Abstract
Although salt tolerance is a feature representative of halophytes, most studies on this topic in plants have been conducted on glycophytes. Transcriptome profiles are also available for only a limited number of halophytes. Hence, the present study was conducted to understand the molecular basis of salt tolerance through the transcriptome profiling of the halophyte Suaeda maritima, which is an emerging plant model for research on salt tolerance. Illumina sequencing revealed 72,588 clustered transcripts, including 27,434 that were annotated using BLASTX. Salt application resulted in the 2-fold or greater upregulation of 647 genes and downregulation of 735 genes. Of these, 391 proteins were homologous to proteins in the COGs (cluster of orthologous groups) database, and the majorities were grouped into the poorly characterized category. Approximately 50% of the genes assigned to MapMan pathways showed homology to S. maritima. The majority of such genes represented transcription factors. Several genes also contributed to cell wall and carbohydrate metabolism, ion relation, redox responses and G protein, phosphoinositide and hormone signaling. Real-time PCR was used to validate the results of the deep sequencing for the most of the genes. This study demonstrates the expression of protein kinase C, the target of diacylglycerol in phosphoinositide signaling, for the first time in plants. This study further reveals that the biochemical and molecular responses occurring at several levels are associated with salt tolerance in S. maritima. At the structural level, adaptations to high salinity levels include the remodeling of cell walls and the modification of membrane lipids. At the cellular level, the accumulation of glycinebetaine and the sequestration and exclusion of Na+ appear to be important. Moreover, this study also shows that the processes related to salt tolerance might be highly complex, as reflected by the salt-induced enhancement of transcription factor expression, including hormone-responsive factors, and that this process might be initially triggered by G protein and phosphoinositide signaling.
Highlights
Plants, being sessile organisms, had to diversify greatly during the course of evolution to colonize the highly heterogeneous abiotic conditions found across the entire globe, such as extremes of temperature, high and low irradiance levels, high salinity, and drought [1]
This study identified many genes involved in biochemical and physiological processes, such as ion transport, osmolyte accumulation, cell wall remodeling, protein modification and degradation, antioxidative defense, and, more importantly, those involved in G protein and phosphoinositide signaling, which could be important for salt tolerance in plants
Deep sequencing to a coverage depth of 120x and 80x for control and treatment samples, respectively, using the Illumina platform yielded a total of 180,443,374 and 123,067,034 reads of 100 bases from the cDNA libraries prepared for the transcriptome sequencing of control and 2.0% NaCl-treated (9 h) S. maritima, respectively (Table 1)
Summary
Plants, being sessile organisms, had to diversify greatly during the course of evolution to colonize the highly heterogeneous abiotic conditions found across the entire globe, such as extremes of temperature, high and low irradiance levels, high salinity, and drought [1]. Because salt affects plants in multiple ways, salinity tolerance is controlled by many genes involved in different processes, such as ion compartmentalization, extrusion and selectivity, the synthesis of compatible solutes, and reactive oxygen species (ROS) scavenging [6,7]. These processes are not universal, and depending upon the metabolic background of the species, the relative importance of each individual biochemical pathway in salt tolerance may vary. The quantitative nature of salt tolerance makes it one of the more complex physiological processes yet to be fully understood
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