Boron Toxicity Research Articles

Association of the benzoxazinoid pathway with boron homeostasis in maize.

Both deficiency and toxicity of the micronutrient boron lead to severe reductions in crop yield. Despite this agricultural importance, the molecular basis underlying boron homeostasis in plants remains unclear. To identify molecular players involved in boron homeostasis in maize (Zea mays L.), we measured boron levels in the Goodman-Buckler association panel and performed genome-wide association studies. These analyses identified a benzoxazinless (bx) gene, bx3, involved in the biosynthesis of benzoxazinoids, such as DIMBOA, which are major defense compounds in maize. Genes involved in DIMBOA biosynthesis are all located in close proximity in the genome, and benzoxazinoid biosynthesis mutants, including bx3, are all DIMBOA deficient. We determined that leaves of the bx3 mutant have a greater boron concentration than those of B73 control plants, which corresponded with enhanced leaf tip necrosis, a phenotype associated with boron toxicity. By contrast, other DIMBOA-deficient maize mutants did not show altered boron levels or the leaf tip necrosis phenotype, suggesting that boron is not associated with DIMBOA. Instead, our analyses suggest that the accumulation of boron is linked to the benzoxazinoid intermediates indolin-2-one (ION) and 3-hydroxy-ION. Therefore, our results connect boron homeostasis to the benzoxazinoid plant defense pathway through bx3 and specific intermediates, rendering the benzoxazinoid biosynthesis pathway a potential target for crop improvement under inadequate boron conditions.

Salicylic Acid and Melatonin Synergy Enhances Boron Toxicity Tolerance via AsA-GSH Cycle and Glyoxalase System Regulation in Fragrant Rice.

Background: Boron is an essential micronutrient for plant growth and productivity, yet excessive boron leads to toxicity, posing significant challenges for agriculture. Fragrant rice is popular among consumers, but the impact of boron toxicity on qualitative traits of fragrant rice, especially aroma, remains largely unexplored. The individual potentials of melatonin and salicylic acid in reducing boron toxicity are less known, while their synergistic effects and mechanisms in fragrant rice remain unclear. Methods: Thus, this study investigates the combined application of melatonin and salicylic acid on fragrant rice affected by boron toxicity. One-week-old seedlings were subjected to boron (0 and 800 µM) and then treated with melatonin and salicylic acid (0 and 100 µM, for 3 weeks). Results: Boron toxicity significantly impaired photosynthetic pigments, plant growth, and chloroplast integrity while increasing oxidative stress markers such as hydrogen peroxide, malondialdehyde, methylglyoxal, and betaine aldehyde dehydrogenase. Likewise, boron toxicity abridged the precursors involved in the 2-acetyl-1-pyrroline (2-AP) biosynthesis pathway. However, individual as well as combined application of melatonin and salicylic acid ameliorated boron toxicity by strengthening the antioxidant defense mechanisms-including the enzymes involved during the ascorbate-glutathione (AsA-GSH) cycle and glyoxalase system-and substantially improved 2-AP precursors including proline, P5C, Δ1-pyrroline, and GABA levels, thereby restoring the 2-AP content and aroma. These findings deduce that melatonin and salicylic acid synergistically alleviate boron toxicity-induced disruptions on the 2-AP biosynthesis pathway by improving the 2-AP precursors and enzymatic activities, as well as modulating the physio-biochemical processes and antioxidant defense system of fragrant rice plants. Conclusions: The findings of this study have the potential to enhance rice productivity and stress tolerance, offering solutions to improve food security and sustainability in agricultural practices, particularly in regions affected by environmental stressors.

Boron toxicity in plants: understanding mechanisms and developing coping strategies; a review.

Boron is essential for plants, but excess can induce toxicity. Boron (B) is a vital micronutrient for plants, but excess B can induce toxicity symptoms and reduce crop yields. B bioavailability depends on soil properties, including clay type, pH, and organic matter content. Symptoms of B toxicity include reduced shoot and root growth, leaf chlorosis and necrosis, impaired photosynthesis, and disrupted pollen development. This review paper examines the current knowledge on B toxicity mechanisms, tolerance strategies, and management approaches in plants. This review covers (1) factors affecting B bioavailability; (2) toxicity symptoms in plants; (3) uptake, transport, and detoxification mechanisms; and (4) strategies. To mitigate toxicity, plants reduce B uptake, activate efflux transporters, compartmentalize B, and enhance antioxidant systems. On the basis of this review, future research should focus on identifying novel tolerance mechanisms, exploring genetic strategies for improved B management, and developing innovative agronomic interventions. These insights will facilitate the breeding and management of crops for enhanced productivity under B toxicity stress.

