NADP-malic Enzyme Research Articles

Light-Dependent Reactions of Photosynthesis in Mesophyll and Bundle Sheath Chloroplasts of C4 Plant Maize. How Our Views Have Changed in Recent Years

Abstract Plants experience a range of light intensities and qualities in their environment. Leaves are subjected to spatial and temporal gradients in incident light, which has major consequences in the photosynthetic carbon assimilation. Plants acclimate to light by developing a range of mechanisms, from adjustments in leaf morphology to changes in the photosynthetic apparatus stoichiometry. In C4 plants, light intensity is a major limiting factor for photosynthesis at optimum temperatures. Under limiting light, it is not clear if all of factors (e.g., temperature, mineral nutrition, water supply) are co-limiting or if there is one primary limitation. Differences in light quality and intensity have a profound impact on C4 photosynthesis, where pathways require metabolic coordination of the mesophyll and bundle sheath cycles. Changes in the linear versus cyclic electron flux in maize (NADP-malic enzyme C4 subtype) in the mesophyll and bundle sheath chloroplasts in response to light may lead to an imbalance in the coordination of the C3 and C4 pathways. Additionally, the rearrangement of the thylakoid complexes of both types of chloroplasts in maize optimizes the light energy distribution between the mesophyll and bundle sheath cells and may also participate in energy dissipation. This review aims to highlight the changes in the understanding of the functions of photosystem II in maize bundle sheath chloroplasts and the role of super and megacomplexes in the thylakoids.

13CO2 labeling kinetics in maize reveal impaired efficiency of C4 photosynthesis under low irradiance.

C4 photosynthesis allows faster photosynthetic rates and higher water and nitrogen use efficiency than C3 photosynthesis, but at the cost of lower quantum yield due to the energy requirement of its biochemical carbon concentration mechanism. It has also been suspected that its operation may be impaired in low irradiance. To investigate fluxes under moderate and low irradiance, maize (Zea mays) was grown at 550 µmol photons m-2 s-l and 13CO2 pulse-labeling was performed at growth irradiance or several hours after transfer to 160 µmol photons m-2 s-1. Analysis by liquid chromatography/tandem mass spectrometry or gas chromatography/mass spectrometry provided information about pool size and labeling kinetics for 32 metabolites and allowed estimation of flux at many steps in C4 photosynthesis. The results highlighted several sources of inefficiency in low light. These included excess flux at phosphoenolpyruvate carboxylase, restriction of decarboxylation by NADP-malic enzyme, and a shift to increased CO2 incorporation into aspartate, less effective use of metabolite pools to drive intercellular shuttles, and higher relative and absolute rates of photorespiration. The latter provides evidence for a lower bundle sheath CO2 concentration in low irradiance, implying that operation of the CO2 concentration mechanism is impaired in this condition. The analyses also revealed rapid exchange of carbon between the Calvin-Benson cycle and the CO2-concentration shuttle, which allows rapid adjustment of the balance between CO2 concentration and assimilation, and accumulation of large amounts of photorespiratory intermediates in low light that provides a major carbon reservoir to build up C4 metabolite pools when irradiance increases.

Differential Impact of Nitric Oxide and Abscisic Acid on the Cellular and Physiological Functioning of sub1A QTL Bearing Rice Genotype under Salt Stress.

Hydroponic culture containing 200 mM NaCl was used to induce oxidative stress in seedlings of cultivars initially primed with 1 mM SNP and 10 µM ABA. Exogenous application of sodium nitroprusside (SNP – a nitric oxide donor) and abscisic acid (ABA) was well sensitized more in cv. Swarna Sub1 than cv. Swarna and also reflected in different cellular responses. The major effects of salinity, irrespective of the cultivar, were lowering the water relation, including relative water content and osmotic potential, and decreasing the compatible solutes like alanine, gamma-aminobutyric acid, and glycine betaine. The accumulated polyamines were reduced more in cv. Swarna with a concomitant decrease in photosynthetic reserves. NADP-malic enzyme activity, sucrose accumulation, ascorbate peroxidase, and glutathione S-transferase activities gradually declined under NaCl stress and the catabolizing enzymes like invertase (both wall and cytosolic forms) also declined. On the contrary, plants suffered from oxidative stress through superoxide, hydrogen peroxide, and their biosynthetic enzymes like NADP(H) oxidase. Moderation of Na+/K+ by both SNP and ABA were correlated with other salt sensitivities in the plants. The maximum effects of SNP and ABA were found in the recovery of antioxidation pathways, osmotic tolerance, and carbohydrate metabolism. Findings predict the efficacy of SNP and ABA either independently or cumulatively in overcoming NaCl toxicity in rice.

