Mechanisms for light-dependent regulation of ribulose-1,5-bisphosphate carboxylase activity and photosynthesis in intact leaves

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The mechanisms involved in the in vivo light-dependent regulation of ribulose-1,5-bisphosphate (RbuP2) carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] activity in intact leaves were studied. In the three species examined, Phaseolus vulgaris, Beta vulgaris, and Spinacea oleracea, the regulated level of RbuP2 carboxylase activity (assayed in vitro with saturating substrate) was highly correlated (r = 0.96) with the rate of net CO2 uptake of the corresponding leaves measured over a wide range of photosynthetic photon flux density (PPFD). However, the mechanisms by which the enzyme was regulated differed between these species. In Phaseolus, the inhibitor 2-carboxyarabinitol 1-phosphate (CAP) accounted for all of the PPFD-dependent regulation of RbuP2 carboxylase activity. A similar compound was detected in Beta, and changes in its concentration accounted for about half of the PPFD-dependent regulation of enzyme activity in this species. No CAP was detected in Spinacea, but evidence we obtained suggests that a different inhibitor (possibly RbuP2) accounts for a significant portion of the PPFD-dependent regulation of enzyme activity in this species. Changes in the activation state of the enzyme were observed with Beta and Spinacea, while in Phaseolus the enzyme was apparently fully activated at all PPFD levels. These results indicate that plant species may differ markedly in the mechanisms they use to regulate RbuP2 carboxylase activity as PPFD changes. The results also suggest that tight binding inhibitors are a more widespread mechanism for regulation of this enzyme than previously thought. Furthermore, the results establish the importance of such inhibitors in regulating both the activity of RbuP2 carboxylase and whole leaf photosynthesis over a range of PPFD.

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