Читать книгу Physiology of Salt Stress in Plants - Группа авторов - Страница 28

Accumulation of salt ions in the plant rhizosphere causes dual stress to the plants, the osmotic imbalance between the soil and root cells, and cellular ionic imbalance due to passive or active intake of salt ions. The salinity‐induced osmotic stress suppresses the water uptake by the plants immediately, which then triggers various physiological and metabolic adjustments in the plants. Osmotic stress inhibits the expansion of the root and shoot cells within minutes due to reduced turgor pressure (Fricke et al. 2004; Munns et al. 2000) and the stress signal transduced rapidly at a speed of the sound through different signaling cascades from the root to the shoot tissue for the closure of stomata to minimize water loss, which seizes shoot metabolism (Christmann et al. 2013; Shabala et al. 2016). The mechanosensitive ion channels of the leaf guard cells sense the drop in xylem pressure and participate in stomatal closure, reducing the CO₂ exchange and photosynthetic assimilation (Shabala et al. 2016). For osmotic adjustment, plants start the intake of the salt ions from the soil, which then enters different root and shoot cells creating the ionic stress to plants (Shabala and Lew 2002).

The NaCl dominates the saline soil, and, therefore, the Na⁺ is the most critical ion, which is assumed to pose toxic effects on the plant cell physiology and metabolism. For their survival and optimal physiological activity, the ionic homeostasis is essential in the plants, and different ion channels and transporters in root or shoot tissues are involved in this process. The Na⁺ enters into the root cells through the glutamate receptor‐like (GLRs) channels, cyclic nucleotide‐gated (CNGCs) nonselective cation channels, high‐affinity K⁺ transporters (HKT2, HAK5), Shaker‐type K⁺ channels (AKT1), the low‐affinity cation transporter (LCT1), and aquaporins (PIP2;1) (Zhao et al. 2020). The entry of salt ions to the root cells and the osmotic stress spikes the Ca²⁺ in the root cells activating the salt overly sensitive (SOS) signaling pathway for the extrusion of the excess Na⁺ out of the cell (Hong et al. 2019; Shi et al. 2002). The function of active Na⁺ extrusion from root cells to the soil is performed by the plasma membrane Na⁺/H⁺ antiporter named salt overly sensitive 1 (SOS1) antiporter (Shi et al. 2002).

However, during salt stress, a higher concentration of salt ions in soil and continuous intrusion of salt ions to the root cells, the extrusion of Na⁺ by SOS1 is not sufficient to maintain the cellular homeostasis of the Na⁺. Thus, the plants sequester the excess Na⁺ into the vacuole by the vacuolar Na⁺/H⁺ antiporter (NHX1), loaded to the xylem vessel to distribute it all over the plants by nonselective cation channel (NCCS), SOS1, high‐affinity K⁺ family transporter 2 (HKT2) or cation‐chloride cotransporter (CCC), or retrieved to the phloem from leaf cells by HKT1 (Zhao et al. 2020). The general limit of external salt concentration for glycophytes is 0.5% of seawater (≈86 mM NaCl) above which the stress‐coping mechanism fails, and severe yield loss in plants are reported (Bose et al. 2017). We discuss the effect of the salt stress on different compartments of the plant cell and their physiology or metabolic activities in the following sections.