Boron toxicity in young cassava plants

Diagnosis of boron (B) toxicity in cassava grown in soil is still a challenge. The objectives of this work were (i) to describe the symptoms of B toxicity in cassava grown in soil and (ii) to establish critical B toxicity concentrations in soil and leaf blades of the plant. In a greenhouse, plants were grown for 68 days in pots filled with sandy soil samples submitted to B rates of 0, 0.5, 1, 2, and 4 mg dm−3. Application of B increased both concentration of B extracted from the soil with hot water and B concentration in leaf blades. Visual symptoms of B toxicity were only observed in the highest rate of applied B. The symptoms were photographed and described as chlorosis followed by necrosis in the tip and margin of older leaves. Despite the symptoms, there were no significant effects on growth of the plant. As a result, critical concentrations could not be obtained. Alternatively, thresholds based on B toxicity symptoms are suggested. The thresholds for concentration of B in younger leaf blades and older leaf blades were calculated to be 99.9 and 130.7 mg kg−1, respectively. These respective thresholds corresponded to the thresholds for B concentrations in soil of 0.875 and 0.891 mg dm−3. Concentrations above these thresholds could be used to support visual diagnosis of suspected B toxicity symptoms. Photos, description, and thresholds provided in this article can help improve the diagnosis of visual B toxicity symptoms in young cassava plants grown in soil.

Evaluation of Enzymatic and Non-enzymatic Antioxidant Defense Responses of Durum Wheat (Triticum durum Desf.) in Coping with Boron Stress

Wheat, one of the world's most important agricultural products, plays a vital role in meeting the nutritional needs of our growing global population. However, arid and semi-arid regions face a potential threat from boron (B) toxicity. While boron is an essential nutrient for plant growth and development, its excessive presence can be toxic. It disrupts physiological processes, causing chlorosis and necrosis, ultimately leading to yield loss or plant death. Although B deficiency is a problem in the soils of many countries, Türkiye is one of the few experiencing B toxicity problems in its agricultural areas. This study investigated the physiological and biochemical responses of durum wheat to various B concentrations (0-20 mg L-1) under controlled air-conditioned cabin conditions. Durum wheat exhibited a decrease in chlorophyll content, phenolic content, and antiradical capacity at B doses exceeding 10 mg L-1. However, carotene content increased steadily with increasing B concentrations. The activities of antioxidant enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione S-transferase (GST), increased up to a B dose of 15 mg L-1. Catalase (CAT) and glutathione reductase (GR) activities increased up to 10 mg L-1 B dose but decreased at higher B levels. Proline content increased tenfold up to a B dose of 10 mg L-1, indicating an attempt to mitigate stress. Conversely, malondialdehyde (MDA) accumulation increased continuously (approximately 150%) with increasing B doses, suggesting membrane damage. Despite being considered B-sensitive, this study demonstrated that durum wheat can effectively cope with B stress up to a B dose of 10 mg L-1 under controlled conditions. Beyond this threshold, physiological and biochemical changes indicate a decline in stress tolerance. Many osmoregulators, carotenes, alkaloids, flavonoids, tocopherols, phenolic compounds, non-protein amino acids, and several unidentified metabolites are activated to support antioxidant defense. The SOS pathway and the released ROS force gene regulatory systems come into play. Following these, the ROS released in the organism are neutralized, and ionic homeostasis and cellular stress resistance are achieved.

