Sulfate Assimilation in C 4 Plants

Abstract

The activity of ATP sulfurylase, cysteine synthase, and cystathionine jl-lyase was measured in crude leaf extracts, bundle sheath strands, and mesophyll and bundle sheath chloroplasts to determine the location of sulfate assimilation of C4 plant leaves. Almost all the ATP sulfurylase activity was located in the bundle sheath chloroplasts while cysteine synthase and cystathionine j-lyase activity was located, in different proportions, in both chloroplast types. A new spectrophotometric assay for measuring ATP sulfurylase activity is also described. The C4 pathway of CO2 assimilation is compartmentalized into specific cell types with atmospheric CO2 initially being fixed into organic acids in the mesophyll cells of C4 plants and then transported to the bundle sheath cells. Within the two types of cells the reactions are compartmentalized into chloroplasts, cy-toplasm and mitochondria. The intercellular compartmentation of sulfur metabolism is less well defined. The intercellular compartmentation of ATP sulfurylase (EC 2.7.7.4) has been reported in a wide variety of C4 plants (8, 9). An hypothesis for the pathway of sulfur assimilation in leaves of C4 plants has been described based upon the location of ATP sulfurylase and, to a lesser extent, thiosulfonate reductase/sulfite reductase (8). Since ATP sulfurylase and sulfite reductase are involved in only the initial stages of assimilatory sulfate pathway in plants, the location of enzymes involved in the synthesis of cysteine and methionine remains unresolved. Although it is generally agreed that in C3 plants, chloroplasts are the main site of S42-assimilation (activation and reduction to S2-) (1) the subcellular distribution of cysteine synthase (EC 4.2.99.8) appears to differ between plant species. In wheat and rape leaves and kidney bean seedlings, the enzyme is apparently cytoplasmic (3, 14, 16) whereas in spinach, white clover, and pea leaves the enzyme is associated with chloroplasts (7, 15). A recent report suggests that cystathionine ,-lyase (EC 4.4.1.8) in barley leaves is at least partially chloroplastic (17). This paper reports on the inter-and intracellular location of ATP sulfurylase, cysteine synthase and cystathionine ,B-lyase in Zea mays. A new spectrophotometric assay for ATP sulfurylase is also described.