Potential benefits and risks of solar photovoltaic power plants on arid and semi-arid ecosystems: an assessment of soil microbial and plant communities.

Exponential increase in photovoltaic installations arouses concerns regarding the impacts of large-scale solar power plants on dryland ecosystems. While the effects of photovoltaic panels on soil moisture content and plant biomass in arid ecosystems have been recognized, little is known about their influence on soil microbial communities. Here, we employed a combination of quantitative PCR, high-throughput sequencing, and soil property analysis to investigate the responses of soil microbial communities to solar panel installation. We also report on the responses of plant communities within the same solar farm. Our findings showed that soil microbial communities responded differently to the shading and precipitation-alternation effects of the photovoltaic panels in an arid ecosystem. By redirecting rainwater to the lower side, photovoltaic panels stimulated vegetation biomass and soil total organic carbon content in the middle and in front of the panels, positively contributing to carbon storage. The shade provided by the panels promoted the co-occurrence of soil microbes but inhibited the abundance of 16S rRNA gene in the soil. Increase in precipitation reduced 18S rRNA gene abundance, whereas decrease in precipitation led to decline in plant aboveground biomass, soil prokaryotic community alpha diversity, and dehydrogenase activity under the panels. These findings highlight the crucial role of precipitation in maintaining plant and soil microbial diversities in dryland ecosystems and are essential for estimating the potential risks of large-scale solar power plants on local and global climate change in the long term.

Integrative transcriptome and metabolome revealed the molecular mechanism of Bacillus megaterium BT22-mediated growth promotion in Arabidopsis thaliana.

Plant growth-promoting rhizobacteria (PGPR) can promote plant growth and protect plants from pathogens, which contributes to sustainable agricultural development. Several studies have reported their beneficial characteristics in facilitating plant growth and development and enhancing plant stress resistance through different mechanisms. However, there is still a challenge to study the molecular mechanism of plant response to PGPR. We integrated the transcriptome and metabolome of Arabidopsis thaliana (Arabidopsis) to understand its responses to the inoculation with an isolated PGPR strain (BT22) of Bacillus megaterium. Fresh shoot weight, dry shoot weight and leaf number of Arabidopsis were increased by BT22 treatment, showing a positive growth-promoting effect. According multi-omics analysis, 878 differentially expressed genes (296 up-regulated, 582 down-regulated) and 139 differentially expressed metabolites (66 up-regulated, 73 down-regulated) response to BT22 inoculation. GO enrichment results indicate that the up-regulated genes mainly enriched in the regulation of growth and auxin response pathways. In contrast, the down-regulated genes mainly enriched in wounding response, jasmonic acid and ethylene pathways. BT22 inoculation regulated plant hormone signal transduction of Arabidopsis, including auxin and cytokinin response genes AUX/IAA, SAUR, and A-ARR related to cell enlargement and cell division. The contents of nine flavonoids and seven phenylpropanoid metabolites were increased, which help to induce systemic resistance in plants. These results suggest that BT22 promoted Arabidopsis growth by regulating plant hormone homeostasis and inducing metabolome reprogramming.

Cytosolic glucose-6-phosphate dehydrogenases play a pivotal role in Arabidopsis seed development.

