Increased α-ketoglutarate links the C3-C4 intermediate state to C4 photosynthesis in the genus Flaveria.

As a complex trait, C4 photosynthesis has multiple independent origins in evolution. Phylogenetic evidence and theoretical analysis suggest that C2 photosynthesis, which is driven by glycine decarboxylation in the bundle sheath cell, may function as a bridge from C3 to C4 photosynthesis. However, the exact molecular mechanism underlying the transition between C2 photosynthesis to C4 photosynthesis remains elusive. Here, we provide evidence suggesting a role of higher α-ketoglutarate (AKG) concentration during this transition. Metabolomic data of 12 Flaveria species, including multiple photosynthetic types, show that AKG concentration initially increased in the C3-C4 intermediate with a further increase in C4 species. Petiole feeding of AKG increases the concentrations of C4-related metabolites in C3-C4 and C4 species but not the activity of C4-related enzymes. Sequence analysis shows that glutamate synthase (Fd-GOGAT), which catalyzes the generation of glutamate using AKG, was under strong positive selection during the evolution of C4 photosynthesis. Simulations with a constraint-based model for C3-C4 intermediate further show that decreasing the activity of Fd-GOGAT facilitated the transition from a C2-dominant to a C4-dominant CO2 concentrating mechanism. All these results provide insight into the mechanistic switch from C3-C4 intermediate to C4 photosynthesis.

Regulatory NADH dehydrogenase-like complex optimizes C4 photosynthetic carbon flow and cellular redox in maize.

C4 plants typically operate a CO2 concentration mechanism from mesophyll (M) cells into bundle sheath (BS) cells. NADH dehydrogenase-like (NDH) complex is enriched in the BS cells of many NADP-malic enzyme (ME) type C4 plants and is more abundant in C4 than in C3 plants, but to what extent it is involved in the CO2 concentration mechanism remains to be experimentally investigated. We created maize and rice mutants deficient in NDH function and then used a combination of transcriptomic, proteomic, and metabolomic approaches for comparative analysis. Considerable decreases in growth, photosynthetic activities, and levels of key photosynthetic proteins were observed in maize but not rice mutants. However, transcript abundance for many cyclic electron transport (CET) and Calvin-Benson cycle components, as well as BS-specific C4 enzymes, was increased in maize mutants. Metabolite analysis of the maize ndh mutants revealed an increased NADPH : NADP ratio, as well as malate, ribulose 1,5-bisphosphate (RuBP), fructose 1,6-bisphosphate (FBP), and photorespiration intermediates. We suggest that by optimizing NADPH and malate levels and adjusting NADP-ME activity, NDH functions to balance metabolic and redox states in the BS cells of maize (in addition to ATP supply), coordinating photosynthetic transcript abundance and protein content, thus directly regulating the carbon flow in the two-celled C4 system of maize.

The evolution of stomatal traits along the trajectory toward C4 photosynthesis.

C4 photosynthesis optimizes plant carbon and water relations, allowing high photosynthetic rates with low stomatal conductance. Stomata have long been considered a part of the C4 syndrome. However, it remains unclear how stomatal traits evolved along the path from C3 to C4. Here, we examined stomata in the Flaveria genus, a model used for C4 evolutionary study. Comparative, transgenic, and semi-in vitro experiments were performed to study the molecular basis that underlies the changes of stomatal traits in C4 evolution. The evolution from C3 to C4 species is accompanied by a gradual rather than an abrupt change in stomatal traits. The initial change appears near the Type I intermediate stage. Co-evolution of the photosynthetic pathway and stomatal traits is supported. On the road to C4, stomata tend to be fewer in number but larger in size and stomatal density dominates changes in anatomical maximum stomatal conductance (gsmax). Reduction of FSTOMAGEN expression underlies decreased gsmax in Flaveria and likely occurs in other C4 lineages. Decreased gsmax contributes to the increase in intrinsic water-use efficiency in C4 evolution. This work highlights the stomatal traits in the current C4 evolutionary model. Our study provides insights into the pattern, mechanism, and role of stomatal evolution along the road toward C4.

