A FUNCTIONAL CALVIN CYCLE IS NOT INDISPENSABLE FOR THE LIGHT ACTIVATION OF C-4 PHOSPHOENOLPYRUVATE CARBOXYLASE KINASE AND ITS TARGET ENZYMEIN THE MAIZE MUTANT BUNDLE-SHEATH DEFECTIVE2-MUTABLE1

We used a pale-green maize (Zea mays L.) mutant that fails to accumula te ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to test t he working hypothesis that the regulatory phosphorylation of C-4 phosp hoenolpyruvate carboxylase (PEPC) by its Ca2+-insensitive protein-seri ne/threonine kinase (PEPC kinase) in the C-4 mesophyrl cytosol depends on cross-talk with a functional Calvin cycle in the bundle sheath. Wi ld-type (W22) and bundle sheath defective2-motable1 (bsd2-m1) seeds we re grown in a controlled environment chamber at 100 to 130 mu mol m(-2 ) s(-1) photosynthetic photon flux density, and leaf tissue was harves ted 11 d after sowing, following exposure to Various light intensities . Immunoblot analysis showed no major difference in the amount of poly peptide present for several mesophyll- and bundle-sheath-specific phot osynthetic enzymes apart from Rubisco, which was either completely abs ent or very much reduced in the mutant. Similarly, leaf net CO2-exchan ge analysis and in vitro radiometric Rubisco assays showed that no app reciable 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 con comitant increase in PEPC kinase activity, similar to that seen in wil d-type leaf tissue. Thus, although bsd2-ml mutant plants lack an opera tive Calvin cycle, light activation of PEPC kinase and its target enzy me are not grossly perturbed.