fpls-11-00345 April 19, 2020 Time: 12:19 # 1 ORIGINAL RESEARCH published: 21 April 2020 doi: 10.3389/fpls.2020.00345 The Pyruvate-Phosphate Dikinase (C4-SmPPDK) Gene From Suaeda monoica Enhances Photosynthesis, Carbon Assimilation, and Abiotic Stress Tolerance in a C3 Plant Under Elevated CO2 Conditions Sonam Yadav, Mangal Singh Rathore and Avinash Mishra* Division of Applied Phycology and Biotechnology, CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar, India A pyruvate-phosphate dikinase (C4-PPDK) gene was cloned from Suaeda monoica, which had a single-cell C4 photosynthesis pathway without Kranz anatomy and was functionally validated in a C3 model plant under different abiotic stress conditions in an Edited by: ambient and elevated CO2 environment. Overexpression of SmPPDK promoted growth Marouane Baslam, Niigata University, Japan of C3 transgenic plants, enhancing their photosynthesis (CO2 assimilation) by lowering Reviewed by: photorespiration under stress conditions. Transgenic plants also showed an improved Petronia Carillo, physiological status, with higher relative water content (RWC), membrane integrity, University of Campania Luigi Vanvitelli, Italy concentration of glycine betaine, total soluble sugars, free amino acids, polyphenols Parvaiz Ahmad, and antioxidant activity, and lower electrolyte leakage, lipid peroxidation, free radical Sri Pratap College Srinagar, India accumulation, and generation of reactive oxygen species (ROS), compared to control *Correspondence: plants. Moreover, SmPPDK transgenic plants exhibited earlier flowering and higher dry Avinash Mishra [email protected]; biomass compared to controls. These results suggested that the C4-PPDK gene was [email protected]; appropriate for improvement of carbon assimilation, and it also played an important role [email protected] in adaption to salinity and severe drought-induced stress. More intriguingly, an elevated Specialty section: CO2 environment alleviated the adverse effects of abiotic stress, particularly caused by This article was submitted to drought through coordination of osmoprotectants and antioxidant defense systems. The Plant Abiotic Stress, a section of the journal molecular, physiological, metabolic, and biochemical indicators ameliorated the overall Frontiers in Plant Science performance of model C3 plants overexpressing the C4-PPDK gene in an elevated Received: 09 December 2019 CO2 environment, by lowering photorespiration metabolic processes, however, further Accepted: 09 March 2020 studies are needed to confirm its precise role in C plants as protection against future Published: 21 April 2020 3 climate change. Citation: Yadav S, Rathore MS and Keywords: abiotic stress, C4 photosynthesis, carbon sequestration, halophyte, salinity, transgenic Mishra A (2020) The Pyruvate-Phosphate Dikinase (C4-SmPPDK) Gene From Suaeda INTRODUCTION monoica Enhances Photosynthesis, Carbon Assimilation, and Abiotic Global climate changes such as increasing temperature, atmospheric CO2 concentration and Stress Tolerance in a C3 Plant Under Elevated CO2 Conditions. salinization, and altered precipitation are among the primary constraints on crop improvement, Front. Plant Sci. 11:345. including biomass production and yield (Dhir, 2018). In addition, the increasing demands in doi: 10.3389/fpls.2020.00345 feeding a fast growing population also impose pressure to enhance the productivity and quality Frontiers in Plant Science| www.frontiersin.org 1 April 2020| Volume 11| Article 345 fpls-11-00345 April 19, 2020 Time: 12:19 # 2 Yadav et al. C4-SmPPDK Enhances CO2-Assimilation in C3-Plants of current crops under limited resources. Global climate changes end, effectively following the same differentiation as the Kranz result in desertification and salinization, with >6% of global type of anatomy (Dengler and Nelson, 1999; Sheen, 1999). As land area and >30% of irrigated farmland being salt affected such, engineering of a single-cell C4 organization into C3 crops (FAO, 2011). Global CO2 concentrations have increased from might provide an alternative approach to achieving Kranz type −1 a preindustrial level of 285 mmol mol to the current level C4 carbon assimilation without the need to establish a dual- of 384 mmol mol−1, and they have been predicted to reach cell system. 