1. Introduction
Salinity is one of the major abiotic stresses affecting plant
agriculture worldwide. High salt stress often causes water deficit,
ion toxicity, nutrient imbalance, and oxidative stress, leading to
cellular damage and growth reduction, and even plant death. To
cope with the detrimental effects of salt stress, plants have evoked
many biochemical and molecular mechanisms to minimize salt
injury, including activation of cascades of molecular networks
involved in stress sensing, signal transduction and the expression of
specific stress-related genes and metabolites [1].
Polyamines (PAs) are low-molecular-weight aliphatic amines
that are ubiquitous in all organisms. Common natural PAs include
the higher PAs, spermine (Spm) and spermidine (Spd), and their
diamine obligate precursor putrescine (Put). PAs are known to
accumulate under salt stress in different plant systems, resulting in
presumed protective effects, acting as free radical scavengers,
stabilizing cellular membranes and maintaining cellular ionic balance
[2]. Among the three major PAs, Spd has been most closely
associated with stress tolerance in plants [3]. Accumulation of Spd
and Spm has been reported in cucumber salt tolerant cultivars in
comparison to the salt sensitive cultivars [4]. During the last few
years, genetic, transcriptomic and metabolomic approaches have
unraveled key functions of different PAs in the regulation of abiotic
stress tolerance. Nevertheless, the precise molecular mechanisms
by which PAs control plant responses to stress stimuli are largely
unknown [5].
Cucumber is one of the most important vegetables worldwide. It
is highly sensitive to salinity, especially in its germination and early
growth phases [6]. Enhancement of salt stress tolerance in cucumber
through exogenous application of PAs has been described
[4,7]. However, a limited number of studies have looked at the
precise mechanism on PAs regulating plant responses to salt stress.
Proteomics is a powerful tool for describing how the proteome is
affected by different physiological conditions, so we investigated
the differentially expressed proteins in cucumber leaves in either
salt stress or normal condition with and without Spd. The aim of
this study is to better understand the underlying mechanism of the
salt resistance by Spd in cucumber seedlings.