Long-Lasting Silver Nanoparticles Synthesized with Tagetes erecta and Their Antibacterial Activity against Erwinia amylovora, a Serious Rosaceous Pathogen.

A rapid, eco-friendly, and simple method for the synthesis of long-lasting (2 years) silver nanoparticles (AgNPs) is reported using aqueous leaf and petal extracts of Tagetes erecta L. The particles were characterized using UV-Visible spectrophotometry and the analytical and crystallographic techniques of transmission electron microscopy (TEM). The longevity of the AgNPs was studied using UV-Vis and high-resolution TEM. The antibacterial activity of the particles against Erwinia amylovora was evaluated using the Kirby-Bauer disk diffusion method. The results were analyzed using ANOVA and Tukey's test (p ≤ 0.05). Both the leaf and petal extracts produced AgNPs, but the leaf extract (1 mL) was long-lasting and quasi-spherical (17.64 ± 8.87 nm), with an absorbance of UV-Vis λmax 433 and a crystalline structure (fcc, 111). Phenols, flavonoids, tannins, and terpenoids which are associated with -OH, C=O, and C=C were identified in the extracts and could act as reducing and stabilizing agents. The best antibacterial activity was obtained with a nanoparticle concentration of 50 mg AgNPs L-1. The main contribution of the present research is to present a sustainable method for producing nanoparticles which are stable for 2 years and with antibacterial activity against E. amylovora, one of most threatening pathogens to pear and apple productions.

Salinity Stress Tolerance in Plants.

Soil salinization negatively impacts plant development and induces land degradation, thus affecting biodiversity, water quality, crop production, farmers' well-being, and the economic situation in the affected region. Plant germination, growth, and productivity are vital processes impaired by salinity stress; thus, it is considered a serious threat to agriculture. The extent to which a plant is affected by salinity depends mainly on the species, but other factors, including soil attributes, water, and climatic conditions, also affect a plant's ability to tolerate salinity stress. Unfortunately, this phenomenon is expected to be exacerbated further by climate change. Consequently, studies on salt stress tolerance in plants represent an important theme for the present Special Issue of Plants. The present Special Issue contains 14 original contributions that have documented novel discoveries regarding induced or natural variations in plant genotypes to cope with salt stress, including molecular biology, biochemistry, physiology, genetics, cell biology, modern omics, and bioinformatic approaches. This Special Issue also includes the impact of biostimulants on the biochemical, physiological, and molecular mechanisms of plants to deal with salt stress and on the effects of salinity on plant nutrient status. We expect that readers and academia will benefit from all the articles included in this Special Issue.

ZnO nanoparticles as potential fertilizer and biostimulant for lettuce.

Zn is an indispensable nutrient for crops that usually presents low bioavailability. Different techniques have been proposed to improve the bioavailability of Zn, including the use of nanofertilizers. The objective of the study was to evaluate the applications of drench (D) and foliar (F) ZnO nanoparticles (NZnO) compared to those of ionic Zn2+ (ZnSO4) in lettuce. The plants cv. Great Lakes 407 was produced in pots of 4 L with perlite-peat moss (1:1) under greenhouse conditions. The treatments consisted of NZnO applications that replaced the total Zn provided with a Steiner solution, as follows: Zn2+ (100%D) (control); Zn2+ (50%D+50%F); NZnO (100%D); NZnO (50%D+50%F); NZnO (75%D); NZnO (50%D); NZnO (75%F) and NZnO (50%F). Four applications of Zn were made with a frequency of 15 days. 75 days after transplant (DAP), the fresh and dry biomass, chlorophyll a, b, and ß-carotene, phenolics, flavonoids, antioxidant capacity, vitamin C, glutathione, H2O2, total protein, and enzymatic activity of PAL, CAT, APX, and GPX were evaluated. The mineral concentrations (N, P, K, Ca, Mg, S, Cu, Fe, Mn, Mo, Zn, Ni, and Si) in the leaves and roots of plants were also determined. The results showed that, compared to Zn2+, NZnO promoted increases in biomass (14-52%), chlorophylls (32-69%), and antioxidant compounds such as phenolics, flavonoids, and vitamin C. The activity of enzymes like CAT and APX, as well as the foliar concentration of Ca, Mg, S, Fe, Mn, Zn, and Si increased with NZnO. A better response was found in the plants for most variables with foliar applications of NZnO equivalent to 50-75% of the total Zn2+ applied conventionally. These results demonstrate that total replacement of Zn2+ with NZnO is possible, promoting fertilizer efficiency and the nutraceutical quality of lettuce.