The receptor-like cytosolic kinase RIPK activates NADP-malic enzyme 2 to generate NADPH for fueling ROS production

Reactive oxygen species (ROS) production is a conserved immune response in Arabidopsis primarily mediated by respiratory burst oxidase homolog D (RBOHD), a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase associated with the plasma membrane. A rapid increase in NADPH is necessary to fuel RBOHD proteins and thus maintain ROS production. However, the molecular mechanism by which NADPH is generated to fuel RBOHD remains unclear. In this study, we isolated a new mutant allele of FLAGELLIN-INSENSITIVE 4 (FIN4), which encodes the first enzyme in de novo NAD biosynthesis. fin4 mutants show reduced NADPH levels and impaired ROS production. However, FIN4 and other genes involved in NAD- and NADPH-generating pathways are not highly upregulated upon elicitor treatment, raising a possibility that a cytosolic NADP-linked dehydrogenase might be post-transcriptionally activated to maintain the NADPH supply close to RBOHD. To verify this possibility, we isolated the proteins associated with RPM1-INDUCED PROTEIN KINASE (RIPK), a receptor-like cytoplasmic kinase that regulates broad-spectrum ROS signaling in plant immunity, and identified NADP-malic enzyme 2 (NADP-ME2), an NADPH-generating enzyme. Compared with wild-type plants, nadp-me2 mutants display decreased NADP-ME activity, lower NADPH levels, and reduced ROS production in response to immune elicitors. Furthermore, we found that RIPK can directly phosphorylate NADP-ME2 and enhance its activity in vitro. The phosphorylation of the NADP-ME2 S371 residue contributes to ROS production upon immune elicitor treatment and susceptibility to the necrotrophic bacterium Pectobacterium carotovorum. Collectively, our study suggests that RIPK phosphorylates and activates NADP-ME2 to rapidly increase cytosolic NADPH, thus fueling RBOHD to sustain ROS production in plant immunity.

Exogenous trehalose differently improves photosynthetic carbon assimilation capacities in maize and wheat under heat stress

ABSTRACT High temperature inhibits the growth and development of maize (Zea mays L.) and wheat (Triticum aestivum L.) seedlings. The current study attempts to elucidate the underlying mechanisms of exogenous trehalose for the improvement of maize and wheat responses to heat stress. Results show that, in maize seedlings, exogenously-supplied trehalose was able to effectively elevate the capacity of photosynthetic carbon assimilation and improve the carbohydrate content via up-regulating the transcript levels of key enzymes including phosphoenol pyruvate carboxylase (PEPC), NADP-malate dehydrogenase (NADP-MDH), NADP-malic enzyme (NADP-ME), pyruvate phosphate dikinase (PPDK) and Ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit. Besides, it was also able to improve C4 pathway enzyme activities, including PEPC, NADP-MDH, NADP-ME and Ribulose-1,5-bisphosphate carboxylase (RUBPCase). In wheat seedlings, exogenously-supplied trehalose promoted photosynthetic carbon assimilation by elevating the activity of RUBPCase. These results show that exogenous trehalose improved photosynthetic carbon assimilation capacities in maize and wheat seedlings under heat stress through different mechanisms.

Two major metabolic factors for an efficient NADP-malic enzymetype C4 photosynthesis.

