**5.2. Proline biosynthesis**

from contributing to maintain osmotic balance, they do protect cell against oxidative stress

The osmoregulators such as protein, sugars, amino acids, and compounds of quaternary ammonium play a vital role in adjusting the osmotic pressure and stabilizing of plant cells and tissues [25]. Drought stress causes osmotic stress in plants which causes a reduction in growth, imbalance ion transport, and a decrease in transpiration rate and an increase in membrane permeability. Such effects result in less water-absorbing capacity of crop plants, and different plant species and genotypes within a species respond differently to adverse environmental conditions. In order to counteract unfavorable environmental conditions, plants accumulate different types of organic and inorganic solutes in cytosol to decrease osmotic

Plant cells lose water and decrease turgor pressure under water-stress conditions. There is an increase in different plant hormones in case of water stress like abscisic acid, which has important roles in the tolerance of plants to drought, high salinity, and cold. Abiotic stresses, which cause depletion of cellular water, are responsible for the greatest agricultural losses. Upon exposure to these prevalent stresses, the accumulation of osmoprotectants is in sufficient quantity to facilitate osmotic adjustment. The increase in cellular osmolarity due to these compatible solutes is accompanied by the influx of water into the cells, providing the turgor necessary for cell expansion [7]. Water deficit develops slowly enough to allow changes in developmental processes as water stress has several adverse effects on plant growth. In this situation, compatible solutes like proline, glycine-betaine, and trehalose produce to counter the unfavorable cellular conditions. The osmotic potential fluctuation of soil solution creating a water stress in plants ultimately leads to plant death due to growth arrest and molecular damage. Osmotic adjustment of cells helps to maintain plant water balance to establish internal milieu [9].

Proline is the most extensively studied osmolyte because of its great importance in stress tolerance [26]. The exogenous application of proline can increase its endogenous levels in plant tissues subjected to waterstress conditions which help maintain osmotic adjustment in plant tissues. It may be a good source of minimizing the adverse effects of water stress on plants, and triggering their growth also depends upon the type of plant species and its concentration [27]. The production of proline is widely present in higher plants and normally accumulates in large quantities in response to environmental stresses [28]. For osmotic adjustment, proline contributes to stabilizing subcellular structures (e.g., membranes and proteins), scavenging free radicals, and buffering cellular redox potential under stress conditions. A rapid breakdown of proline upon relief of stress may provide sufficient reducing agents that support mitochondrial oxidative phosphorylation and generation of ATP for recovery from stress and repairing of stress-induced damages [29]. Iqbal et al. [30] have reported that the accumulation of proline in drought-tolerant and drought-sensitive cultivars has revealed the significance of this osmolyte. The role of proline in induced PEG experiment gave evidence that the higher levels of proline are due to the emergent need of stressed plant. This osmolyte is able to control the osmotic regulation of the cellular environment because of its high water solubility and

as scavengers of "reactive oxygen species" (ROS) [23, 24].

potential by which they can maintain cell turgor.

**5.1. Proline**

58 Global Wheat Production

Proline biosynthesis in plants is initiated with the ATP-dependent phosphorylation of the carboxy group of glutamate by glutamyl kinase (GK).The resulting glutamyl phosphate (GP) is reduced to glutamic semi-aldehyde (GSA) by GSA dehydrogenase and glutamyl kinase which forms obligatory enzyme complex [32]. The accumulation of proline under stress in many plant species has been correlated with stress tolerance, and its concentration has been shown to be generally higher in stresstolerant than in stresssensitive plants. In wheat, an assessment of the effects of drought stress on proline accumulation in a drought-tolerant and a droughtsensitive cultivar revealed that the rate of proline accumulation and utilization was significantly higher in the drought-tolerant cultivar [33]. Furthermore, in *B. juncea* plants grown under stress conditions, activities of proline biosynthetic enzymes P5CR and ornithine-aminotransferase (OAT) increased mainly intolerant lines though the activity of prolinedegrading enzyme "proline oxidase" decreased in all lines.
