Photosynthesis In Arabidopsis Research Articles

Inactivation of photosynthetic cyclic electron transports upregulates photorespiration for compensation of efficient photosynthesis in Arabidopsis.

Plants have multiple mechanisms to maintain efficient photosynthesis. Photosynthetic cyclic electron transports around photosystem I (CET), which includes the PGR5/PGRL1 and NDH pathways, and photorespiration play a crucial role in photosynthetic efficiency. However, how these two mechanisms are functionally linked is not clear. In this study, we revealed that photorespiration could compensate for the function of CET in efficient photosynthesis by comparison of the growth phenotypes, photosynthetic properties monitored with chlorophyll fluorescence parameters and photosynthetic oxygen evolution in leaves and photorespiratory activity monitored with the difference of photosynthetic oxygen evolution rate under high and low concentration of oxygen conditions between the deleted mutant PGR5 or PGRL1 under NDH defective background (pgr5 crr2 or pgrl1a1b crr2). Both CET mutants pgr5 crr2 and pgrl1a1b crr2 displayed similar suppression effects on photosynthetic capacities of light reaction and growth phenotypes under low light conditions. However, the total CET activity and photosynthetic oxygen evolution of pgr5 crr2 were evidently lower than those of pgrl1a1b crr2, accompanied by the upregulation of photorespiratory activity under low light conditions, resulting in severe suppression of photosynthetic capacities of light reaction and finally photodamaged phenotype under high light or fluctuating light conditions. Based on these findings, we suggest that photorespiration compensates for the loss of CET functions in the regulation of photosynthesis and that coordination of both mechanisms is essential for maintaining the efficient operation of photosynthesis, especially under stressed conditions.

Light acclimation interacts with thylakoid ion transport to govern the dynamics of photosynthesis in Arabidopsis.

Understanding photosynthesis in natural, dynamic light environments requires knowledge of long-term acclimation, short-term responses, and their mechanistic interactions. To approach the latter, we systematically determined and characterized light-environmental effects on thylakoid ion transport-mediated short-term responses during light fluctuations. For this, Arabidopsis thaliana wild-type and mutants of the Cl- channel VCCN1 and the K+ exchange antiporter KEA3 were grown under eight different light environments and characterized for photosynthesis-associated parameters and factors in steady state and during light fluctuations. For a detailed characterization of selected light conditions, we monitored ion flux dynamics at unprecedented high temporal resolution by a modified spectroscopy approach. Our analyses reveal that daily light intensity sculpts photosynthetic capacity as a main acclimatory driver with positive and negative effects on the function of KEA3 and VCCN1 during high-light phases, respectively. Fluctuations in light intensity boost the accumulation of the photoprotective pigment zeaxanthin (Zx). We show that KEA3 suppresses Zx accumulation during the day, which together with its direct proton transport activity accelerates photosynthetic transition to lower light intensities. In summary, both light-environment factors, intensity and variability, modulate the function of thylakoid ion transport in dynamic photosynthesis with distinct effects on lumen pH, Zx accumulation, photoprotection, and photosynthetic efficiency.

Cold Tolerance of ScCBL6 Is Associated with Tonoplast Transporters and Photosynthesis in Arabidopsis.

Plants that are adapted to harsh environments offer enormous opportunity to understand stress responses in ecological systems. Stipa capillacea is widely distributed in the frigid and arid region of the Tibetan Plateau, but its signal transduction system under cold stress has not been characterized. In this study, we isolated a cDNA encoding the signal transduction protein, ScCBL6, from S. capillacea, and evaluated its role in cold tolerance by ectopically expressing it in Arabidopsis. Full-length ScCBL6 encode 227 amino acids, and are clustered with CBL6 in Stipa purpurea and Oryza sativa in a phylogenetic analysis. Compared with tolerance in wild-type (WT) plants, ScCBL6-overexpressing plants (ScCBL6-OXP) were more tolerant to cold stress but not to drought stress, as confirmed by their high photosynthetic capacity (Fv/Fm) and survival rate under cold stress. We further compared their cold-responsive transcriptome profiles by RNA sequencing. In total, 3931 genes were differentially expressed by the introduction of ScCBL6. These gene products were involved in multiple processes such as the immune system, lipid catabolism, and secondary metabolism. A KEGG pathway analysis revealed that they were mainly enriched in plant hormone signal transduction and biomacromolecule metabolism. Proteins encoded by differentially expressed genes were predicted to be localized in chloroplasts, mitochondria, and vacuoles, suggesting that ScCBL6 exerts a wide range of functions. Based on its tonoplast subcellular location combined with integrated transcriptome and physiological analyses of ScCBL6-OXP, we inferred that ScCBL6 improves plant cold stress tolerance in Arabidopsis via the regulation of photosynthesis, redox status, and tonoplast metabolite transporters.

