Putative role of glutamine in the activation of CBL/CIPK signalling pathways during salt stress in sorghum.

The salt overly sensitive (SOS) pathway is the only mechanism known for Na+ extrusion in plant cells. SOS pathway activation involves Ca2+-sensing proteins, such as calcineurin B-like (CBL) proteins, and CBL-interacting protein kinases (CIPKs). In this signalling mechanism, a transit increase in cytosolic Ca2+ concentration triggered by Na+ accumulation is perceived by CBL (also known as SOS3). Afterward, SOS3 physically interacts with a CIPK (also known as SOS2), forming the SOS2/SOS3 complex, which can regulate the number downstream targets, controlling ionic homeostasis. For instance, the SOS2/SOS3 complex phosphorylates and activates the SOS1 plasmalemma protein, which is a Na+/H+ antiporter that extrudes Na+ out of the cell. The CBL-CIPK networking system displays specificity, complexity and diversity, constituting a critical response against salt stress and other abiotic stresses. In a study reported in the journal Plant and Cell Physiology, we showed that NH4+ induces the robust activation of transporters for Na+ homeostasis in root cells, especially the SOS1 antiporter and plasma membrane H+-ATPase, differently than does NO3-. Despite some studies having shown that external NH4+ ameliorates salt-induced effects on ionic homeostasis, there is no evidence that NH4+ per se or some product of its assimilation is responsible for these responses. Here, we speculate about the signalling role behind glutamine in CBL-CIPK modulation, which could effectively activate the SOS pathway in NH4+-fed stressed plants.

Integrative Control Between Proton Pumps and SOS1 Antiporters in Roots is Crucial for Maintaining Low Na+ Accumulation and Salt Tolerance in Ammonium-Supplied Sorghum bicolor.

An effective strategy for re-establishing K+ and Na+ homeostasis is a challenge for the improvement of plant performance in saline soil. Specifically, attempts to understand the mechanisms of Na+ extrusion from plant cells, the control of Na+ loading in the xylem and the partitioning of the accumulated Na+ between different plant organs are ongoing. Our goal was to provide insight into how an external nitrogen source affects Na+ accumulation in Sorghum bicolor under saline conditions. The NH4+ supply improved the salt tolerance of the plant by restricting Na+ accumulation and improving the K+/Na+ homeostasis in shoots, which was consistent with the high activity and expression of Na+/H+ antiporters and proton pumps in the plasma membrane and vacuoles in the roots, resulting in low Na+ loading in the xylem. Conversely, although NO3--grown plants had exclusion and sequestration mechanisms for Na+, these responses were not sufficient to reduce Na+ accumulation. In conclusion, NH4+ acts as an efficient signal to activate co-ordinately responses involved in the regulation of Na+ homeostasis in sorghum plants under salt stress, which leads to salt tolerance.

NH(4)(+)-stimulated low-K(+) uptake is associated with the induction of H(+) extrusion by the plasma membrane H(+)-ATPase in sorghum roots under K(+) deficiency.

The effect of external inorganic nitrogen and K(+) content on K(+) uptake from low-K(+) solutions and plasma membrane (PM) H(+)-ATPase activity of sorghum roots was studied. Plants were grown for 15 days in full-nutrient solutions containing 0.2 or 1.4mM K(+) and inorganic nitrogen as NO(3)(-), NO(3)(-)/NH(4)(+) or NH(4)(+) and then starved of K(+) for 24, 48 and 72 h. NH(4)(+) in full nutrient solution significantly affected the uptake efficiency and accumulation of K(+), and this effect was less pronounced at the high K(+) concentration. In contrast, the translocation rate of K(+) to the shoot was not altered. Depletion assays showed that plants grown with NH(4)(+) more efficiently depleted the external K(+) and reached higher initial rates of low-K(+) uptake than plants grown with NO(3)(-). One possible influence of K(+) content of shoot, but not of roots, on K(+) uptake was evidenced. Enhanced K(+)-uptake capacity was correlated with the induction of H(+) extrusion by PM H(+)-ATPase. In plants grown in high K(+) solutions, the increase in the active H(+) gradient was associated with an increase of the PM H(+)-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2mM K(+), only the initial rate of H(+)-pumping and ATP hydrolysis were affected. Under these conditions, two specific isoforms of PM H(+)-ATPase were detected, independent of the nitrogen source and deficiency period. No change in enzyme activity was observed in NO(3)(-)-grown plants. The results suggest that K(+) homeostasis in NH(4)(+)-grown sorghum plants may be regulated by a high capacity for K(+) uptake, which is dependent upon the H(+)-pumping activity of PM H(+)-ATPase.