Phosphorus (P) is an essential mineral nutrient for plant growth and development. Low availability of inorganic phosphate (orthophosphate; Pi) in soil seriously restricts the crop production, while excessive fertilization has caused environmental pollution. Pi acquisition and homeostasis depend on transport processes controlled Pi transporters, which are grouped into five families so far: PHT1, PHT2, PHT3, PHT4, and PHT5. This review summarizes the current understanding on plant PHT families, including phylogenetic analysis, function, and regulation. The potential application of Pi transporters and the related regulatory factors for developing genetically modified crops with high phosphorus use efficiency (PUE) are also discussed in this review. At last, we provide some potential strategies for developing high PUE crops under salt or drought stress conditions, which can be valuable for improving crop yields challenged by global scarcity of water resources and increasing soil salinization.

High salinity and nitrogen (N) deficiency in soil are two key factors limiting crop productivity, and they usually occur simultaneously. Here we firstly found that H(+) -PPase is involved in salt-stimulated NO3 (-) uptake in the euhalophyte Salicornia europaea. Then, two genes (named SeVP1 and SeVP2) encoding H(+) -PPase from S. europaea were characterized. The expression of SeVP1 and SeVP2 was induced by salt stress and N starvation. Both SeVP1 or SeVP2 transgenic Arabidopsis and wheat plants outperformed the wild types (WTs) when high salt and low N occur simultaneously. The transgenic Arabidopsis plants maintained higher K(+) /Na(+) ratio in leaves and exhibited increased NO3 (-) uptake, inorganic pyrophosphate-dependent vacuolar nitrate efflux and assimilation capacity under this double stresses. Furthermore, they had more soluble sugars in shoots and roots and less starch accumulation in shoots than WT. These performances can be explained by the up-regulated expression of ion, nitrate and sugar transporter genes in transgenic plants. Taken together, our results suggest that up-regulation of H(+) -PPase favours the transport of photosynthates to root, which could promote root growth and integrate N and carbon metabolism in plant. This work provides potential strategies for improving crop yields challenged by increasing soil salinization and shrinking farmland.

The V-ATPase subunit A participates in vacuolar Na compartmentalization in Salicornia europaea regulating V-ATPase and V-PPase activities. Na sequestration into the vacuole is an efficient strategy in response to salinity in many halophytes. However, it is not yet fully understood how this process is achieved. Particularly, the role of vacuolar H-ATPase (V-ATPase) in this process is controversial. Our previous proteomic investigation in the euhalophyte Salicornia europaea L. found a significant increase of the abundance of V-ATPase subunit A under salinity. Here, the gene encoding this subunit named SeVHA-A was characterized, and its role in salt tolerance was demonstrated by RNAi directed downregulation in suspension-cultured cells of S. europaea. The transcripts of genes encoding vacuolar H-PPase (V-PPase) and vacuolar Na/H antiporter (SeNHX1) also decreased significantly in the RNAi cells. Knockdown of SeVHA-A resulted in a reduction in both V-ATPase and vacuolar H-PPase (V-PPase) activities. Accordingly, the SeVHA-A-RNAi cells showed increased vacuolar pH and decreased cell viability under different NaCl concentrations. Further Na staining showed the reduced vacuolar Na sequestration in RNAi cells. Taken together, our results evidenced that SeVHA-A participates in vacuolar Na sequestration regulating V-ATPase and V-PPase activities and thereby vacuolar pH in S. europaea. The possible mechanisms underlying the reduction of vacuolar V-PPase activity in SeVHA-A-RNAi cells were also discussed.

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Salinity-induced lipid alterations have been reported in many plant species, however, how lipid biosynthesis and metabolism are regulated and how lipids work in plant salt tolerance are much less studied. Here a constitutively much higher phosphatidylserine (PS) content in plasma membrane (PM) was found in the euhalophyte Salicornia europaea than Arabidopsis. A gene encoding phosphatidylserine synthase (PSS) was subsequently isolated from S. europaea, named SePSS, which was induced by salinity. Multiple alignments and phylogenetic analysis suggested SePSS belong to base-exchange-type PSS, which locates in endoplasmic reticulum. Knockdown of SePSS in S. europaea suspension cells resulted in reduced PS content, decreased cell survival rate, increased PM depolarization and K+ efflux under 400 or 800 mM NaCl. By contrast, upregulation of SePSS leads to increased PS and phosphatidylethanolamine (PE) levels and enhanced salt tolerance in Arabidopsis, along with lower accumulation of reactive oxygen species, less membrane injury, less PM depolarization and higher K+/Na+ in the transgenic lines than WT. These results suggest the positive correlation between PS levels and plant salt tolerance, and SePSS participates in plant salt tolerance by regulating PS levels, hence PM potential and permeability, which help maintain ion homeostasis. Our work provides a potential strategy for improving plant growth under multiple stresses.

Salt cress (Eutrema salsugineum) presents relatively high phosphate (Pi) use efficiency cy in its natural habitat. Phosphate Transporters (PHTs) play critical roles in Pi acquisition and homeostasis. Here, a comparative study of PHT families between salt cress and Arabidopsis was performed. A total of 27 putative PHT genes were identified in E. salsugineum genome. Notably, seven tandem genes encoding PHT1;3 were found, and function analysis in Arabidopsis indicated at least six EsPHT1;3s participated in Pi uptake. Meanwhile, different expression profiles of PHT genes between the two species under Pi limitation and salt stress were documented. Most PHT1 genes were down-regulated in Arabidopsis while up-regulated in salt cress under salinity, among which EsPHT1;9 was further characterized. EsPHT1;9 was involved in root-to-shoot Pi translocation. Particularly, the promoter of EsPHT1;9 outperformed that of AtPHT1;9 in promoting Pi translocation, K + /Na + ratio, thereby salt tolerance. Through cis-element analysis, we identified a bZIP transcription factor EsABF5 negatively regulating EsPHT1;9 and plant tolerance to low-Pi and salt stress. Altogether, more copies and divergent transcriptional regulation of PHT genes contribute to salt cress adaptation to the co-occurrence of salinity and Pi limitation, which add our knowledge on the evolutionary and molecular component of multistress-tolerance of this species.

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