Uridylation and the SKI complex orchestrate the Calvin cycle of photosynthesis through RNA surveillance of TKL1 in Arabidopsis.

RNA uridylation, catalyzed by terminal uridylyl transferases (TUTases), represents a conserved and widespread posttranscriptional RNA modification in eukaryotes that affects RNA metabolism. In plants, several TUTases, including HEN1 SUPPRESSOR 1 (HESO1) and UTP: RNA URIDYLYLTRANSFERASE (URT1), have been characterized through genetic and biochemical approaches. However, little is known about their physiological significance during plant development. Here, we show that HESO1 and URT1 act cooperatively with the cytoplasmic 3'-5' exoribonucleolytic machinery component SUPERKILLER 2 (SKI2) to regulate photosynthesis through RNA surveillance of the Calvin cycle gene TRANSKETOLASE1 (TKL1) in Arabidopsis. Simultaneous dysfunction of HESO1, URT1, and SKI2 resulted in leaf etiolation and reduced photosynthetic efficiency. In addition, we detected massive illegitimate short interfering RNAs (siRNAs) from the TKL1 locus in heso1 urt1 ski2, accompanied by reduced TKL1/2 expression and attenuated TKL activities. Consequently, the metabolic analysis revealed that the abundance of many Calvin cycle intermediates is dramatically disturbed in heso1 urt1 ski2. Importantly, all these molecular and physiological defects were largely rescued by the loss-of-function mutation in RNA-DEPENDENT RNA POLYMERASE 6 (RDR6), demonstrating illegitimate siRNA-mediated TKL silencing. Taken together, our results suggest that HESO1- and URT1-mediated RNA uridylation connects to the cytoplasmic RNA degradation pathway for RNA surveillance, which is crucial for TKL expression and photosynthesis in Arabidopsis.

The Transcription Factor MYB37 Positively Regulates Photosynthetic Inhibition and Oxidative Damage in Arabidopsis Leaves Under Salt Stress.

MYB transcription factors (TFs) mediate plant responses and defenses to biotic and abiotic stresses. The effects of overexpression of MYB37, an R2R3 MYB subgroup 14 transcription factors in Arabidopsis thaliana, on chlorophyll content, chlorophyll fluorescence parameters, reactive oxygen species (ROS) metabolism, and the contents of osmotic regulatory substances were studied under 100 mM NaCl stress. Compared with the wild type (Col-0), MYB37 overexpression significantly alleviated the salt stress symptoms in A. thaliana plants. Chlorophyll a (Chl a) and chlorophyll b (Chl b) contents were significantly decreased in OE-1 and OE-2 than in Col-0. Particularly, the Chl a/b ratio was also higher in OE-1 and OE-2 than in Col-0 under NaCl stress. However, MYB37 overexpression alleviated the degradation of chlorophyll, especially Chl a. Salt stress inhibited the activities of PSII and PSI in Arabidopsis leaves, but did not affect the activity of PSII electron donor side oxygen-evolving complex (OEC). MYB37 overexpression increased photosynthesis in Arabidopsis by increasing PSII and PSI activities. MYB37 overexpression also promoted the transfer of electrons from QA to QB on the PSII receptor side of Arabidopsis under NaCl stress. Additionally, MYB37 overexpression increased Y(II) and Y(NPQ) of Arabidopsis under NaCl stress and decreased Y(NO). These results indicate that MYB37 overexpression increases PSII activity and regulates the proportion of energy dissipation in Arabidopsis leaves under NaCl stress, thus decreasing the proportion of inactivated reaction centers. Salt stress causes excess electrons and energy in the photosynthetic electron transport chain of Arabidopsis leaves, resulting in the release of reactive oxygen species (ROS), such as superoxide anion and hydrogen peroxide, leading to oxidative damage. Nevertheless, MYB37 overexpression reduced accumulation of malondialdehyde in Arabidopsis leaves under NaCl stress and alleviated the degree of membrane lipid peroxidation caused by ROS. Salt stress also enhanced the accumulation of soluble sugar (SS) and proline (Pro) in Arabidopsis leaves, thus reducing salt stress damage to plants. Salt stress also degraded soluble protein (SP). Furthermore, the accumulation of osmoregulation substances SS and Pro in OE-1 and OE-2 was not different from that in Col-0 since MYB37 overexpression in Arabidopsis OE-1, and OE-2 did not significantly affect plants under NaCl stress. However, SP content was significantly higher in OE-1 and OE-2 than in Col-0. These results indicate that MYB37 overexpression can alleviate the degradation of Arabidopsis proteins under NaCl stress, promote plant growth and improve salt tolerance.

