Boron inhibits aluminum-induced toxicity to citrus by stimulating antioxidant enzyme activity

Insights into physiological and molecular mechanisms underlying efficient utilization of boron in different boron efficient Beta vulgaris L. varieties

Boron (B) deficiency and consequent limitation of plant yield and quality, particularly of sugar beet (Beta vulgaris L.) has emerged as a maior problem,which is exacerbating due to cultivar dependent variability in B deficiency tolerance. Pertinently, the current study was designed to elucidate the physiological and molecular mechanisms of B deficiency tolerance of sugar beet varieties KWS1197 (B-efficient variety) and KWS0143 (B-inefficient variety). A hydroponic experiment was conducted employing two B levels B0.1 (0.1 μM L^-1 H₃BO₃, deficiency) and B50 (50 μM L^-1 H₃BO₃, adequacy). Boron deficiency greatly inhibited root elongation and dry matter accumulation; however, formation of lateral roots stimulated and average root diameter was increased. Results exhibited that by up-regulating the expression of NIP5-1, NIP6-1, and BOR2, and suppressing the expression of BOR4, cultivar KWS1197, in contrast to KWS0143, managed to transfer sufficient amount of B to the aboveground plant parts, facilitating its effective absorption and utilization. Accumulation of malondialdehyde (MDA) and reactive oxygen species (ROS) was also mellowed in KWS1197, as well as the oxidative damage to root cells via preservation of the antioxidant enzyme system. Additionally, the expression of essential enzymes for biosynthesis of phytohormone (PYR/PYL) and lignin (COMT, POX, and CCoAOMT) were found to be highly up-regulated in KWS1197. Deductively, through effective B absorption and transportation, balanced nutrient accumulation, and an activated antioxidant enzyme system, B-efficient cultivars may cope with B deficiency while retaining a superior cellular structure to enable root development.

Increasing media pH contribute to the absorption of boron via roots to promote the growth of citrus

Low pH is an important limiting factor for plant development in the south of China due to problems of acid red soil and boron (B) deficiency. Whereas, there is very limited information on the relationship between media pH and B distribution in plant, the physiological process changed by the interaction of pH and B in citrus growth also unclear. This experiment was conducted on trifoliate rootstock by employing two different concentrations of B (0 or 10 μM B) under three pH levels: pH 4, pH 5, and pH 6. Our results illustrate that low pH inhibite plant growth and cause oxidative stress in the roots, resulting in cell membrane injury. The increase of pH and B addition reduce the accumulation of ROS (O₂^.- and H₂O₂) by regulating the activity of Class Ⅲ peroxidases (CIII Prxs). Moreover, increased pH improves the internal circulation of B in plants and decrease the content of lignin and cellulose in cell wall (CW). In summary, our investigation demonstrated that the increase of pH in nutrient solution can accelerate the re-distribution of B by roots to promote citrus growth. The accumulation of B in roots can protect plants from the damage of ROS by regulating the activity of CIII Prxs as well as decrease the content of lignin and cellulose are to promotes roots elongation.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

The aluminum distribution and translocation in two citrus species differing in aluminum tolerance

BACKGROUND

Many citrus orchards of south China suffer from soil acidification, which induces aluminum (Al) toxicity. The Al-immobilization in vivo is crucial for Al detoxification. However, the distribution and translocation of excess Al in citrus species are not well understood.

