Gene expression and metabolite profiling of Populus euphratica growing in the Negev desert

BACKGROUND: Plants growing in their natural habitat represent a valuable resource for elucidating mechanisms of acclimation to environmental constraints. Populus euphratica is a salt-tolerant tree species growing in saline semi-arid areas. To identify genes involved in abiotic stress responses under natural conditions we constructed several normalized and subtracted cDNA libraries from control, stress-exposed and desert-grown P. euphratica trees. In addition, we identified several metabolites in desert-grown P. euphratica trees. RESULTS: About 14,000 expressed sequence tag (EST) sequences were obtained with a good representation of genes putatively involved in resistance and tolerance to salt and other abiotic stresses. A P. euphratica DNA microarray with a uni-gene set of ESTs representing approximately 6,340 different genes was constructed. The microarray was used to study gene expression in adult P. euphratica trees growing in the desert canyon of Ein Avdat in Israel. In parallel, 22 selected metabolites were profiled in the same trees. CONCLUSION: Of the obtained ESTs, 98% were found in the sequenced P. trichocarpa genome and 74% in other Populus EST collections. This implies that the P. euphratica genome does not contain different genes per se, but that regulation of gene expression might be different and that P. euphratica expresses a different set of genes that contribute to adaptation to saline growth conditions. Also, all of the five measured amino acids show increased levels in trees growing in the more saline soil

Populus euphratica Displays Apoplastic Sodium Accumulation, Osmotic Adjustment by Decreases in Calcium and Soluble Carbohydrates, and Develops Leaf Succulence under Salt Stress

Populus euphratica Olivier is known to exist in saline and arid environments. In this study we investigated the physiological mechanisms enabling this species to cope with stress caused by salinity. Acclimation to increasing Na(+) concentrations required adjustments of the osmotic pressure of leaves, which were achieved by accumulation of Na(+) and compensatory decreases in calcium and soluble carbohydrates. The counterbalance of Na(+)/Ca(2+) was also observed in mature leaves from field-grown P. euphratica trees exposed to an environmental gradient of increasing salinity. X-ray microanalysis showed that a primary strategy to protect the cytosol against sodium toxicity was apoplastic but not vacuolar salt accumulation. The ability to cope with salinity also included maintenance of cytosolic potassium concentrations and development of leaf succulence due to an increase in cell number and cell volume leading to sodium dilution. Decreases in apoplastic and vacuolar Ca(2+) combined with suppression of calcineurin B-like protein transcripts suggest that Na(+) adaptation required suppression of calcium-related signaling pathways. Significant increases in galactinol synthase and alternative oxidase after salt shock and salt adaptation point to shifts in carbohydrate metabolism and suppression of reactive oxygen species in mitochondria under salt stress

Linking the Salt Transcriptome with Physiological Responses of a Salt-Resistant Populus Species as a Strategy to Identify Genes Important for Stress Acclimation1[W][OA]

To investigate early salt acclimation mechanisms in a salt-tolerant poplar species (Populus euphratica), the kinetics of molecular, metabolic, and physiological changes during a 24-h salt exposure were measured. Three distinct phases of salt stress were identified by analyses of the osmotic pressure and the shoot water potential: dehydration, salt accumulation, and osmotic restoration associated with ionic stress. The duration and intensity of these phases differed between leaves and roots. Transcriptome analysis using P. euphratica-specific microarrays revealed clusters of coexpressed genes in these phases, with only 3% overlapping salt-responsive genes in leaves and roots. Acclimation of cellular metabolism to high salt concentrations involved remodeling of amino acid and protein biosynthesis and increased expression of molecular chaperones (dehydrins, osmotin). Leaves suffered initially from dehydration, which resulted in changes in transcript levels of mitochondrial and photosynthetic genes, indicating adjustment of energy metabolism. Initially, decreases in stress-related genes were found, whereas increases occurred only when leaves had restored the osmotic balance by salt accumulation. Comparative in silico analysis of the poplar stress regulon with Arabidopsis (Arabidopsis thaliana) orthologs was used as a strategy to reduce the number of candidate genes for functional analysis. Analysis of Arabidopsis knockout lines identified a lipocalin-like gene (AtTIL) and a gene encoding a protein with previously unknown functions (AtSIS) to play roles in salt tolerance. In conclusion, by dissecting the stress transcriptome of tolerant species, novel genes important for salt endurance can be identified