Glycinebetaine in saline conditions: an assessment of the current state of knowledge

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.