Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 28

35]. Scavenging of reactive oxygen species (ROS) to restore redox metabolism, preservation of
cellular turgor by restitution of osmotic balance, and associated protection and stabilization
of proteins and cellular structures are among the multiple protective functions of compatible
osmoprotectants during environmental stress [36-38].
A large amount of research has been done on the beneficial effects of compatible solutes on
plant tolerance to environmental stress. Correlation between amino acid accumulation (mainly
proline) and stress tolerance was described in the mid-1960s in Bermuda grass during water
stress [39]. Since then, extensive work has proven that proline serves as an osmoprotectant, a
cryoprotectant, a signaling molecule, a protein structure stabilizer, and an ROS scavenger in
response to stresses that cause dehydration; including salinity, freezing, heavy metals, and
drought (low water potential) [40, 41]. Proline oxidation may also provide energy to sustain
metabolically demanding programs of plant reproduction, once the stress has passed [42].
Proline metabolism and its regulation are processes well characterized in plants. Proline is
synthesized from glutamate in the cytoplasm or chloroplasts: Δ-1-pyrroline-5-carboxylate
synthetase (P5CS) reduces glutamate to glutamate semialdehyde (GSA). Then GSA sponta‐
neously cyclizes into pyrroline-5-carboxylate (P5C), which is further reduced by P5C reductase
(P5CR) to proline. Conversely, proline is catabolized within the mitochondrial matrix by action
of proline dehydrogenase (ProDH) and P5C dehydrogenase (P5CDH) to glutamate. In an
alternative pathway, proline can be synthesized from ornithine in a pathway involving
ornithine δ-aminotransferase (OAT). Core enzymes P5CS, P5C, P5CR, ProDH, and OAT are
responsible for maintaining the balance between biosynthesis and catabolism of proline.
Regulation comes at transcriptional level of genes encoding the key enzymes. Transcriptional
up-regulation of genes for P5CS and P5C to increase proline synthesis from glutamate and
down-regulation of genes for P5CR and ProDH to arrest proline catabolism is observed during
dehydration/osmotic stress [43]. Also, post-translational regulation of core enzymes is closely
associated with proline levels and environmental signals. For example, the Arabidopsis P5CS1
enzyme is subjected to feedback inhibition by proline, controlling the carbon influx into the
biosynthetic pathway [44, 45]. Considering that proline accumulation is associated with stress
tolerance, that core enzymes regulate proline biosynthesis, and that these core enzymes are
likely rate-limiting steps for its accumulation, logic dictates that overexpression of biosynthetic
proline enzymes might increase the levels of the compatible solute and thus improve the
tolerance in plants against abiotic stress. Several studies have tested this by overexpressing
genes for P5CS or P5C enzymes in different plant species, reporting the expected rise in proline
levels and the associated resistance to dehydration, salinity, or freezing [46-53]. Furthermore,
deletion of genes coding ProDH [54] or P5CDH [55, 56], expression of a feedback-insensitive
P5CS [45], or the overexpression of OAT [57, 58] increase the cellular levels of proline and
osmoprotection to some abiotic stresses.
Comparable extensive work has been done for other compatible solutes such as γ-aminobu‐
tyric acid [59], glycine betaine [60], trehalose [61], mannitol, and sorbitol [36]; these solutes are
efficient protectors against some abiotic stressors. Metabolic pathways for biosynthesis and
catabolism of compatible solutes, their regulation, participant enzymes, and compartmental‐
ization are well characterized in most important plant species. This knowledge has led to