Titanium Increases the Antioxidant Activity and Macronutrient Concentration in Tomato Seedlings Exposed to Salinity in Hydroponics.

Global climate change affects agriculture and tends to aggravate the effect of various environmental stress factors including soil salinity. Beneficial elements such as titanium (Ti) may improve the performance of plants facing restrictive environments such as saline soils. This research work evaluated the individual effect of sodium chloride (0, 50, and 100 mM NaCl) in solution, that of leaf-applied Ti (0, 500, and 1000 mg L-1 Ti), and their interactions on physiological, biochemical, and nutritional variables of tomato (Solanum lycopersicum L.) seedlings cv. Rio Grande in a factorial design in greenhouse hydroponics. NaCl reduced seedling height, stem diameter, leaf area, SPAD units, and sugar and K concentrations, and increased antioxidant activity in stems and roots, photosynthetic pigments, sugars. Titanium increased the N, P, K, Ca, Mg, and Ti concentrations in leaves, but the concentration of total sugars in leaves was reduced when applying 500 mg Ti L-1. Under moderate salinity conditions (50 mM NaCl) the application of Ti increased the antioxidant activity in roots, while, at all salinity levels tested, Ti increased the concentrations of macro-nutrients and Ti in leaves. Titanium is concluded to have a positive effect on the antioxidant activity and nutrition of seedlings under saline stress conditions.

Endogenous Hormone Profile and Sugars Display Differential Distribution in Leaves and Pseudobulbs of Laelia anceps Plants Induced and Non-Induced to Flowering by Exogenous Gibberellic Acid.

A profile of endogenous hormones and sugars in leaves and pseudobulbs of Laelia anceps subsp. anceps (Orchidaceae) plants induced and non-induced to flowering by the effect of different doses of exogenous gibberellic acid (GA3), considering the current and back growth structures (CGS and BGS), were investigated. A factorial experiment with five doses of GA3 and two growth structures was designed. Adult plants with undifferentiated vegetative buds were selected and sprayed with doses of 0, 400, 600, 800, and 1000 mg GA3 L-1. The main results showed a strong interaction between GA3 dose and growth structures, which promoted the highest kinetin (KIN) concentration in CGS. Exogenous GA3 increased endogenous GA3 in leaves and pseudobulbs induced (I-Leaf and I-PSB) and non-induced (NI-Leaf and NI-PSB) to flowering. For sugar concentration, the 400 mg L-1 GA3 dose promotes significant interaction with the CGS in NI-PSB. In general, the hormone profile revealed opposite balances of endogenous hormone concentrations for KIN, zeatin (ZEA), trans-zeatin (T-ZEA), indoleacetic acid (IAA), indole-3-butyric acid (IBA) and GA3, not only for growth structures but also for vegetative organs analyzed, depending on whether the plants were induced or not induced to flowering, with the highest concentration of endogenous hormones in pseudobulbs. Likewise, different sugar concentration balances were observed. These balances of both endogenous hormones and sugars are likely to be involved in the flowering of L. anceps.

Silver Nanoparticles Increase Nitrogen, Phosphorus, and Potassium Concentrations in Leaves and Stimulate Root Length and Number of Roots in Tomato Seedlings in a Hormetic Manner.

BACKGROUND: Silver nanoparticles (AgNPs) display unique biological activities and may serve as novel biostimulators. Nonetheless, their biostimulant effects on germination, early growth, and major nutrient concentrations (N, P, and K) in tomato (Solanum lycopersicum) have been little explored. METHODS: Tomato seeds of the Vengador and Rio Grande cultivars were germinated on filter paper inside plastic containers in the presence of 0, 5, 10, and 20 mg/L AgNPs. Germination parameters were recorded daily, while early growth traits of seedlings were determined 20 days after applying the treatments (dat). To determine nutrient concentrations in leaves, a hydroponic experiment was established, adding AgNPs to the nutrient solution. Thirty-day-old plants were established in the hydroponic system and kept there for 7 days, and subsequently, leaves were harvested and nutrient concentrations were determined. RESULTS: The AgNPs applied did not affect germination parameters, whereas their application stimulated length and number of roots in a hormetic manner. In 37-day-old plants, low AgNP applications increased the concentrations of N, P, and K in leaves. CONCLUSION: As novel biostimulants, AgNPs promoted root development, especially when applied at 5 mg/L. Furthermore, they increased N, P, and K concentration in leaves, which is advantageous for seedling performance during the early developmental stages.