Transcriptome and protein-protein interaction analysis reveals the tolerance of poplar to high boron toxicity regulated by transport and cell wall synthesis pathways

Soil contamination with high levels of boron (B) destroys the balance of the soil ecosystem, reduces crop yields and poses a potential threat to human health and safety. Hyperaccumulator plants such as poplar can be used to mitigate high B soil contamination and its negative effects. Despite having a certain level of understanding of the physiological response of poplars to high boron stress tolerance, the differences in boron accumulation efficiency among different clones and their underlying molecular mechanisms are still unclear. The effects of high B toxicity on growth, B accumulation and physiological parameters were investigated in this research to compare the high B tolerance and high B accumulation abilities of five poplar clones (717, SXY, NL895, 84 K, and T89). Then two poplar clones (SXY and T89) were selected, due to their differences in B content and accumulation under high B toxicity, for transcriptomic and protein-protein interactions (PPI) analysis. The plant biomass and root-to-shoot ratio were reduced, the leaves exhibited symptoms of chlorosis and shrinkage of high B stressed five poplar clones. Boron toxicity significantly decreased the net photosynthetic rates, disturbed the balance of redox, induced the accumulation of H2O2 and malondialdehyde (MDA), and caused the increase in free proline levels in roots and leaves. B accumulation was significantly increased in the high B treated new leaves. The SXY clones showed the highest B accumulation, making it a potential hyperaccumulator for B contaminated soils. The transcriptome analysis of SXY and T89 revealed differential expression of genes involved in cell wall organization, active transmembrane transporter activity, cell wall synthesis, and B transport. The analysis of PPI indicates that these proteins work together in group functions. We identified some key differentially expressed genes (DEGs) related to transport and cell wall degradation. Our findings suggest that poplar is a plant that can tolerate high levels of boron, and it is possible to select poplar clones that accumulate high levels of boron to reduce soil boron pollution. The potential candidate genes, involved in transport and cell wall synthesis, can be focused on to improve poplar tolerance to high B in future breeding programs.

Metabolic responses to boron toxicity in different citrus cultivars

Metabolic responses to boron toxicity in different citrus cultivars

Evaluating Ecological Nano-Calcium from Eggshells: Effects on Calcium Nutrition and Oxidative Stress in Lettuce Under Saline and Boron Toxicity

AbstractNano-fertilizers with higher efficacy compared to conventional fertilizers can provide advantage for plant cultivation in both productive and problematic soils. Therefore, this study aimed to determine the effect of nano-calcium (nano-Ca) on lettuce plants grown in saline-boron toxic soil. Nano-calcium fertilizer was prepared from eggshells. Functional and structural properties of nano-Ca was determined by scanning electron microscopy (SEM), x-ray diffraction (XRD) and fourier transform infrared spectroscopy (FTIR) before plant experiment. The treatments was; control, 40 mM NaCl and 20 mg kg−1 B (NaCl + B), and 40 mM NaCl and 20 mg kg−1 B with 4 mM Nano-Ca (NaCl + B + nano-Ca). The nano-Ca significantly increased the dry weight and calcium (Ca) concentration of lettuce plants under saline-B toxic conditions. Although there was a decrease in the concentrations of sodium (Na), chloride (Cl), and boron (B) with nano-Ca treatment, it was not statistically significant. Salinity and boron toxicity lead to increased lipid peroxidation. In the present study, the production of malondialdehyde (MDA) as a marker for lipid peroxidation, along with a significant decrease in hydrogen peroxide (H2O2) concentration, was observed with the application of nano-Ca. There was no significant alteration in superoxide dismutase activity (SOD) observed in lettuce grown under saline and boron toxic conditions. However, catalase activity (CAT) increased with nano-Ca application, while the activity of ascorbate peroxidase (APX) decreased. The study results suggest that nano-Ca serves a protective function for lettuce plants cultivated under saline and boron toxic conditions.

Role of silicon in alleviating boron toxicity and enhancing growth and physiological traits in hydroponically cultivated Zea mays var. Merit