Alterations in stomatal response to fluctuating light increase biomass and yield of rice under drought conditions.

The acceleration of stomatal closure upon high to low light transition could improve plant water use efficiency and drought tolerance. Herein, using genome-wide association study, we showed that the genetic variation in OsNHX1 was strongly associated with the changes in τcl , the time constant of stomatal closure, in 206 rice accessions. OsNHX1 overexpression in rice resulted in a decrease in τcl , and an increase in biomass, grain yield under drought. Conversely, OsNHX1 knockout by CRISPR/CAS9 shows opposite trends for these traits. We further found three haplotypes spanning the OsNHX1 promoter and CDS regions. Two among them, HapII and HapIII, were found to be associated with a high and low τcl , respectively. A near-isogenic line (NIL, S464) was developed through replacing the genomic region harboring HapII (~10 kb) from MH63 (recipient) rice cultivar by the same sized genomic region containing Hap III from 02428 (donor). Compared with MH63, S464 shows a reduction by 35% in τcl and an increase by 40% in the grain yield under drought. However, under normal conditions, S464 maintains closely similar grain yield as MH63. The global distribution of the two OsNHX1 haplotypes is associated with the local precipitation. Taken together, the natural variation in OsNHX1 could be utilized to manipulate the stomatal dynamics for an improved rice drought tolerance.

Overexpression of maize transcription factor mEmBP-1 increases photosynthesis, biomass, and yield in rice.

Identifying new options to improve photosynthetic capacity is a major approach to improve crop yield potential. Here we report that overexpression of the gene encoding the transcription factor mEmBP-1 led to simultaneously increased expression of many genes in photosynthesis, including genes encoding Chl a,b-binding proteins (Lhca and Lhcb), PSII (PsbR3 and PsbW) and PSI reaction center subunits (PsaK and PsaN), chloroplast ATP synthase subunit, electron transport reaction components (Fd1 and PC), and also major genes in the Calvin-Benson-Bassham cycle, including those encoding Rubisco, glyceraldehyde phosphate dehydrogenase, fructose bisphosphate aldolase, transketolase, and phosphoribulokinase. These increased expression of photosynthesis genes resulted in increased leaf chlorophyll pigment, photosynthetic rate, biomass growth, and grain yield both in the greenhouse and in the field. Using EMSA experiments, we showed that mEmBP-1a protein can directly bind to the promoter region of photosynthesis genes, suggesting that the direct binding of mEmBP-1a to the G-box domain of photosynthetic genes up-regulates expression of these genes. Altogether, our results show that mEmBP-1a is a major regulator of photosynthesis, which can be used to increase rice photosynthesis and yield in the field.

Genome-Wide Association Study Unravels LRK1 as a Dark Respiration Regulator in Rice (Oryza sativa L.).

Respiration is a major plant physiological process that generates adenosine triphosphate (ATP) to support the various pathways involved in the plant growth and development. After decades of focused research on basic mechanisms of respiration, the processes and major proteins involved in respiration are well elucidated. However, much less is known about the natural variation of respiration. Here we conducted a survey on the natural variation of leaf dark respiration (Rd) in a global rice minicore diversity panel and applied a genome-wide association study (GWAS) in rice (Oryza sativa L.) to determine candidate loci associated with Rd. This rice minicore diversity panel consists of 206 accessions, which were grown under both growth room (GR) and field conditions. We found that Rd shows high single-nucleotide polymorphism (SNP) heritability under GR and it is significantly affected by genotype-environment interactions. Rd also exhibits strong positive correlation to the leaf thickness and chlorophyll content. GWAS results of Rd collected under GR and field show an overlapped genomic region in the chromosome 3 (Chr.3), which contains a lead SNP (3m29440628). There are 12 candidate genes within this region; among them, three genes show significantly higher expression levels in accessions with high Rd. Particularly, we observed that the LRK1 gene, annotated as leucine rich repeat receptor kinase, was up-regulated four times. We further found that a single significantly associated SNPs at the promoter region of LRK1, was strongly correlated with the mean annual temperature of the regions from where minicore accessions were collected. A rice lrk1 mutant shows only ~37% Rd of that of WT and retarded growth following exposure to 35 °C for 30 days, but only 24% reduction in growth was recorded under normal temperature (25 °C). This study demonstrates a substantial natural variation of Rd in rice and that the LRK1 gene can regulate leaf dark respiratory fluxes, especially under high temperature.

