Simultaneous expression of serine acetyltransferase and cysteine synthase results in enhanced sulfate uptake and increased biomass in Ipomaea aquatica

Abstract

The long range goal of the present study is to practically utilize Ipomoea aquatica (water spinach) for phytoremediation of polluted water with sulfuric compounds. In higher plants, the sulfate assimilation pathway consists of 5 key enzymes, among which serine acetyltransferase (SAT) and cysteine synthase (CS) constitute one of the rate limiting steps. Subsequently we have attempted to improve the sulfur assimilation capacity of I. aquatica using genes encoding these two enzymes. Cotyledon segments of seedlings were transformed with Arabidopsis SAT and rice CS genes under the control of the cauliflower mosaic virus 35S promoter. Among 3,245 cotyledon explants, 325 regenerated shoots, and two showed a high tolerance to hygromycin, designated as SR3 and SR10. In transgenic lines, the SAT activity was over 2-fold, and the CS was 3-fold higher than those in the wild type control. The cysteine and glutathione contents were also 6- and 2-fold higher than the control, respectively. When cultured in the presence of 1 g l � 1 (7 mM) sulfate, they accumulated sulfate as much as 20 mg g � 1 fresh weight, being 5-fold higher than the control. Under standard culture conditions, transgenic lines grew faster than the control, showing a 20% increase in fresh weight within 5 weeks cultivation. These results suggested that strengthening of SAT and CS resulted in increase not only in sulfate uptake, but also in total biomass.