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.     

Is the release of phytosiderophores in zinc-deficient wheat plants a response to impaired iron utilization?

The effect of zinc nutritional status on the time course of phytosiderophore release, and uptake of iron and translocation of iron to the shoot, was studied in nutrient solution cultures for two cultivars of wheat ( Triticum aestivum . cv. Aroona: T. durum , cv. Duratit) differing in their susceptibility to zinc deficiency. In the zinc-efficient cultivar Aroona, under zinc deficiency translocation of iron from roots to shoot was significantly decreased in 13- and 15-day-old plants, whereas release of phytosiderophores was enhanced when the plants were 16 days old. As zinc deficiency became more severe in older plains, translocation of iron to the shoot was further decreased and release of phytosiderophores was further enhanced. Resupplying zinc in nutrient solution to zinc-deficient plants significantly increased the translocation of iron to the shoot after 48 and 72 h. Concomitantly the release of phytosiderophores was repressed. The other cultivar Durati classified as zinc-inefficient in field observations differed from cv. Aroona by showing a lower rate of phytosiderophore release under Zinc deficiency, and a less impaired translocation of iron to the shoot. Foliar application of iron citrate to zinc-deficient Aroona plants repressed the release of phytosiderophores and increased iron concentrations in shoot and roots. Application of ⁵⁵Fe to the leaves demonstrated that retranslocation of iron from the shoot to the roots was not affected by the zinc nutritional status. It is concluded that enhanced release of phytosiderophores in zinc-deficient wheat plants was induced primarily by impaired trans-location of iron lo the shoot.     

Timing of canopy closure influences carbon translocation and seed production of an understorey herb, Trillium apetalon (Trilliaceae)

BACKGROUND AND AIMS: The light availability on a temperate, deciduous-forest floor varies greatly, reflecting the seasonal leaf dynamics of the canopy trees. The growth and/or reproductive activity of understorey plants should be influenced by the length of the high-irradiance period from snowmelt to canopy closure. The aim of the present study was to clarify how spring-blooming species regulate the translocation of photosynthetic products to current reproduction and storage organs during a growing season in accordance with the changing light conditions. METHODS: Growth pattern, net photosynthetic rate, seed production, and shoot and flower production in the next year of Trillium apetalon were compared between natural and experimentally shaded conditions. Furthermore, translocation of current photosynthetic products within plants was assessed by a labelled carbon-chase experiment. KEY RESULTS: During the high-irradiance period, plants showed high photosynthetic ability, in which current products were initially used for shoot growth, then reserved in the rhizome. Carbon translocation to developing fruit occurred after canopy closure, but this was very small due to low photosynthetic rates under the darker conditions. The shading treatment in the early season advanced the time of carbon translocation to fruit, but reduced seed production in the current year and flower production of the next year. CONCLUSIONS: Carbon translocation to the storage organ had priority over seed production under high-irradiance conditions. A shortened bright period due to early canopy closure effectively restricts carbon assimilation, which greatly reduces subsequent reproductive output owing to low photosynthetic products for fruit development and small carbon storage for future reproduction. As populations of this species are maintained by seedling recruitment, acceleration of canopy closure timing may influence the maintenance and dynamics of populations.     

Silicon Induced Antioxidative Responses and Expression of BOR2 and Two PIP Family Aquaporin Genes in Barley Grown Under Boron Toxicity

In this study, the effects of boron stress and the application of silicon were investigated on the expression levels of barley homologues of three transporter genes, namely BOR2, PIP1, and PIP1;1, which have potential in transferring boron and silicon into or out of tissues. Boron toxicity in shoot tissues was observed as early as 1-day-long exposure by means of several stress indicators including ion leakage, malondialdehyde (MDA) and H₂O₂ levels. Elemental analysis showed that presence of Si under B stress reduces tissue B levels, whereas B presence increased Si levels in tissues. Presence of silicon induced BOR2 gene expression in shoots during early stress. Presence of both elements simultaneously increased BOR2 expression in both shoot and root tissues, which might be attributed to element similarity. Expression levels of both aquaporin genes PIP1 and PIP1;1 increased in shoots under short term B and Si applications, and levels were more responsive to B when compared to Si. Similar to BOR2 expression, silicon increased both aquaporin gene expressions in shoot tissues under short term boron stress. Investigation of the response of BOR2 and aquaporin genes under boron stress and in the presence of silicon revealed their sensitivity to silicon and their potential function in transporting silicon into tissues. Based on the present work, stress mitigating effects of silicon can be attributed to the competitive role of silicon for the transport via boron transporters under toxic boron levels.