Silicon Stimulates Plant Growth and Metabolism in Rice Plants under Conventional and Osmotic Stress Conditions.

Exogenous silicon (Si) can enhance plant resistance to various abiotic factors causing osmotic stress. The objective of this research was to evaluate the application of 1 and 2 mM Si to plants under normal conditions and under osmotic stress. Morelos A-98 rice seedlings, were treated with 1 and 2 mM SiO2 for 28 d. Subsequently, half of the plants were subjected to osmotic stress with the addition of 10% polyethylene glycol (PEG) 8000; and continued with the addition of Si (0, 1 and 2 mM SiO2) for both conditions. The application of Si under both conditions increased chlorophyll b in leaves, root volume, as well as fresh and dry biomass of roots. Interestingly, the number of tillers, shoot fresh and dry biomass, shoot water content, concentration of total chlorophyll, chlorophyll a/b ratio, and the concentration of total sugars and proline in shoot increased with the addition of Si under osmotic stress conditions. The addition of Si under normal conditions decreased the concentration of sugars in the roots, K and Mn in roots, and increased the concentration of Fe and Zn in shoots. Therefore, Si can be used as a potent inorganic biostimulant in rice Morelos A-98 since it stimulates plant growth and modulates the concentration of vital biomolecules and essential nutrients.

Silicon flow from root to shoot in pepper: a comprehensive in silico analysis reveals a potential linkage between gene expression and hormone signaling that stimulates plant growth and metabolism.

BACKGROUND: Silicon (Si) is categorized as a quasi-essential element for plants thanks to the benefits on growth, development and metabolism in a hormetic manner. Si uptake is cooperatively mediated by Lsi1 and Lsi2. Nevertheless, Lsi channels have not yet been identified and characterized in pepper (Capsicum annuum), while genes involved in major physiological processes in pepper are Si-regulated. Furthermore, Si and phytohormones may act together in regulating plant growth, metabolism and tolerance against stress. Our aim was to identify potential synergies between Si and phytohormones stimulating growth and metabolism in pepper, based on in silico data. METHODS: We established a hydroponic system to test the effect of Si (0, 60, 125 and 250 mg L-1 Si) on the concentrations of this element in different pepper plant tissues. We also performed an in silico analysis of putative Lsi genes from pepper and other species, including tomato (Solanum lycopersicum), potato (Solanum tuberosum) and Arabidopsis thaliana, to look for cis-acting elements responsive to phytohormones in their promoter regions. With the Lsi1 and Lsi2 protein sequences from various plant species, we performed a phylogenetic analysis. Taking into consideration the Lsi genes retrieved from tomato, potato and Arabidopsis, an expression profiling analysis in different plant tissues was carried out. Expression of Si-regulated genes was also analyzed in response to phytohormones and different plant tissues and developmental stages in Arabidopsis. RESULTS: Si concentrations in plant tissues exhibited the following gradient: roots > stems > leaves. We were able to identify 16 Lsi1 and three Lsi2 genes in silico in the pepper genome, while putative Lsi homologs were also found in other plant species. They were mainly expressed in root tissues in the genomes analyzed. Both Lsi and Si-regulated genes displayed cis-acting elements responsive to diverse phytohormones. In Arabidopsis, Si-regulated genes were transcriptionally active in most tissues analyzed, though at different expressed levels. From the set of Si-responsive genes, the NOCS2 gene was highly expressed in germinated seeds, whereas RABH1B, and RBCS-1A, were moderately expressed in developed flowers. All genes analyzed showed responsiveness to phytohormones and phytohormone precursors. CONCLUSION: Pepper root cells are capable of absorbing Si, but small amounts of this element are transported to the upper parts of the plant. We could identify putative Si influx (Lsi1) and efflux (Lsi2) channels that potentially participate in the absorption and transport of Si, since they are mainly expressed in roots. Both Lsi and Si-regulated genes exhibit cis-regulatory elements in their promoter regions, which are involved in phytohormone responses, pointing to a potential connection among Si, phytohormones, plant growth, and other vital physiological processes triggered by Si in pepper.

