Interaction Between ABA Signaling and Copper Homeostasis in Arabidopsis thaliana

The Role of Blue and Red Light in the Orchestration of Secondary Metabolites, Nutrient Transport and Plant Quality

Light is a fundamental environmental parameter for plant growth and development because it provides an energy source for carbon fixation during photosynthesis and regulates many other physiological processes through its signaling. In indoor horticultural cultivation systems, sole-source light-emitting diodes (LEDs) have shown great potential for optimizing growth and producing high-quality products. Light is also a regulator of flowering, acting on phytochromes and inducing or inhibiting photoperiodic plants. Plants respond to light quality through several light receptors that can absorb light at different wavelengths. This review summarizes recent progress in our understanding of the role of blue and red light in the modulation of important plant quality traits, nutrient absorption and assimilation, as well as secondary metabolites, and includes the dynamic signaling networks that are orchestrated by blue and red wavelengths with a focus on transcriptional and metabolic reprogramming, plant productivity, and the nutritional quality of products. Moreover, it highlights future lines of research that should increase our knowledge to develop tailored light recipes to shape the plant characteristics and the nutritional and nutraceutical value of horticultural products.

Deficient copper availability on organoleptic and nutritional quality of tomato fruit

Copper (Cu) is an essential micronutrient for plants because it functions as a redox-active cofactor in vital processes inside the cells. Arable lands are often deficient in micronutrient contents and require the application of enriched fertilisers, whose overuse poses a high risk for human health, the environment and the food safety. Here, we aimed to decipher the effects of Cu deficiency during fruit growth on Cu and other micronutrients contents and on the fruit nutritional value and quality of tomato, the most consumed fruit worldwide, throughout the maturation process. Changes in the contents of important micronutrients for fruit physiology and human health, such as Fe and Mn, occurred in response to Cu deficient growing conditions at different fruit ripening stages, while lower Cu levels were detected in those fruit along the whole maturation process. Cu deficiency delayed changes in lycopene content and fruit colour, but increased acidity, and advanced the rise in antioxidant capacity and vitamin C content during fruit colour change from green to light red in the Moneymaker tomato; although this time lag eventually caught up in the most mature fruit stage. Cu deficiency also increased total phenolic and flavonoid contents only in green fruit.

Biochemical Insights into the Ability of Lemna minor L. Extract to Counteract Copper Toxicity in Maize

Metal trace elements (MTE) can damage crops if present in excessive amounts in the environment. This research investigated the effect of a plant extract of an aquatic species, Lemna minor L. (duckweed) (LE), on the ability of maize to cope with copper (Cu) toxicity. LE reversed the effects of Cu²⁺ on photosynthetic activity (Pn), evapotranspiration (E), stomatal conductance (gs), sub-stomatal CO₂ concentration (Ci) and biomass which did not differ from the untreated controls. LE did not regulate the amount of copper in maize leaves, but compared to Cu-treated samples, the extract decreased the hydrogen peroxide (H₂O₂; -26% on average) and malondialdehyde (MDA; -47% on average) content, regardless of the dosage applied. Furthermore, the activity of antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) was significantly increased by LE compared to samples treated with Cu alone. Untargeted metabolomic profiling revealed that LE activated maize secondary metabolism, eliciting the content of non-enzymatic antioxidants (flavonoids, glutathione and glutathione-related compounds, tocopherols and tocotrienols) and modulating plant stress-related hormones (brassinosteroids and ABA derivatives). The results of this study are promising and pave the way for using duckweed as a biostimulant to trigger beneficial effects in maize and increase its resistance to MTEs.

Implication of exogenous abscisic acid (ABA) application on phytoremediation: plants grown in co-contaminated soil

