Research Article

Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field

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Science  04 Jan 2019:
Vol. 363, Issue 6422, eaat9077
DOI: 10.1126/science.aat9077

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  • RE: Where did the carbon go?
    • John R Evans, Professor in Plant Science, Australian National University

    A huge effort went into creating the material for this study which claims the exciting prospect of increasing plant growth by redirecting photorespiration. However, the data presented raise questions about the fate of the glycolate formed. Normally it is assumed that 0.5 CO2 are released in the mitochondria for each oxygenation reaction catalysed by Rubisco in the chloroplast and there is substantial evidence for the value being very close to 0.5. The CO2 compensation point in the absence of non-photorespiratory mitochondrial CO2 release, Gamma* = FO/S, where F is the number of moles of CO2 released per oxygenation, O is the partial pressure of oxygen and S is the specificity factor for Rubisco (Farquhar, von Caemmerer & Berry, 1980, Planta 149, 78-90). If 2 CO2 were released per oxygenation instead of 0.5, then the CO2 compensation point for photosynthesis would increase fourfold. This was not observed (Fig. 5) and mistakenly it was claimed that theory predicts a lower compensation point. The slightly lower compensation point only applies to the case where the 0.5 CO2 normally released in mitochondria during photorespiration, is relocated to within the chloroplast. It does not apply if glycolate is fully converted to 2 CO2. The data presented in Fig.5 are consistent with only 0.5 CO2 being released, which raises the question of what happens to the remaining carbon? As this is not satisfactorily revealed from metabolite pool size measurements, we eagerly await the re...

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    Competing Interests: None declared.
  • Down Regulation of Photorespiration?
    • Richard B. Peterson, Emeritus Scientist, The Connecticut Agricultural Experiment Station
    • Other Contributors:
      • Israel Zelitch, Emeritus Scientist, The Connecticut Agricultural Experiment Station

    We have participated in the discovery of photorespiration (PR) and investigations of glycolic acid metabolism in C3 and C4 plants (PR is repressed in the latter). Thus we were pleased to learn of the innovative approach by South et al to reduce PR and thereby increase CO2 assimilation in a C3 plant.that resulted in a 40% increase in dry weight in field experiments. However we do not believe the paper shows (Fig. 5) that PR was reduced in the transgenic plants to the extent necessary to produce such a dramatic stimulation of yield. Key to this reservation is the lack of evidence for reduced inhibition of photosynthesis by atmospheric O2 which is essential for P-glycolate biosynthesis. Measurements of photosynthesis at varying levels of O2 can provide unequivocal quantitative information on the diversion of photosynthetic energy to photorespiration and thereby establish whether contrasting properties in wild type and transgenic plants are attributable to altered PR. We therefore believe the yield increases may be better explained by other factors that were changed besides PR, possibly by somaclonal variation that often occurs when plants are regenerated from somatic cells.

    Competing Interests: None declared.
  • Improving photosynthetic efficiency using non-synthetic pathways?

    South et al. provide robust evidence of the benefits of synthetic biology in improving photosynthetic efficiency as an approach to increasing agricultural yields to meet the challenge of global food security. While outlining their rationale for the introduction of a synthetic glycolate metabolic pathway, they note that some algae, bacteria and plants do indeed have natural mechanisms and metabolic pathways to promote photosynthetic efficiency. The addition of functional cyanobacterial (blue-green algae) components into plant chloroplasts has been proposed as a mechanism for improving photosynthetic efficiency including through C3 RuBisCO suppression (1).

    In the light of the substantial interest generated by the reported improvements in biomass yield and photosynthetic efficiency achieved through the application of synthetic glycolyate metabolism pathways, we feel that there is some utility in presenting our preliminary data that could possibly support the hypothesis that similar improvements in both parameters can be obtained without the application of "genetic engineering".

    We recently conducted a series of dose-finding, controlled, small-scale observational trials on the potential of purified, organic phycocyanin-rich extract of Spirulina (a blue-green algae or cyanobacterium) as an agricultural biostimulant. Our initial results based on a dosing regimen of 1-5mg per plant per day undertaken on lettuce, basil and medical cannabis (indoors) and...

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    Competing Interests: I hold equity in Back of the Yards Algae Sciences LLC.
  • 40% increased growth with genetic engineering
    • Tony Fischer, Honorary Fellow, CSIRO Food andAgriculture, Australia
    • Other Contributors:
      • Richard Richards, Chief Scientist, CSIRO Food and Agriculture, Australia
      • Victor Sadras, Principal Scientist, South Australia Research & Development Institute, Australia

    This paper convincingly describes elegant and complex genetic engineering (GE) of tobacco to enhance leaf photosynthesis, an important target to improve crop productivity. Unlike many other claims in high impact journals of crop yield-boosting GE events, this paper also evaluated the material in field plots under supposed crop-like conditions, the critical step for us as crop scientists. However on careful examination of the methodology, we suggest that the reported 40% increase in biomass production is strongly biased upwards.

    Referring to the key replicated field experiments in 2016 and 2017, the central 16 GE plants in each plot which were sampled for biomass measurement were bordered by a single row of wild type (WT) plants; the WT plots had a similar arrangement. The former creates two biases due to interrow competition between GE and WT plants: (1) the GE plants had greater early growth and would have outcompeted the WT ones for light in the common interrow, and (2) the GE plants were also taller and this would have boosted their light interception further as plants formed canopies. The emergence of hierarchies in plant stands resulting from this kind of plant-plant interactions are well established (Harper 1977). Since the harvested plots were only 1.2 x 1.2 m2, the extra intercepted light could easily have explained the 40% extra final biomass of the best GE line, even when the photosynthetic advantage at the individual leaf level (and the expected advant...

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    Competing Interests: None declared.
  • RE: Increased photosynthesis?

    A change like this has catastrophic repercussions if the genetic code were to escape into wild plants. No one is considering 40% enhanced superweeds? If this were to escape into the wild, would it lead to a 40% increase of oxygen in the atmosphere? The Earth would burn. Perhaps that is why nature did not settle on this photosynthetic pathway.

    Competing Interests: None declared.

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