Compared to the large number of studies focused on the factors controlling C3 photosynthesis efficiency, there are relatively fewer studies of the factors controlling photosynthetic efficiency in C4 leaves. Here, we used a dynamic systems model of C4 photosynthesis based on maize (Zea mays) to identify features associated with high photosynthetic efficiency in NADP-malic enzyme (NADP-ME) type C4 photosynthesis. We found that two additional factors related to coordination between C4 shuttle metabolism and C3 metabolism are required for efficient C4 photosynthesis: (1) accumulating a high concentration of phosphoenolpyruvate through maintaining a large PGA concentration in the mesophyll cell chloroplast and (2) maintaining a suitable oxidized status in bundle sheath cell chloroplasts. These identified mechanisms are in line with the current cellular location of enzymes/proteins involved in the starch synthesis, the Calvin–Benson cycle and photosystem II of NADP-ME type C4 photosynthesis. These findings suggested potential strategies for improving C4 photosynthesis and engineering C4 rice.

Photosynthetic nitrogen- and water-use efficiencies in C3 and C4 subtype grasses grown under two nitrogen supply levels

ABSTRACT C4 plants have higher photosynthetic nitrogen- and water-use efficiencies (PNUE and PWUE) than C3 plants. Some studies on C4 grasses have reported that NADP-malic enzyme (NADP-ME) grasses have higher PNUE than NAD-malic enzyme (NAD-ME) grasses, whereas other studies have suggested that this difference is caused by the traits of phylogenetic lineage. Further extensive studies are required to properly understand the PNUE and PWUE of C4 grasses. We investigated the PNUE and PWUE for 22 grass species of four photosynthetic types [4 NADP-ME, 6 NAD-ME, 6 phosphoenolpyruvate carboxykinase (PCK), and 6 C3 species] grown under two N supply levels. Under the same N level, the three C4 types showed higher photosynthetic rate and lower stomatal conductance and transpiration rate than the C3 type, but these parameters were similar in the C4 types. Although there were no significant differences in leaf mass per area and N content per leaf area among the four types, these traits tended to be somewhat lower in the NADP-ME type than in other types. As a result, PNUE was highest in the NADP-ME type, lowest in the C3 type, and intermediate in the NAD-ME and PCK types. PWUE was much higher in the C4 types than in the C3 type but similar in the C4 types. The N supply level did not greatly affect the trends of PNUE and PWUE. This study suggests that the NAD-ME and PCK grasses have the N-related physiological traits similar to each other but differing from the NADP-ME grasses.

Metabolic flux analysis of the non-transitory starch tradeoff for lipid production in mature tobacco leaves

The metabolic plasticity of tobacco leaves has been demonstrated via the generation of transgenic plants that can accumulate over 30% dry weight as triacylglycerols. In investigating the changes in carbon partitioning in these high lipid-producing (HLP) leaves, foliar lipids accumulated stepwise over development. Interestingly, non-transient starch was observed to accumulate with plant age in WT but not HLP leaves, with a drop in foliar starch concurrent with an increase in lipid content. The metabolic carbon tradeoff between starch and lipid was studied using 13CO2-labeling experiments and isotopically nonstationary metabolic flux analysis, not previously applied to the mature leaves of a crop. Fatty acid synthesis was investigated through assessment of acyl-acyl carrier proteins using a recently derived quantification method that was extended to accommodate isotopic labeling. Analysis of labeling patterns and flux modeling indicated the continued production of unlabeled starch, sucrose cycling, and a significant contribution of NADP-malic enzyme to plastidic pyruvate production for the production of lipids in HLP leaves, with the latter verified by enzyme activity assays. The results suggest an inherent capacity for a developmentally regulated carbon sink in tobacco leaves and may in part explain the uniquely successful leaf lipid engineering efforts in this crop.

Corrigendum to: Posttranslational Modification of the NADP-Malic Enzyme Involved in C4 Photosynthesis Modulates the Enzymatic Activity during the Day.

