Photosynthetic carbon reduction pathway is absent in chloroplasts of Vicia faba guard cells

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Abstract
Four cell types from V. faba Linnaeus cv. Long Pod leaflets were assayed for 3 enzymes unique to the photosynthetic carbon reduction pathway. The enzymes were ribulosebisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39], phosphoribulokinase (ATP:D-ribulose-5-phosphate 1-phosphotransferase, EC 2.7.1.19), and glyceraldehyde-phosphate dehydrogenase (NADP+) (phosphorylating) [D-glyceraldehyde-3-phosphate:NADP+ oxidoreductase (phosphorylating), EC 1.2.1.13]. On a dry weight basis, the enzyme activities were about twice as high in palisade as in spongy parenchyma. Two enzymes were not detected in epidermal cells and the other was present in only a trace amount. In guard cells, the enzyme activities were absent or present at less than 1% of the amount in palisade cells. Immunoelectrophoresis showed that ribulosebisphosphate carboxylase was absent in extracts of guard cell protoplasts. Microscopy confirmed the abundance of typical guard cell chloroplasts. These results demonstrate the absence of the photosynthetic carbon reduction pathway in guard cell chloroplasts. This is the only chloroplast type known to be deficient in this pathway in plants whose primary CO2 acceptor is ribulose bisphosphate. Possible reasons for the absence of this pathway in guard cells are discussed.