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.