Lanthanum delays senescence and improves postharvest quality in cut tulip (Tulipa gesneriana L.) flowers.

We tested two sources of lanthanum (La), LaCl3 and La(NO3)3 × 6H2O at a concentration of 40 µM each, in the treatment solution of cut flowers of 15 tulip (Tulipa gesneriana L.) cultivars. Ascorbic acid (AsA; 0.2 g/L) was used as a reference solution, while distilled water was evaluated as an absolute control. With both La sources, bud length and diameter, and stem length were increased; as a result, stem curvature was also significantly increased with La treatments. The cultivars Laura Fygi and Rosario registered the highest relative stem elongation. Lalibela and Acropolis displayed the greatest stem curvature on the last day in vase. At 3, 5, 7, 9 and 11 days after cutting, the highest solution uptake was recorded in flower stems treated with LaCl3, surpassing the control by 5, 11, 15, 18 and 24%, respectively. The relative stem elongations observed were 21.3, 27.4, 35.2 and 35.5% in the control, AsA, LaCl3 and La(NO3)3, respectively. The mean solution uptake per gram of stem fresh biomass weight was 1.44, 1.44, 1.71 and 1.54 mL in the control, AsA, LaCl3 and La(NO3)3, respectively. LaCl3 significantly increased the bud length and solution uptake of flower stems, while La(NO3)3 × 6H2O increased stem fresh weight.

Silicon induces hormetic dose-response effects on growth and concentrations of chlorophylls, amino acids and sugars in pepper plants during the early developmental stage.

BACKGROUND: Silicon (Si) is a beneficial element that has been proven to influence plant responses including growth, development and metabolism in a hormetic manner. METHODS: In the present study, we evaluated the effect of Si on the growth and concentrations of chlorophylls, total amino acids, and total sugars of pepper plants (Capsicum annuum L.) during the early developmental stage in a hydroponic system under conventional (unstressed) conditions. We tested four Si concentrations (applied as calcium silicate): 0, 60, 125 and 250 mg L-1, and growth variables were measured 7, 14, 21 and 28 days after treatment (dat), while biochemical variables were recorded at the end of the experiment, 28 dat. RESULTS: The application of 125 mg L-1 Si improved leaf area, fresh and dry biomass weight in leaves and stems, total soluble sugars, and concentrations of chlorophylls a and b in both leaves and stems. The amino acids concentration in leaves and roots, as well as the stem diameter were the highest in plants treated with 60 mg L-1 Si. Nevertheless, Si applications reduced root length, stem diameter and total free amino acids in leaves and stems, especially when applied at the highest concentration (i.e., 250 mg L-1 Si). CONCLUSION: The application of Si has positive effects on pepper plants during the early developmental stage, including stimulation of growth, as well as increased concentrations of chlorophylls, total free amino acids and total soluble sugars. In general, most benefits from Si applications were observed in the range of 60-125 mg L-1 Si, while some negative effects were observed at the highest concentration applied (i.e., 250 mg L-1 Si). Therefore, pepper is a good candidate crop to benefit from Si application during the early developmental stage under unstressed conditions.

Lanthanum Prolongs Vase Life of Cut Tulip Flowers by Increasing Water Consumption and Concentrations of Sugars, Proteins and Chlorophylls.

We evaluated the effect of separately adding two sources of lanthanum (La), LaCl3 and La(NO3)3 × 6H2O at a concentration of 40 µM each, to the preservative solution of 15 cut tulip flower varieties. Ascorbic acid (AsA; 0.2 g/L) was used as a reference solution, while distilled water was used as control. The variety Laura Fygi recorded the longest vase life with 13 days. The highest water consumption per gram of stem fresh biomass weight (FBW) (2.5 mL) was observed in the variety Violet Beauty, whereas the lowest (1.098 mL) was recorded in Pink Impression. At the end of the vase life period, higher concentrations of total soluble sugars in petals and total soluble proteins in leaves were recorded in La-treated stems, compared to the AsA treatment and the control. Additionally, La(NO3)3 × 6H2O supply increased the fresh weight of stems in vase and prolonged vase life. Moreover, this treatment resulted in the highest foliar concentration of chlorophylls at the end of vase life. Therefore, La increases tulip flower vase life as a consequence of improving the concentrations of some vital biomolecules.

