Progress in Understanding the Physiological and Molecular Responses of Populus to Salt Stress

Abstract

Salt stress (SS) has become an important factor limiting afforestation programs. Because of their salt tolerance and fully sequenced genomes, poplars (Populus spp.) are used as model species to study SS mechanisms in trees. Here, we review recent insights into the physiological and molecular responses of Populus to SS, including ion homeostasis and signaling pathways, such as the salt overly sensitive (SOS) and reactive oxygen species (ROS) pathways. We summarize the genes that can be targeted for the genetic improvement of salt tolerance and propose future research areas.

Keywords: ROS; SOS; molecular mechanisms; poplars (Populus); salt tolerance.

Figure 1

Schematic model showing multiple signaling networks active in Populus in response to NaCl stress. NaCl induces the efflux of intracellular ATP (iATP) and an increase in extracellular ATP (eATP), which is sensed by P2K₁ in the plasma membrane (PM) and leads to the induction of H₂O₂ production. This stimulates the movement of Ca²⁺ into the cells via Ca²⁺-permeable channels. The elevated cytosolic Ca²⁺ concentration initiates the SOS pathway by stimulating Na⁺/H⁺ antiporters, such as SOS1, localized in the PM to extrude Na⁺, or activates CBL10, forming the CBL10–SOS2 complex, which may indirectly target the NHX type antiporters to the tonoplast to compartmentalize Na⁺ into vacuoles in green tissues. The elevated cytosolic Ca²⁺ also stimulates tonoplast-localized NHX1/3 to accumulate Na⁺(K⁺) into vacuoles. Besides, the elevated cytosolic Ca²⁺ increases H⁺-ATPase activity in the PM, which activates a H⁺ pump to supply a proton gradient for the Na⁺/H⁺ antiporters, stimulating the extrusion of Na⁺. A proton gradient supplied by the H⁺ pump contributes to the activation of NADPH oxidases, which leads to H₂O₂ production. H⁺-ATPases can also inhibit the efflux of K⁺ by further polarizing the PM. All these signaling components help to maintain K⁺/Na⁺ homeostasis in Populus cells.