions even at high concentrations. GB has been
assumed to be involved in osmotic adjustment and/or
osmoprotection of cellular functional macromolecules and,
hence, can improve tolerance to saline conditions. However,
the exact mechanism and direct evidences for such
correlative data are still lacking despite many attempts to
improve growth under saline conditions by exogenous
application as well as genetic engineering of metabolic
pathways involved in metabolism of GB. Despite the
enormous amount of information accumulated in this
regard, the exact function of GB in the adaptation to saline
environments is not fully clear to this point, and even GB
functions have been argued. Because of that, inconsistencies
exist in the published data regarding GB accumulation
and functions under salt stress. In this review, we provide
an update on evidence supporting each of these arguments
to reassess how GB affects plant growth and physiological
traits under salt imposition, and whether its effects correlate
with salt tolerance.