Flavonoid, Nitrate and Glucosinolate Concentrations in Brassica Species Are Differentially Affected by Photosynthetically Active Radiation, Phosphate and Phosphite.

We evaluated the effects of phosphate (Pi-deficiency: 0.1 mM; Pi-sufficiency: 0.5 mM), phosphite (low-Phi: 0.1 mM; medium-Phi: 0.5 mM; and high-Phi: 2.5 mM), and two mean daily photosynthetically active radiations (lower PAR: 22.2 mol ⋅ m-2 ⋅ d-1; higher PAR: 29.7 mol ⋅ m-2 ⋅ d-1), as well as their interactions, on flavonoid, nitrate and glucosinolate (GL) concentrations and growth characteristics in hydroponically grown Brassica campestris cv. Mibuna Early and Brassica juncea cv. Red Giant. As expected, higher PAR increased dry matter and contrariwise decreased number of leaves but only in B. campestris. Total flavonoid and individual flavonoid compounds increased with the higher PAR value in B. campestris. Pi-sufficiency resulted in a lower quercetin concentration in both species, the isorhamnetin and total flavonoid concentrations in B. campestris, and the cyanidin concentration in B. juncea, in comparison to Pi-deficiency. Similarly, Pi-sufficient plants exhibited lower GL concentration, especially alkyl-GLs in B. campestris and alkenyl-GLs and an aryl-GL in B. juncea. Pi did not affect the nitrate concentration in either species, and nor did Phi influence the flavonoid concentrations in either species. In B. campestris, medium Phi (0.5 mM) increased the 1-methoxyindol-3-ylmethyl GL concentration by 28.3%, as compared to that observed at low Phi. In B. juncea, high Phi level increased the but-3-enyl-GL concentration by 18.9%, in comparison to values recorded at medium Phi. B. campestris plants exposed to higher PAR increased total flavonoids concentration. In both Brassica species, higher PAR stimulated the alkyl-, alkenyl-, and indole-GLs. The interaction of lower PAR and increasing Phi significantly decreased flavonoid concentration in B. juncea, whereas increasing Phi at higher PAR increased such concentration in this species. The same combination reduced the concentration of 2-phenylethyl- and indol-3-ylmethyl-GL in B. juncea. The highest indol-3-ylmethyl-GL concentration was observed when Pi was deficient combined with medium Phi in B. juncea. Thus, PAR, Pi and Phi may modulate flavonoid, GL and nitrate concentrations in Brassica species, which may be a useful tool to improve the nutraceutical quality of these leafy vegetables if properly managed.

Vanadium stimulates pepper plant growth and flowering, increases concentrations of amino acids, sugars and chlorophylls, and modifies nutrient concentrations.

Vanadium (V) can be absorbed by plants and regulate their growth and development, although contrasting effects have been reported among species and handling conditions. The objective of this work was to evaluate the beneficial effect of V on pepper plants (Capsicum annuum L.). The plants were grown in a hydroponic system with the application of four V concentrations (0, 5, 10, and 15 µM NH4VO3). Four weeks after the beginning of the treatments, growth, flowering, biomass, chlorophyll concentration, total amino acids, total soluble sugars, and nutrients were determined in leaves, stems, and roots. The application of 5 µM V increased plant growth, induced floral bud development, and accelerated flowering. The chlorophyll concentration varied according to the type of plant part analyzed. The concentrations of amino acids and sugars in leaves and roots were higher with 5 µM. With 10 and 15 µM V, the plants were smaller and showed toxicity symptoms. The K concentration in leaves decreased as the V dose increased (0 to 15 µM). However, 5 µM V increased the concentrations of N, P, K, Ca, Mg, Cu, Mn, and B, exclusively in stems. The application of 15 µM V decreased the concentrations of Mg and Mn in leaves, but increased those of P, Ca, Mg, Cu, and B in roots. We conclude that V has positive effects on pepper growth and development, as well as on the concentrations of amino acids and total sugars. V was antagonistic with K, Mg, and Mn in leaves, while in stems and roots, there was synergism with macro and micronutrients. Vanadium is a beneficial element with the potential to be used in biostimulation approaches of crops like pepper.