Combined Proteomics and Metabolism Analysis Unravels Prominent Roles of Antioxidant System in the Prevention of Alfalfa (Medicago sativa L.) against Salt Stress.

Alfalfa is the most extensively cultivated forage legume worldwide, and salinity constitutes the main environmental scourge limiting its growth and productivity. To unravel the potential molecular mechanism involved in salt tolerance in alfalfa, we accomplished a combined analysis of parallel reaction monitoring-based proteomic technique and targeted metabolism. Based on proteomic analysis, salt stress induced 226 differentially abundant proteins (DAPs). Among them, 118 DAPs related to the antioxidant system, including glutathione metabolism and oxidation-reduction pathways, were significantly up-regulated. Data are available via ProteomeXchange with identifier PXD017166. Overall, 107 determined metabolites revealed that the tricarboxylic acid (TCA) cycle, especially the malate to oxaloacetate conversion step, was strongly stimulated by salt stress. This leads to an up-regulation by about 5 times the ratio of NADPH/NADP+, as well as about 3 to 5 times in the antioxidant enzymes activities, including those of catalase and peroxidase and proline contents. However, the expression levels of DAPs related to the Calvin-Benson-Bassham (CBB) cycle and photorespiration pathway were dramatically inhibited following salt treatment. Consistently, metabolic analysis showed that the metabolite amounts related to carbon assimilation and photorespiration decreased by about 40% after exposure to 200 mM NaCl for 14 d, leading ultimately to a reduction in net photosynthesis by around 30%. Our findings highlighted also the importance of the supplied extra reducing power, thanks to the TCA cycle, in the well-functioning of glutathione to remove and scavenge the reactive oxygen species (ROS) and mitigate subsequently the oxidative deleterious effect of salt on carbon metabolism including the CBB cycle.

Light-harvesting regulation from leaf to molecule with the emphasis on rapid changes in antenna size.

In the sunlight-fluctuating environment, plants often encounter both light-deficiency and light-excess cases. Therefore, regulation of light harvesting is absolutely essential for photosynthesis in order to maximize light utilization at low light and avoid photodamage of the photosynthetic apparatus at high light. Plants have developed a series of strategies of light-harvesting regulation during evolution. These strategies include rapid responses such as leaf movement and chloroplast movement, state transitions, and reversible dissociation of some light-harvesting complex of the photosystem II (LHCIIs) from PSII core complexes, and slow acclimation strategies such as changes in the protein abundance of light-harvesting antenna and modifications of leaf morphology, structure, and compositions. This review discusses successively these strategies and focuses on the rapid change in antenna size, namely reversible dissociation of some peripheral light-harvesting antennas (LHCIIs) from PSII core complex. It is involved in protective role and species dependence of the dissociation, differences between the dissociation and state transitions, relationship between the dissociation and thylakoid protein phosphorylation, and possible mechanism for thermal dissipation by the dissociated LHCIIs.

Evolution of gene regulatory network of C4 photosynthesis in the genus Flaveria reveals the evolutionary status of C3-C4 intermediate species.

C4 photosynthesis evolved from ancestral C3 photosynthesis by recruiting pre-existing genes to fulfill new functions. The enzymes and transporters required for the C4 metabolic pathway have been intensively studied and well documented; however, the transcription factors (TFs) that regulate these C4 metabolic genes are not yet well understood. In particular, how the TF regulatory network of C4 metabolic genes was rewired during the evolutionary process is unclear. Here, we constructed gene regulatory networks (GRNs) for four closely evolutionarily related species from the genus Flaveria, which represent four different evolutionary stages of C4 photosynthesis: C3 (F. robusta), type I C3-C4 (F. sonorensis), type II C3-C4 (F. ramosissima), and C4 (F. trinervia). Our results show that more than half of the co-regulatory relationships between TFs and core C4 metabolic genes are species specific. The counterparts of the C4 genes in C3 species were already co-regulated with photosynthesis-related genes, whereas the required TFs for C4 photosynthesis were recruited later. The TFs involved in C4 photosynthesis were widely recruited in the type I C3-C4 species; nevertheless, type II C3-C4 species showed a divergent GRN from C4 species. In line with these findings, a 13CO2 pulse-labeling experiment showed that the CO2 initially fixed into C4 acid was not directly released to the Calvin-Benson-Bassham cycle in the type II C3-C4 species. Therefore, our study uncovered dynamic changes in C4 genes and TF co-regulation during the evolutionary process; furthermore, we showed that the metabolic pathway of the type II C3-C4 species F. ramosissima represents an alternative evolutionary solution to the ammonia imbalance in C3-C4 intermediate species.