Abscisic acid (ABA) may play an important role in alleviating negative effects of heavy metal stress on growth performance of plants. A pot experiment was conducted to investigate differential effects of exogenous ABA with different concentrations (0, 20, 40, and 60 μmol/L) on heavy metal accumulation and physiological response of Cd/Zn hyperaccumulator Sedum alfredii Hance and non-hyperaccumulator Hylotelephium spectabile (Boreau) H. Ohba grown in co-contaminated soil. In the experiment, Cd, Zn, or Pb concentration in stem and leaf of H. spectabile was significantly increased by exogenous ABA application than control. However, the opposite pattern was observed for S. alfredii. With decrease of Cd concentration, Zn or Pb concentration in root of H. spectabile grown in co-contaminated soil was significantly increased by exogenous ABA application than control. Cd, Zn, or Pb concentration in root of S. alfredii was significantly increased by exogenous ABA application than control. Compared with S. alfredii, BCF and TF of Cd, Zn, or Pb for H. spectabile were significantly increased by exogenous ABA application. With negative effect on root growth, total biomass of the two species, especially H. spectabile, was significantly increased by exogenous ABA application than control. With increase of their total chlorophyll content, antioxidant capacity of the two species subjected to heavy metal stress was improved by exogenous ABA application than control. Heavy metal-induced growth inhibition was significantly alleviated by exogenous ABA application when the two species were grown in co-contaminated soil. We tentatively concluded that differential effects of exogenous ABA application on transport pathway of ions incurred different patterns of heavy metal accumulation between Cd/Zn hyperaccumulator S. alfredii and non-hyperaccumulator H. spectabile. It is suggested that compared with Cd/Zn hyperaccumulator S. alfredii, exogenous ABA application may improve heavy metal uptake in root and transport of heavy metal ions between different organs for non-hyperaccumulator H. spectabile grown in co-contaminated soil. Our results provide insight into effects of exogenous ABA application on phytoremediation of Cd-, Pb-, and Zn-co-contaminated soil.

Identification and molecular characterization of the high-affinity copper transporters family in Solanum lycopersicum

Copper (Cu) plays a key role as cofactor in the plant proteins participating in essential cellular processes, such as electron transport and free radical scavenging. Despite high-affinity Cu transporters (COPTs) being key participants in Cu homeostasis maintenance, very little is known about COPTs in tomato (Solanum lycopersicum) even though it is the most consumed fruit worldwide and this crop is susceptible to suboptimal Cu conditions. In this study, a six-member family of COPT (SlCOPT1-6) was identified and characterized. SlCOPTs have a conserved architecture consisting of three transmembrane domains and β-strains. However, the presence of essential methionine residues, a methionine-enriched amino-terminal region, an Mx₃Mx₁₂Gx₃G Cu-binding motif and a cysteine rich carboxy-terminal region, all required for their functionality, is more variable among members. Accordingly, functional complementation assays in yeast indicate that SlCOPT1 and SlCOPT2 are able to transport Cu inside the cell, while SlCOPT3 and SlCOPT5 are only partially functional. In addition, protein interaction network analyses reveal the connection between SlCOPTs and Cu P_IB-type ATPases, other metal transporters, and proteins related to the peroxisome. Gene expression analyses uncover organ-dependency, fruit vasculature tissue specialization and ripening-dependent gene expression profiles, as well as different response to Cu deficiency or toxicity in an organ-dependent manner.

Effect of Cadmium and Copper Exposure on Growth, Physio-Chemicals and Medicinal Properties of Cajanus cajan L. (Pigeon Pea)

Soil contamination with heavy metals is an emerging concern in the modern era, affecting all forms of life. Pigeon pea is a multi-use shrub with medicinal and nutritional values. On the basis of a randomized complete design, we investigated in the current project the combined cadmium (Cd) and copper (Cu) effect on plant growth and physio-chemical/medicinal properties of pigeon pea. Three-week-old seedlings were grown in combined Cd and Cu amended soil with increasing metal concentrations (control, 20 + 30 mg/kg, 40 + 60 mg/kg, and 60 + 90 mg/kg) for three months. At high-dose metal cumulative stress (60 + 90 mg/kg), plant shoot and root growth in terms of plant height as well as fresh and dry weight were significantly inhibited in association with decreased photosynthetic attributes (chlorophyll a and b contents, net photosynthesis, transpiration rate, stomatal conductance, intercellular CO₂ concentrations) and diminished nutrient contents. Cd and Cu at high amounts inflicted oxidative stresses as assessed in elevated lipid peroxidation (MDA), hydrogen peroxide (H₂O₂), and electrolyte leakage contents. Antioxidant enzyme activities, namely, those of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and glutathione peroxidase (GPX), were enhanced, along with proline content with increasing metal quantity. Phenolics and flavonoids exhibited a diverse response regarding metal concentration, and their biosynthesis was significantly suppressed at high Cd and Cu cumulative stress. The reduction in secondary metabolites may account for declined medicinal properties of pigeon pea as appraised in reduced antibacterial, 2, 2-diphenyl-1-picrylhydrazyl (DPPH), and ferric-reducing antioxidant potential (FRAP) activities. Our results clearly demonstrate that the exposure of pigeon pea to Cd- and Cu-contaminated soil might affect consumers due to the presence of metals and the negligible efficacy of the herbal products.

