title: Faster Rubisco is the key to superior nitrogen-use efficiency in NADP-malic enzyme relative to NAD-malic enzyme C4 grasses.
abstract: 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 (k(cat); 3.8 versus 5.7 s(-1) at 25 degrees 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 k(cat).
authors.affiliation: Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia. ghannoum@rsbs.anu.edu.au
authors.forename: Oula
authors.initials: O
authors.lastname: Ghannoum
authors.forename: John R
authors.initials: JR
authors.lastname: Evans
authors.forename: Wah Soon
authors.initials: WS
authors.lastname: Chow
authors.forename: T John
authors.initials: TJ
authors.lastname: Andrews
authors.forename: Jann P
authors.initials: JP
authors.lastname: Conroy
authors.forename: Susanne
authors.initials: S
authors.lastname: von Caemmerer
mesh_terms: D000222:Adaptation, Physiological; D000596:Amino Acids; D002734:Chlorophyll; D000077194:Chlorophyll A; D045348:Cytochromes f; D008027:Light; D008291:Malate Dehydrogenase / Q000378:metabolism*; D050538:Malate Dehydrogenase (NADP+); D009566:Nitrates; D009584:Nitrogen / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D018515:Plant Leaves / Q000201:enzymology; D006109:Poaceae / Q000201:enzymology*; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D020524:Thylakoids / Q000378:metabolism
chemical_list: D000596:Amino Acids; D009566:Nitrates; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll; D045348:Cytochromes f; D008291:Malate Dehydrogenase; D050538:Malate Dehydrogenase (NADP+); D012273:Ribulose-Bisphosphate Carboxylase; D009584:Nitrogen; D000077194:Chlorophyll A