Analyze and summarize the important findings RESEARCH ARTICLE SUMMARYPLANT SCIENCESynthetic glycolate metabolismpathways stimulate crop growth andproductivity in the fieldPaul F. South, Amanda P. Cavanagh, Helen W. Liu, Donald R. Ort*altered photorespiratory pathways within thechloroplast have shown promising results, in-cluding increased photosynthetic rates andplant size.INTRODUCTION: Meeting food demands forthe growing global human population requiresimproving crop productivity, and large gainsare possible through enhancing photosyntheticefficiency. Photosynthesis requires the carbox-ylation of ribulose-1,5-bisphosphate (RuBP) byribulose-1,5-bisphosphate carboxylase-oxygenase(RuBisCO), but photorespiration occurs in mostplants such as soybean, rice, and wheat (knownas C, crops) when RuBisCO oxygenates RuBPinstead, requiring costly processing of toxicbyproducts such as glycolate. Photorespirationcan reduce C, crop photosynthetic efficiencyby 20 to 50%. Although various strategies existfor lowering the costs of photorespiration,chamber- and greenhouse-grown plants withRATIONALE: To determine if alternative pho-torespiratory pathways could effectively im-prove Cs field crop productivity, we tested theperformance of three alternative photorespi-ratory pathways in field-grown tobacco. Onepathway used five genes from the Escherichiacoli glycolate oxidation pathway; a second path-way used glycolate oxidase and malate syn-thase from plants and catalase from E. coli; andthe third pathway used plant malate synthaseand a green algal glycolate dehydrogenase. AllChloroplastPeroxisomeGlycerateGlycerate HydroxypyruvateGlycolate GlyoxylateGlycineTartronic-semialdehydeCO₂Glyoxylate2-P-Glycolate GlycolateH₂0₂GlyoxylateGlyoxylateNH,MalatePyruvateMitochondriaAlternate pathwaysAlternative photorespiratory pathways in tobacco. Three alternative pathways [1 (red).2 (dark blue), and 3(light blue)] introduced into tobacco chloroplasts for more efficient recyclingof glycolate. RNAi suppresses the native glycolate/glycerate transporter PLGG1 to prevent glycolatefrom leaving the chloroplast and entering the native pathway (gray).3-P-GlycerateRuBisCORuBP10014Acetyl-CoAGlycineSerineCO₂Serineenzymes in the alternative pathway designswere directed to the chloroplast. RNA inter-ference (RNAI) was also used to down-regulatea native chloroplast glycolate transporter inthe photorespiratory pathway, thereby limitingmetabolite flux through the native pathway.The three pathways were introduced withand without the trans-porter RNAi constructinto tobacco, which is anideal model field crop be-cause it is easily trans-formed, has a short lifeON OUR WEBSITERead the full articleat http://dx.doi.org/10.1126/science.aat9077cycle, produces large quan-tities of seed, and develops a robust canopysimilar to that of other field crops.RESULTS: Using a synthetic biology approachto vary promoter gene combinations, we gen-erated a total of 17 construct designs of thethree pathways with and without the trans-porter RNAi construct. Initial screens for pho-toprotection by alternative pathway functionunder high-photorespiratory stress conditionsidentified three to five independent transfor-mants of each design for further analysis. Geneand protein expression analyses confirmed ex-pression of the introduced genes and suppres-sion of the native transporter in RNAi plants.In greenhouse screens, pathway 1 increasedbiomass by nearly 13%. Pathway 2 showed nobenefit compared to wild type. Introduction ofpathway 3 increased biomass by 18% withoutRNAi and 24% with RNAi, which were con-sistent with changes in photorespiratory me-tabolism and higher photosynthetic rates.Ultimately, field testing across two differentgrowing seasons showed significant increasesin biomass of pathway 3 plants with RNAicompared to WT of 20 % in 2016 ( P = 0.04) andby 24% in 2017 (P = 0.018). In addition, thispathway increased the light-use efficiency ofphotosynthesis by 17% in the field.CONCLUSION: Engineering more efficientphotorespiratory pathways into tobacco whileinhibiting the native pathway markedly in-creased both photosynthetic efficiency andvegetative biomass. We are optimistic that sim-ilar gains may be achieved and translated intoincreased yield in C, grain crops because pho-torespiration is common to all C, plants andhigher photosynthetic rates under elevated CO₂,which suppresses photorespiration and in-creases harvestable yield in C, crops.The list of author affiliations is available in the full article online.*Corresponding author. Email: d-ort@illinois.eduThis is an open-access article distributed under theterms of the Creative Commons Attribution license(https://creativecommons.org/licenses/by/4.0/), whichpermits unrestricted use, distribution, and reproduction inany medium, provided the original work is properly cited.Cite this article as P. F. South et al, Science 363, east9077(2019). DOI: 10.1126/science.ast9077

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The necessity to increase agricultural production to meet rising food demand is discussed in the…