Interplay between Hormones and Several Abiotic Stress Conditions on Arabidopsis thaliana Primary Root Development

As sessile organisms, plants must adjust their growth to withstand several environmental conditions. The root is a crucial organ for plant survival as it is responsible for water and nutrient acquisition from the soil and has high phenotypic plasticity in response to a lack or excess of them. How plants sense and transduce their external conditions to achieve development, is still a matter of investigation and hormones play fundamental roles. Hormones are small molecules essential for plant growth and their function is modulated in response to stress environmental conditions and internal cues to adjust plant development. This review was motivated by the need to explore how Arabidopsis thaliana primary root differentially sense and transduce external conditions to modify its development and how hormone-mediated pathways contribute to achieve it. To accomplish this, we discuss available data of primary root growth phenotype under several hormone loss or gain of function mutants or exogenous application of compounds that affect hormone concentration in several abiotic stress conditions. This review shows how different hormones could promote or inhibit primary root development in A. thaliana depending on their growth in several environmental conditions. Interestingly, the only hormone that always acts as a promoter of primary root development is gibberellins.

Early responses of maize seedlings to Cu stress include sharp decreases in gibberellins and jasmonates in the root apex

Copper (Cu) interferes with numerous biological functions in plants, including plant growth, which is partly governed by plant hormones. In the present study, Cu stress effect on the roots of pre-emerging maize seedlings in terms of growth, nutrient composition, protein modifications, and root hormone homeostasis was investigated, focusing on possible metabolic differences between the root apex and the rest of the root tissues. Significant decreases in root length and root biomass after 72 h of Cu exposure (50 and 100 μM CuCl₂), accompanied by reductions in Ca, Mg, and P root contents, were found. Cu also generated cell redox imbalance in both root tissues and revealed by altered enzymatic and non-enzymatic antioxidant defenses. Oxidative stress was evidenced by an increased protein carbonylation level in both tissues. Copper also induced protein ubiquitylation and SUMOylation and affected 20S proteasome peptidase activities in both tissues. Drastic reductions in ABA, IAA, JA (both free and conjugated), GA3, and GA4 levels in the root apex were detected under Cu stress. Our results show that Cu exposure generated oxidative damage and altered root hormonal homeostasis, mainly at the root apex, leading to a strong root growth inhibition. Severe protein post-translational modifications upon Cu exposure occurred in both tissues, suggesting that even when hormonal adjustments to cope with Cu stress occurred mainly at the root apex, the entire root is compromised in the protein turnover that seems to be necessary to trigger and/or to sustain defense mechanisms against Cu toxicity.

Excess Copper-Induced Alterations of Protein Profiles and Related Physiological Parameters in Citrus Leaves

This present study examined excess copper (Cu) effects on seedling growth, leaf Cu concentration, gas exchange, and protein profiles identified by a two-dimensional electrophoresis (2-DE) based mass spectrometry (MS) approach after Citrus sinensis and Citrus grandis seedlings were treated for six months with 0.5 (control), 200, 300, or 400 μM CuCl2. Forty-one and 37 differentially abundant protein (DAP) spots were identified in Cu-treated C. grandis and C. sinensis leaves, respectively, including some novel DAPs that were not reported in leaves and/or roots. Most of these DAPs were identified only in C. grandis or C. sinensis leaves. More DAPs increased in abundances than DAPs decreased in abundances were observed in Cu-treated C. grandis leaves, but the opposite was true in Cu-treated C. sinensis leaves. Over 50% of DAPs were associated with photosynthesis, carbohydrate, and energy metabolism. Cu-toxicity-induced reduction in leaf CO2 assimilation might be caused by decreased abundances of proteins related to photosynthetic electron transport chain (PETC) and CO2 assimilation. Cu-effects on PETC were more pronounced in C. sinensis leaves than in C. grandis leaves. DAPs related to antioxidation and detoxification, protein folding and assembly (viz., chaperones and folding catalysts), and signal transduction might be involved in Citrus Cu-toxicity and Cu-tolerance.