A mutation of OSOTP 51 leads to impairment of photosystem I complex assembly and serious photo-damage in rice.

Gene expression in chloroplasts is regulated by many nuclear-encoded proteins. In this study, we isolated a rice (Oryza sativa subsp. japonica) mutant osotp51 with significant reduction in photosystem I (PSI). The osotp51 is extremely sensitive to light and accumulates a higher level of reactive oxygen species. Its leaves are almost albino when grown at 40 µmol photons/m(2) per s. However, grown at 4 µmol photons/m(2) per s, osotp51 has a similar phenotype to the wild-type. 77K chlorophyll fluorescence analysis showed a blue shift in the highest peak emission from PSI in osotp51. In addition, the level of PSI and PSII dimer is dramatically reduced in osotp51. OSOTP 51 encodes a pentatricopeptide repeats protein, homologous to organelle transcript processing 51 in Arabidopsis. Loss-of-function OSOTP51 affects intron splicing of a number of plastid genes, particularly the ycf3 coding a protein involved in the assembly of PSI complex. OSOTP51 is functionally conserved in higher plants. The mutation of osotp51 indirectly leads to a widespread change in the structure and functions of PSI, results in severe photoinhibition, and finally dies, even when grown under very low light intensity.

An integrated isotopic labeling and freeze sampling apparatus (ILSA) to support sampling leaf metabolomics at a centi-second scale.

BACKGROUND: Photosynthesis close interacts with respiration and nitrogen assimilation, which determine the photosynthetic efficiency of a leaf. Accurately quantifying the metabolic fluxes in photosynthesis, respiration and nitrogen assimilation benefit the design of photosynthetic efficiency improvement. To accurately estimate metabolic fluxes, time-series data including leaf metabolism and isotopic abundance changes should be collected under precisely controlled environments. But for isotopic labelled leaves under defined environments the, time cost of manually sampling usually longer than the turnover time of several intermediates in photosynthetic metabolism. In this case, the metabolic or physiological status of leaf sample would change during the sampling, and the accuracy of metabolomics data could be compromised. RESULTS: Here we developed an integrated isotopic labeling and freeze sampling apparatus (ILSA), which could finish freeze sampling automatically in 0.05 s. ILSA can not only be used for sampling of photosynthetic metabolism measurement, but also suit for leaf isotopic labeling experiments under controlled environments ([CO2] and light). Combined with HPLC-MS/MS as the metabolic measurement method, we demonstrated: (1) how pool-size of photosynthetic metabolites change in dark-accumulated rice leaf, and (2) variation in photosynthetic metabolic flux between rice and Arabidopsis thaliana. CONCLUSIONS: The development of ILSA supports the photosynthetic research on metabolism and metabolic flux analysis and provides a new tool for the study of leaf physiology.

Ubiquitin-based pathway acts inside chloroplasts to regulate photosynthesis.

Photosynthesis is the energetic basis for most life on Earth, and in plants it operates inside double membrane-bound organelles called chloroplasts. The photosynthetic apparatus comprises numerous proteins encoded by the nuclear and organellar genomes. Maintenance of this apparatus requires the action of internal chloroplast proteases, but a role for the nucleocytosolic ubiquitin-proteasome system (UPS) was not expected, owing to the barrier presented by the double-membrane envelope. Here, we show that photosynthesis proteins (including those encoded internally by chloroplast genes) are ubiquitinated and processed via the CHLORAD pathway: They are degraded by the 26S proteasome following CDC48-dependent retrotranslocation to the cytosol. This demonstrates that the reach of the UPS extends to the interior of endosymbiotically derived chloroplasts, where it acts to regulate photosynthesis, arguably the most fundamental process of life.