Cerium enhances germination and shoot growth, and alters mineral nutrient concentration in rice.

Cerium (Ce) belongs to the rare earth elements (REEs), and although it is not essential for plants, it can stimulate growth and other physiological processes. The objective of this research was to evaluate the effect of Ce on seed germination, initial seedling growth, and vegetative growth in rice (Oryza sativa L.) cv. Morelos A-98. During the germination process, the seeds were treated with Ce concentrations of 0, 4, 8, and 12 µM; after 5 d, germination percentage was recorded and after 10 d seedling growth was measured. For vegetative growth, a hydroponic system was established where 14-d-old plants without previous Ce treatment were transferred into nutrient solution. After two weeks of acclimatizing, 0, 25, 50, and 100 µM Ce were added to the nutrient solution for 28 d. Ce significantly increased germination and the initial growth variables of the seedlings. During vegetative growth, Ce increased plant height, number of tillers, root volume, and shoot fresh and dry biomass, without affecting root biomass weight. With low Ce concentrations (25 and 50 µM), the concentrations of chlorophylls and amino acids in the shoots were similar to those in the control, like amino acid concentration in the roots at a concentration of 25 µM Ce. Conversely, the concentration of total sugars increased in the shoot with the application of 25, 50, and 100 µM Ce, and in the roots with the application of 50 µM Ce. Also, Ce did not affect the concentration of macro or micronutrients in the shoots. However, in the roots, the high Ce concentration decreased the concentrations of Ca, Fe, Mn, and Zn, while the Mg concentration increased. Our results indicate that Ce, at the right concentrations, can function as a biostimulant in rice germination and growth.

Lead phytoextraction from printed circuit computer boards by Lolium perenne L. and Medicago sativa L.

This work assessed the ability of Lolium perenne and Medicago sativa for extracting lead (Pb) from particulate printed circuit computer boards (PCB) mixed in sand with the following concentrations: 0.5, 1.0 and 1.5 g of PCB, and including a control treatment without PCB. The PCB were obtained from computers, and grinded in two particle sizes: 0.0594 mm (PCB1) and 0.0706 mm (PCB2). The PCB particle sizes at their corresponding concentrations were applied to L. perenne and M. sativa by using three experimental assays. In assay II, PCB2 affected the biomass production for both plants. For assay III, the PCB1 increased the biomass of M. sativa (236.5%) and L. perenne (142.2%) when applying either 0.5 or 1.0 g, respectively. In regards to phytoextraction, assay I showed the highest Pb-extraction by roots of L. perenne (4.7%) when exposed to 1.5 g of PCB1. At assay I, L. perenne showed a Pb-bioconcentration factor higher than 1.0 when growing at 0.5 g of PCB1, and when HNO3 was used as digestion solution; moreover, in assay III both plants showed a Pb-translocation factor higher than 1.0. Therefore, Lolium perenne and Medicago sativa are able to recover Pb from electronic wastes (PCB).

Expression patterns and promoter analyses of aluminum-responsive NAC genes suggest a possible growth regulation of rice mediated by aluminum, hormones and NAC transcription factors.

In acid soils, the solubilized form of aluminum, Al+3, decreases root growth and affects the development of most crops. However, like other toxic elements, Al can have hormetic effects on plant metabolism. Rice (Oryza sativa) is one of the most tolerant species to Al toxicity, and when this element is supplied at low doses, growth stimulation has been observed, which could be due to combined mechanisms that are partly triggered by NAC transcription factors. This protein family can regulate vital processes in plants, including growth, development, and response to environmental stimuli, whether biotic or abiotic. Under our experimental conditions, 200 µM Al stimulated root growth and the formation of tillers; it also caused differential expression of a set of NAC genes. The promoter regions of the genes regulated by Al were analyzed and the cis-acting elements that are potentially involved in the responses to different stimuli, including environmental stress, were identified. Through the Genevestigator platform, data on the expression of NAC genes were obtained by experimental condition, tissue, and vegetative stage. This is the first study on NAC genes where in vivo and in silico data are complementarily analyzed, relating the hormetic effect of Al on plant growth and gene expression with a possible interaction in the response to phytohormones in rice. These findings could help to elucidate the possible convergence between the signaling pathways mediated by phytohormones and the role of the NAC transcription factors in the regulation of growth mediated by low Al doses.