Abstract Plant Cell (2019) 29: 2525-2539 doi: 10.1105/tpc.19.00406

Selenomethionine regulates the sugar-acid ratio of strawberry fruit by modulating the activities of related enzymes

ABSTRACT To study the effects of exogenous organic selenium on the sugar-acid ratio (SAR) of strawberry (Fragaria x ananassa) fruit, we investigated the effects of different concentrations of selenomethionine (SeMet) on the activities of key enzymes in sugar and organic acid metabolism in fruit during Large fruit period (LP, 20 d after flowering), Colour changed period (CP, 24 d after flowering) and Mature period (MP, 28 d after flowering) of fruit. The results showed that SeMet increased the contents of total sugars (TS), sucrose (Suc), glucose (Glu) and fructose (Fru) by improving the activities of acid invertase (AI), sucrose synthase (SS) and sucrose phosphate synthase (SPS), and reducing hexokinase (HK) activity. Meanwhile, SeMet markedly decreased the contents of total acids (TA), citric acid (CA) and malic acid (MA) by reducing the activities of NAD-malic dehydrogenase (NAD-MDH), NADP-malic enzyme (NADP-ME) and citrate synthase (CS). By this way, SeMet improved the SAR of fruit. Among different concentrations, 60 mg L−1 SeMet was the best concentration in improving the value of SAR in ripe fruit and could be used in the production of ‘Sweet Charlie’ strawberry to improve fruit SAR.

Chloroplastic SaNADP-ME4 of C3-C4 Woody Desert Species Salsola laricifolia Confers Drought and Salt Stress Resistance to Arabidopsis.

The NADP-malic enzyme (NADP-ME) catalyzes the reversible decarboxylation of L-malate to produce pyruvate, CO2, and NADPH in the presence of a bivalent cation. In addition, this enzyme plays crucial roles in plant developmental and environment responses, especially for the plastidic isoform. However, this isoform is less studied in C3–C4 intermediate species under drought and salt stresses than in C3 and C4 species. In the present study, we characterized SaNADP-ME4 from the intermediate woody desert species Salsola laricifolia. SaNADP-ME4 encoded a protein of 646 amino acids, which was found to be located in the chloroplasts based on confocal imaging. Quantitative real-time PCR analysis showed that SaNADP-ME4 was highly expressed in leaves, followed by stems and roots, and SaNADP-ME4 expression was improved and reached its maximum under the 200 mm mannitol and 100 mm NaCl treatments, respectively. Arabidopsis overexpressing SaNADP-ME4 showed increased root length and fresh weight under mannitol and salt stress conditions at the seedling stage. In the adult stage, SaNADP-ME4 could alleviate the decreased in chlorophyll contents and PSII photochemical efficiency, as well as improve the expression of superoxide dismutase, peroxidase, and pyrroline-5-carboxylate synthase genes to enhance reactive oxygen species scavenging capability and proline levels. Our results suggest that SaNADP-ME4 overexpression in Arabidopsis increases drought and salt stress resistance.

The C4 cycle and beyond: diverse metabolic adaptations accompany dual-cell photosynthetic functions in Setaria

C4 photosynthesis is typically characterized by the spatial compartmentalization of the photosynthetic reactions into mesophyll (M) and bundle sheath (BS) cells. Initial carbon fixation within M cells gives rise to C4 acids, which are transported to the BS cells. There, C4 acids are decarboxylated so that the resulting CO2 is incorporated into the Calvin cycle. This work is focused on the study of Setaria viridis, a C4 model plant, closely related to several major feed and bioenergy grasses. First, we performed the heterologous expression and biochemical characterization of Setaria isoforms for chloroplastic NADP-malic enzyme (NADP-ME) and mitochondrial NAD-malic enzyme (NAD-ME). The kinetic parameters obtained agree with a major role for NADP-ME in the decarboxylation of the C4 acid malate in the chloroplasts of BS cells. In addition, mitochondria-located NAD-ME showed regulatory properties that could be important in the context of the operation of the C4 carbon shuttle. Secondly, we compared the proteomes of M and BS compartments and found 825 differentially accumulated proteins that could support different metabolic scenarios. Most interestingly, we found evidence of metabolic strategies to insulate the C4 core avoiding the leakage of intermediates by either up-regulation or down-regulation of chloroplastic, mitochondrial, and peroxisomal proteins. Overall, the results presented in this work provide novel data concerning the complexity of C4 metabolism, uncovering future lines of research that will undoubtedly contribute to the expansion of knowledge on this topic.