Decreasing nitrogen assimilation under drought stress by suppressing DST-mediated activation of Nitrate Reductase 1.2 in rice.

Nitrogen is an essential nutrient for plant growth and development, and plays vital roles in crop yield. Assimilation of nitrogen is thus fine-tuned in response to heterogeneous environments. However, the regulatory mechanism underlying this essential process remains largely unknown. Here, we report that a zinc-finger transcription factor, drought and salt tolerance (DST), controls nitrate assimilation in rice by regulating the expression of OsNR1.2. We found that loss of function of DST results in a significant decrease of nitrogen use efficiency (NUE) in the presence of nitrate. Further study revealed that DST is required for full nitrate reductase activity in rice and directly regulates the expression of OsNR1.2, a gene showing sequence similarity to nitrate reductase. Reverse genetics and biochemistry studies revealed that OsNR1.2 encodes an NADH-dependent nitrate reductase that is required for high NUE of rice. Interestingly, the DST-OsNR1.2 regulatory module is involved in the suppression of nitrate assimilation under drought stress, which contributes to drought tolerance. Considering the negative role of DST in stomata closure, as revealed previously, the positive role of DST in nitrogen assimilation suggests a mechanism coupling nitrogen metabolism and stomata movement. The discovery of this coupling mechanism will aid the engineering of drought-tolerant crops with high NUE in the future.

Proteomic study of Carissa spinarum in response to combined heat and drought stress.

Carissa spinarum is one of the secondary advantage plants grown in dry-hot valleys in China, which can survive under stress conditions of high temperature and extreme low humidity. Here, we studied the physiological and proteomic changes of C. spinarum in response to 42 degrees C heat stress treatment in combination with drought stress. Dynamic changes in the leaf proteome were analyzed at four time points during the stress treatment and recovery stages. Approximately, 650 protein spots were reproducibly detected in each gel. Forty-nine spots changed their expression levels upon heat and drought treatment, and 30 proteins were identified by MS and 2-D Western blot. These proteins were classified into several categories including HSP, photosynthesis-related protein, RNA-processing protein and proteins involved in metabolism and energy production. The potential roles of these stress-responsive proteins are discussed.

Reversible association of ribulose-1, 5-bisphosphate carboxylase/oxygenase activase with the thylakoid membrane depends upon the ATP level and pH in rice without heat stress.

Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activase (RCA) in the thylakoid membrane (TM) has been shown to play a role in protection and regulation of photosynthesis under moderate heat stress. However, the physiological significance of RCA bound to the TM (TM-RCA) without heat stress remains unknown. In this study, it is first shown, using experiments in vivo, that the TM-RCA varies in rice leaves at different development stages, under different environmental conditions, and in a rice mutant. Furthermore, it is shown that the amount of TM-RCA always increased when the Rubisco activation state and the pH gradient across the TM (DeltapH) decreased. It was then demonstrated in vitro that the RCA bound dynamically to TM and the amount of TM-RCA increased during Rubisco activation. A high level of ATP and a high pH value promoted the dissociation of RCA from the TM. Both the RCA association with and dissociation from the TM showed conformational changes related to the ATP level or pH as indicated by the changes in fluorescence intensity of 1-anilinonaphthalene-8-sulphonic acid (ANS) binding to RCA. These results suggest that the reversible association of RCA with the TM is ATP and pH (or DeltapH) dependent; it might be involved in the RCA activation of Rubisco, in addition to the previously discovered role in the protection and regulation of photosynthesis under heat stress.

Analytical dataset of short-term heat stress induced reshuffling of metabolism and transcriptomes in maize grown under elevated CO2.