BackgroundBoron (B) is a micronutrient, but excessive levels can cause phytotoxicity, impaired growth, and reduced photosynthesis. B toxicity arises from over-fertilization, high soil B levels, or irrigation with B-rich water. Conversely, silicon (Si) is recognized as an element that mitigates stress and alleviates the toxic effects of certain nutrients. In this study, to evaluate the effect of different concentrations of Si on maize under boron stress conditions, a factorial experiment based on a randomized complete block design was conducted with three replications in a hydroponic system. The experiment utilized a nutrient solution for maize var. Merit that contained three different boron (B) concentrations (0.5, 2, and 4 mg L−1) and three Si concentrations (0, 28, and 56 mg L−1).ResultsOur findings unveiled that exogenous application of B resulted in a substantial escalation of B concentration in maize leaves. Furthermore, B exposure elicited a significant diminution in fresh and dry plant biomass, chlorophyll index, chlorophyll a (Chl a), chlorophyll b (Chl b), carotenoids, and membrane stability index (MSI). As the B concentration augmented, malondialdehyde (MDA) content and catalase (CAT) enzyme activity exhibited a concomitant increment. Conversely, the supplementation of Si facilitated an amelioration in plant fresh and dry weight, total carbohydrate, and total soluble protein. Moreover, the elevated activity of antioxidant enzymes culminated in a decrement in hydrogen peroxide (H2O2) and MDA content. In addition, the combined influence of Si and B had a statistically significant impact on the leaf chlorophyll index, total chlorophyll (a + b) content, Si and B accumulation levels, as well as the enzymatic activities of guaiacol peroxidase (GPX), ascorbate peroxidase (APX), and H2O2 levels. These unique findings indicated the detrimental impact of B toxicity on various physiological and biochemical attributes of maize, while highlighting the potential of Si supplementation in mitigating the deleterious effects through modulation of antioxidant machinery and biomolecule synthesis.ConclusionsThis study highlights the potential of Si supplementation in alleviating the deleterious effects of B toxicity in maize. Increased Si consumption mitigated chlorophyll degradation under B toxicity, but it also caused a significant reduction in the concentrations of essential micronutrients iron (Fe), copper (Cu), and zinc (Zn). While Si supplementation shows promise in counteracting B toxicity, the observed decrease in Fe, Cu, and Zn concentrations warrants further investigation to optimize this approach and maintain overall plant nutritional status.

Evaluation of Possible Toxic Effects of Boric Acid in Palourde Clam (Ruditapes decussatus) Through Histological Changes and Oxidative Responses.

The extensive utilization of boric acid, particularly in industrial and agricultural sectors, also engenders concerns regarding the toxicity of boron and its derivatives. Particularly, the behavior of boric acid at increasing concentrations in aquatic ecosystems remains poorly understood. In light of these concerns, this study aimed to investigate the toxicity of boric acid in bivalves, which occupy a critical position in the food chain. Specimens of Ruditapes decussatus, which had not been previously exposed to any pollutants and were cultivated under controlled conditions, were subjected to three different concentrations of boric acid (0.05mg/L, 0.5mg/L, and 5mg/L) in vitro for 96h. Following the exposure period, the specimens were assessed for histological changes (the mantle, gill, and digestive gland) and specific oxidative parameters (the gill and digestive gland), including superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase, and lipid peroxidation (LPO). The research findings indicated that boric acid primarily induced oxidative damage at the applied concentrations and increased antioxidant levels (p < 0.05). Moreover, although no significant histopathological abnormalities were observed in the examined histological sections, subtle changes were noted. This study evaluated the potential adverse effects of boric acid on bivalves, which are crucial components of the aquatic food chain, utilizing histological and specific physiological parameters following its introduction into aquatic environments. It is anticipated that the findings of this study will contribute to the development of new insights and perspectives regarding the extensive use of boric acid.

Effects of Claroideoglomus etunicatum Fungus on the Growth Parameters of Maize (Zea mays L.) Plants under Boron Toxicity and Salt Stress

Soil salinity is an emerging phenomenon threatening arid and semiarid areas due to changing climatic events. Salinity, in combination with other elemental contaminants, can often harm crop performance and productivity. This experiment was conducted to evaluate the mitigating effect of Claroideoglomus etunicatum, an arbuscular mycorrhizal fungus (AMF), on combined boron (B) toxicity and salt stress symptoms in maize plants. After the stress and AMF treatments, plants were subjected to a wide range of analyses, such as AMF colonization rates, ion leakage, plant biomass, and concentration of B, phosphorus, sodium, potassium, iron, zinc, copper, and manganese in root and shoot tissues. The results showed that the combined stress did not affect the AMF colonization rate. AMF inoculation significantly increased plant biomass, the K+/Na+ ratio, and shoot B, sodium, and copper concentrations, but reduced root B concentrations and ion leakage. AMF inoculation slightly increased root dry weight and the sodium, potassium, zinc, copper and Mn contents in shoots under combined B and salinity stress, while AMF reduced the electrolyte leakage in leaves. It is inferred that AMF can ameliorate B toxicity in maize by improving biomass and reducing B concentration in plant tissues. Our research implies that C. etunicatum could be a valuable candidate for assisting in the remediation of boron-contaminated and saline soils.