The Absence of the AtSYT1 Function Elevates the Adverse Effect of Salt Stress on Photosynthesis in Arabidopsis.

Arabidopsis thaliana SYNAPTOTAGMIN 1 (AtSYT1) was shown to be involved in responses to different environmental and biotic stresses. We investigated gas exchange and chlorophyll a fluorescence in Arabidopsis wild-type (WT, ecotype Col-0) and atsyt1 mutant plants irrigated for 48 h with 150 mM NaCl. We found that salt stress significantly decreases net photosynthetic assimilation, effective photochemical quantum yield of photosystem II (ΦPSII), stomatal conductance and transpiration rate in both genotypes. Salt stress has a more severe impact on atsyt1 plants with increasing effect at higher illumination. Dark respiration, photochemical quenching (qP), non-photochemical quenching and ΦPSII measured at 750 µmol m−2 s−1 photosynthetic photon flux density were significantly affected by salt in both genotypes. However, differences between mutant and WT plants were recorded only for qP and ΦPSII. Decreased photosynthetic efficiency in atsyt1 under salt stress was accompanied by reduced chlorophyll and carotenoid and increased flavonol content in atsyt1 leaves. No differences in the abundance of key proteins participating in photosynthesis (except PsaC and PsbQ) and chlorophyll biosynthesis were found regardless of genotype or salt treatment. Microscopic analysis showed that irrigating plants with salt caused a partial closure of the stomata, and this effect was more pronounced in the mutant than in WT plants. The localization pattern of AtSYT1 was also altered by salt stress.

Effectiveness of Light-Quality and Dark-White Growth Light Shifts in Short-Term Light Acclimation of Photosynthesis in Arabidopsis.

Photosynthesis needs to run efficiently under permanently changing illumination. To achieve this, highly dynamic acclimation processes optimize photosynthetic performance under a variety of rapidly changing light conditions. Such acclimation responses are acting by a complex interplay of reversible molecular changes in the photosynthetic antenna or photosystem assemblies which dissipate excess energy and balance uneven excitation between the two photosystems. This includes a number of non-photochemical quenching processes including state transitions and photosystem II remodeling. In the laboratory such processes are typically studied by selective illumination set-ups. Two set-ups known to be effective in a highly similar manner are (i) light quality shifts (inducing a preferential excitation of one photosystem over the other) or (ii) dark-light shifts (inducing a general off-on switch of the light harvesting machinery). Both set-ups result in similar effects on the plastoquinone redox state, but their equivalence in induction of photosynthetic acclimation responses remained still open. Here, we present a comparative study in which dark-light and light-quality shifts were applied to samples of the same growth batches of plants. Both illumination set-ups caused comparable effects on the phosphorylation of LHCII complexes and, hence, on the performance of state transitions, but generated different effects on the degree of state transitions and the formation of PSII super-complexes. The two light set-ups, thus, are not fully equivalent in their physiological effectiveness potentially leading to different conclusions in mechanistic models of photosynthetic acclimation. Studies on the regulation of photosynthetic light acclimation, therefore, requires to regard the respective illumination test set-up as a critical parameter that needs to be considered in the discussion of mechanistic and regulatory aspects in this subject.

