Effects of rhizosphere sulfur concentration on sulfur content and sulfate transporter gene expression in Japanese heirloom vegetables of the Brassicaceae family

Several region-specific vegetable varieties in Japan have an extensive cultivation history. Distinct tastes and aromas make them suitable for use in spices and pickles, and varieties with a high content of functional sulfur-containing compounds have been preferentially selected over the years. In the present study, we investigated the effects of different rhizosphere sulfate concentrations (0, 0.2, 0.4, and 0.8 mM) on the dry weight and sulfur content of 40 varieties of Brassicaceae heirloom vegetables. We also analyzed the expression levels of sulfate transporter genes, such as SULTR1s (sum of SULTR1;1, SULTR1;2, and SULTR1;3) and SULTR2;1, in 20 vegetable varieties. At 0 mM sulfate, plants have small leaves, as well as thin and scarce roots, whereas the mean shoot sulfur content per plant was 10% of that in control. Sulfur content increased by 29% at 0.4 mM and by 66% at 0.8 mM sulfate compared with that in control (0.2 mM sulfate); however, dry weight was not significantly affected. Mean shoot sulfur concentration was 0.12% at 0 mM, 0.52% at 0.2 mM, 0.65% at 4 mM, and 0.86% at 0.8 mM of sulfate. Shoot sulfur concentration exceeded 1% at 0.4 mM sulfate in several varieties. Shoot sulfur content and dry weight were positively correlated, suggesting that the capacity for sulfate ion absorption had a strong effect on biomass production in plants of the Brassicaceae family. Sulfur content and SULTR1s and SULTR2;1 expression were poorly correlated regardless of rhizosphere sulfur concentration. These genes did not respond to shoot sulfur content but responded to sulfate levels in the rhizosphere. Overall, our data indicated that sulfate absorption response to sulfate concentration in the rhizosphere by sulfate transporters in Brassicaceae plants depends on the inherent characteristics of the variety.