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Pyruvate uptake by mesophyll and bundle sheath chloroplasts of a 4 carbon pathway plant panicum miliaceum l



Pyruvate uptake by mesophyll and bundle sheath chloroplasts of a 4 carbon pathway plant panicum miliaceum l



Plant & Cell Physiology 28(1): 1-10



Intact chloroplasts were isolated from mesophyll and bundle sheath protoplasts of a C4 plant, Panicum miliaceum L., to measure the uptake of [1-14C]pyruvate into their sorbitol-impermeable spaces at 4.degree. C by the silicone oil filtering centrifugation method. When incubated in the dark, both chloroplasts showed similar slow kinetics of pyruvate uptake, and the equilibrium internal concentrations were almost equal to the external levels. When incubated in the light, only mesophyll chloroplasts showed remarkable enhancement of the uptake, the internal concentration reaching 10-30 times of the external level after 5 min incubation. The initial uptake rate of the mesophyll chloroplasts was enhanced about ten fold by light and was saturated with increasing pyruvate concentration; Km and Vmax were 0.2-0.4 mM and 20-40 .mu.mol (mg Chl)-1 h-1, respectively. The light enhancement was abolished by DCMU and uncoupling reagents such as carbonylcyanide-m-chlorophenylhydrazone and nigericin. These results indicate the existence of a light-dependent pyruvate transport system in the envelope of mesophyll chloroplasts of P. miliaceum. The uptake activity of mesophyll chloroplasts both in the light and the dark was inhibited by sulfhydryl reagents such as mersalyl and p-chloromercuriphenylsulfonate, but the bundle sheath activity was insensitive to the reagents. These findings are further evidence for the differentiation of mesophyll and bundle sheath chloroplasts of a C4 plant with respect to metabolite transport.

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