Cytosolic CTP Production Limits the Establishment of Photosynthesis in Arabidopsis.

CTP synthases (CTPS) comprise a protein family of the five members CTPS1-CTPS5 in Arabidopsis, all located in the cytosol. Specifically, downregulation of CTPS2 by amiRNA technology results in plants with defects in chlorophyll accumulation and photosynthetic performance early in development. CTP and its deoxy form dCTP are present at low levels in developing seedlings. Thus, under conditions of fast proliferation, the synthesis of CTP (dCTP) can become a limiting factor for RNA and DNA synthesis. The higher sensitivity of ami-CTPS2 lines toward the DNA-Gyrase inhibitor ciprofloxacin, together with reduced plastid DNA copy number and 16S and 23S chloroplast ribosomal RNA support this view. High expression and proposed beneficial biochemical features render CTPS2 the most important isoform for early seedling development. In addition, CTPS2 was identified as an essential enzyme in embryo development before, as knock-out mutants were embryo lethal. In line with this, ami-CTPS2 lines also exhibited reduced seed numbers per plant.

Joint expression of Zmpepc, Zmppdk, and Zmnadp-me is more efficient than expression of one or two of those genes in improving the photosynthesis of Arabidopsis

This study assessed the effects of seven combinations of maize (Zea mays) genes phosphoenolpyruvate carboxylase (pepc), pyruvate phosphate dikinase (ppdk), and NADP-malic enzyme (nadp-me), on the photosynthesis of Arabidopsis. The photosynthetic rate, carboxylation efficiency, and shoot-dry-weight of Zmpepc (PC), Zmpepc + Zmppdk (PCK), Zmpepc + Zmnadp-me (PCM), and Zmpepc + Zmppdk + Zmnadp-me (PCKM) were significantly higher than those of the control wild-type (WT), with a trends to be PCKM > PCK > PC and PCM > WT. This indicated that Zmpepc was a prerequisite for improved photosynthetic performance; Zmppdk had a positive effect on Zmpepc, and the triple gene combination had the most significant synergistic effects. PCKM significantly enhanced activity of photosystem (PS)II (K, J phase) and PSI, light energy absorption (ABS/CSm) and conversion (TRo/ABS), and electron transfer (ETo/TRo). PCKM up-regulated 18 photosynthesis-related proteins, among which, 11 were involved in light reaction resulting in improved light-energy absorption and conversion efficiency, electron transfer, activity and stability of PSII and PSI, and the ATP and NADPH production. The remaining seven proteins were involved in dark reaction. The up-regulation of these proteins in PCKM improved the coordinated operation of light and dark reaction, increasing the photosynthesis and dry weight ultimately. These results also provide a promising strategy for the genetic improvement of the photosynthetic performance of C3 crops by inserting major C4 photosynthetic genes.

Transit Peptides From Photosynthesis-Related Proteins Mediate Import of a Marker Protein Into Different Plastid Types and Within Different Species.

Nucleus-encoded plastid proteins are synthesized as precursors with N-terminal targeting signals called transit peptides (TPs), which mediate interactions with the translocon complexes at the outer (TOC) and inner (TIC) plastid membranes. These complexes exist in multiple isoforms in higher plants and show differential specificity and tissue abundance. While some show specificity for photosynthesis-related precursor proteins, others distinctly recognize nonphotosynthetic and housekeeping precursor proteins. Here we used TPs from four Arabidopsis thaliana proteins, three related to photosynthesis (chlorophyll a/b binding protein, Rubisco activase) and photo-protection (tocopherol cyclase) and one involved in the assimilation of ammonium into amino-acids, and whose expression is most abundant in the root (ferredoxin dependent glutamate synthase 2), to determine whether they were able to mediate import of a nuclear-encoded marker protein into plastids of different tissues of a dicot and a monocot species. In A. thaliana, import and processing efficiency was high in all cases, while TP from the rice Rubisco small chain 1, drove very low import in Arabidopsis tissues. Noteworthy, our results show that Arabidopsis photosynthesis TPs also mediate plastid import in rice callus, and in leaf and root tissues with almost a 100% efficiency, providing new biotechnological tools for crop improvement strategies based on recombinant protein accumulation in plastids by the expression of nuclear-encoded transgenes.