Amelioration of heavy metal stress by endophytic Bacillus amyloliquefaciens RWL-1 in rice by regulating metabolic changes: potential for bacterial bioremediation

This study aimed to investigate the bioremediation efficiency of phytohormone-producing endophytic Bacillus amyloliquefaciens RWL-1 isolated from rice seeds. In this study, we tested RWL-1 against various heavy metals (Cu, Cr, Pb, and Cd). Among the tested heavy metals, RWL-1 showed the highest tolerance for Cu stress and we observed alterations in growth kinetics with various Cu concentrations (1, 2.5, and 5 mM). We confirmed the biosorption potential of RWL-1 by scanning electron microscopy coupled with energy-dispersive X-ray spectrometry showing that Cu ions were adsorbed on RWL-1 cell surfaces. We further tested RWL-1 for its plant growth promoting and stress reliance efficiency in response to a dose-dependent increase in soil Cu (1, 2.5, and 5 mM). The RWL-1 inoculation significantly increased seedling biomass and growth attributes compared with non-inoculated control seedlings with and without Cu stress. Moreover, RWL-1 inoculation significantly promoted a physiochemical response in seedlings with and without Cu stress by reducing Cu uptake, improving carbohydrate levels (glucose, sucrose, fructose, and raffinose), enhancing amino acids regulation, and augmenting antioxidant levels (POD, PPO, and GHS). Levels of stress-responsive phytohormones such as abscisic acid (ABA) and jasmonic acid were significantly reduced in RWL-1-inoculated seedlings as compared with non-inoculated control seedlings under normal condition and same levels of Cu stress. In conclusion, the inoculation of B. amyloliquefaciens RWL-1 can significantly improve plant growth in Cu-contaminated soil and reduce metal accumulation, thus making plants safer for consumption. This approach could be tremendously helpful for safe and sustainable agriculture in heavy metal-contaminated areas.

Overexpression of native Musa-miR397 enhances plant biomass without compromising abiotic stress tolerance in banana

Plant micro RNAs (miRNAs) control growth, development and stress tolerance but are comparatively unexplored in banana, whose cultivation is threatened by abiotic stress and nutrient deficiencies. In this study, a native Musa-miR397 precursor harboring 11 copper-responsive GTAC motifs in its promoter element was identified from banana genome. Musa-miR397 was significantly upregulated (8-10) fold in banana roots and leaves under copper deficiency, correlating with expression of root copper deficiency marker genes such as Musa-COPT and Musa-FRO2. Correspondingly, target laccases were significantly downregulated (>-2 fold), indicating miRNA-mediated silencing for Cu salvaging. No significant expression changes in the miR397-laccase module were observed under iron stress. Musa-miR397 was also significantly upregulated (>2 fold) under ABA, MV and heat treatments but downregulated under NaCl stress, indicating universal stress-responsiveness. Further, Musa-miR397 overexpression in banana significantly increased plant growth by 2-3 fold compared with wild-type but did not compromise tolerance towards Cu deficiency and NaCl stress. RNA-seq of transgenic and wild type plants revealed modulation in expression of 71 genes related to diverse aspects of growth and development, collectively promoting enhanced biomass. Summing up, our results not only portray Musa-miR397 as a candidate for enhancing plant biomass but also highlight it at the crossroads of growth-defense trade-offs.

Copper transporter COPT5 participates in the crosstalk between vacuolar copper and iron pools mobilisation

Copper (Cu) deficiency affects iron (Fe) homeostasis in several plant processes, including the increased Fe requirements due to cuproprotein substitutions for the corresponding Fe counterpart. Loss-of-function mutants from Arabidopsis thaliana high affinity copper transporter COPT5 and Fe transporters NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 3/4 (NRAMP3 and NRAMP4) were used to study the interaction between metals internal pools. A physiological characterisation showed that the copt5 mutant is sensitive to Fe deficiency, and that nramp3nramp4 mutant growth was severely affected under limiting Cu. By a transcriptomic analysis, we observed that NRAMP4 expression was highly induced in the copt5 mutant under Cu deficiency, while COPT5 was overexpressed in the nramp3nramp4 mutant. As a result, an enhanced mobilisation of the vacuolar Cu or Fe pools, when the other metal export through the tonoplast is impaired in the mutants, has been postulated. However, metals coming from internal pools are not used to accomplish the increased requirements that derive from metalloprotein substitution under metal deficiencies. Instead, the metal concentrations present in aerial parts of the copt5 and nramp3nramp4 mutants conversely show compensated levels of these two metals. Together, our data uncover an interconnection between Cu and Fe vacuolar pools, whose aim is to fulfil interorgan metal translocation.

