Ameliorative role of boron to toxicity of aluminum in trifoliate orange roots.

Aluminum (Al) toxicity is a major limiting factor for plant productivity. Boron (B) could mitigate Al toxicity in many plant species. However, information about the mechanisms of B alleviating Al toxicity in citrus is lacking. Trifoliate orange rootstock (Poncirus trifoliate L. Raf.) seedlings were irrigated with a nutrient solution containing two B and two Al levels. Results showed that exposure to Al severely impeded plant growth-related parameters. However, B supply improved plant biomass, root activity and relative root elongation under Al stress. Furthermore, B reduced the Al-induced H2O2 accumulation in roots as evidenced by lower fluorescence intensity of H2O2 staining. Boron decreased the Al-stimulated ascorbate (AsA) synthesis by down-regulated AsA synthesis-related metabolites in the L-galactose pathway. Boron alleviated some of the toxic effects of Al by decreasing redox states of AsA and enzyme activities involved in ascorbate-glutathione (AsA-GSH) cycle, ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase and glutathione peroxidase while increased glutathione (GSH) content and γ-glutamylcysteine synthetase (γ-GCS) activity. Overall, our results suggest that B protects roots against Al-induced oxidative stress possibly by reducing metabolites accumulation in the L-galactose pathway of AsA synthesis and regulating AsA-GSH cycle.

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

Aluminum (Al) toxicity is a major factor limiting plant productivity. The objective of the present study was to develop the mechanisms of boron (B) alleviating aluminum toxicity in citrus. The results showed that aluminum toxicity severely hampered root elongation. Interestingly, under aluminum exposure, boron supply improved superoxide dismutase activity while reducing peroxidase, catalase and polyphenol oxidase activities. Likewise, the contents of H2O2, lipid peroxidation, protein and proline in roots were markedly decreased by boron application under aluminum exposure. Our results demonstrated that boron could alleviate aluminum toxicity by regulating antioxidant enzyme activities in the roots.

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.

Ameliorative effects of boron on aluminum induced variations of cell wall cellulose and pectin components in trifoliate orange (Poncirus trifoliate (L.) Raf.) rootstock.

Aluminum (Al) phytotoxicity is a major limitation in the production of crops in the soils with pH ≤ 5. Boron (B) is indispensable nutrient for the development of higher plants and B role has been reported in the alleviation Al toxicity. Trifoliate orange rootstock was grown in two B and two Al concentrations. The results of the present study showed that Al toxicity adversely inhibited root elongation and exhibited higher oxidative stress in terms of H2O2 and O2- under B-deficiency. Additionally, the X-ray diffraction (XRD) analysis confirmed the increase of the cellulose crystallinity in the cell wall (CW). Al-induced remarkable variations in the CW components were prominent in terms of alkali-soluble pectin, 2-keto-3-deoxyoctonic acid (KDO) and the degree of methyl-esterification (DME) of pectin. Interesting, B supply reduced the pectin (alkali-soluble) under Al toxicity. Moreover, the results of FTIR (Fourier transform infrared spectroscopy) and 13C-NMR (13C nuclear magnetic resonance) spectra revealed the decrease of carboxyl groups and cellulose by B application during Al exposure. Furthermore, B supply tended to decrease the Al uptake, CW thickness and callose formation. The study concluded that B could mitigate Al phytotoxicity by shielding potential Al binding sites and by reducing Al induced alterations in the CW cellulose and pectin components.

Boron Deficiency in Trifoliate Orange Induces Changes in Pectin Composition and Architecture of Components in Root Cell Walls.

Boron (B) is a micronutrient indispensable for citrus and B deficiency causes a considerable loss of productivity and quality in China. However, studies on pectin composition and architecture of cell wall components in trifoliate orange roots under B deficiency condition are not sufficient. In this study, we investigated the alteration in pectin characteristics and the architecture of cell wall components in trifoliate orange [Poncirus trifoliata (L.) Raf.] roots under B starvation. The results showed that B-deficient roots resulted in a significant enlargement of root tips and an obvious decrease in cell wall B and uronic acid content in Na2CO3-soluble pectin compared with B-adequate roots. Meanwhile, they showed a decrease of 2-keto-3-deoxyoctanoic acid in CDTA-soluble and Na2CO3-soluble pectin in cell walls, while the degree of methylation (DM) of CDTA-soluble pectin was significantly increased under B deficiency. Transmission electron microscope (TEM) micrographs of B deficient plants showed a distinct thickening of the cell walls, with the thickness 1.82 times greater than that of control plant roots. The results from Fourier-transform infrared spectroscopy (FTIR) showed that B deficiency changed the mode of hydrogen bonding between protein and carbohydrates (cellulose and hemicellulose). The FTIR spectra exhibited a destroyed protein structure and accumulation of wax and cellulose in the cell walls under B starvation. The 13C nuclear magnetic resonance (13C-NMR) spectra showed that B starvation changed the organic carbon structure of cell walls, and enhanced the contents of amino acid, cellulose, phenols, and lignin in the cell wall. The results reveal that the swelling and weakened structural integrity of cell walls, which induced by alteration on the network of pectin and cell wall components and structure in B-deficient roots, could be a major cause of occurrence of the rapid interruption of growth and significantly enlarged root tips in trifoliate orange roots under B-insufficient condition.