This data article describes the analysis of sudden heat stress (SHS) induced transcriptomes and metabolism in SQ maize cultivar (Zea mays L. cv. Silver Queen). Plants were grown under elevated CO2 in both field based open top chambers (OTCs) and indoor growth chamber conditions [1]. After 20 days after radicle emergence, intact leaf section of maize was exposed for 2 hours to SHS treatment. Samples were stored in liquid nitrogen immediately and used thereafter for metabolism and transcriptomes determinations. Metabolism consisting of 37 targeted metabolites together with corresponding reference standard were determined by gas chromatography coupled to mass spectrometry (GC-MS). Total RNA was extracted using TRIzol® reagent according to the manufacturer's instructions (Invitrogen, Carlsbad, CA). RNA integrity was assessed using RNA Nano 6000 Assay Kit of the Agilent Bioanalyzer 2100 system (Agilent Technologies, CA, USA). Transcriptomes were determined by Illumina Hiseq 4000 platform. Further interpretation and discussion on these datasets can be found in the related article entitled "Elevated CO2 concentrations may alleviate the detrimental effects of sudden heat stress on photosynthetic carbon metabolism in maize" [1].

Photosynthetic and transcriptomic responses of two C4 grass species with different NaCl tolerance.

This report reveals the effects of salt on the photosynthetic electron transport and transcriptome of the glycophyte Setaria viridis (S. viridis) and its salt-tolerant close relative halophyte Spartina alterniflora (S. alterniflora). S. viridis was unable to survive exposed to sodium chloride (NaCl) levels higher than 100 mM, in contrast, S. alterniflora could tolerate NaCl up to 550 mM, with negligible effect on gas exchange related parameters and conductance of electrons transport chain (gETC). Under salt, the prompt fluorescence (OJIP-curves) exhibits an increase in the O- and J-steps in S. viridis and much less for S. alterniflora. Flowing NaCl stress, a dramatic decline in the photosystem II (PSII) primary photochemistry was observed for S. viridis, as reflected by the drastic drop in Fv/Fm, Fv/Fo and ΦPSII; however, no substantial change was recorded for these parameters in S. alterniflora. Interestingly, we found an increase in the primary PSII photochemistry (ΦPSII) for S. alterniflora with increasing either NaCl concentration or NaCl treatment duration. The NPQ magnitude was strongly enhanced for S. viridis even at a low NaCl (50 mM); however, it remains unchangeable or slightly increased for S. alterniflora at NaCl levels above 400 mM. After NaCl treatment, we found an increase in both the proportion of oxidized P700 and the amount of active P700 in S. viridis and almost no change for S. alterniflora. Under salt, the net photosynthetic rate (A) and stomatal conductance (gs) measurements demonstrate that A decreases earlier in S. viridis, even after one week exposure to only 50 mM NaCl; in contrast, in S. alterniflora, the effect of NaCl on A and gs was minor even after exposure for two weeks to high NaCl levels. For S. viridis exposed to 50 mM NaCl for 12 d, carbon dioxide (CO2) at a concentration of 2000 µL L-1 could not fully restore A to the control (Ctrl) level. Conversely, in S. alterniflora, high CO2 can fully restore A for all NaCl treatments except at 550 mM. RNA-seq data shows a major impact of NaCl on metabolic pathways in S. viridis and we found a number of transcription factors potentially related to NaCl responses. For S. alterniflora, no major changes in the transcriptomic levels were recorded under NaCl stress. To confirm our data analysis of RNA-seq, we performed quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis for randomly selected four genes for each species (8 genes in total) and we found that our results (up- and/or down-regulated genes) are fully consistent and match well our RNA-seq data. Overall, this study showed drastically different photosynthetic and transcriptomic responses of a salt-tolerant C4 grass species and one salt-sensitive C4 grass species to NaCl stress, which suggests that S. alterniflora could be used as a promising model species to study salt tolerance in C4 or monocot species.

What Matters for C4 Transporters: Evolutionary Changes of Phosphoenolpyruvate Transporter for C4 Photosynthesis.