Structural insights into the allosteric site of Arabidopsis NADP-malic enzyme 2: role of the second sphere residues in the regulatory signal transmission.

NADP-ME2 from Arabidopsis thaliana exhibits a distinctive and complex regulation by fumarate, acting as an activator or an inhibitor according to substrate and effector concentrations. In this work, we used molecular modeling approach and site-directed mutagenesis to characterized the NADP-ME2 structural determinants necessary for allosteric regulation providing new insights for enzyme optimization. Structure-function studies contribute to deciphering how small modifications in the primary structure could introduce desirable characteristics into enzymes without affecting its overall functioning. Malic enzymes (ME) are ubiquitous and responsible for a wide variety of functions. The availability of a high number of ME crystal structures from different species facilitates comparisons between sequence and structure. Specifically, the structural determinants necessary for fumarate allosteric regulation of ME has been of particular interest. NADP-ME2 from Arabidopsis thaliana exhibits a distinctive and complex regulation by fumarate, acting as an activator or an inhibitor according to substrate and effector concentrations. However, the 3D structure for this enzyme is not yet reported. In this work, we characterized the NADP-ME2 allosteric site by structural modeling, molecular docking, normal mode analysis and mutagenesis. The regulatory site model and its docking analysis suggested that other C4 acids including malate, NADP-ME2 substrate, could also fit into fumarate's pocket. Besides, a non-conserved cluster of hydrophobic residues in the second sphere of the allosteric site was identified. The substitution of one of those residues, L62, by a less flexible residue as tryptophan, resulted in a complete loss of fumarate activation and a reduction of substrate affinities for the active site. In addition, normal mode analysis indicated that conformational changes leading to the activation could originate in the region surrounding L62, extending through the allosteric site till the active site. Finally, the results in this work contribute to the understanding of structural determinants necessary for allosteric regulation providing new insights for enzyme optimization.

Efficient Regulation of CO2 Assimilation Enables Greater Resilience to High Temperature and Drought in Maize.

Increasing temperatures and extended drought episodes are among the major constraints affecting food production. Maize has a relatively high temperature optimum for photosynthesis compared to C3 crops, however, the response of this important C4 crop to the combination of heat and drought stress is poorly understood. Here, we hypothesized that resilience to high temperature combined with water deficit (WD) would require efficient regulation of the photosynthetic traits of maize, including the C4–CO2 concentrating mechanism (CCM). Two genotypes of maize with contrasting levels of drought and heat tolerance, B73 and P0023, were acclimatized at high temperature (38°C versus 25°C) under well-watered (WW) or WD conditions. The photosynthetic performance was evaluated by gas exchange and chlorophyll a fluorescence, and in vitro activities of key enzymes for carboxylation (phosphoenolpyruvate carboxylase), decarboxylation (NADP-malic enzyme), and carbon fixation (Rubisco). Both genotypes successfully acclimatized to the high temperature, although with different mechanisms: while B73 maintained the photosynthetic rates by increasing stomatal conductance (gs), P0023 maintained gs and showed limited transpiration. When WD was experienced in combination with high temperatures, limited transpiration allowed water-savings and acted as a drought stress avoidance mechanism. The photosynthetic efficiency in P0023 was sustained by higher phosphorylated PEPC and electron transport rate (ETR) near vascular tissues, supplying chemical energy for an effective CCM. These results suggest that the key traits for drought and heat tolerance in maize are limited transpiration rate, allied with a synchronized regulation of the carbon assimilation metabolism. These findings can be exploited in future breeding efforts aimed at improving maize resilience to climate change.

Improving C4 photosynthesis to increase productivity under optimal and suboptimal conditions.