Embryo development is essential for seed yield and post-germination growth. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in oxidative pentose phosphate pathway (OPPP), is widely involved in plant development and stress tolerance by providing nicotinamide adenine dinucleotide phosphate (NADPH). In this study, the double mutant (g6pd5/6), overexpression line (G6PD5/6OE) and complementation line (g6pd5/6Comp) of cytosolic glucose-6-phosphate dehydrogenases (Cyt-G6PD) were used to investigate Cyt-G6PD roles in embryo development of Arabidopsis. The results showed that the germination rate of g6pd5/6 seeds was delayed in comparison with that of Col-0; moreover, 11.5% of g6pd5/6 seeds did not germinate. The dysfunction of Cyt-G6PD resulted in decreased fresh weight and primary root length of g6pd5/6 seedlings. The height and silique length of g6pd5/6 plants were also decreased. Moreover, the abortion rate of siliques and seeds of g6pd5/6 plants were increased compared with those of Col-0, G6PD5/6OE and g6pd5/6Comp lines. However, the dysfunction of Cyt-G6PD did not affect pollen activity; but in g6pd5/6, the embryo development was partially delayed or inhibited. The contents of fatty acids and storage proteins, two main storage materials in Arabidopsis seeds, were decreased in g6pd5/6 seeds. Exogenous application of fatty acids (C18:2; C18:3) alleviated the delayed germination of g6pd5/6 seeds. RT-qPCR results further demonstrated that the early embryo development genes were down-regulated in g6pd5/6. Taken together, Cyt-G6PD plays a pivotal role in plant seed development by regulating the transcriptions of early embryo development genes and the accumulation of storage materials (especially fatty acids).

UCP1 and AOX1a contribute to regulation of carbon and nitrogen metabolism and yield in Arabidopsis under low nitrogen stress.

Nitrogen (N) availability is a critical factor for plant development and crop yield, and it closely correlates to carbon (C) metabolism. Uncoupling protein (UCP) and alternative oxidase (AOX) exhibit a strong correlation with N and C metabolism. Here, we investigated the functions of UCP1 and AOX1a using their mutants and complementation lines in Arabidopsis adaptation to low N. Low N markedly increased AOX1a and UCP1 expression, alternative pathway capacity and UCP activity. Eight-day-old aox1a/ucp1 seedlings were more sensitive to low N than Col-0 and single mutants, exhibiting lower primary root length and higher anthocyanin accumulation. The net photosynthetic rate, electron transport rate, PSII actual photochemical efficiency, stomatal conductance and carboxylation efficiency were markedly decreased in ucp1 and aox1a/ucp1 compared to those in Col-0 and aox1a under low N stress; comparatively, chlorophyll content and non-photochemical quenching coefficient were the lowest and highest in aox1a/ucp1, respectively. Nitrate acquisition rate was accelerated in aox1a/ucp1, but its transport activity was decreased, which resulted in low nitrate content and nitrate reductase activity under low N condition. The C/N ratio in seeds, but not in leaves, is higher in aox1a/ucp1 than that in Col-0, aox1a and ucp1 under low N condition. RNA-seq analysis revealed that many genes involved in photosynthesis and C/N metabolism were markedly down-regulated in aox1a/ucp1 under low N stress. These results highlight the key roles of UCP1 and AOX1a in modulating photosynthetic capacity, C/N assimilation and distribution under low N stress.

PIF4-PAP1 interaction affects MYB-bHLH-WD40 complex formation and anthocyanin accumulation in Arabidopsis.

Anthocyanin accumulation is a marked phenotype of plants under environmental stresses. PHYTOCHROME-INTERACTING FACTORs (PIFs) are involved in environment-induced anthocyanin biosynthesis through interacting with the MYB-bHLH-WD40 (MBW) complex. However, the molecular mechanism of this interaction remains unclear. The present study demonstrated that PIF3 and PIF5 can slightly repress anthocyanin accumulation under NaCl, low nitrogen (-N), or 6-BA treatments; in contrast, PIF4 can significantly repress anthocyanin accumulation. Bimolecular fluorescence complementation and yeast two-hybrid assays showed that PIF4 directly interacts with PRODUCTION OF ANTHOCYANIN PIGMENT 1 (PAP1), a MYB transcription factor in the MBW complex. Further analysis revealed that the active phytochrome binding (APB) domain in the N terminus of PIF4 is necessary for the interaction between PIF4 and PAP1. Yeast three-hybrid analysis showed that PIF4 competes with TRANSPARENT TESTA 8 (TT8) to bind PAP1, thereby interfering with the regulation of the MBW protein complex in anthocyanin synthesis. Consistently, the anthocyanin content in pap1-D/35S::PIF4 and 35S::PAP1/35S::PIF4 seedlings was markedly lower than that in pap1-D and 35S::PAP1 under 6-BA, MeJA, -N, and NaCl stresses, implying that overexpression of PIF4 suppresses anthocyanin accumulation in pap1-D and 35S::PAP1. Thus, PIF4 is genetically epistatic to PAP1. Taken together, PIF4 plays a negative role in modulating anthocyanin biosynthesis in Arabidopsis under different stress environments, and PIF4 interacts with PAP1 to affect the integrity of the MBW complex.

