Sulfur Assimilation in C4 Plants

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Abstract
The activity of ATP sulfurylase was determined in crabgrass [D. sanguinalis (L.) Scop.] mesophyll cells, bundle sheath strands and whole leaf extracts. The enzyme was assayed by following molybdate-dependent pyrophosphate release from ATP, 35SO42- incorporation into adenosine 5'' phosphosulfate, and ATP synthesis dependent on adenosine 5'' phosphosulfate and PPi. With all assays, > than 90% of the activity was found in extracts from bundle sheath strands. The activities in whole leaf extracts were consistently intermediate between the activities of mesophyll and bundle sheath extracts and extract-mixing expeirments gave no indication of enzyme activation or inhibition in vitro. Whole leaf activities were several hundred-fold less than concurrent measurements of ribulose 1,5-bisphosphate and phosphoenolpyruvate carboxylase activities, which is interpreted as being consistent with the relative amounts of elemental C and S found in higher plants. A hypothesis is presented for the intercellular compartmentation of S assimilation in relationship to NO3- and CO2 assimilation in leaves of C4 plants.