RESULTS

The seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] and 'Shatianyou' [Citrus grandis (L.) Osbeck], that differ in Al tolerance, were hydroponically treated with a nutrient solution (Control) or supplemented by 1.0 mM Al³⁺ (Al toxicity) for 21 days after three months of pre-culture. The Al distribution at the tissue level of citrus species followed the order: lateral roots > primary roots > leaves > stems. The concentration of Al extracted from the cell wall (CW) of lateral roots was found to be about 8 to 10 times higher than in the lateral roots under Al toxicity, suggesting that the CW was the primary Al-binding site at the subcellular level. Furthermore, the Al distribution in CW components of the lateral roots showed that pectin had the highest affinity for binding Al. The relative expression level of genes directly relevant to Al transport indicated a dominant role of Cs6g03670.1 and Cg1g021320.1 in the Al distribution of two citrus species. Compared to C. grandis, C. sinensis had a significantly higher Al concentration on the CW of lateral roots, whereas remarkably lower Al levels in the leaves and stems. Furthermore, Al translocation revealed by the absorption kinetics of the CW demonstrated that C. sinensis had a higher Al retention and stronger Al affinity on the root CW than C. grandis. According to the FTIR (Fourier transform infrared spectroscopy) analysis, the Al distribution and translocation might be affected by a modification in the structure and components of the citrus lateral root CW.

CONCLUSIONS

A higher Al-retention, mainly attributable to pectin of the root CW, and a lower Al translocation efficiency from roots to shoots contributed to a higher Al tolerance of C. sinensis than C. grandis. The aluminum distribution and translocation of two citrus species differing in aluminum tolerance were associated with the transcriptional regulation of genes related to Al transport and the structural modification of root CW.

Enhancing Salt Tolerance in Soybean by Exogenous Boron: Intrinsic Study of the Ascorbate-Glutathione and Glyoxalase Pathways

Boron (B) performs physiological functions in higher plants as an essential micronutrient, but its protective role in salt stress is poorly understood. Soybean (Glycine max L.) is planted widely throughout the world, and salinity has adverse effects on its physiology. Here, the role of B (1 mM boric acid) in salt stress was studied by subjecting soybean plants to two levels of salt stress: mild (75 mM NaCl) and severe (150 mM NaCl). Exogenous B relieved oxidative stress by enhancing antioxidant defense system components, such as ascorbate (AsA) levels, AsA/dehydroascorbate ratios, glutathione (GSH) levels, the GSH and glutathione disulfide ratios, and ascorbate peroxidase, monodehydroascorbate reductase, and dehydroascorbate reductase activities. B also enhanced the methylglyoxal detoxification process by upregulation of the components of the glyoxalase system in salt-stressed plants. Overall, B supplementation enhanced antioxidant defense and glyoxalase system components to alleviate oxidative stress and MG toxicity induced by salt stress. B also improved the physiology of salt-affected soybean plants.

Recent Advances in Understanding Mechanisms of Plant Tolerance and Response to Aluminum Toxicity

Aluminum (Al) toxicity is a major environmental stress that inhibits plant growth and development. There has been impressive progress in recent years that has greatly increased our understanding of the nature of Al toxicity and its mechanisms of tolerance. This review describes the transcription factors (TFs) and plant hormones involved in the adaptation to Al stress. In particular, it discusses strategies to confer plant resistance to Al stress, such as transgenic breeding, as well as small molecules and plant growth-promoting rhizobacteria (PGPRs) to alleviate Al toxicity. This paper provides a theoretical basis for the enhancement of plant production in acidic soils.

Boron reduces aluminum deposition in alkali-soluble pectin and cytoplasm to release aluminum toxicity

Boron (B) is indispensable for plant growth and has been reported in the mitigation of aluminum (Al) toxicity in different plants. This study focused on the efficacy of B in reducing Al toxicity to trifoliate orange seedlings in a hydroponic experiment. Boron supply had a positive effect on root length and plant growth-related parameters and attenuated Al-induced inhibition of plasma membrane H⁺-ATPase activity. X-ray photoelectron spectroscopy (XPS) in conjunction with scanning electron microscope-energy dispersive x-ray spectrometer (SEM-EDS) revealed that B reduced Al accumulation in root cell wall, especially on pectin fractions (alkali-soluble pectin), accompanied by suppressing pectin synthesis, pectin methylesterase (PME) activity and PME expression. Furthermore, B application inhibited NRAT1 expression while increased ALS1 expression, indicating restraining Al transport from external cells to cytoplasm and accelerating accelerating vacuolar sequestration. The results were further demonstrated by transmission electron microscope-energy dispersive x-ray spectrometer (TEM-EDS) analysis. Taken together, our results indicated that B mainly promoted the efflux of H⁺ by regulating the plasma membrane H⁺-ATPase activity, and reduced the demethylation of pectin to weaken Al binding to carboxyl. More importantly, B alleviated some of the toxic effects of Al by compartmentalizing Al into vacuoles and decreasing the deposition of Al in cytoplasm.