Two chloroplast thioredoxin systems differentially modulate photosynthesis in Arabidopsis depending on light intensity and leaf age.

SUMMARYVarious regulatory mechanisms have evolved in plants to optimize photosynthetic activity under fluctuating light. Thioredoxins (TRX) are members of the regulatory network balancing activities of light and carbon fixation reactions in chloroplasts. We have studied the impact of two chloroplast TRX systems, the ferredoxin‐dependent TRX reductase (FTR) and the NADPH‐dependent TRX reductase C (NTRC) on regulation of photosynthesis by mutants lacking or overexpressing a component of either system. Plants were subjected to image‐based phenotyping and chlorophyll fluorescence measurements that allow long‐term monitoring of the development and photosynthetic activity of the rosettes, respectively. Our experiments demonstrate that NTRC and FTR systems respond differently to variation of light intensity. NTRC was an indispensable regulator of photosynthesis in young leaves, at light‐intensity transitions and under low light intensities limiting photosynthesis, whereas steady‐state exposure of plants to growth or higher light intensities diminished the need of NTRC in regulation of photosynthesis. In fluctuating light, overexpression of NTRC increased the quantum yield of Photosystem II (YII) at low light and stimulated the relaxation of non‐photochemical quenching (NPQ) after high light exposure, indicating that overexpression of NTRC improves leaf capacity to convert light energy to chemical energy under these conditions. Overexpression of chimeric protein (NTR‐TRXf) containing both the thioredoxin reductase and TRXf activity on an ntrc mutant background, did not completely recover either growth or steady‐state photosynthetic activity, whereas OE‐NTR‐TRXf plants exposed to fluctuating light regained the wild‐type level of Y(II) and NPQ.

Photosynthesis in Arabidopsis Is Unaffected by the Function of the Vacuolar K+ Channel TPK3.

Photosynthesis is limited by the slow relaxation of nonphotochemical quenching, which primarily dissipates excess absorbed light energy as heat. Because the heat dissipation process is proportional to light-driven thylakoid lumen acidification, manipulating thylakoid ion and proton flux via transport proteins could improve photosynthesis. However, an important aspect of the current understanding of the thylakoid ion transportome is inaccurate. Using fluorescent protein fusions, we show that the Arabidopsis (Arabidopsis thaliana) two-pore K+ channel TPK3, which had been reported to mediate thylakoid K+ flux, localizes to the tonoplast, not the thylakoid. The localization of TPK3 outside of the thylakoids is further supported by the absence of TPK3 in isolated thylakoids as well as the inability of isolated chloroplasts to import TPK3 protein. In line with the subcellular localization of TPK3 in the vacuole, we observed that photosynthesis in the Arabidopsis null mutant tpk3-1, which carries a transfer DNA insertion in the first exon, remains unaffected. To gain a comprehensive understanding of how thylakoid ion flux impacts photosynthetic efficiency under dynamic growth light regimes, we performed long-term photosynthesis imaging of established and newly isolated transthylakoid K+- and Cl--flux mutants. Our results underpin the importance of the thylakoid ion transport proteins potassium cation efflux antiporter KEA3 and voltage-dependent chloride channel VCCN1 and suggest that the activity of yet unknown K+ channel(s), but not TPK3, is critical for optimal photosynthesis in dynamic light environments.

The Iron Deficiency Response Regulators IAA-LEUCINE RESISTANT3 and bHLH104 Possess Different Targets and Have Distinct Effects on Photosynthesis in Arabidopsis