Integrated Physiological, Transcriptomic, and Metabolomic Analyses Revealed Molecular Mechanism for Salt Resistance in Soybean Roots.

Salinity stress is a threat to yield in many crops, including soybean (Glycine max L.). In this study, three soybean cultivars (JD19, LH3, and LD2) with different salt resistance were used to analyze salt tolerance mechanisms using physiology, transcriptomic, metabolomic, and bioinformatic methods. Physiological studies showed that salt-tolerant cultivars JD19 and LH3 had less root growth inhibition, higher antioxidant enzyme activities, lower ROS accumulation, and lower Na+ and Cl- contents than salt-susceptible cultivar LD2 under 100 mM NaCl treatment. Comparative transcriptome analysis showed that compared with LD2, salt stress increased the expression of antioxidant metabolism, stress response metabolism, glycine, serine and threonine metabolism, auxin response protein, transcription, and translation-related genes in JD19 and LH3. The comparison of metabolite profiles indicated that amino acid metabolism and the TCA cycle were important metabolic pathways of soybean in response to salt stress. In the further validation analysis of the above two pathways, it was found that compared with LD2, JD19, and LH3 had higher nitrogen absorption and assimilation rate, more amino acid accumulation, and faster TCA cycle activity under salt stress, which helped them better adapt to salt stress. Taken together, this study provides valuable information for better understanding the molecular mechanism underlying salt tolerance of soybean and also proposes new ideas and methods for cultivating stress-tolerant soybean.

Alternative Pathway Is Involved in Hydrogen Peroxide-Enhanced Cadmium Tolerance in Hulless Barley Roots.

Hulless barley, grown in the Qinghai Tibet Plateau, has a wide range of environmental stress tolerance. Alternative pathway (AP) and hydrogen peroxide (H2O2) are involved in enhancing plant tolerance to environmental stresses. However, the relationship between H2O2 and AP in hulless barley tolerance to cadmium (Cd) stress remains unclear. In the study, the role and relationship of AP and H2O2 under Cd stress were investigated in hulless barley (Kunlun14) and common barley (Ganpi6). Results showed that the expression level of alternative oxidase (AOX) genes (mainly AOX1a), AP capacity (Valt), and AOX protein were clearly induced more in Kunlun14 than in Ganpi 6 under Cd stress; moreover, these parameters were further enhanced by applying H2O2. Malondialdehyde (MDA) content, electrolyte leakage (EL) and NAD(P)H to NAD(P) ratio also increased in Cd-treated roots, especially in Kunlun 14, which can be markedly alleviated by exogenous H2O2. However, this mitigating effect was aggravated by salicylhydroxamic acid (SHAM, an AOX inhibitor), suggesting AP contributes to the H2O2-enhanced Cd tolerance. Further study demonstrated that the effect of SHAM on the antioxidant enzymes and antioxidants was minimal. Taken together, hulless barley has higher tolerance to Cd than common barley; and in the process, AP exerts an indispensable function in the H2O2-enhanced Cd tolerance. AP is mainly responsible for the decrease of ROS levels by dissipating excess reducing equivalents.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD.

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Cytoplasmic glucose-6-phosphate dehydrogenase plays an important role in the silicon-enhanced alkaline tolerance in highland barley.