Although improving photosynthetic efficiency is widely recognized as an underutilized strategy to increase crop yields, research in this area is strongly biased towards species with C3 photosynthesis relative to C4 species. Here, we outline potential strategies for improving C4 photosynthesis to increase yields in crops by reviewing the major bottlenecks limiting the C4 NADP-malic enzyme pathway under optimal and suboptimal conditions. Recent experimental results demonstrate that steady-state C4 photosynthesis under non-stressed conditions can be enhanced by increasing Rubisco content or electron transport capacity, both of which may also stimulate CO2 assimilation at supraoptimal temperatures. Several additional putative bottlenecks for photosynthetic performance under drought, heat, or chilling stress or during photosynthetic induction await further experimental verification. Based on source-sink interactions in maize, sugarcane, and sorghum, alleviating these photosynthetic bottlenecks during establishment and growth of the harvestable parts are likely to improve yield. The expected benefits are also shown to be augmented by the increasing trend in planting density, which increases the impact of photosynthetic source limitation on crop yields.

C4 Photosynthetic Enzymes Engaged in the Habituation of Allium cepa L. Seedlings to Drought Stress

Drought is one of the major stress which affects agricultural crop productivity around the world. In this study the role of Phos phoenol pyruvate carboxylase (PEPC, EC 4.1.1.31), NADP-malic enzyme (NADP-ME, EC 1.1.1.40), and pyruvate, phosphate dikinase (PPDK, EC 2.7.9.1) in different onion cultivars (Allium cepa L.) under drought stress was investigated.An elevated specific activity of these three enzymes was monitored in onion cultivars after 8 h of drought stress. With the increasing concentration of drought there was an increase in C4 enzyme activities. PEPC showed 3.5-fold increase after 24 h, NADP-ME 3.9-fold, and PPDK 3.2-fold increase was observed in Arkalalima (AL)onion after 48 h when compared to control. Statistical analysis including regression analysis suggested significant results to onion drought resistant cultivars under drought stress condition. The results revealed that the variety AL exhibited best response followed by Arkakirthaman (AK) and Bellary(BL). Therefore PEPC, PPDK and NADPME participate in the drought response of onion, a C3 Plant. Future research will allow further understanding of the drought stress mechanism of adaptation of the onion germplasm to drought.

Photosynthetic Linear Electron Flow Drives CO2 Assimilation in Maize Leaves.

Photosynthetic organisms commonly develop the strategy to keep the reaction center chlorophyll of photosystem I, P700, oxidized for preventing the generation of reactive oxygen species in excess light conditions. In photosynthesis of C4 plants, CO2 concentration is kept at higher levels around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) by the cooperation of the mesophyll and bundle sheath cells, which enables them to assimilate CO2 at higher rates to survive under drought stress. However, the regulatory mechanism of photosynthetic electron transport for P700 oxidation is still poorly understood in C4 plants. Here, we assessed gas exchange, chlorophyll fluorescence, electrochromic shift, and near infrared absorbance in intact leaves of maize (a NADP-malic enzyme C4 subtype species) in comparison with mustard, a C3 plant. Instead of the alternative electron sink due to photorespiration in the C3 plant, photosynthetic linear electron flow was strongly suppressed between photosystems I and II, dependent on the difference of proton concentration across the thylakoid membrane (ΔpH) in response to the suppression of CO2 assimilation in maize. Linear relationships among CO2 assimilation rate, linear electron flow, P700 oxidation, ΔpH, and the oxidation rate of ferredoxin suggested that the increase of ΔpH for P700 oxidation was caused by the regulation of proton conductance of chloroplast ATP synthase but not by promoting cyclic electron flow. At the scale of intact leaves, the ratio of PSI to PSII was estimated almost 1:1 in both C3 and C4 plants. Overall, the photosynthetic electron transport was regulated for P700 oxidation in maize through the same strategies as in C3 plants only except for the capacity of photorespiration despite the structural and metabolic differences in photosynthesis between C3 and C4 plants.

Dynamic changes of genome sizes and gradual gain of cell-specific distribution of C4 enzymes during C4 evolution in genus Flaveria.

