Properties of Guard-Cell Chloroplasts and their Role in Stomatal Opening

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Enzyme activities of Calvin-Benson cycle were investigated in guard cell protoplasts from Vicia faba L. Evidence for the key enzyme of ribulose 1,5-bisphosphate carboxylase (RuBPase) was obtained in guard cell protoplasts by immunoblotting using the antibody of RuBPase, by the activity measured by methods of enzyme coupling and ^<14>CO_2 incorporation. All these measurements revealed that the activity of RuBPase in guard cells was about 40 % of that in mesophyll cells on a chlorophyll basis. Other enzymes which constitute the Calvin-Benson cycle, such as Phosphoglycerate Kinase (PGAK), NADP-glyceraldehyde phosphate dehydrogenase (NADP-GAPD), Triose Phosphate Isomerase (TPI), Fructose Bisphosphatase (FBPase) were also found in Vicia guard cell protoplasts. Among these enzymes, the enzymes related closely to the PGA/DHAP shuttle which mediate the transfer of ATP and reducing equivalents from chloroplasts to cytosol had very high activities, indicating that guard cells utilize the shuttle very largely. The RuBPase activity of guard cell protoplasts was 30% of that of mesophyll protoplasts on a chlorophyll basis; however, the O_2 evolution rate was 3 times higher in guard cell protoplasts than in mesophyll protoplasts. Irreversible NADP-GAPD was found in high activity in guard cell protoplasts. From these results I concluded that guard cells utilize a considerable proportion of reducing equivalents from water for reactions other than CO_2 fixation.