Glucose-6-phosphate dehydrogenase (G6PDH), as a key enzyme in the pentose phosphate pathway, extensively responds to the biotic and abiotic stresses. In this study we focussed on the G6PDH role in the alleviation of alkaline stress induced by silicon (Si) in highland barley. Application of Si reduced the water loss and malondialdehyde (MDA) and reactive oxygen species (ROS) contents, improved the fresh weight, photosynthesis, K+ content, and the superoxide dismutase (SOD) and catalase (CAT) activities, thus alleviating the damage caused by alkaline stress. The G6PDH activity, especially the cytoplasmic G6PDH, significantly increased under alkaline stress, and was further stimulated by the addition of exogenous Si. Meanwhile, the levels of NADPH and reduced glutathione (GSH) showed similar profiles to G6PDH activity under NaHCO3 and NaHCO3 + Si treatments. The inhibition of G6PDH activity by glucosamine abolished the relieving effect of Si on alkaline stress, which was manifested in the increase of ROS and the decrease of GSH content. Together, our results suggest that Si-enhanced tolerance of alkaline stress may be related to the regulation of GSH levels by the cytoplasmic G6PDH in highland barley.

Identification, Characterization, and Stress Responsiveness of Glucose-6-phosphate Dehydrogenase Genes in Highland Barley.

G6PDH provides intermediate metabolites and reducing power (nicotinamide adenine dinucleotide phosphate, NADPH) for plant metabolism, and plays a pivotal role in the cellular redox homeostasis. In this study, we cloned five G6PDH genes (HvG6PDH1 to HvG6PDH5) from highland barley and characterized their encoded proteins. Functional analysis of HvG6PDHs in E. coli showed that HvG6PDH1 to HvG6PDH5 encode the functional G6PDH proteins. Subcellular localization and phylogenetic analysis indicated that HvG6PDH2 and HvG6PDH5 are localized in the cytoplasm, while HvG6PDH1, HvG6PDH3, and HvG6PDH4 are plastidic isoforms. Analysis of enzymatic activities and gene expression showed that HvG6PDH1 to HvG6PDH4 are involved in responses to salt and drought stresses. The cytosolic HvG6PDH2 is the major isoform against oxidative stress. HvG6PDH5 may be a house-keeping gene. In addition, HvG6PDH1 to HvG6PDH4 and their encoded enzymes responded to jasmonic acid (JA) and abscisic acid (ABA) treatments, implying that JA and ABA are probably critical regulators of HvG6PDHs (except for HvG6PDH5). Reactive oxygen species analysis showed that inhibition of cytosolic and plastidic G6PDH activities leads to increased H2O2 and O2- contents in highland barley under salt and drought stresses. These results suggest that G6PDH can maintain cellular redox homeostasis and that cytosolic HvG6PDH2 is an irreplaceable isoform against oxidative stress in highland barley.

Involvement of active MKK9-MAPK3/MAPK6 in increasing respiration in salt-treated Arabidopsis callus.

Mitogen-activated protein kinase kinase 9 (MKK9) is an upstream activator of mitogen-activated protein kinase 3 (MAPK3) and MAPK6 in planta. To investigate MKK9 roles in mitochondrial respiration in Arabidopsis, MKK9DD, the active allele with mutations of Thr-201 and Ser-205 to Asp, and MKK9KR, the allele lacking MKK9 activity with a mutation of Lys-76 to Arg, were used. Results showed that the total respiratory rate (Vt), alternative pathway capacity (Valt) and cytochrome pathway capacity (Vcyt) increased under 0-100 mM NaCl treatments but decreased under 150-300 mM NaCl treatments in Col-0 callus. However, the activation of MKK9 by dexamethasone (DEX) increased Vt, Valt and Vcyt under 200 mM NaCl treatment; moreover, Valt showed more increase than Vcyt. The activation of MKK9 in MKK9DD callus sharply increased AOX protein expression under normal and NaCl conditions, but the increase was not observed in MKK9KR callus. Further results indicated that MAPK3 and MAPK6 were involved in the MKK9-induced increase of AOX protein levels. qRT-PCR results showed that MKK9-MAPK3/MAPK6 was involved in the NaCl-induced AOX1b and AOX1d expression, but only MKK9-MAPK3 was necessary for AOX2 expression; in addition, MAPK3 regulated the AOX1a transcription in an MKK9-independent manner. MKK9 positively regulated SOD and CAT activities by affecting MAPK3 and MAPK6 and negatively regulated APX and POD activities by affecting MAPK3. Moreover, MKK9 functions as a positive factor in H2O2 accumulation under salt stress. The regulation of ethylene on alternative respiration was also associated with MKK9 under salt stress. Taken together, the MKK9-MAPK3/MAPK6 pathway plays a pivotal role in increasing alternative respiration in the salt-treated Arabidopsis callus.

