Effect of foliar GSH treatment on gene expression profile in the root of Brassica napus

Glutathione is a tripeptide involved in diverse aspects of plant metabolism. We investigated how the reduced form of glutathione, GSH, applied site-specifically to plants, affects zinc (Zn) distribution and behavior in oilseed rape plants (Brassica napus) cultured hydroponically. Foliar-applied GSH significantly increased the Zn content in shoots and the root-to-shoot Zn translocation ratio; furthermore, this treatment raised the Zn concentration in the cytosol of root cells and substantially enhanced Zn xylem loading. Notably, microarray analysis revealed that the gene encoding pectin methylesterase was upregulated in roots following foliar GSH treatment. We conclude that certain physiological signals triggered in response to foliar-applied GSH were transported via sieve tubes and functioned in root cells, which, in turn, increased Zn availability in roots by releasing Zn from their cell wall. Consequently, root-to-shoot translocation of Zn was activated and Zn accumulation in the shoot was markedly increased. Seeds of oilseed rape (Brassica napus L. cv. Nourin No.16, Kaneko Seed Co. Ltd., Gumma, Japan) were germinated in vermiculite by using bottom irrigation. Modified Hoagland solution (nutrient solution), described previously [Soil Sci. Plant Nutr. 54 (2008) 118–127.], was used as the irrigation solution. At 10 days after germination, young seedlings were transferred to 1.5-L containers (4 seedlings/container) containing 1.2 L of the nutrient solution, and the containers were maintained in a growth chamber for a 16-h photoperiod (~200μmol m-2s-1) at 24 °C, and at 16°C at night. Nutrient solutions were aerated continuously and renewed twice weekly. Each tested plant was grown for 4 weeks before being harvested. Plants were exposed to glutathione for 4 days. GSH (glutathione, reduced form) was applied to source leaves (fully expanded leaves). In the leaf GSH treatment, 50 µL of 100 mM GSH in a foliar-application solution (10 mM MES-KOH, pH6.1, 0.01% [w/v] Triton X-100) was applied to each leaf with a painting brush; GSH was applied to 3 expanded leaves per plant daily before harvest.