Salinity stress, which induces both ionic and osmotic damage, impairs plant growth and causes severe reductions in crop yield. Plants are equipped with defense responses against salinity stress such as regulation of ion transport including Na+ and K+, accumulation of compatible solutes and stress-related gene expression. The initial Ca2+ influx mediated by plasma membrane ion channels has been suggested to be crucial for the adaptive signaling. NADPH oxidase (Nox)-mediated deliberate production of reactive oxygen species (ROS) has also been suggested to play crucial roles in regulating adaptation to salinity stress in several plant species including halophytes. Respiratory burst oxidase homolog (Rboh) proteins show the ROS-producing Nox activity, which are synergistically activated by the binding of Ca2+ to EF-hand motifs as well as Ca2+-dependent phosphorylation. We herein review molecular identity, structural features and roles of the Ca2+-permeable channels involved in early salinity and osmotic signaling, and comparatively discuss the interrelationships among spatiotemporal dynamic changes in cytosolic concentrations of free Ca2+, Rboh-mediated ROS production, and downstream signaling events during salinity adaptation in planta.

Suc, an end product of photosynthesis, is metabolized by Suc synthase in sink organs as an initial step in the biosynthesis of storage products. Suc synthase activity is known to be regulated by reversible phosphorylation, but the details of this process are unclear at present. Rice SPK, a calcium-dependent protein kinase, is expressed uniquely in the endosperm of immature seed, and its involvement in the biosynthetic pathways of storage products was suggested. Antisense SPK transformants lacked the ability to accumulate storage products such as starch, but produced watery seed with a large amount of Suc instead, as the result of an inhibition of Suc degradation. Analysis of in vitro phosphorylation indicated that SPK phosphorylated specifically a Ser residue in Suc synthase that has been shown to be important for its activity in the degradation of Suc. This finding suggests that SPK is involved in the activation of Suc synthase. It appears that SPK is a Suc synthase kinase that may be important for supplying substrates for the biosynthesis of storage products.

The turf grass Sporobolus virginicus is halophyte and has high salinity tolerance. To investigate the molecular basis of its remarkable tolerance, we performed Illumina high-throughput RNA sequencing on roots and shoots of a S. virginicus genotype under normal and saline conditions. The 130 million short reads were assembled into 444,242 unigenes. A comparative analysis of the transcriptome with rice and Arabidopsis transcriptome revealed six turf grass-specific unigenes encoding transcription factors. Interestingly, all of them showed root specific expression and five of them encode bZIP type transcription factors. Another remarkable transcriptional feature of S. virginicus was activation of specific pathways under salinity stress. Pathway enrichment analysis suggested transcriptional activation of amino acid, pyruvate, and phospholipid metabolism. Up-regulation of several unigenes, previously shown to respond to salt stress in other halophytes was also observed. Gene Ontology enrichment analysis revealed that unigenes assigned as proteins in response to water stress, such as dehydrin and aquaporin, and transporters such as cation, amino acid, and citrate transporters, and H+-ATPase, were up-regulated in both shoots and roots under salinity. A correspondence analysis of the enriched pathways in turf grass cells, but not in rice cells, revealed two groups of unigenes similarly up-regulated in the turf grass in response to salt stress; one of the groups, showing excessive up-regulation under salinity, included unigenes homologous to salinity responsive genes in other halophytes. Thus, the present study identified candidate genes involved in salt tolerance of S. virginicus. This genetic resource should be valuable for understanding the mechanisms underlying high salt tolerance in S. virginicus. This information can also provide insight into salt tolerance in other halophytes.

To identify key proteins in the regulation of salt tolerance in the mangrove plant Bruguiera gymnorhiza, proteome analysis of samples grown under conditions of salt stress was performed. Comparative two-dimensional electrophoresis revealed that two, three and one protein were differentially expressed in the main root, lateral root and leaf, respectively, in response to salt stress. Among these, three proteins were identified by internal peptide sequence analysis: fructose-1,6-bisphosphate (FBP) aldolase and a novel protein in the main root, and osmotin in the lateral root. These results suggest that FBP aldolase and osmotin play roles in salt tolerance mechanisms common to both glycophytes and mangrove plants. Osmotin was abundant at early time points following salt treatment and seems to play a role in initial osmotic adaptation in lateral roots of B. gymnorhiza under salt stress, but does not contribute towards adaptation to prolonged or continuous exposure to salt stress. The amounts of these proteins were not correlated with those of the respective mRNAs, as determined by microarray analysis. A novel salt-responsive protein, not previously detected by expressed sequence tag analysis or transcriptome analysis, was also identified in this proteomic approach, and may provide insight into the salt tolerance mechanism of the mangrove plant. This is the first report of proteome analysis with detailed analysis of main and lateral roots of mangrove plants under salt stress conditions.