植物对硼元素的吸收转运机制

硼是植物生长发育所必需的微量元素,但是在世界范围内,土壤中硼含量过高或者过低都会对植物生长产生影响,是农业生产上的主要问题.近来人们对硼的吸收转运机制的研究取得了突破性进展,鉴定了一些硼的转运通道和转运蛋白,例如：NIP5;1、NIP6;1、BOR1和BOR4,并对它们的转运机制有了一些了解.植物在硼缺少的情况下首先通过转运通道NIP5;1把硼吸收到共质体,然后通过转运蛋白BOR1运入中柱;在高硼毒害时,通过转运蛋白BOR4把过多的硼转出植物体,同时在植物中增加糖醇的含量,过表达BOR1或BOR4都能改变植物对硼含量变化的耐受性.因此,对植物中硼吸收转运机制的研究将有利于人们通过生物学手段提高作物对土壤中硼过高或过低的抗性.     

植物硼营养研究的重要进展与展望

硼是高等植物必需的矿质营养元素,但是人们对硼行使生理功能及其分子机理的认识远落后于其它必需营养元素。近几年国际上植物硼营养的研究取得了一些重要的突破。首先是进一步明确了B-RG-II复合物的形成及其影响细胞壁结构和功能的分子机制,并且发现B-RG-II的形成及其含量与陆生植物的进化密切相关。其次是在拟南芥中克隆了第一个植物硼转运子基因BOR1,并揭示了它的作用机理;通过转基因实验证明了植物硼的高效吸收与水通道基因NIP5;1密切相关。进而通过大量的种质筛选,从油菜、小麦、大麦及棉花等农作物中获得一批硼高效吸收利用的优异种质材料,并开展了硼高效QTL定位和克隆。本文详细综述了以上几个方面的研究进展,并对进一步的研究做了展望。     

植物硼营养研究的重要进展与展望

硼是高等植物必需的矿质营养元素, 但是人们对硼行使生理功能及其分子机理的认识远落后于其它必需营养元素。近几年国际上植物硼营养的研究取得了一些重要的突破。首先是进一步明确了B-RG-II复合物的形成及其影响细胞壁结构和功能的分子机制, 并且发现B-RG-II的形成及其含量与陆生植物的进化密切相关。其次是在拟南芥中克隆了第一个植物硼转运子基因 BOR1, 并揭示了它的作用机理; 通过转基因实验证明了植物硼的高效吸收与水通道基因NIP5;1密切相关。进而通过大量的种质筛选, 从油菜、小麦、大麦及棉花等农作物中获得一批硼高效吸收利用的优异种质材料, 并开展了硼高效QTL定位和克隆。本文详细综述了以上几个方面的研究进展, 并对进一步的研究做了展望。     

Roles of BOR1, DUR3, and FPS1 in boron transport and tolerance in Saccharomyces cerevisiae

The roles of three membrane proteins, BOR1, DUR3, and FPS1, in boron (B) transport in yeast were examined. The boron concentration in yeast cells lacking BOR1 was elevated upon exposure to 90 mM boric acid, whereas cells lacking DUR3 or FPS1 showed lower boron concentrations. Compared with control cells, cells overexpressing BOR1 or FPS1 had a lower boron concentration, and cells overexpressing DUR3 had a higher boron concentration. These results suggest that, in addition to the efflux boron transporter BOR1, DUR3 and FPS1 play important roles in regulating the cellular boron concentration. Analysis of the yeast transformants for tolerance to a high boric acid concentration revealed an apparent negative correlation between the protoplasmic boron concentration and the degree of tolerance to a high external boron concentration. Thus, BOR1, DUR3, and FPS1 appear to be involved in tolerance to boric acid and the maintenance of the protoplasmic boron concentration.     