C4 photosynthesis is a complex trait that evolved from its ancestral C3 photosynthesis by recruiting pre-existing genes. These co-opted genes were changed in many aspects compared to their counterparts in C3 species. Most of the evolutionary changes of the C4 shuttle enzymes are well characterized, however, evolutionary changes for the recruited metabolite transporters are less studied. Here we analyzed the evolutionary changes of the shuttle enzyme phosphoenolpyruvate (PEP) transporter (PPT) during its recruitment from C3 to C4 photosynthesis. Our analysis showed that among the two PPT paralogs PPT1 and PPT2, PPT1 was the copy recruited for C4 photosynthesis in multiple C4 lineages. During C4 evolution, PPT1 gained increased transcript abundance, shifted its expression from predominantly in root to in leaf and from bundle sheath cell to mesophyll cell, and gained more rapid and long-lasting responsiveness to light. Modifications occurred in both regulatory and coding regions in C4 PPT1 as compared to C3 PPT1, however, the PEP transporting function of PPT1 remained. We found that PPT1 of a Flaveria C4 species recruited a MEM1 B submodule in the promoter region, which might be related to the increased transcript abundance of PPT1 in C4 mesophyll cells. The case study of PPT further suggested that high transcript abundance in a proper location is of high priority for PPT to support C4 function.

Contrasting Responses of Plastid Terminal Oxidase Activity Under Salt Stress in Two C4 Species With Different Salt Tolerance.

The present study reveals contrasting responses of photosynthesis to salt stress in two C4 species: a glycophyte Setaria viridis (SV) and a halophyte Spartina alterniflora (SA). Specifically, the effect of short-term salt stress treatment on the photosynthetic CO2 uptake and electron transport were investigated in SV and its salt-tolerant close relative SA. In this experiment, at the beginning, plants were grown in soil then were exposed to salt stress under hydroponic conditions for two weeks. SV demonstrated a much higher susceptibility to salt stress than SA; while, SV was incapable to survive subjected to about 100 mM, SA can tolerate salt concentrations up to 550 mM with slight effect on photosynthetic CO2 uptake rates and electrons transport chain conductance (gETC ). Regardless the oxygen concentration used, our results show an enhancement in the P700 oxidation with increasing O2 concentration for SV following NaCl treatment and almost no change for SA. We also observed an activation of the cyclic NDH-dependent pathway in SV by about 2.36 times upon exposure to 50 mM NaCl for 12 days (d); however, its activity in SA drops by about 25% compared to the control without salt treatment. Using PTOX inhibitor (n-PG) and that of the Qo-binding site of Cytb6/f (DBMIB), at two O2 levels (2 and 21%), to restrict electrons flow towards PSI, we successfully revealed the presence of a possible PTOX activity under salt stress for SA but not for SV. However, by q-PCR and western-blot analysis, we showed an increase in PTOX amount by about 3-4 times for SA under salt stress but not or very less for SV. Overall, this study provides strong proof for the existence of PTOX as an alternative electron pathway in C4 species (SA), which might play more than a photoprotective role under salt stress.

Morphological and physiological factors contributing to early vigor in the elite rice cultivar 9,311.

Huanghuazhan (HHZ) and 9,311 are two elite rice cultivars in China. They have achieved high yield through quite different mechanisms. One of the major features that gives high yield capacity to 9,311 is its strong early vigor, i.e., faster establishment of its seedling as well as its better growth in its early stages. To understand the mechanistic basis of early vigor in 9,311, as compared to HHZ the cultivar, we have examined, under controlled environmental conditions, different morphological and physiological traits that may contribute to its early vigor. Our results show that the fresh weight of the seeds, at germination, not only determined the seedling biomass at 10 days after germination (DAG), but was also responsible for ~ 80% of variations in plant biomass between the two cultivars even up to 30 DAG. Furthermore, the 9,311 cultivar had a larger root system, which led to its higher nitrogen uptake capacity. Other noteworthy observations about 9,311 being a better cultivar than HHZ are: (i) Ten out of 15 genes involved in nitrogen metabolism were much more highly expressed in its roots; (ii) it had a higher water uptake rate, promoting better root-to-shoot nitrogen transfer; and (iii) consistent with the above, it had higher leaf photosynthetic rate and stomatal conductance. All of the above identified features explain, to a large extent, why the 9,311, as compared to HHZ, exhibits much more vigorous early growth.