Zinc Oxide Nanoparticles and Zinc Sulfate Alleviate Boron Toxicity in Cotton (Gossypium hirsutum L.).

Boron toxicity significantly hinders the growth and development of cotton plants, therefore affecting the yield and quality of this important cash crop worldwide. Limited studies have explored the efficacy of ZnSO4 (zinc sulfate) and ZnO nanoparticles (NPs) in alleviating boron toxicity. Nanoparticles have emerged as a novel strategy to reduce abiotic stress directly. The precise mechanism underlying the alleviation of boron toxicity by ZnO NPs in cotton remains unclear. In this study, ZnO NPs demonstrated superior potential for alleviating boron toxicity compared to ZnSO4 in hydroponically cultivated cotton seedlings. Under boron stress, plants supplemented with ZnO NPs exhibited significant increases in total fresh weight (75.97%), root fresh weight (39.64%), and leaf fresh weight (69.91%). ZnO NPs positively affected photosynthetic parameters and SPAD values. ZnO NPs substantially reduced H2O2 (hydrogen peroxide) by 27.87% and 32.26%, MDA (malondialdehyde) by 27.01% and 34.26%, and O2- (superoxide anion) by 41.64% and 48.70% after 24 and 72 h, respectively. The application of ZnO NPs increased the antioxidant activities of SOD (superoxide dismutase) by 82.09% and 76.52%, CAT (catalase) by 16.79% and 16.33%, and POD (peroxidase) by 23.77% and 21.66% after 24 and 72 h, respectively. ZnO NP and ZnSO4 application demonstrated remarkable efficiency in improving plant biomass, mineral nutrient content, and reducing boron levels in cotton seedlings under boron toxicity. A transcriptome analysis and corresponding verification revealed a significant up-regulation of genes encoding antioxidant enzymes, photosynthesis pathway, and ABC transporter genes with the application of ZnO NPs. These findings provide valuable insights for the mechanism of boron stress tolerance in cotton and provide a theoretical basis for applying ZnO NPs and ZnSO4 to reduce boron toxicity in cotton production.

Exploring the Impact of Phosphorus Application on Boron Toxicity Tolerance in Maize and Sunflower Under Aerobic Conditions, and Rice Under Anaerobic Conditions

ABSTRACT Both phosphorus (P) and boron (B) can interact with each other due to their similar properties and P can hinder the uptake of boron by plants. In this study, the effect of P and B concentrations on the growth of rice, sunflower, and maize plants grown under greenhouse conditions in an environment that would induce boron toxicity was investigated. To create a toxic environment, 40 mg kg−1 B was applied to rice plants, and 30 and 25 mg kg−1 B were applied to sunflower and maize plants, respectively. To alleviate the toxicity, 500 mg kg−1 P was added to the respective pots. The results of the study showed that B application reduced the fresh and dry weight of all plant species, while significant increases were observed with the addition of P. Among the applied P sources, TSP was found to be more effective than H3PO4 in terms of fresh and dry weight of maize plants. Plant-B concentrations increased with B application but were significantly reduced by the applied P sources, with no difference observed among the P sources. Plant-P concentrations, which decreased with B applications, increased as a result of P applications. In particular, H3PO4 was found to be more effective than TSP in terms of plant-P concentration, especially in sunflower and maize plants. This study determined that P fertilization can be beneficial in reducing the damage caused by boron toxicity, which is a stress factor that severely limits agricultural production in regions with arid and semi-arid climates.