C4 plants are believed to have evolved from C3 plants through various C3 -C4 intermediate stages in which a photorespiration-dependent CO2 concentration system known as C2 photosynthesis operates. Genes involved in the C4 cycle were thought to be recruited from orthologs present in C3 species and developed cell-specific expression during C4 evolution. To understand the process of establishing C4 photosynthesis, we performed whole-genome sequencing and investigated expression and mesophyll- or bundle-sheath-cell-specific localization of phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), pyruvate, orthophosphate dikinase (PPDK) in C3 , C3 -C4 intermediate, C4 -like, and C4 Flaveria species. While genome sizes vary greatly, the number of predicted protein-coding genes was similar among C3 , C3 -C4 intermediate, C4 -like, and C4 Flaveria species. Cell-specific localization of the PEPC, NADP-ME, and PPDK transcripts was insignificant or weak in C3 -C4 intermediate species, whereas these transcripts were expressed cell-type specific in C4 -like species. These results showed that elevation of gene expression and cell-specific control of pre-existing C4 cycle genes in C3 species was involved in C4 evolution. Gene expression was gradually enhanced during C4 evolution, whereas cell-specific control was gained independently of quantitative transcriptional activation during evolution from C3 -C4 intermediate to C4 photosynthesis in genus Flaveria.

Adaptation responses in C4 photosynthesis of sweet maize (Zea mays L.) exposed to nicosulfuron

Nicosulfuron is an ingredient in photosynthesis-inhibiting herbicides and has been widely used in corn post-emergence weed control. In the current study, a pair of sister lines, HK301 (nicosulfuron-tolerence, NT) and HK320 (nicosulfuron-sensitive, NS), was used to study the effect of nicosulfuron in sweet maize seedlings on C4 photosynthetic enzymes and non-enzymatic substances, expression levels of key enzymes, and chloroplast structure. Nicosulfuron was sprayed at the four-leaf stage, and water was sprayed as a control. After nicosulfuron treatment, phosphoenolpyruvate carboxylase (PEPC), NADP-malic dehydrogenase (NADP-MDH), NADP-malic enzyme (NADP-ME), pyruvate orthophosphate dikinase (PPDK), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities of NT were significantly higher than those of NS. Compared to NT, malate, oxaloacetic acid, and pyruvic acid significantly decreased as exposure time increased in NS. Compared to NS, nicosulfuron treatment significantly increased the expression levels of PEPC, NADP-MDH, NADP-ME, PPDK, and Rubisco genes in NT. Under nicosulfuron treatment, chloroplast ultrastructure of NS, compared to that of NT, nicosulfuron induced swelling of the chloroplast volume and reduced starch granules in NS. In general, our results indicate that in different resistant sweet maize, C4 photosynthetic enzymes activity and key genes expression play a critical role in enhancing the adaptability of plants to nicosulfuron stress at a photosynthetic physiological level.

Potential importance of malate diffusion in the response of maize photosynthesis to heterogeneous light

It is well known that the photosynthetic performance of a leaf is highly dependent on the systemic regulation from distal parts within a plant under light heterogeneity. However, there are few studies focusing on C4-specific processes. In the present study, two cultivars of maize (Zea mays L.), ‘Rongyu 1210’ (RY) and ‘Zhongdan 808’ (ZD), were treated with heterogeneous light (HL). The net photosynthetic rate (Pn) of newly developed leaves was found to increase in HL-treated RY, while it decreased in HL-treated ZD. Result also showed a negative correlation between the Pn and the content of malate, a key metabolite in C4 photosynthesis, in these two cultivars. In HL-treated ZD, malate content increased with a decline in the abundance of NADP-malic enzyme (EC 1.1.1.40), suggesting that less malate was decarboxylated. Moreover, a restriction of malate diffusion is proposed in HL-treated ZD, since the interface length between mesophyll cells (MC) and bundle sheath cells (BSC) decreased. In contrast, malate diffusion and subsequent decarboxylation in HL-treated RY should be stimulated, due to an increase in the abundance of NADP-malate dehydrogenase (EC 1.1.1.82) and a decline in the content of malate. In this case, malate diffusion from MC to BSC should be systemically stimulated, thereby facilitating C4 photosynthesis of a maize leaf in heterogeneous light. While if it is systemically restricted, C4 photosynthesis would be suppressed.