A Functional Calvin Cycle Is Not Indispensable for
the Light Activation of C4
Phosphoenolpyruvate Carboxylase Kinase and Its Target
Enzyme in the Maize Mutant
bundle sheath
defective2-mutable1 |
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Authors: | Lucy H. Smith Jane A. Langdale Raymond Chollet |
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Affiliation: | Department of Biochemistry, University of Nebraska-Lincoln, G.W. Beadle Center, Lincoln, Nebraska 68588–0664 (L.H.S., R.C.);Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, United Kingdom (J.A.L.) |
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Abstract: | We used a pale-green maize (Zea mays L.) mutant that fails to accumulate ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to test the working hypothesis that the regulatory phosphorylation of C4 phosphoenolpyruvate carboxylase (PEPC) by its Ca2+-insensitive protein-serine/threonine kinase (PEPC kinase) in the C4 mesophyll cytosol depends on cross-talk with a functional Calvin cycle in the bundle sheath. Wild-type (W22) and bundle sheath defective2-mutable1 (bsd2-m1) seeds were grown in a controlled environment chamber at 100 to 130 μmol m−2 s−1 photosynthetic photon flux density, and leaf tissue was harvested 11 d after sowing, following exposure to various light intensities. Immunoblot analysis showed no major difference in the amount of polypeptide present for several mesophyll- and bundle-sheath-specific photosynthetic enzymes apart from Rubisco, which was either completely absent or very much reduced in the mutant. Similarly, leaf net CO2-exchange analysis and in vitro radiometric Rubisco assays showed that no appreciable carbon fixation was occurring in the mutant. In contrast, the sensitivity of PEPC to malate inhibition in bsd2-m1 leaves decreased significantly with an increase in light intensity, and there was a concomitant increase in PEPC kinase activity, similar to that seen in wild-type leaf tissue. Thus, although bsd2-m1 mutant plants lack an operative Calvin cycle, light activation of PEPC kinase and its target enzyme are not grossly perturbed. |
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