Nitric oxide and hydrogen peroxide increase glucose-6-phosphate dehydrogenase activities and expression upon drought stress in soybean roots.

KEY MESSAGE: Changes in glucose-6-phosphate dehydrogenase (G6PD) isoforms activities and expression were investigated in soybean roots under drought, suggesting that cytosolic G6PD plays a main role by regulating H2O2 signal and redox homeostasis. G6PD acts a vital role in plant growth, development and stress adaptation. Drought (PEG6000 treatment) could markedly increase the enzymatic activities of cytosolic G6PD (Cyt-G6PD) and compartmented G6PD (mainly plastidic P2-G6PD) in soybean roots. Application of G6PD inhibitor upon drought condition dramatically decreased the intracellular NADPH and reduced glutathione levels in soybean roots. Nitric oxide (NO) and hydrogen peroxide (H2O2) participated in the regulation of Cyt-G6PD and P2-G6PD enzymatic activities under drought stress. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, abolished the drought-induced accumulation of H2O2. The exogenous application of H2O2 and its production inhibitor (DPI) could stimulate and inhibit the NO accumulation, respectively, but not vice versa. qRT-PCR analysis confirmed that NO, as the downstream signal of H2O2, positively regulated the transcription of genes encoding Cyt-G6PD (GPD5, G6PD6, G6PD7) under drought stress in soybean roots. Comparatively, NO and H2O2 signals negatively regulated the gene expression of compartmented G6PD (GPD1, G6PD2, G6PD4), indicating that a post-transcriptional mechanism was involved in compartmented G6PD regulation. Taken together, the high Cyt-G6PD activity is essential for maintaining redox homeostasis upon drought condition in soybean roots, and the H2O2-dependent NO cascade signal is differently involved in Cyt-G6PD and compartmented G6PD regulation.

Alternative Pathway is Involved in Nitric Oxide-Enhanced Tolerance to Cadmium Stress in Barley Roots.

Alternative pathway (AP) has been widely accepted to be involved in enhancing tolerance to various environmental stresses. In this study, the role of AP in response to cadmium (Cd) stress in two barley varieties, highland barley (Kunlun14) and barley (Ganpi6), was investigated. Results showed that the malondialdehyde (MDA) content and electrolyte leakage (EL) level under Cd stress increased in two barley varieties. The expressions of alternative oxidase (AOX) genes (mainly AOX1a), AP capacity (Valt), and AOX protein amount were clearly induced more in Kunlun14 under Cd stress, and these parameters were further enhanced by applying sodium nitroprussid (SNP, a NO donor). Moreover, H2O2 and O2- contents were raised in the Cd-treated roots of two barley varieties, but they were markedly relieved by exogenous SNP. However, this mitigating effect was aggravated by salicylhydroxamic acid (SHAM, an AOX inhibitor), suggesting that AP contributes to NO-enhanced Cd stress tolerance. Further study demonstrated that the effect of SHAM application on reactive oxygen species (ROS)-related scavenging enzymes and antioxidants was minimal. These observations showed that AP exerts an indispensable function in NO-enhanced Cd stress tolerance in two barley varieties. AP was mainly responsible for regulating the ROS accumulation to maintain the homeostasis of redox state.