Aluminum Enhances Growth and Sugar Concentration, Alters Macronutrient Status and Regulates the Expression of NAC Transcription Factors in Rice.

Aluminum (Al) is a beneficial element for some plant species, especially when used at low concentrations. Though some transcription factors are induced by exposure to this element, no data indicate that Al regulates the expression of NAC genes in rice. In this study we tested the effect of applying 200 µM Al on growth, chlorophyll, amino acids, sugars, macronutrient concentration and regulation of NAC transcription factors gene expression in 24-day-old plants of four rice (Oryza sativa ssp. indica) cultivars: Cotaxtla, Tres Ríos, Huimanguillo and Temporalero, grown hydroponically under greenhouse conditions. Twenty days after treatment, we observed that Al enhanced growth in the four cultivars studied. On average, plants grown in the presence of Al produced 140% more root dry biomass and were 30% taller than control plants. Cotaxtla and Temporalero showed double the root length, while Huimanguillo and Cotaxtla had three times more root fresh biomass and 2.5 times more root dry biomass. Huimanguillo plants showed 1.5 times more shoot height, while Cotaxtla had almost double the root dry biomass. With the exception of Tres Ríos, the rest of the cultivars had almost double the chlorophyll concentration when treated with Al, whereas amino acid and proline concentrations were not affected by Al. Sugar concentration was also increased in plants treated with Al, almost 11-fold in comparison to the control. Furthermore, we observed a synergic response of Al application on P and K concentration in roots, and on Mg concentration in shoots. Twenty-four hours after Al treatment, NAC transcription factors gene expression was measured in roots by quantitative RT-PCR. Of the 57 NAC transcription factors genes primer-pairs tested, we could distinguish that 44% (25 genes) showed different expression patterns among rice cultivars, with most of the genes induced in Cotaxtla and Temporalero plants. Of the 25 transcription factors up-regulated, those showing differential expression mostly belonged to the NAM subfamily (56%). We conclude that Al improves growth, increases sugar concentration, P and K concentrations in roots, and Mg concentration in shoots, and report, for the first time, that Al differentially regulates the expression of NAC transcription factors in rice.

Arabidopsis AtDGK7, the smallest member of plant diacylglycerol kinases (DGKs), displays unique biochemical features and saturates at low substrate concentration: the DGK inhibitor R59022 differentially affects AtDGK2 and AtDGK7 activity in vitro and alters plant growth and development.

Diacylglycerol kinase (DGK) regulates the level of the second messenger diacylglycerol and produces phosphatidic acid (PA), another signaling molecule. The Arabidopsis thaliana genome encodes seven putative diacylglycerol kinase isozymes (named AtDGK1 to -7), structurally falling into three major clusters. So far, enzymatic activity has not been reported for any plant Cluster II DGK. Here, we demonstrate that a representative of this cluster, AtDGK7, is biochemically active when expressed as a recombinant protein in Escherichia coli. AtDGK7, encoded by gene locus At4g30340, contains 374 amino acids with an apparent molecular mass of 41.2 kDa. AtDGK7 harbors an N-terminal catalytic domain, but in contrast to various characterized DGKs (including AtDGK2), it lacks a cysteine-rich domain at its N terminus, and, importantly, its C-terminal DGK accessory domain is incomplete. Recombinant AtDGK7 expressed in E. coli exhibits Michaelis-Menten type kinetics with 1,2-dioleoyl-sn-glycerol as substrate. AtDGK7 activity was affected by pH, detergents, and the DGK inhibitor R59022. We demonstrate that both AtDGK2 and AtDGK7 phosphorylate diacylglycerol molecular species that are typically found in plants, indicating that both enzymes convert physiologically relevant substrates. AtDGK7 is expressed throughout the Arabidopsis plant, but expression is strongest in flowers and young seedlings. Expression of AtDGK2 is transiently induced by wounding. R59022 at approximately 80 mum inhibits root elongation and lateral root formation and reduces plant growth, indicating that DGKs play an important role in plant development.