Improvement of seed yields under boron-limiting conditions through overexpression of BOR1, a boron transporter for xylem loading, in Arabidopsis thaliana

Soil fertilization is a common practice in modern agriculture, undertaken to prevent nutrient deficiency in crops. However, fertilization is costly and causes environmental pollution. The cultivation of plants that tolerate low nutrient supplies may circumvent this problem. Here, we report the generation of Arabidopsis thaliana plants that tolerate boron (B)-deficient conditions due to the overexpression of BOR1, an efflux B transporter that is required for efficient xylem loading of B. In several independently generated transgenic plants expressing BOR1 or BOR1-GFP under the control of the cauliflower mosaic virus 35S RNA promoter, root-to-shoot translocation of B was enhanced and shoot growth was greater under B-limiting conditions compared with wild-type plants. In addition, the transgenic plants showed increased translocation of B, especially to the shoot apex, and set seed normally under B-limiting conditions, under which wild-type plants failed to set seed. This study therefore reports plants that show improved seed yields compared with wild-type under nutrient-deficient conditions as a result of increased production of an essential mineral nutrient transporter.     

Boron transport in plants: co-ordinated regulation of transporters

Background

The essentiality of boron (B) for plant growth was established >85 years ago. In the last decade, it has been revealed that one of the physiological roles of B is cross-linking the pectic polysaccharide rhamnogalacturonan II in primary cell walls. Borate cross-linking of pectic networks serves both for physical strength of cell walls and for cell adhesion. On the other hand, high concentrations of B are toxic to plant growth. To avoid deficiency and toxicity problems, it is important for plants to maintain their tissue B concentrations within an optimum range by regulating transport processes. Boron transport was long believed to be a passive, unregulated process, but the identification of B transporters has suggested that plants sense and respond to the B conditions and regulate transporters to maintain B homeostasis.

Scope

Transporters responsible for efficient B uptake by roots, xylem loading and B distribution among leaves have been described. These transporters are required under B limitation for efficient acquisition and utilization of B. Transporters important for tolerating high B levels in the environment have also been identified, and these transporters export B from roots back to the soil. Two types of transporters are involved in these processes: NIPs (nodulin-26-like intrinsic proteins), boric acid channels, and BORs, B exporters. It is demonstrated that the expression of genes encoding these transporters is finely regulated in response to B availability in the environment to ensure tissue B homeostasis. Furthermore, plants tolerant to stress produced by low B or high B in the environment can be generated through altered expression of these transporters.

Conclusions

The identification of the first B transporter led to the discovery that B transport was a process mediated not only by passive diffusion but also by transporters whose activity was regulated in response to B conditions. Now it is evident that plants sense internal and external B conditions and regulate B transport by modulating the expression and/or accumulation of these transporters. Results obtained in model plants are applicable to other plant species, and such knowledge may be useful in designing plants or crops tolerant to soils containing low or high B.     

Mitochondrial function and toxicity: role of the B vitamin family on mitochondrial energy metabolism

The B vitamins are water-soluble vitamins required as coenzymes for enzymes essential for cell function. This review focuses on their essential role in maintaining mitochondrial function and on how mitochondria are compromised by a deficiency of any B vitamin. Thiamin (B1) is essential for the oxidative decarboxylation of the multienzyme branched-chain ketoacid dehydrogenase complexes of the citric acid cycle. Riboflavin (B2) is required for the flavoenzymes of the respiratory chain, while NADH is synthesized from niacin (B3) and is required to supply protons for oxidative phosphorylation. Pantothenic acid (B5) is required for coenzyme A formation and is also essential for alpha-ketoglutarate and pyruvate dehydrogenase complexes as well as fatty acid oxidation. Biotin (B7) is the coenzyme of decarboxylases required for gluconeogenesis and fatty acid oxidation. Pyridoxal (B6), folate and cobalamin (B12) properties are reviewed elsewhere in this issue. The experimental animal and clinical evidence that vitamin B therapy alleviates B deficiency symptoms and prevents mitochondrial toxicity is also reviewed. The effectiveness of B vitamins as antioxidants preventing oxidative stress toxicity is also reviewed.     

