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BIOENERGETICS AND PHOTOSYNTHESIS

Cool C₄ Photosynthesis: Pyruvate P_i Dikinase Expression and Activity Corresponds to the Exceptional Cold Tolerance of Carbon Assimilation in Miscanthus x giganteus^1,2,[W],[OA]

Institute for Genomic Biology (D.W., S.P.L.), Department of Plant Biology (D.W., A.R.P., S.P.L.), and Department of Crop Sciences (A.R.P., S.P.M., S.P.L.), University of Illinois, Urbana, Illinois 61801; and United States Department of Agriculture, Photosynthesis Research Unit, Agricultural Research Service, Urbana Illinois 61801 (A.R.P.)

The bioenergy feedstock grass Miscanthus x giganteus is exceptional among C₄ species for its high productivity in cold climates. It can maintain photosynthetically active leaves at temperatures 6°C below the minimum for maize (Zea mays), which allows it a longer growing season in cool climates. Understanding the basis for this difference between these two closely related plants may be critical in adapting maize to colder weather. When M. x giganteus and maize grown at 25°C were transferred to 14°C, light-saturated CO₂ assimilation and quantum yield of photosystem II declined by 30% and 40%, respectively, in the first 48 h in these two species. The decline continued in maize but arrested and then recovered partially in M. x giganteus. Within 24 h of the temperature transition, the pyruvate phosphate dikinase (PPDK) protein content per leaf area transiently declined in M. x giganteus but then steadily increased, such that after 7 d the enzyme content was significantly higher than in leaves growing in 25°C. By contrast it declined throughout the chilling period in maize leaves. Rubisco levels remained constant in M. x giganteus but declined in maize. Consistent with increased PPDK protein content, the extractable PPDK activity per unit leaf area (V_max,ppdk) in cold-grown M. x giganteus leaves was higher than in warm-grown leaves, while V_max,ppdk was lower in cold-grown than in warm-grown maize. The rate of light activation of PPDK was also slower in cold-grown maize than M. x giganteus. The energy of activation (E_a) of extracted PPDK was lower in cold-grown than warm-grown M. x giganteus but not in maize. The specific activities and E_a of purified recombinant PPDK from M. x giganteus and maize cloned into Escherichia coli were similar. The increase in PPDK protein in the M. x giganteus leaves corresponded to an increase in PPDK mRNA level. These results indicate that of the two enzymes known to limit C₄ photosynthesis, increase of PPDK, not Rubisco content, corresponds to the recovery and maintenance of photosynthetic capacity. Functionally, increased enzyme concentration is shown to increase stability of M. x giganteus PPDK at low temperature. The results suggest that increases in either PPDK RNA transcription and/or the stability of this RNA are important for the increase in PPDK protein content and activity in M. x giganteus under chilling conditions relative to maize.

² Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Stephen P. Long (stevel{at}life.uiuc.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.120709

^* Corresponding author; e-mail stevel{at}life.uiuc.edu.

Received April 10, 2008; accepted May 28, 2008; published June 6, 2008.

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