Transcript Analysis of Two Spinach Cultivars Reveals the Complexity of Salt Tolerance Mechanisms

Increasing soil salinization threatens global crop productivity. An understanding of the genetic networks involved in salinity tolerance mechanisms of high-value crops, such as spinach, is lacking. RNA-Seq analysis of leaves and roots of two spinach genotypes, Monstrans Viroflag and Palek, subjected to high-salinity irrigation, revealed that a higher degree of differential gene expression was caused by salinity rather than by genotype. Genotypic comparisons suggested that the low salt tolerance index for root and shoot biomass of Palek, compared to Monstrans Viroflag, was due to the differential expression of genes involved in water/nutrient uptake rather than tissue salt accumulation. Montrans Viroflag displayed a better Cl- exclusion than Palek and was more efficient in restricting Na+ from entering its roots, thus protecting leaves from ion toxicity. In addition, differentially expressed genes (DEGs) involved in MAPK signaling, hormonal signaling, and transport revealed salinity- and genotype-specific differences and resulted in the identification of candidate genes that may function to mediate ion influx across cell membranes to maintain osmotic homeostasis when plants are under salt stress. The quantitative reverse transcription assay validated the overall expression trends of the selected RNA-Seq-based DEGs among different spinach samples. Collectively, the assays used in this study highlighted the complexity of the salinity tolerance mechanism and isolated several putative genes with the potential to improve salinity tolerance in spinach.