Physiological effect of application of compatible solutes as a means of increasing tolerance to abiotic stress
The aim of this short text is to familiarize you with the issue of compatible solutes and their use in plants grown in stressful conditions, which is the topic of my dissertation thesis.
Water deficit is one of the main environmental factors that affects the primary productivity of plants. More than 10 % of arable land is threatened by drought. Globally, rates of desertification and associated salinization are increasing rapidly, reducing average yields of most utilized crops by more than 50 %. Ways are currently being sought to limit the effects of abiotic stressors on the physiological state and yield of plants. In addition to the selection of suitable genetic material, the optimization of agricultural techniques and the protection of field crops, the use of not only native or synthetic growth stimulators, but also compatible solutes appears to be another possibility. Compatible solutes are low-molecular compounds that do not interfere with biochemical reactions in the cell and can replace water molecules in stressful situations. They do not have an ionic character, but can be polar. They work as osmolytes in the osmotic adaptation of the cell, as osmoprotectants that stabilize proteins, multiprotein complexes and membrane structures. These are, for example, proline, sucrose, polyols or trehalose.
Stress
During their life cycle, plants are exposed to various types of environmental stress, such as salinity, lack or excess of water, temperature extremes, concentrations of toxic metal ions and UV radiation. These environmental factors limit plant growth and productivity to varying degrees, depending on the severity of the stress. As an example of the severity of the effect of stressors on plants, we can mention the stimulated production of reactive oxygen species (ROS), e.g. OH ·, O2 ·, H2O2, etc. These radicals cause significant damage by peroxidation of the lipid components of the membrane and also as a result of direct interaction with various macromolecules. Cells have adapted various mechanisms to keep ROS levels under control (Foyer & Noctor 2005).
Drought stress, or water deficit, is primarily caused by zero or little precipitation and its uneven distribution during the growing season (Alghabari et al. 2016). The influence of water deficit causes morphological, physiological, anatomical and molecular changes in plants. Water stress conditions affect crop morphology by, for example, inhibiting seed germination and early plant growth (Manivannan et al. 2007; Harris et al. 2002). Drought reduces the accumulation of assimilates, their transport and dry matter formation, harvest index and crop productivity (Challinor et al. 2004). Approximately 70% of crop yield is reduced by water deficit (Akram et al. 2013; Kaur et al. 2008). Earl & Davis (2003) state that physiological and metabolic processes such as osmotic adjustment, water regime, photosynthesis and respiration are affected by water deficit. The mechanism of yield reduction under water deficit conditions is related to reduced light absorption, low photosynthetic rate, water use efficiency (WUE) and harvest index.
Compatible solutes
When plants encounter adverse environmental conditions associated with drought, high salt concentration or low temperature, their cells protect themselves from stress by accumulating various small organic metabolites, which are collectively referred to as compatible solutes (Bohnert et al. 1995). Compatible solutes are low molecular weight, highly soluble organic compounds that are usually nontoxic at high cellular concentrations. These substances provide protection to plants against stress by contributing to cellular osmotic adjustment and helping to maintain cell turgor, detoxify ROS, protect membrane integrity and stabilize enzymes/proteins (Ashraf & Foolad 2007; Bohnert & Jensen 1996; Yancey 1994).
Proline is one of several small molecules classified as an osmolyte or osmoprotectant (Csonka 1989). Other biologically important osmolytes are glycerol, trehalose, sorbitol, sucrose, taurine, sarcosine, glycine betaine, and trimethylamine N-oxide (Yancey et al. 1982). These osmolytes accumulate in response to conditions of drought, salt and temperature extremes. Osmolytes help alleviate water stress and balance turgor pressure during stress (Csonka 1989).
The strategy of exogenous application of various organic osmolytes, compatible solutes and growth regulators appears to be an effective practice for improving plant condition (Anjum et al. 2011). This approach is generally used to mitigate the adverse effects of environmental stress and can lead to improved plant health (Wang et al. 2010; Senaratna et al. 2003).
The possibility of foliar application of compatible solutes to plants as a means of limiting the impact of stress on plants appears promising (Janmohammadi et al. 2017).
References:
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