High salinity stress promotes plant ethylene biosynthesis and triggers the ethylene signalling response. However, the precise mechanism underlying how plants transduce ethylene signalling in response to salt stress remains largely unknown. In this study, we discovered that SALT OVERLY SENSITIVE 2 (SOS2) inhibits the kinase activity of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) by phosphorylating the 87th serine (S87). This phosphorylation event activates the ethylene signalling response, leading to enhanced plant salt resistance. Furthermore, through genetic analysis, we determined that the loss of CTR1 or the gain of SOS2-mediated CTR1 phosphorylation both contribute to improved plant salt tolerance. Additionally, in the sos2 mutant, we observed compromised proteolytic processing of ETHYLENE INSENSITIVE 2 (EIN2) and reduced nuclear localization of EIN2 C-terminal fragments (EIN2-C), which correlate with decreased accumulation of ETHYLENE INSENSITIVE 3 (EIN3). Collectively, our findings unveil the role of the SOS2-CTR1 regulatory module in promoting the activation of the ethylene signalling pathway and enhancing plant salt tolerance.

Soil salinization is an essential environmental stressor, threatening agricultural yield and ecological security worldwide. Saline soils accumulate excessive soluble salts which are detrimental to most plants by limiting plant growth and productivity. It is of great necessity for plants to efficiently deal with the adverse effects caused by salt stress for survival and successful reproduction. Multiple determinants of salt tolerance have been identified in plants, and the cellular and physiological mechanisms of plant salt response and adaption have been intensely characterized. Plants respond to salt stress signals and rapidly initiate signaling pathways to re-establish cellular homeostasis with adjusted growth and cellular metabolism. This review summarizes the advances in salt stress perception, signaling, and response in plants. A better understanding of plant salt resistance will contribute to improving crop performance under saline conditions using multiple engineering approaches. The rhizosphere microbiome-mediated plant salt tolerance as well as chemical priming for enhanced plant salt resistance are also discussed in this review.

The world\'s population continues to increase and thus requires more food production to take place in nonarable land, such as saline soil; therefore, it is urgent to find solutions to enhance the salinity tolerance of crops. As the second genome of plants, the rhizosphere microbiome plays critical roles in plant fitness under stress conditions. Many beneficial microbes that help plants cope with salinity stress have been identified, highlighting their roles in mitigating salt stress-induced negative effects on plants. However, a comprehensive review of the microbial species that are able to confer plant salt tolerance and the underlying mechanisms is still lacking. In this review, we compared the representative fungal and bacterial taxa that demonstrate the ability to enhance plant growth in saline soil. We also reviewed the mechanisms by which rhizosphere microbes enhance plant salt stress tolerance, i.e., by re-establishing ion and osmotic homeostasis, preventing damage to plant cells, and resuming plant growth under salt stress. Finally, future research efforts to explore the rhizosphere microbiome for agricultural sustainability are proposed.

Salt stress significantly induces accumulation of misfolded or unfolded proteins in plants. Endoplasmic reticulum (ER)-associated protein degradation (ERAD) and other degradative machineries function in the degradation of these abnormal proteins, leading to enhanced salt tolerance in plants. Here we characterise that a novel receptor-like kinase, Salt-Induced Malectin-like domain-containing Protein1 (SIMP1), elevates ERAD efficiency during salt stress through UMP1A, a putative proteasome maturation factor in Arabidopsis. SIMP1 loss-of-function caused a salt-hypersensitive phenotype. SIMP1 interacts and phosphorylates UMP1A, and the protein stability of UMP1A is positively regulated by SIMP1. SIMP1 modulates the 26S proteasome maturation possibly through enhancing the recruitment of specific β subunits of the core catalytic particle to UMP1A. Functionally, the SIMP1-UMP1A module plays a positive role in ERAD efficiency in Arabidopsis. The degradation of misfolded/unfolded proteins was impaired in both simp1 and ump1a mutants during salt stress. Consistently, both simp1 and ump1a plants exhibited reduced ER stress tolerance. Phenotypic analysis revealed that SIMP1 regulates salt tolerance through UMP1A at least in part. Taken together, our work demonstrated that SIMP1 modulates plant salt tolerance by promoting proteasome maturation via UMP1A, therefore mitigating ER stress through enhanced ERAD efficiency under saline conditions.

UNASSIGNED: This work demonstrates that PpCIPK1, a putative protein kinase, participates in regulating plant salt tolerance in moss Physcomitrella patens. Calcineurin B-Like protein (CBL)-interacting protein kinases (CIPKs) have been reported to be involved in multiple signaling networks and function in plant growth and stress responses, however, their biological functions in non-seed plants have not been well characterized. In this study, we report that PpCIPK1, a putative protein kinase, participates in regulating plant salt tolerance in moss Physcomitrella patens (P. patens). Phylogenetic analysis revealed that PpCIPK1 shared high similarity with its homologs in higher plants. PpCIPK1 transcription level was induced upon salt stress in P. patens. Using homologous recombination, we constructed PpCIPK1 knockout mutant lines (PpCIPK1 KO). Salt sensitivity analysis showed that independent PpCIPK1 KO plants exhibited severe growth inhibition and developmental deficiency of gametophytes under salt stress condition compared to that of wild-type P. patens (WT). Consistently, ionic homeostasis was disrupted in plants due to PpCIPK1 deletion, and high level of H2O2 was accumulated in PpCIPK1 KO than that in WT. Furthermore, PpCIPK1 functions in regulating photosynthetic activity in response to salt stress. Interestingly, we observed that PpCIPK1 could completely rescue the salt-sensitive phenotype of sos2-1 to WT level in Arabidopsis, indicating that AtSOS2 and PpCIPK1 are functionally conserved. In conclusion, our work provides evidence that PpCIPK1 participates in salt tolerance regulation in P. patens.

Salt stress adversely affects plant growth and development. Multiple adaptive mechanisms have been used for plant salt tolerance. We previously reported that membrane trafficking-related protein patellin1 (PATL1) negatively regulates plant salt tolerance. Here, we characterized that Arabidopsis PATL1 negatively modulates nitric oxide (NO) accumulation upon salt exposure. Our work revealed a functional link between salt response and NO signaling.

Alternative oxidase (AOX) has been reported to be involved in mitochondrial function and redox homeostasis, thus playing an essential role in plant growth as well as stress responses. However, its biological functions in nonseed plants have not been well characterized. Here, we report that AOX participates in plant salt tolerance regulation in moss Physcomitrella patens (P. patens). AOX is highly conserved and localizes to mitochondria in P. patens. We observed that PpAOX rescued the impaired cyanide (CN)-resistant alternative (Alt) respiratory pathway in Arabidopsis thaliana (Arabidopsis) aox1a mutant. PpAOX transcription and Alt respiration were induced upon salt stress in P. patens. Using homologous recombination, we generated PpAOX-overexpressing lines (PpAOX OX). PpAOX OX plants exhibited higher Alt respiration and lower total reactive oxygen species accumulation under salt stress condition. Strikingly, we observed that PpAOX OX plants displayed decreased salt tolerance. Overexpression of PpAOX disturbed redox homeostasis in chloroplasts. Meanwhile, chloroplast structure was adversely affected in PpAOX OX plants in contrast to wild-type (WT) P. patens. We found that photosynthetic activity in PpAOX OX plants was also lower compared with that in WT. Together, our work revealed that AOX participates in plant salt tolerance in P. patens and there is a functional link between mitochondria and chloroplast under challenging conditions.