936 mmol mol−1 by the year 2100, with an increase in PPDK (EC 2.7.9.1) is nuclear encoded, abundantly present in global atmospheric temperature of about 6.4◦C(IPCC, 2014). the chloroplasts of mesophyll cells, an important rate-limiting These stresses will likely affect the productivity of plants, enzyme of the C4 pathway (Edwards et al., 1985; Hatch, influencing the sequestration and assimilation of atmospheric 1987), and is regulated by light-dependent gene expression CO2 through photosynthesis. (Sheen and Bogorad, 1987). It catalyzes the interconversion Various studies on elevated CO2 concentrations have shown of pyruvate to phosphoenolpyruvate (PEP), the acceptor of a close relationship between improvements to photosynthesis atmospheric CO2 in C4 plants, and C4 photosynthesis rate and increases in yield and total biomass of plants (Kellner is correlated (r = 0.96) with the activity of PPDK (Edwards et al., 2019; Pastore et al., 2019). Therefore, this provides et al., 1985). Its structural and functional properties have been an attractive approach to enhancing yield by increasing studied in the bacterium Clostridium symbiosum (Chastain photosynthesis of a particular crop species (Zhang et al., 2014; et al., 2011). The phylogenetic distribution of PPDK is Yadav et al., 2018). Among the three categories of photosynthesis confined to selected groups of bacteria, archaea, protozoa, lower in carbon-assimilating plants, productivity and biomass of fungi, and green plants (Liapounova et al., 2006; Slamovits C4 plants are not limited by atmospheric CO2; therefore, and Keeling, 2006). C4 PPDK is highly homologous to C3 they are considered the most efficient CO2 sequestrators. C4 PPDK (Agarie et al., 1997), with the regions and residues photosynthesis inhibits the oxygenation reaction and acts as involved in catalysis also being highly conserved among a CO2 pump, increasing the intracellular CO2/O2 ratio at bacteria (Fißlthaler et al., 1995). PPDK can also contribute to the active site of ribulose bisphosphate carboxylase/oxygenase the conversion of NADH to NADPH, which is an essential (Rubisco) (Hatch, 1987; Sage, 2004). It has been well documented coenzyme of antioxidant enzymes, and is also involved in that C3 plants lose about 50% of total available CO2 to nitrogen assimilation and synthesis of fatty acids and osmotically photorespiration compared to C4 plants. In addition, C4 active compounds (Doubnerová and Ryšlavá, 2011). PPDK photosynthesis has an advantage over that of C3 plants, exhibiting is critically involved during early seedling growth and seed a higher assimilation efficiency and yield of biomass under development in both C3 and C4 plants (Chastain et al., different abiotic stress conditions, including drought, high 2006; Yu et al., 2016; Chang et al., 2017). In Arabidopsis temperature, and high light intensity, by improving water thaliana, PPDK is overexpressed during leaf senescence by and nitrogen use efficiency (Hatch, 1987; Sage and Pearcy, significantly accelerating nitrogen remobilization from leaves 2000). Enzymes involved in the C4 assimilation pathway (Taylor et al., 2010; Wang et al., 2019). PPDK also plays a could also play a significant role in tolerance to abiotic and key role in host stress defense mechanisms, as it is induced biotic stress (Ku et al., 2000; Gu et al., 2013). Enhancing by various abiotic stresses, such as drought, salinity, heavy photosynthesis efficiency in C3 plants using an enzyme from metals, submergence, low oxygen and cold (Doubnerová and the C4 pathway could provide an alternative approach for Ryšlavá, 2011; Hýsková et al., 2014; Wang et al., 2018), and improving productivity, biomass, and yield (Yadav and Mishra, biotic stress, including viral infection (Spoustová et al., 2015). In 2019). Genera Suaeda and Bienertia have broken the dual- nonphotosynthetic parts of the plant, PPDK plays a significant cell paradigm that Kranz anatomy is essential for C4 carbon role in the maintenance of pH and rejuvenation of citric assimilation (Freitag and Stichler, 2002; Voznesenskaya et al., acid intermediates, which are involved in the synthesis of 2002; Akhani et al., 2005). amino acids (Hýsková et al., 2014). PPDK homologs have Suaeda species belong to the family Amaranthaceae and been reported from bacteria and unicellular parasitic protists, have a unique approach to C4 carbon assimilation within a such as Giardia lamblia, Trichomonas vaginalis, and Entamoeba single chlorenchyma cell (Voznesenskaya et al., 2002, 2004), histolytica, however, they have not been found in insects or in which chloroplasts are dimorphic. A single-cell process is vertebrates. As such, PPDK is a potential target for antiparasitic the most common in the nicotinamide adenine dinucleotide and antimicrobial drugs, as well as C4-specific herbicides phosphate-dependent malic enzyme (NADP-ME) subtype of C4 (Minges and Groth, 2017). photosynthesis species, where the chloroplasts of mesophyll cells The advantages of a C4-based physiology are observed during have well-developed grana, while the chloroplasts of bundle stress triggered by high temperatures, drought, salinity, and sheath lack grana. However, single-cell C4 photosynthesis is not elevated CO2 levels.