In higher plants, iron (Fe) is an essential element for photosynthesis and growth. Two basic helix-loop-helix (bHLH) transcription factors, bHLH104 and IAA-LEUCINE RESISTANT3 (ILR3), positively regulate Fe uptake in Arabidopsis. Furthermore, an E3 ubiquitin ligase, BRUTUS (BTS) functions as a negative regulator upstream of bHLH104 and ILR3 in the Fe homeostasis pathway. Interestingly, we characterized a lesion of BRUTUS (BTS), which exhibited pale-green leaves and retarded in growth. Genetic analyses showed that only loss of ILR3 could entirely compromise the chlorosis in bts-2 mutants. To further investigate whether bHLH104 and ILR3 have different roles in functioning downstream of BTS other than Fe absorption, we obtained bHLH104-, and ILR3-overexpressing plants. Interestingly, similar to bts-2, overexpression of ILR3 displayed chlorotic leaves. Moreover, overexpressing ILR3 also caused the abundance of thylakoid proteins reduced, along with photosynthetic genes decreased. However, there was no visible difference between bHLH104-overexpressing plants and WT. Furthermore, we found that bHLH104 and ILR3 may act independently on different downstream targets. bHLH104, but not ILR3, could bind to the promoter of At- NEET, which acts as a Fe-S/Fe cluster donor in chloroplasts. Collectively, our data demonstrate that bHLH104 and ILR3 possess different downstream targets that may have distinct effects on photosynthesis, although they share a common function in Fe deficiency responses.

Small One-Helix Proteins Are Essential for Photosynthesis in Arabidopsis

The extended superfamily of chlorophyll a/b binding proteins comprises the Light-Harvesting Complex Proteins (LHCs), the Early Light-Induced Proteins (ELIPs) and the Photosystem II Subunit S (PSBS). The proteins of the ELIP family were proposed to function in photoprotection or assembly of thylakoid pigment-protein complexes and are further divided into subgroups with one to three transmembrane helices. Two small One-Helix Proteins (OHPs) are expressed constitutively in green plant tissues and their levels increase in response to light stress. In this study, we show that OHP1 and OHP2 are highly conserved in photosynthetic eukaryotes, but have probably evolved independently and have distinct functions in Arabidopsis. Mutations in OHP1 or OHP2 caused severe growth deficits, reduced pigmentation and disturbed thylakoid architecture. Surprisingly, the expression of OHP2 was severely reduced in ohp1 T-DNA insertion mutants and vice versa. In both ohp1 and ohp2 mutants, the levels of numerous photosystem components were strongly reduced and photosynthetic electron transport was almost undetectable. Accordingly, ohp1 and ohp2 mutants were dependent on external organic carbon sources for growth and did not produce seeds. Interestingly, the induction of ELIP1 expression and Cu/Zn superoxide dismutase activity in low light conditions indicated that ohp1 mutants constantly suffer from photo-oxidative stress. Based on these data, we propose that OHP1 and OHP2 play an essential role in the assembly or stabilization of photosynthetic pigment-protein complexes, especially photosystem reaction centers, in the thylakoid membrane.

Functional and RNA-Sequencing Analysis Revealed Expression of a Novel Stay-Green Gene from Zoysia japonica (ZjSGR) Caused Chlorophyll Degradation and Accelerated Senescence in Arabidopsis.

Senescence is not only an important developmental process, but also a responsive regulation to abiotic and biotic stress for plants. Stay-green protein plays crucial roles in plant senescence and chlorophyll degradation. However, the underlying mechanisms were not well-studied, particularly in non-model plants. In this study, a novel stay-green gene, ZjSGR, was isolated from Zoysia japonica. Subcellular localization result demonstrated that ZjSGR was localized in the chloroplasts. Quantitative real-time PCR results together with promoter activity determination using transgenic Arabidopsis confirmed that ZjSGR could be induced by darkness, ABA and MeJA. Its expression levels could also be up-regulated by natural senescence, but suppressed by SA treatments. Overexpression of ZjSGR in Arabidopsis resulted in a rapid yellowing phenotype; complementary experiments proved that ZjSGR was a functional homolog of AtNYE1 from Arabidopsis thaliana. Over expression of ZjSGR accelerated chlorophyll degradation and impaired photosynthesis in Arabidopsis. Transmission electron microscopy observation revealed that overexpression of ZjSGR decomposed the chloroplasts structure. RNA sequencing analysis showed that ZjSGR could play multiple roles in senescence and chlorophyll degradation by regulating hormone signal transduction and the expression of a large number of senescence and environmental stress related genes. Our study provides a better understanding of the roles of SGRs, and new insight into the senescence and chlorophyll degradation mechanisms in plants.