Formation of rhamnogalacturonan II-borate dimer in pectin determines cell wall thickness of pumpkin tissue

Boron (B) deficiency results in inhibition of pumpkin (Cucurbia moschata Duchesne) growth that is accompanied by swelling of the cell walls. Monomeric rhamnogalacturonan II (mRG-II) accounted for 80% to 90% of the total RG-II in B-deficient walls, whereas the borate ester cross-linked RG-II dimer (dRG-II-B) accounted for more than 80% of the RG-II in control plants. The results of glycosyl residue and glycosyl linkage composition analyses of the RG-II from control and B-deficient plants were similar. Thus, B deficiency does not alter the primary structure of RG-II. The addition of (10)B-enriched boric acid to B-deficient plants resulted within 5 h in the conversion of mRG-II to dRG-II-(10)B. The wall thickness of the (10)B-treated plants and control plants was similar. The formation and possible functions of a borate ester cross-linked RG-II in the cell walls are discussed.     

High boron-induced ubiquitination regulates vacuolar sorting of the BOR1 borate transporter in Arabidopsis thaliana

Boron homeostasis is important for plants, as boron is essential but is toxic in excess. Under high boron conditions, the Arabidopsis thaliana borate transporter BOR1 is trafficked from the plasma membrane (PM) to the vacuole via the endocytic pathway for degradation to avoid excess boron transport. Here, we show that boron-induced ubiquitination is required for vacuolar sorting of BOR1. We found that a substitution of lysine 590 with alanine (K590A) in BOR1 blocked degradation. BOR1 was mono- or diubiquitinated within several minutes after applying a high concentration of boron, whereas the K590A mutant was not. The K590A mutation abolished vacuolar transport of BOR1 but did not apparently affect polar localization to the inner PM domains. Furthermore, brefeldin A and wortmannin treatment suggested that Lys-590 is required for BOR1 translocation from an early endosomal compartment to multivesicular bodies. Our results show that boron-induced ubiquitination of BOR1 is not required for endocytosis from the PM but is crucial for the sorting of internalized BOR1 to multivesicular bodies for subsequent degradation in vacuoles.     

The boron transporter BnaC4.BOR1;1c is critical for inflorescence development and fertility under boron limitation in Brassica napus

Boron (B) is an essential micronutrient for plants, but the molecular mechanisms underlying the uptake and distribution of B in allotetraploid rapeseed (Brassica napus) are unclear. Here, we identified a B transporter of rapeseed, BnaC4.BOR1;1c, which is expressed in shoot nodes and involved in distributing B to the reproductive organs. Transgenic Arabidopsis plants containing a BnaC4.BOR1;1c promoter‐driven GUS reporter gene showed strong GUS activity in roots, nodal regions of the shoots and immature floral buds. Overexpressing BnaC4.BOR1;1c in Arabidopsis wild type or in bor1‐1 mutants promoted wild‐type growth and rescued the bor1‐1 mutant phenotype. Conversely, knockdown of BnaC4.BOR1;1c in a B‐efficient rapeseed line reduced B accumulation in flower organs, eventually resulting in severe sterility and seed yield loss. BnaC4.BOR1;1c RNAi plants exhibited large amounts of disintegrated stigma papilla cells with thickened cell walls accompanied by abnormal proliferation of lignification under low‐B conditions, indicating that the sterility may be a result of altered cell wall properties in flower organs. Taken together, our results demonstrate that BnaC4.BOR1;1c is a AtBOR1‐homologous B transporter gene expressing in both roots and shoot nodes that is essential for the developing inflorescence tissues, which highlights its diverse functions in allotetraploid rapeseed compared with diploid model plant Arabidopsis.     