Response of Winter Canola Varieties to Boron Stress during Germination and Seedling Growth Stage

The objective of the study was to determine the effects of boron concentrations on germination and seedling growth of winter canola varieties under laboratory conditions. Seeds of four winter canola varieties (KWS Cyrill CL, Miranda, PT264, and NK Caravel) were germinated between papers with different boron levels (0, 20, 40, 60, 80, and 100 mg B L-1) consisting of sodium borate (Na2B8O13.4H2O) at 20°C for 7 days. The germination percentage, mean germination time, germination index, seedling growth parameters, and dry matter were measured. The results showed that germination percentage, mean germination time, and germination index were negatively affected by increasing B concentrations. When B levels increased, root and shoot lengths and weights were also inhibited, while the responses of canola varieties differed. B levels had a significant effect on shoot length, which decreased from 5.15 cm to 1.82 cm and root length from 4.99 cm to 2.59 cm. Under boron stress, KWS Cyrill CL germinated higher and developed longer roots and shoots. Differences in both germination and seedling growth among canola cultivars were observed at 80 mg B L-1 and higher. It was concluded that there was a genotypic variation among canola varieties concerning boron toxicity and that KWS Cyrill CL was more tolerant to high boron concentrations than the other varieties.

Understanding boron toxicity in aquatic plants (Salvinia natans and Lemna minor) in the presence and absence of EDTA

Even though boron is a widely used element in various industries and a contributor to water pollution worldwide, few studies have examined the toxicity of boron in aquatic plants. EDTA is used to maintain aquatic plants cultures, however it is possible to modify the toxicity of metals. The objective of this study is to assess the toxicity of boron in aquatic plants and explore the impact of EDTA presence on the resulting toxic responses. Floating watermoss Salvinia natans and duckweed Lemna minor were exposed to concentrations ranging from 5 to 100 mg/L for 7 days and 1 to 60 mg/L for 3 days, respectively. Growth and photosynthetic activity parameters were investigated in the presence and absence of EDTA. Growth inhibitions in both aquatic plants were observed in a concentration-dependent manner, irrespective of the presence or absence of EDTA. For instance, based on the specific growth rate (leaves coverage), EC10 values for S. natans were calculated as 12.7 (9.9–15.3) mg/L and 8.0 (5.8–10.3) mg/L with and without EDTA, respectively. In the case of L. minor, EC10 values were calculated as 1.3 (0.8–1.89) mg/L and 2.0 (0.4–4.3) mg/L with EDTA without EDTA, respectively. Significant effects were also observed on the photosynthetic capacity, however there was no change in the increase of boron concentration. Generally, negligible effects of EDTA to the toxicity of boron were observed in the present study. By comparing toxicity results based on the presence and absence of EDTA, which is an essential element in the test medium, the results of this study are expected to be utilized for the ecological risk assessment of boron in aquatic ecosystems.

Knock-Out Mutatıon of Cat2 Drastıcally Reduced The Melatonın-Medıated Resıstance to Boron Toxıcıty in Arabıdopsıs thalıana

The role of catalase in resistance to boron toxicity after melatonin application (MEL) was investigated in Arabidopsis thaliana plants. Col-0 and cat2-2 plants were exposed to 50 μM MEL followed by boron toxicity (BT) in a medium containing 10 mM H 3BO 3. Pigment loss and accordingly chlorosis were reduced by melatonin under BT conditions, while they were more prominent in cat2-2 mutants. Moreover, TBARS and H 2O 2 contents, which increased due to BT, decreased as a result of melatonin application and the levels of these parameters in cat2-2 mutants were higher than the values in Col-0. Antioxidant enzyme activity of SOD and SOD1 gen transcript were induced by MEL under BT. Conversely, APX4, PER10 and CAT1 transcripts were down-regulated by MEL under BT. In addition, antioxidant enzyme activities and their transcript levels were lower than those of Col-0. Thus, we suggested that MEL scavenged ROS directly under BT. Melatonin also reduced the accumulation of boric acid in leaf tissues of Col-0, but not cat2-2. Finally, even though melatonin application provided a degree of endurance, the cat2 mutation resulted in increased sensitivity to BT.