The Altered Expression of microRNA408 Influences the Arabidopsis Response to Iron Deficiency

MicroRNAs contribute to the adaptation of plants to varying environmental conditions by affecting systemic mineral nutrient homeostasis. Copper and iron deficiencies antagonistically control the expression of Arabidopsis thaliana microRNA408 (miR408), which post-transcriptionally regulates laccase-like multicopper oxidase family members LAC3, LAC12, and LAC13. In this work, we used miR408 T-DNA insertion mutants (408-KO1 and 408-KO2) and a previously characterized transgenic line overexpressing miR408 (35S:408-14) to explore how miR408 influences copper- and iron-dependent metabolism. We observed that the altered expression of miR408 diminished plant performance and the activation of the iron-regulated genes under iron-deficient conditions. Consistently with the low expression of the miR408-target laccases, we showed that the vascular bundle lignification of the 35S:408-14 plants diminished. The decrease in the phenoloxidase and ferroxidase activities exhibited by wild-type plants under iron deficiency did not occur in the 408-KO1 plants, probably due to the higher expression of laccases. Finally, we observed that the hydrogen peroxide levels under iron starvation were altered in both the 408-KO1 and 35S:408-14 lines. Taken together, these results suggest that Arabidopsis plants with modified miR408 levels undergo multiple deregulations under iron-deficient conditions.

Identification of miRNA-mediated drought responsive multi-tiered regulatory network in drought tolerant rice, Nagina 22

Comparative characterization of microRNA-mediated stress regulatory networks in contrasting rice cultivars is critical to decipher plant stress response. Consequently, a multi-level comparative analysis, using sRNA sequencing, degradome analysis, enzymatic and metabolite assays and metal ion analysis, in drought tolerant and sensitive rice cultivars was conducted. The study identified a group of miRNAs "Cultivar-specific drought responsive" (CSDR)-miRNAs (osa-miR159f, osa-miR1871, osa-miR398b, osa-miR408-3p, osa-miR2878-5p, osa-miR528-5p and osa-miR397a) that were up-regulated in the flag-leaves of tolerant cultivar, Nagina 22 (N22) and Vandana, but down-regulated in the sensitive cultivar, Pusa Basmati 1 (PB1) and IR64, during drought. Interestingly, CSDR-miRNAs target several copper-protein coding transcripts like plantacyanins, laccases and Copper/Zinc superoxide dismutases (Cu/Zn SODs) and are themselves found to be similarly induced under simulated copper-starvation in both N22 and PB1. Transcription factor OsSPL9, implicated in Cu-homeostasis also interacted with osa-miR408-3p and osa-miR528-5p promoters. Further, N22 flag leaves showed lower SOD activity, accumulated ROS and had a higher stomata closure. Interestingly, compared to PB1, internal Cu levels significantly decreased in the N22 flag-leaves, during drought. Thus, the study identifies the unique drought mediated dynamism and interplay of Cu and ROS homeostasis, in the flag leaves of drought tolerant rice, wherein CSDR-miRNAs play a pivotal role.

Endophytic Paecilomyces formosus LHL10 Augments Glycine max L. Adaptation to Ni-Contamination through Affecting Endogenous Phytohormones and Oxidative Stress

This study investigated the Ni-removal efficiency of phytohormone-producing endophytic fungi Penicillium janthinellum, Paecilomyces formosus, Exophiala sp., and Preussia sp. Among four different endophytes, P. formosus LHL10 was able to tolerate up to 1 mM Ni in contaminated media as compared to copper and cadmium. P. formosus LHL10 was further assessed for its potential to enhance the phytoremediation of Glycine max (soybean) in response to dose-dependent increases in soil Ni (0.5, 1.0, and 5.0 mM). Inoculation with P. formosus LHL10 significantly increased plant biomass and growth attributes as compared to non-inoculated control plants with or without Ni contamination. LHL10 enhanced the translocation of Ni from the root to the shoot as compared to the control. In addition, P. formosus LHL10 modulated the physio-chemical apparatus of soybean plants during Ni-contamination by reducing lipid peroxidation and the accumulation of linolenic acid, glutathione, peroxidase, polyphenol oxidase, catalase, and superoxide dismutase. Stress-responsive phytohormones such as abscisic acid and jasmonic acid were significantly down-regulated in fungal-inoculated soybean plants under Ni stress. LHL10 Ni-remediation potential can be attributed to its phytohormonal synthesis related genetic makeup. RT-PCR analysis showed the expression of indole-3-acetamide hydrolase, aldehyde dehydrogenase for indole-acetic acid and geranylgeranyl-diphosphate synthase, ent-kaurene oxidase (P450-4), C13-oxidase (P450-3) for gibberellins synthesis. In conclusion, the inoculation of P. formosus can significantly improve plant growth in Ni-polluted soils, and assist in improving the phytoremediation abilities of economically important crops.