Regulation of sulfur metabolism in seedlings of the C₄ plant maize upon sulfate deprivation and atmospheric H₂S exposure

The increased cultivation of highly productive C₄ crop plants may contribute to a second green revolution in agriculture. However, the regulation of mineral nutrition is rather poorly understood in C₄ plants. To understand the impact of C₄ photosynthesis on the regulation of sulfate uptake by the root and sulfate assimilation into cysteine at the whole plant level, seedlings of the monocot C₄ plant maize (Zea mays) were exposed to a non-toxic level of 1.0 µl l⁻¹ atmospheric H₂S at sulfate-sufficient and sulfate-deprived conditions. Sulfate deprivation not only affected growth and the levels of sulfur- and nitrogen-containing compounds, but it also enhanced the expression and activity of the sulfate transporters in the root and the expression and activity of APS reductase (APR) in the root and shoot. H₂S exposure alleviated the establishment of sulfur deprivation symptoms and seedlings switched, at least partly, from sulfate to H₂S as sulfur source. Moreover, H₂S exposure resulted in a downregulation of the expression and activity of APR in both shoot and root, though it hardly affected that of the sulfate transporters in the root. These results indicate that maize seedlings respond similarly to sulfate deprivation and atmospheric H₂S exposure as C₃ monocots, implying that C₄ photosynthesis in maize is not associated with a distinct whole plant regulation of sulfate uptake and assimilation into cysteine.