Carbon partitioning in soybean (Glycine max) leaves by combined ¹¹C and ¹³C labeling

Abstract

SummaryWe labeled soybean (Glycine max) leaves with 200 and 600 ppm 13 CO 2 spiked with 11 CO 2 and examined the effects of light intensity and water stress on metabolism by using a combination of direct positron imaging and solid-state 13 C nuclear magnetic resonance (NMR) of the same leaf.We first made 60-min movies of the transport of photosynthetically assimilated 11 C labels. The positron imaging identified zones or patches within which variations in metabolism could be probed later by NMR. At the end of each movie, the labeled leaf was frozen in liquid nitrogen to stop metabolism, the leaf was lyophilized, and solid-state NMR was used either on the whole leaf or on various leaf fragments.The NMR analysis determined total 13 C incorporation into sugars, starch, proteins, and protein precursors.The combination of 11 C and 13 C analytical techniques has led to three major conclusions regarding photosynthetically heterogeneous soybean leaves: transient starch deposition is not the temporary storage of sucrose excluded from a saturated sugar-transport system; peptide synthesis is reduced under high-light, high CO 2 conditions; and all glycine from the photorespiratory pathway is routed to proteins within photosynthetically active zones when the leaf is water stressed and under high-light and low CO 2 conditions.

Tracking the metabolic pulse of plant lipid production with isotopic labeling and flux analyses: Past, present and future

“…In most labeling experiments, a portion of the most abundant isotope of an element (e.g., 12 C, 16 35 S, 32 P, 33 P) form. Other radioactive tracers with very short half-lives (e.g., 11 C, half-life 20 min relative to 14 C, half-life 5730 years) can be synthesized just prior to use and assessed by positron emission tomography (PET) analysis [21][22][23]. Such short-lived radioisotope investigations are more specialized and will not be further considered though their application in photosynthesis, long distance transport and other plant processes are becoming more common [22,[24][25][26].…”

Net photosynthetic CO2 assimilation: more than just CO2 and O2 reduction cycles

“…It is worth noting that despite the amino acid build‐up, photorespiration per se does not represent a nitrogen assimilation pathway since it strictly depends on the glutamine synthetase/glutamine‐2‐oxoglutarate amino transferase cycle (Wingler et al ., ). Photorespiratory recycling (or lack thereof) of glycerate, glycine and serine has been controversial for a long time (Harley & Sharkey, ; Dirks et al ., ; Tcherkez, ; Busch et al ., ). The abstraction of serine and glycine molecules from the photorespiratory cycle might be detrimental for photosynthesis because it leads to an extra‐demand in electrons (for nirogen assimilation in glyoxylate amination to glycine) and PGA, potentially impeding RuBP regeneration.…”

Fate of xylem‐transported 11C‐ and 13C‐labeled CO2 in leaves of poplar

“…Previously, 14 C autoradiography has been applied to detached leaves (Stringer and Kimmerer , Hibberd and Quick ), hydroponically grown tree seedlings and cuttings (Zelawski et al , Vapaavuori and Pelkonen , Vuorinen et al ) and herbaceous plants (Amiro and Ewing , Hibberd and Quick ) for studying xylem CO 2 fixation at a detailed spatial resolution. On the other hand, 11 C‐based studies have almost exclusively traced the carbon export from labeled leaves, by applying 11 C as gaseous 11 CO 2 (Minchin and Thorpe , Thorpe et al , Dirks et al ). The higher energy of the β + particle emitted during 11 C decay than that from 14 C decay allows a more efficient in vivo detection of 11 C decay relative to 14 C decay (Minchin and Thorpe ).…”