β-carbonic anhydrases and carbonic ions uptake positively influence Arabidopsis photosynthesis, oxidative stress tolerance and growth in light dependent manner

Carbonic anhydrases (CAs) catalyse reversible interconversion of CO2 and water into bicarbonate and protons and regulate concentration of CO2 around photosynthetic enzymes. In higher plants the CAs are divided into three distinct classes α, β and γ, with members off each of them being involved in CO2 uptake, fixation or recycling. The most abundant group is βCAs. In C4 plants they are localized in the cytosol of mesophyll cells and catalyse first step of carbon concentration pathway. C3 plants contain orthologues genes encoding βCAs’s, however their functions are unknown. Given the importance of βCAs in the present study we analysed the effect of carbonic ions, selected orthologues βCAs’s gene expression and βCAs enzymatic activity on Arabidopsis photosynthesis, growth and cell death in different light conditions. Plants fertilised with 0.5–3mM sodium bicarbonate had a significantly increased number of leaves, improved fresh and dry weight and reduced cell death (cellular ion leakage). This effect was dependent on provided photon flux density and photoperiod. Higher content of carbonic ions also stimulated photoprotective mechanisms such as non-photochemical quenching and foliar content of photoprotective pigments (neoxanthin, violaxanthin and carotenes). Function of various βCAs genes examined in null βcas mutants showed to be complementary and additive, and confirm results of fertilizing experiments. Taken together, regulation of βCAs gene expression and enzymatic activities are important for optimal plant growth and probably can be one of the factor influencing a switch between C3 and C4 photosynthesis mode in variable light conditions. Therefore, biotechnological amelioration of βCAs activity in economically important plants and their fertilisation with carbonic ions may lead to improved photosynthetic efficiency and further crop productivity.

Zinc Oxide Nanoparticles Affect Biomass Accumulation and Photosynthesis in Arabidopsis.

Dramatic increase in the use of nanoparticles (NPs) in a variety of applications greatly increased the likelihood of the release of NPs into the environment. Zinc oxide nanoparticles (ZnO NPs) are among the most commonly used NPs, and it has been shown that ZnO NPs were harmful to several different plants. We report here the effects of ZnO NPs exposure on biomass accumulation and photosynthesis in Arabidopsis. We found that 200 and 300 mg/L ZnO NPs treatments reduced Arabidopsis growth by ∼20 and 80%, respectively, in comparison to the control. Pigments measurement showed that Chlorophyll a and b contents were reduced more than 50%, whereas carotenoid contents remain largely unaffected in 300 mg/L ZnO NPs treated Arabidopsis plants. Consistent with this, net rate of photosynthesis, leaf stomatal conductance, intercellular CO2 concentration and transpiration rate were all reduced more than 50% in 300 mg/L ZnO NPs treated plants. Quantitative RT-PCR results showed that expression levels of chlorophyll synthesis genes including CHLOROPHYLL A OXYGENASE (CAO), CHLOROPHYLL SYNTHASE (CHLG), COPPER RESPONSE DEFECT 1 (CRD1), MAGNESIUM-PROTOPORPHYRIN IX METHYLTRANSFERASE (CHLM) and MG-CHELATASE SUBUNIT D (CHLD), and photosystem structure gene PHOTOSYSTEM I SUBUNIT D-2 (PSAD2), PHOTOSYSTEM I SUBUNIT E-2 (PSAE2), PHOTOSYSTEM I SUBUNIT K (PSAK) and PHOTOSYSTEM I SUBUNIT K (PSAN) were reduced about five folds in 300 mg/L ZnO NPs treated plants. On the other hand, elevated expression, though to different degrees, of several carotenoids synthesis genes including GERANYLGERANYL PYROPHOSPHATE SYNTHASE 6 (GGPS6), PHYTOENE SYNTHASE (PSY) PHYTOENE DESATURASE (PDS), and ZETA-CAROTENE DESATURASE (ZDS) were observed in ZnO NPs treated plants. Taken together, these results suggest that toxicity effects of ZnO NPs observed in Arabidopsis was likely due to the inhibition of the expression of chlorophyll synthesis genes and photosystem structure genes, which results in the inhibition of chlorophylls biosynthesis, leading to the reduce in photosynthesis efficiency in the plants.

