Ethylene: A Master Regulator of Salinity Stress Tolerance in Plants.

Salinity stress is one of the major threats to agricultural productivity across the globe. Research in the past three decades, therefore, has focused on analyzing the effects of salinity stress on the plants. Evidence gathered over the years supports the role of ethylene as a key regulator of salinity stress tolerance in plants. This gaseous plant hormone regulates many vital cellular processes starting from seed germination to photosynthesis for maintaining the plants' growth and yield under salinity stress. Ethylene modulates salinity stress responses largely via maintaining the homeostasis of Na+/K+, nutrients, and reactive oxygen species (ROS) by inducing antioxidant defense in addition to elevating the assimilation of nitrates and sulfates. Moreover, a cross-talk of ethylene signaling with other phytohormones has also been observed, which collectively regulate the salinity stress responses in plants. The present review provides a comprehensive update on the prospects of ethylene signaling and its cross-talk with other phytohormones to regulate salinity stress tolerance in plants.

Sodium Influx and Potassium Efflux Currents in Sunflower Root Cells Under High Salinity.

Helianthus annuus L. is an important oilseed crop, which exhibits moderate salt tolerance and can be cultivated in areas affected by salinity. Using patch-clamp electrophysiology, we have characterized Na+ influx and K+ efflux conductances in protoplasts of salt-tolerant H. annuus L. hybrid KBSH-53 under high salinity. This work demonstrates that the plasma membrane of sunflower root cells has a classic set of ionic conductances dominated by K+ outwardly rectifying channels (KORs) and non-selective cation channels (NSCCs). KORs in sunflower show extreme Na+ sensitivity at high extracellular [Ca2+] that can potentially have a positive adaptive effect under salt stress (decreasing K+ loss). Na+ influx currents in sunflower roots demonstrate voltage-independent activation, lack time-dependent component, and are sensitive to Gd3+. Sunflower Na+-permeable NSCCs mediate a much weaker Na+ influx currents on the background of physiological levels of Ca2+ as compared to other species. This suggests that sunflower NSCCs have greater Ca2+ sensitivity. The responses of Na+ influx to Ca2+ correlates well with protection of sunflower growth by external Ca2+ in seedlings treated with NaCl. It can be, thus, hypothesized that NaCl tolerance in sunflower seedling roots is programmed at the ion channel level via their sensitivity to Ca2+ and Na+.

Deciphering the nitric oxide, cyanide and iron-mediated actions of sodium nitroprusside in cotyledons of salt stressed sunflower seedlings.

Nitric oxide (NO) is an endogenous signaling molecule in plants. Sodium nitroprusside (SNP), an established NO donor used in plant science research, simultaneously releases NO, cyanide (CN-) and iron (Fe) in solution. Since cyanide and iron mask NO effect of SNP, its use in NO research is debatable. Deciphering the action of SNP through NO, CN- or Fe has been undertaken in the present work. Cotyledons from salt stressed sunflower seedlings grown in the presence of NO donors were subjected to spectrofluorometric analysis of NO, CN- and Fe contents, and proteome and biochemical analyses. Diethylenetriamine NONOate (DETA) proved to be a better NO source since SNP enhanced ROS accumulation in the tissue. Abundance of 127 proteins is modulated by salt stress. SNP and exhausted SNP (exSNP) alter the abundance of 117 and 129 proteins, respectively. These proteins belong to primary metabolism, stress-response, transport, translation, proteolysis, chaperone, regulatory, and storage. Salt-responsive proteins, such as, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK) and isocitrate dehydrogenase are negatively modulated. DETA and SNP lower the activities of GAPDH and S-adenosylmethionine synthase (SAMS). Abundance of heat shock 70 kDa protein and actin are sensitive to both NaCl and NO. SNP affects plant growth by modulating proteome though iron, cyanide and NO. Its use only as an NO donor is thus debatable. exSNP control also releases substantial amount of cyanide and iron, thus questioning its use as control in NO research.

Mechanisms of Sodium Transport in Plants-Progresses and Challenges.

Understanding the mechanisms of sodium (Na⁺) influx, effective compartmentalization, and efflux in higher plants is crucial to manipulate Na⁺ accumulation and assure the maintenance of low Na⁺ concentration in the cytosol and, hence, plant tolerance to salt stress. Na⁺ influx across the plasma membrane in the roots occur mainly via nonselective cation channels (NSCCs). Na⁺ is compartmentalized into vacuoles by Na⁺/H⁺ exchangers (NHXs). Na⁺ efflux from the plant roots is mediated by the activity of Na⁺/H⁺ antiporters catalyzed by the salt overly sensitive 1 (SOS1) protein. In animals, ouabain (OU)-sensitive Na⁺, K⁺-ATPase (a P-type ATPase) mediates sodium efflux. The evolution of P-type ATPases in higher plants does not exclude the possibility of sodium efflux mechanisms similar to the Na⁺, K⁺-ATPase-dependent mechanisms characteristic of animal cells. Using novel fluorescence imaging and spectrofluorometric methodologies, an OU-sensitive sodium efflux system has recently been reported to be physiologically active in roots. This review summarizes and analyzes the current knowledge on Na⁺ influx, compartmentalization, and efflux in higher plants in response to salt stress.