Overexpression of miR408 influences the cotton response to boron toxicity

Overexpression of miR408 influences the cotton response to boron toxicity

Insight of Oxidative Stress, Ultrastructural Transformations, and Elemental Variations in Rice Seedlings Exposed to Boron Toxicity: Unraveling the Role of Fe-SOD in Boron Tolerance

Rice (Oryza sativa L.) seedlings differing in boron tolerance were grown in hydroponics containing varying boron concentration. (as boric acid) for 8 days. Yoshida Nutrient Solution (Yoshida et al.1976) served as control, whereas 0.5 mM boron (moderately toxic) and 1.5 mM boron (highly toxic). Boron excess (B) caused marked reduction in length, biomass and relative water content of the seedlings, with more reductions in B-sensitive cv. Malaviya-36 as compared with B-tolerant cv. Brown Gora. B-sensitive seedlings showed higher B uptake in roots and shoots compared to the tolerant. Scanning electron microscopy (SEM) revealed ultrastructural damage to the guard cells with excess boron. Energy dispersive X-ray analysis (EDXA) of rice leaves showed decline in concentration of P, S, Ca and Mg in seedlings on B treatment. Increased production of reactive oxygen species O2˙−, H2O2, lipid peroxides, alteration in activity of antioxidative enzymes and increased membrane permeability was observed in B treated seedlings compared to controls. Real time PCR analysis of stress regulatory genes indicated differential expression of SOD isoforms in the two sets of cultivars with B treatment. Interestingly seedlings of B-tolerant cultivar were characterized by higher level of expression of Fe-SOD and its further increased expression on B treatment. Results suggest that B toxicity involves ultrastructural and elemental changes, increased generation of ROS and altered antioxidative enzyme activities in rice seedlings and increased expression Fe-SOD isoform appears to be associated with B tolerance.

The effect of arbuscular mycorrhizal inoculation and plant growth-promoting rhizobacteria on maize (Zea mays L.) under boron toxicity stress

A greenhouse study was conducted to investigate the effect of arbuscular mycorrhiza fungus and plant growth-promoting rhizobacteria on maize (Zea mays L.) yield and boron (B) uptake under excessive B concentrations. Four levels of B (0, 10, 20 and 40 mg kg-1 soil) and four levels of microbial inoculation (non-inoculation, Rhizophagus intraradices, Micrococcus yunnanensis and co-inoculation) were applied. In general, elevated B decreased the maize relative yield, plant dry weight, plant height, total leaf area and chlorophyll content. The highest B uptake was recorded in the 20 mg kg-1 B treatments, whereas the lowest uptake was in B treatments of 0 and 40 mg B kg-1. Excessive B supply increased the proline content and electrolyte leakage of maize leaves. Microbial inoculation did not change the yield of B-stressed plants. In addition, mycorrhiza inoculation and co-inoculation increased the shoot and root B uptake. Maize plants growing at high B concentrations with or without microbial inoculation showed leaf edge burning and necrosis when compared with control treatments. Microbial inoculation improved the maize yield under B stress up to 20 mg B kg-1. Thus, inoculation with arbuscular mycorrhiza and plant growth promoting rhizobacteria can increase the maize tolerance under B toxicity. Nonetheless, the inoculation of bacterium and arbuscular mycorrhiza had a more alleviative effect when the plants were exposed to excess B.

Functional characterization of polyol/monosaccharide transporter 1 in Lotus japonicus

Polyol/Monosaccharide Transporters (PLTs/PMTs) localized in the plasma membrane have previously been identified in plants. The physiological role and the functional properties of these proteins in legume plants are, however, unclear. Here we describe the functional analysis of LjPLT1, a plasma membrane-localized PLT protein from Lotus japonicus. The LjPLT1 gene was strongly expressed in the vascular tissue of roots, stems and leaves. Expression of the LjPLT1 cDNAs in yeast revealed that the protein functions as a broad-spectrum H+ -symporter for both linear polyols of sorbitol and mannitol, and cyclic polyol myo-inositol. It also catalyzes the transport of different hexoses, including fructose, glucose, galactose and mannose. Overexpression of LjPLT1 (OELjPLT1) results in inhibition of plant growth and a decrease in nodule nitrogenase activity in L. japonicus. The soluble sugars were increased in newly expanded leaves, roots and nodules but decreased in mature leaves in OELjPLT1 plants. In addition, the OELjPLT1 seedlings displayed an increased sensitivity to high content mannitol and boron toxicity, but neither drought nor salinity stresses. Taken together, the present study indicates that the LjPLT1 protein may participate in the translocation of hexoses/polyols to regulate multiple physiological and growth processes in L. japonicus.