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. 2009;60(14):4077-88.
doi: 10.1093/jxb/erp242. Epub 2009 Aug 6.

Rubisco in planta kcat is regulated in balance with photosynthetic electron transport

H Eichelmann  1 , E Talts, V Oja, E Padu, A Laisk
Affiliations

Rubisco in planta kcat is regulated in balance with photosynthetic electron transport

H Eichelmann et al. J Exp Bot. 2009.

Abstract

Site turnover rate (k(cat)) of Rubisco was measured in intact leaves of different plants. Potato (Solanum tuberosum L.) and birch (Betula pendula Roth.) leaves were taken from field-growing plants. Sunflower (Helianthus annuus L.), wild type (wt), Rubisco-deficient (-RBC), FNR-deficient (-FNR), and Cyt b(6)f deficient (-CBF) transgenic tobacco (Nicotiana tabacum L.) were grown in a growth chamber. Rubisco protein was measured with quantitative SDS-PAGE and FNR protein content with quantitative immunoblotting. The Cyt b(6)f level was measured in planta by maximum electron transport rate and the photosystem I (PSI) content was assessed by titration with far-red light. The CO(2) response of Rubisco was measured in planta with a fast-response gas exchange system at maximum ribulose 1,5-bisphosphate concentration. Reaction site k(cat) was calculated from V(m) and Rubisco content. Biological variation of k(cat) was significant, ranging from 1.5 to 4 s(-1) in wt, but was >6 s(-1) at 23 degrees C in -RBC leaves. The lowest k(cat) of 0.5 s(-1) was measured in -FNR and -CBF plants containing sufficient Rubisco but having slow electron transport rates. Plotting k(cat) against PSI per Rubisco site resulted in a hyperbolic relationship where wt plants are on the initial slope. A model is suggested in which Rubisco Activase is converted into an active ATP-form on thylakoid membranes with the help of a factor related to electron transport. The activation of Rubisco is accompanied by the conversion of the ATP-form into an inactive ADP-form. The ATP and ADP forms of Activase shuttle between thylakoid membranes and stromally-located Rubisco. In normal wt plants the electron transport-related activation of Activase is rate-limiting, maintaining 50-70% Rubisco sites in the inactive state.

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Figures

Fig. 1.
Fig. 1.
Steady-state CO2 response curves of CO2 uptake at 210 mmol O2 mol−1 (filled data points, solid line) and kinetic curves of Rubisco with respect to active site CO2 concentration at 20 mmol O2 mol−1 (empty data points, dotted line) in wild-type (diamonds) and –RBC (circles) tobacco. Jumps of CO2 concentration down and up were made from the steady-state (duplicate empty diamond 4th from zero).
Fig. 2.
Fig. 2.
The dependence of Rubisco content on leaf N content. Filled black and grey data points, wild type; empty data points, transgenic plants. Filled triangles, sunflower; filled circles, potato; filled squares, wt tobacco; filled diamonds, wt –N tobacco; open squares, –RBC tobacco; multi, –FNR tobacco; plus, –CBF tobacco. Dashed line indicates the maximum slope of regression.
Fig. 3.
Fig. 3.
Average in planta catalytic turnover rate, kcat, of Rubisco sites as a function of the Rubisco content in leaves. Grey diamonds, data for birch from Laisk et al. (2005) and Eichelmann et al. (2004b); the meaning of the other symbols is given in Fig. 2. Sunflower leaves (grey triangles) with exceptionally high kcat were transferred from low to high growth light. The line was calculated from equation (3).
Fig. 4.
Fig. 4.
Average in planta catalytic turnover rate, kcat, of Rubisco sites as a function of relative FNR content in the –FNR (A) and of maximum electron transport rate ETRMAX in –CBF (B) transgenic tobacco (empty symbols). Filled symbols present comparative wt plants from the same growth series.
Fig. 5.
Fig. 5.
Dependence of PSI content on leaf N content. The meaning of the symbols is given in Fig. 2. Dashed line indicates the maximum slope of regression.
Fig. 6.
Fig. 6.
Average in planta catalytic turnover rate, kcat, of a Rubisco site as a function of PSI per Rubisco site (A) or as a function of Rubisco sites per PSI (B). The meaning of the symbols is given in Figs 2 and 3. The lines were calculated from equation (4), in (A) and equation (5) in (B).
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