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Advances in Proteomics Research in Environmental Stress Response in Plants

13.4.4 OSMOPROTECTANT REGULATORS

Osmolyte molecules like sugar, mannitol, or amino acids (proline) and their

N-methyl derivatives can be controlled by osmoprotectant regulator proteins

(betaines). The solutes raise the cell’s osmotic pressure, preventing further

water loss and preserving turgor (McNeil et al., 1999). Plant responses to

abiotic stress have indicated that the expression of genes for sugar syntheses

and sugar transporters is frequently up-regulated during abiotic stress,

and that buildup of proline and glycine-betaine helps plants overcome the

stressed condition, improving tolerance capacity. Commonly osmoprotec­

tants were found majorly in the cytoplasm and all organelles except vacuoles

(Maruyama et al., 2004; Ashraf et al., 2006; Aghaei et al., 2009). Soybeans

under osmotic stress and stress affecting the ER have been found to have the

osmoprotectant biosynthetic protein 1-pyrroline-5-carboxylate synthase, as

well as other enzymes including sucrose synthase and sugar transporters.

Under stress, osmoprotectants, and water channels are typically responsible

for maintaining cell water content.

13.4.5 ION TRANSPORTER PROTEINS

The changes in the passage of H⁺, K⁺, Cl^–, and Ca²⁺ ions via the plasma

membrane impact cytosolic pH and transmembrane electrical potential.

Accumulation of these ions in plant tissues might cause stress. Na⁺ and Cl^–

ions are usually retained at low concentrations in plant cells, whereas nutri­

tionally essential components like K⁺ are stored at high quantities (Munns

& Tester, 2008; Sun et al., 2009). Ion balance is maintained by the plasma

membrane and vacuoles, which are two important subcellular components

of cells. Regulator proteins, tonoplast Na⁺/H⁺ antiporters, and plasma

membrane Na⁺/H⁺ antiporters all play a role in adsorbing Na⁺ in vacuoles or

extruding it to the external environment (Xiong & Zhu, 2002; Waditee et al.,

2006). Under drought stress, Proton pumps coupled with the cell membrane

regulate the outflow of K⁺ and other anions from guard cells that facilitates

stomatal closure. Ca²⁺ ions are involved in maintaining the outflow of anions

by regulating the activity of plasma membrane H⁺-ATPase (Kinoshita et al.,

1995). Plasma membrane proton pumps are essential membrane proteins

that help the cell maintain its ion balance during osmotic stress. Because

intracellular ion homeostasis is critical for the functioning of many cellular

enzymes, effective ion efflux control is essential during stressful conditions

(Hossain et al., 2012).