Boron in plant cell walls

Boron is an essential element for higher plants, yet the primary functions remain unclear. In intact tissues of higher plants, this element occurs as both water soluble and water insoluble forms. In this review, the intracellular localisation of B and possible function of B in cell walls of higher plants are discussed. The majority of the water soluble B seems to be localised in the apoplastic region as boric acid. The water insoluble B is associated with rhamnogalacturonan II (RG-II) and the complex is ubiquitous in higher plants. In the Brassicaceae, Apiaceae, Chenopodiaceae, Asteraceae, Amaryllidaceae, and Liliaceae, nearly all the cell wall B is associated with RG-II, while in the Cucurbitaceae, only half of the cell wall B is in this complex. In duckweed, a different type of B-polysaccharide complex has been identified.Analysis of the structure of the B–RG-II complex reveals that the complex is composed of boric acid and two chains of monomeric RG-II. Boric acid does not merely bind to sugars but crosslinks two chains of pectic polysaccharide at the RG-II region through borate-diester bonding, thus forming a network of pectic polysaccharides in cell walls. The B–RG-II complex is reconstituted in vitro only by mixing monomeric RG-II and boric acid at pH 4.0. In the in vitro reconstitution, germanic acid can substitute for boric acid to some extent. The RG-II epitope, which cross reacts with the antibody toward the B-RG-II complex, is detected in walls of every cell in radish roots. The epitope is also detected in growing pollen tube cell walls, which are claimed to require B.Whilst it is now clear that boric acid links some cell wall components, it is not yet clear whether there is a structural requirement for B in cell wall function.     

Transport-coupled ubiquitination of the borate transporter BOR1 for its boron-dependent degradation

Plants take up and translocate nutrients through transporters. In Arabidopsis thaliana, the borate exporter BOR1 acts as a key transporter under boron (B) limitation in the soil. Upon sufficient-B supply, BOR1 undergoes ubiquitination and is transported to the vacuole for degradation, to avoid overaccumulation of B. However, the mechanisms underlying B-sensing and ubiquitination of BOR1 are unknown. In this study, we confirmed the lysine-590 residue in the C-terminal cytosolic region of BOR1 as the direct ubiquitination site and showed that BOR1 undergoes K63-linked polyubiquitination. A forward genetic screen identified that amino acid residues located in vicinity of the substrate-binding pocket of BOR1 are essential for the vacuolar sorting. BOR1 variants that lack B-transport activity showed a significant reduction of polyubiquitination and subsequent vacuolar sorting. Coexpression of wild-type (WT) and a transport-defective variant of BOR1 in the same cells showed degradation of the WT but not the variant upon sufficient-B supply. These findings suggest that polyubiquitination of BOR1 relies on its conformational transition during the transport cycle. We propose a model in which BOR1, as a B transceptor, directly senses the B concentration and promotes its own polyubiquitination and vacuolar sorting for quick and precise maintenance of B homeostasis.

The borate transporter BOR1 senses the boron concentration through its borate transport activity for K63-linked polyubiquitination and subsequent degradation.     

Comparative toxicology of borates

Inorganic borates, including boric acid, Na, ammonium, K, and Zn borates generally display low acute toxicity orally, dermally, and by inhalation. They are either not irritant or mild skin and eye irritants. Exceptions owing to physiochemical properties do occur. Longer-term toxicological studies have been reported mainly on boric acid or borax where the properties are generally similar on an equivalent boron (B) basis. The critical effects in several species are male reproductive toxicity and developmental toxicity. The doses that cause these effects are far higher than any levels to which the human population could be exposed. Humans would need to consume daily some 3.3 g of boric acid (or 5.0 g borax) to ingest the same dose level as the lowest animal NOAEL. No effects on fertility were seen in a population of workers exposed to borates or to a population exposed to high environmental borate levels. There is remarkable similarity in the toxicological effects of boric acid and borax across different species. Other inorganic borates that simply dissociate to boric acid are expected to display similar toxicity, whereas those that do not dissociate simply to boric acid may display a different toxicological profile.