Cytosolic Glucose-6-Phosphate Dehydrogenase Is Involved in Seed Germination and Root Growth Under Salinity in Arabidopsis.

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) is the key regulatory enzyme in the oxidative pentose phosphate pathway (OPPP). The cytosolic isoforms including G6PD5 and G6PD6 account for the major part of the G6PD total activity in plant cells. Here, we characterized the Arabidopsis single null mutant g6pd5 and g6pd6 and double mutant g6pd5/6. Compared to wild type, the mutant seeds showed a reduced germination rate and root elongation under salt stress. The seeds and seedlings lacking G6PD5 and G6PD6 accumulate more reactive oxygen species (ROS) than the wild type under salt stress. Cytosolic G6PD (cy-G6PD) affected the expression of NADPH oxidases and the G6PD enzymatic activities in the mutant atrbohD/F, in which the NADPH oxidases genes are disrupted by T-DNA insertion and generation of ROS is inhibited, were lower than that in the wild type. The NADPH level in mutants was decreased under salt stress. In addition, we found that G6PD5 and G6PD6 affected the activities and transcript levels of various antioxidant enzymes in response to salt stress, especially the ascorbate peroxidase and glutathione reductase. Exogenous application of ascorbate acid and glutathione rescued the seed and root phenotype of g6pd5/6 under salt stress. Interestingly, the cytosolic G6PD negatively modulated the NaCl-blocked primary root growth under salt stress in the root meristem and elongation zone.

Involvement of G6PD5 in ABA response during seed germination and root growth in Arabidopsis.

BACKGROUND: Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) functions in supply of NADPH, which is required for plant defense responses to stresses. However, whether G6PD functions in the abscisic acid (ABA) signaling pathway remains to be elucidated. In this study, we investigated the involvement of the cytosolic G6PD5 in the ABA signaling pathway in Arabidopsis. RESULTS: We characterized the Arabidopsis single null mutant g6pd5. Phenotypic analysis showed that the mutant is more sensitive to ABA during seed germination and root growth, whereas G6PD5-overexpressing plants are less sensitive to ABA compared to wild type (WT). Furthermore, ABA induces excessive accumulation of reactive oxygen species (ROS) in mutant seeds and seedlings. G6PD5 participates in the reduction of H2O2 to H2O in the ascorbate-glutathione cycle. In addition, we found that G6PD5 suppressed the expression of Abscisic Acid Insensitive 5 (ABI5), the major ABA signaling component in dormancy control. When G6PD5 was overexpressed, the ABA signaling pathway was inactivated. Consistently, G6PD5 negatively modulates ABA-blocked primary root growth in the meristem and elongation zones. Of note, the suppression of root elongation by ABA is triggered by the cell cycle B-type cyclin CYCB1. CONCLUSIONS: This study showed that G6PD5 is involved in the ABA-mediated seed germination and root growth by suppressing ABI5.

Alternative respiration pathway is involved in the response of highland barley to salt stress.