Harpin Hpa1 Interacts with Aquaporin PIP1;4 to Promote the Substrate Transport and Photosynthesis in Arabidopsis.

Harpin proteins produced by plant-pathogenic Gram-negative bacteria are the venerable player in regulating bacterial virulence and inducing plant growth and defenses. A major gap in these effects is plant sensing linked to cellular responses, and plant sensor for harpin Hpa1 from rice bacterial blight pathogen points to plasma membrane intrinsic protein (PIP). Here we show that Arabidopsis AtPIP1;4 is a plasma membrane sensor of Hpa1 and plays a dual role in plasma membrane permeability of CO2 and H2O. In particular, AtPIP1;4 mediates CO2 transport with a substantial contribute to photosynthesis and further increases this function upon interacting with Hpa1 at the plasma membrane. As a result, leaf photosynthesis rates are increased and the plant growth is enhanced in contrast to the normal process without Hpa1-AtPIP1;4 interaction. Our findings demonstrate the first case that plant sensing of a bacterial harpin protein is connected with photosynthetic physiology to regulate plant growth.

Candidate Gene Identification for a Lethal Chlorophyll-Deficient Mutant in Soybean

Chlorophyll-deficient mutants have been studied persistently to understand genetic mechanisms controlling metabolic pathways. A spontaneous chlorophyll-deficient lethal mutant was observed in self-pollinated progeny of a soybean cultivar “BSR 101”. Observed segregation patterns indicated single-gene recessive inheritance for this lethal-yellow mutant. The objectives of this investigation were to develop a genetic linkage map of the region containing the lethal-yellow (YL_PR350) gene and identify putative candidate genes for this locus. The YL_PR350 gene was mapped to chromosome 15 and is flanked by BARCSOYSSR_15_1591 and BARCSOYSSR_15_1597. This region physically spans ~153 kb and there are 14 predicted genes that lie in this region. The predicted gene Glyma.15g275900 is an excellent candidate for the YL_PR350 gene as it is homologous to an Arabidopsis gene, At3g08010, which codes for a chloroplast-localized protein (ATAB2) involved in the biogenesis of Photosystem I and II. This thylakoid membrane protein is crucial for photosynthesis in Arabidopsis. Future characterization of the candidate gene may enhance our knowledge about photosynthesis, a complex metabolic process critical for sustainability of plants.

Mitogen-Activated Protein Kinase 4 Is a Salicylic Acid-Independent Regulator of Growth But Not of Photosynthesis in Arabidopsis

Mitogen-activated protein kinase (MAPK) pathways regulate signal transduction from different cellular compartments and from the extracellular environment to the nucleus in all eukaryotes. One of the best-characterized MAPKs in Arabidopsis thaliana is MPK4, which was shown to be a negative regulator of systemic-acquired resistance. The mpk4 mutant accumulates salicylic acid (SA), possesses constitutive expression of pathogenesis-related (PR) genes, and has an extremely dwarf phenotype. We show that suppression of SA and phylloquinone synthesis in chloroplasts by knocking down the ICS1 gene (by crossing it with the ics1 mutant) in the mpk4 mutant background did not revert mpk4-impaired growth. However, it did cause changes in the photosynthetic apparatus and severely impaired the quantum yield of photosystem II. Transmission microscopy analysis revealed that the chloroplasts' structure was strongly altered in the mpk4 and mpk4/ics1 double mutant. Analysis of reactive oxygen species (ROS)-scavenging enzymes expression showed that suppression of SA and phylloquinone synthesis in the chloroplasts of the mpk4 mutant caused imbalances in ROS homeostasis which were more pronounced in mpk4/ics1 than in mpk4. Taken together, the presented results strongly suggest that MPK4 is an ROS/hormonal rheostat hub that negatively, in an SA-dependent manner, regulates immune defenses, but at the same time positively regulates photosynthesis, ROS metabolism, and growth. Therefore, we concluded that MPK4 is a complex regulator of chloroplastic retrograde signaling for photosynthesis, growth, and immune defenses in Arabidopsis.