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More than one third of the world's irrigated lands are affected by salinity, which has great impact on plant growth and yield worldwide. Proline accumulation under salt stress has been indicated to correlate with salt tolerance. Exogenous application as well as genetic engineering of metabolic pathways involved in the metabolism of proline has been successful in improving tolerance to salinity. Correlation between proline accumulation as well as its proposed roles and salt adaptation, however, has not been clearly confirmed in several plant species. In addition, the studies relating proline functions and plant salt tolerance are always carried out in growth chambers, and are not successfully verified in field conditions. Further, plant salt tolerance is a complex trait, and studies based solely on proline accumulation do not adequately explain its functions in salinity tolerance, and thus it is difficult to interpret the discrepancies among different data. Moreover, several reports indicate that Pro role in salt tolerance is a matter of debates, as whether Pro accumulation has adaptive significance or is a consequence of alterations in cellular metabolism induced by salinity. As no consensus is obtained on the exact roles of proline production, proline exact roles in the adaptation to saline environments is therefore still lacking and is even a matter of debates. It is obvious that comprehensive future research is needed to establish the proline exact mechanism by which it enhances plant salt tolerance. We propose, however, that proline might be essential for improving salinity tolerance in some species/cultivars, but may not be relevant in others. Evidence supporting both arguments has been presented in order to reassess the feasibility of the proposed roles of Pro in plant salt tolerance mechanism.

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.