Multiple stresses occurring with boron toxicity and deficiency in plants

Boron (B), an essential nutrient for plants, participates in many physiological processes, with emphasis its role in the formation of the plant's cell wall. In soil, the range between deficiency and toxicity of B is very narrow as compared to other nutrients, which makes its management in agriculture very difficult, as it depends on the soil and environmental conditions. B stress simultaneously acts with others (extreme temperatures, excess of light, high concentration of CO₂, drought, salinity or heavy metal contamination, etc.). The effects of these other stresses could increase the sensitivity of plants to B toxicity or deficiency. The simultaneous combination (B stress × other abiotic stresses) is a complex interaction that should be analyzed in detail if the resistance of crops to climate change is needed. This article reviews the response of plants when facing a combination of B stress with other stresses, and compares this response with the individual stresses. Also, in the last few years, the role of B has been described in multiple plant functions that can improve its resilience to specific stresses. Thus, this article also analyses in what conditions the application of B can be efficient for the improvement of the plant's response to other stresses.

Boron mitigates citrus root injuries by regulating intracellular pH and reactive oxygen species to resist H+-toxicity

Boron (B)-deficiency and H⁺-toxicity are important limiting factors for plants growth in acid soils. High B supply may reduce H⁺-toxicity-induced inhibition of growth in citrus. Trifoliate orange rootstock seedlings were irrigated with nutrient solution containing either 0 μM or 10 μM H₃BO₃ at two pH levels (pH4 (H⁺-toxicity) and pH6 (normal)). The results showed that H⁺-toxicity without B severely hampered main root elongation. Simultaneously, oxidative damage caused by H⁺-toxicity led to severe damage to the apical structure of root such as root crown abscission. However, B application promoted the root length, root cell viability and reduced cell wall (CW) thickness of root tips under H⁺-toxicity. Additionally, B application reduced the H⁺-toxicity-induced reactive oxygen species (ROS) accumulation in roots as characterized by lower fluorescence intensity of H₂O₂ and O₂^- staining. Moreover, ³¹P-NMR (³¹P nuclear magnetic resonance) spectra revealed B application regulated the pH of vacuoles and cytoplasm in root tips by reducing phosphoenolpyruvate carboxykinase (PEPCase) activity while enhancing NADP malic enzyme (NADP-ME) activity during H⁺-toxicity. Collectively, our results demonstrate that B supply alleviates H⁺-toxicity and promotes root growth by reducing ROS accumulation, attenuating intracellular acidic microenvironment to ensure normal chemical reactions in root tip cells.

Boron supply maintains efficient antioxidant system, cell wall components and reduces aluminum concentration in roots of trifoliate orange

Aluminum (Al) toxicity in the acid soils (pH ≤ 5) is the major limiting abiotic factor affecting the productivity of crops. Boron (B) has been reported to alleviate Al toxicity. In spite of recent advances, it is not clear how B relieves Al toxicity. Results demonstrated that Al toxicity hampered the root elongation. Moreover, lumogallion fluorescent molecular probe unequivocally localized mostly bound Al to the periphery of the cell wall (CW) and to the nuclei. Additionally, Al toxicity induced variations in the CW components through the accumulation of pectin and hemicellulose. Nevertheless, B supply reduced callose deposition, increased root growth and reduced changes in the CW components under Al toxicity. Moreover, B supply reduced the un-methylated pectin while increased the degree of methyl esterification of pectin. These results imply that B due to its role in the CW formation could reduce aluminum-induced negative effects on plant growth by attenuating apoplastic Al³⁺ and changes in the CW components which ultimately results in the improved root growth.