Plant Physiology Preview
Published on January 21, 2005; 10.1104/pp.104.054759
Received October 12, 2004
Returned for revision November 25, 2004
Accepted November 29, 2004
Faster Rubisco Is the Key to Superior Nitrogen-Use Efficiency in NADP-Malic Enzyme Relative to NAD-Malic Enzyme C4 Grasses
Oula Ghannoum *, John R. Evans , Wah Soon Chow , T. John Andrews , Jann P. Conroy , and Susanne von Caemmerer
Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
Environmental Biology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
Photobioenergetics Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
Centre for Horticulture and Plant Sciences, University of Western Sydney, Penrith South DC, New South Wales 1797, Australia
* Corresponding author; email: ghannoum{at}rsbs.anu.edu.au.
In 27 C4 grasses grown under adequate or deficient nitrogen (N) supplies, N-use efficiency at the photosynthetic (assimilation rate per unit leaf N) and whole-plant (dry mass per total leaf N) level was greater in NADP-malic enzyme (ME) than NAD-ME species. This was due to lower N content in NADP-ME than NAD-ME leaves because neither assimilation rates nor plant dry mass differed significantly between the two C4 subtypes. Relative to NAD-ME, NADP-ME leaves had greater in vivo (assimilation rate per Rubisco catalytic sites) and in vitro Rubisco turnover rates (kcat; 3.8 versus 5.7 s-1 at 25°C). The two parameters were linearly related. In 2 NAD-ME (Panicum miliaceum and Panicum coloratum) and 2 NADP-ME (Sorghum bicolor and Cenchrus ciliaris) grasses, 30% of leaf N was allocated to thylakoids and 5% to 9% to amino acids and nitrate. Soluble protein represented a smaller fraction of leaf N in NADP-ME (41%) than in NAD-ME (53%) leaves, of which Rubisco accounted for one-seventh. Soluble protein averaged 7 and 10 g (mmol chlorophyll)-1 in NADP-ME and NAD-ME leaves, respectively. The majority (65%) of leaf N and chlorophyll was found in the mesophyll of NADP-ME and bundle sheath of NAD-ME leaves. The mesophyll-bundle sheath distribution of functional thylakoid complexes (photosystems I and II and cytochrome f) varied among species, with a tendency to be mostly located in the mesophyll. In conclusion, superior N-use efficiency of NADP-ME relative to NAD-ME grasses was achieved with less leaf N, soluble protein, and Rubisco having a faster kcat.
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