KEY MESSAGE: Alternative respiration pathway is involved in the response of highland barley to salt stress. The response of two barley seedlings to salt stress was investigated. Results showed that the growth of highland barley (Kunlun 14) and barley (Ganpi 6) had no obvious difference under low concentrations (50, 100 and 200 mM) of NaCl treatment. However, high concentrations of NaCl treatment (300 and 400 mM) severely affected the growth of two barley cultivars. Under 300 mM NaCl treatment, the fresh weight, relative water content (RWC), pigments and K+ content reduced more in Ganpi 6 than in Kunlun 14. In contrast, the electrolyte leakage and the content of MDA, Na+, H2O2 and O2- increased more in Ganpi 6 than in Kunlun 14. The gene expression of AOX1a, HvNHX1, HvNHX3, HvHVP1, HvHVA, H+-ATPase, the alternative respiration capacity (Valt) and the enzymatic activity of SOD, POD, CAT, APX and H+-ATPase increased more in Kunlun14 than in Ganpi6 under 300 mM NaCl treatment, whereas the cytochrome respiration capacity (Vcyt) decreased similarly in both barley cultivars. Western blot analysis showed that the protein level of the alternative oxidase (AOX) increased more in Kunlun 14 than in Ganpi 6 under 300 mM NaCl treatment. Inhibition of the alternative respiration by salicylhydroxamic acid (SHAM) decreased the fresh weight, K+ content, Valt, H+-ATPase activity and the gene expression of AOX1a, HvNHX1, HvNHX3, HvHVP1, HvHVA, H+-ATPase, but increased the electrolyte leakage, MDA and Na+ content in both cultivars under 300 mM NaCl treatment. In short, alternative respiration is involved in the tolerance of highland barley to salt stress.

Alternative pathway is involved in the tolerance of highland barley to the low-nitrogen stress by maintaining the cellular redox homeostasis.

KEY MESSAGE: Alternative pathway (AP) is involved in the tolerance of highland barley seedlings to the low-nitrogen stress by dissipating excessive reducing equivalents generated by photosynthesis and maintaining the cellular redox homeostasis. Low nitrogen (N) is a major limiting factor for plant growth and crop productivity. In this study, we investigated the roles of the alternative pathway (AP) in the tolerance of two barley seedlings, highland barley (Kunlun12) and barley (Ganpi6), to low-N stress. The results showed that the chlorophyll content and the fresh weight decreased more in Ganpi6 than those in Kunlun12 under low-N stress, suggesting that Kunlun12 has higher tolerance to low-N stress than Ganpi6. AP capacity was markedly induced by low-N stress; and it was higher in Kunlun12 than in Ganpi6. Comparatively, the cytochrome pathway capacity was not affected under all conditions. Western-blot analysis showed that the protein level of the alternative oxidase (AOX) increased under low-N stress in Kunlun12 but not in Ganpi6. Under low-N stress, the NAD(P)H content and the NAD(P)H to NAD(P)(+)+NAD(P)H ratio in Ganpi6 increased more than those in Kunlun12. Furthermore, photosynthetic parameters (Fv/Fm, qP, ETR and Yield) decreased markedly and qN increased, indicating photoinhibition occurred in both barley seedlings, especially in Ganpi6. When AP was inhibited by salicylhydroxamic acid (SHAM), the NAD(P)H content and the NAD(P)H to NAD(P)(+)+NAD(P)H ratio dramatically increased under all conditions, resulting in the marked accumulation of H(2)O(2) and malondialdehyde in leaves of both barley seedlings. Meanwhile, the malate-oxaloacetate shuttle activity and the photosynthetic efficiency were further inhibited. Taken together, AP is involved in the tolerance of highland barley seedlings to low-N stress by dissipating excess reducing equivalents and maintaining the cellular redox homeostasis.

Calcium alleviates cadmium-induced inhibition on root growth by maintaining auxin homeostasis in Arabidopsis seedlings.

Cadmium (Cd) toxicity has been widely studied in different plant species. However, the mechanism involved in its toxicity and the cell response to Cd has not been well established. In the present study, we investigated the possible mechanism of calcium (Ca) in protecting Arabidopsis from Cd toxicity. The results showed that 50 µM Cd significantly inhibited the seedling growth and decreased the chlorophyll content in Arabidopsis. Specifically, the primary root (PR) length was decreased but the lateral root (LR) number was increased under Cd stress. Furthermore, Cd enhanced the hydrogen peroxide (H2O2) content and lipid peroxidation as indicated by malondialdehyde (MDA) accumulation. Cd also altered the level and the distribution of auxin in PR tips (as evidenced by DR5::GUS and PIN:GFP reporter expression) and the expression of several putative auxin biosynthetic, catabolic, and transport pathway-related genes. Application of 3 mM Ca alleviated the inhibition of Cd on the root growth. Ca application not only led to reducing oxidative injuries but also restoring the normal auxin transport and distribution in Arabidopsis root under Cd stress. Taken together, these results suggest that Ca alleviates the root growth inhibition caused by Cd through maintaining auxin homeostasis in Arabidopsis seedlings.

Narciclasine, a potential allelochemical, affects subcellular trafficking of auxin transporter proteins and actin cytoskeleton dynamics in Arabidopsis roots.

MAIN CONCLUSION: The present study documented the action of a potential allelochemical, narciclasine, on auxin transport in Arabidopsis by mainly affecting subcellular trafficking of PIN and AUX1 proteins and through interfering actin cytoskeletal organization. Narciclasine (NCS), an Amaryllidaceae alkaloid isolated from Narcissus tazetta bulbs, has potential allelopathic activity and affects auxin transport. However, little is known about the cellular mechanism of this inhibitory effect of NCS on auxin transport. The present study characterizes the effects of NCS at the cellular level using transgenic Arabidopsis plants harboring the promoters of PIN, in combination with PIN-GFP proteins or AUX1-YFP fusions. NCS treatment caused significant reduction in the abundance of PIN and AUX1 proteins at the plasma membrane (PM). Analysis of the subcellular distribution of PIN and AUX1 proteins in roots revealed that NCS induced the intracellular accumulation of auxin transporters, including PIN2, PIN3, PIN4, PIN7 and AUX1. However, other PM proteins, such as PIP2, BRI1, and low temperature inducible protein 6b (LTI6b), were insensitive to NCS treatment. NCS-induced PIN2 compartments were further defined using endocytic tracer FM 4-64 labeled early endosomes and suggested that this compound affects the endocytosis trafficking of PIN proteins. Furthermore, pharmacological analysis indicated that the brefeldin A (BFA)-insensitive pathway is employed in the cellular effects of NCS on PIN2 trafficking. Although NCS did not alter actin dynamics in vitro, it resulted in the depolymerization of the actin cytoskeleton in vivo. This disruption of actin filaments by NCS subsequently influences the actin-based vesicle motility. Hence, the elucidation of the specific role of NCS is useful for further understanding the mechanisms of allelopathy at the phytohormone levels.

Phytochrome-interacting factors PIF4 and PIF5 negatively regulate anthocyanin biosynthesis under red light in Arabidopsis seedlings.

Light is an important environmental factor inducing anthocyanin accumulation in plants. Phytochrome-interacting factors (PIFs) have been shown to be a family of bHLH transcription factors involved in light signaling in Arabidopsis. Red light effectively increased anthocyanin accumulation in wild-type Col-0, whereas the effects were enhanced in pif4 and pif5 mutants but impaired in overexpression lines PIF4OX and PIF5OX, indicating that PIF4 and PIF5 are both negative regulators for red light-induced anthocyanin accumulation. Consistently, transcript levels of several genes involved in anthocyanin biosynthesis and regulatory pathway, including CHS, F3'H, DFR, LDOX, PAP1 and TT8, were significantly enhanced in mutants pif4 and pif5 but decreased in PIF4OX and PIF5OX compared to in Col-0, indicating that PIF4 and PIF5 are transcriptional repressor of these gene. Transient expression assays revealed that PIF4 and PIF5 could repress red light-induced promoter activities of F3'H and DFR in Arabidopsis protoplasts. Furthermore, chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) test and electrophoretic mobility shift assay (EMSA) showed that PIF5 could directly bind to G-box motifs present in the promoter of DFR. Taken together, these results suggest that PIF4 and PIF5 negatively regulate red light-induced anthocyanin accumulation through transcriptional repression of the anthocyanin biosynthetic genes in Arabidopsis.