     

Expression of the Arabidopsis borate efflux transporter gene, AtBOR4, in rice affects the xylem loading of boron and tolerance to excess boron

Boron is an essential nutrient for plants, but it is toxic in excess. Transgenic rice plants expressing an Arabidopsis thaliana borate efflux transporter gene, AtBOR4, at a low level exhibited increased tolerance to excess boron. Those lines with high levels of expression exhibited reduced growth. These findings suggest a potential of the borate transporter BOR4 for the generation of high-boron tolerant rice.     

Adaptation of potassium translocation into the shoot of maize (Zea mays) to shoot demand: Evidence for xylem loading as a regulating step

In order to manipulate the shoot demand for mineral nutrients per unit root weight, maize ( Zea mays L.) seedlings were grown in nutrient solution with different temperatures in the root zone and at the shoot base. The aerial temperature was kept uniform at 24/20°C day/night. At a root zone temperature (RZT) of 24°C, shoot growth was reduced by decreasing the shoot base temperature (SBT) to 12°C; at a RZT of 12°C, shoot growth was increased by raising the SBT to 24°C. At both RZT root growth was not affected by the SBT. Thus, the shoot demand for nutrients per unit root was either increased by raising, or decreased by lowering the SBT. The net uptake rate of potassium (K), as determined from accumulation rates between sequential harvests, was not affected within the first 3 days after lowering the SBT, whereas net translocation rates of K into the shoot and translocation rates in the xylem exudate of decapitated plants were markedly reduced. Obviously, translocation of K into the shoot seems to be regulated independently from K uptake into the root cells. Translocation rates of K in the xylem exudate of decapitated plants were markedly reduced when the nutrient solution was replaced by CaCl₂ solution during exudation. But, depending on the SBT before decapitation, significant differences remained in the translocation rates of K even when K uptake from the nutrient solution was prevented.
From the results it is suggested that xylem loading of K is regulated separately from K uptake from the external solution and that the adaptation of K translocation to shoot demand is coupled with an altered capacity of the root for xylem loading.     

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.     

A zinc finger-containing glycine-rich RNA-binding protein, atRZ-1a, has a negative impact on seed germination and seedling growth of Arabidopsis thaliana under salt or drought stress conditions

Despite the fact that glycine-rich RNA-binding proteins (GRPs) have been implicated in the responses of plants to changing environmental conditions, the reports demonstrating their biological roles are severely limited. Here, we examined the functional roles of a zinc finger-containing GRP, designated atRZ-1a, in Arabidopsis thaliana under drought or salt stress conditions. Transgenic Arabidopsis plants overexpressing atRZ-1a displayed retarded germination and seedling growth compared with the wild-type plants under salt or dehydration stress conditions. In contrast, the loss-of-function mutants of atRZ-1a germinated earlier and grew faster than the wild-type plants under the same stress conditions. Germination of the transgenic plants and mutant lines was influenced by the addition of ABA or glucose, implying that atRZ-1a affects germination in an ABA-dependent way. H(2)O(2) was accumulated at higher levels in the transgenic plants compared with the wild-type plants under stress conditions. The expression of several germination-responsive genes was modulated by atRZ-1a, and proteome analysis revealed that the expression of different classes of genes, including those involved in reactive oxygen species homeostasis and functions, was affected by atRZ-1a under dehydration or salt stress conditions. Taken together, these results suggest that atRZ-1a has a negative impact on seed germination and seedling growth of Arabidopsis under salt or dehydration stress conditions, and imply that atRZ-1a exerts its function by modulating the expression of several genes under stress conditions.     

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.     

Over-Expression of an Arabidopsis Zinc Transporter in Hordeum Vulgare Increases Short-Term Zinc Uptake after Zinc Deprivation and Seed Zinc Content

Increasing the zinc content of cereal grains will be important for improving human nutrition. Improved plant zinc efficiency will lead to increased yields when available zinc is limiting plant growth. The aim of our work was to test how the over-expression of zinc transporters in cereals affects plant growth, seed mineral content, and zinc transport rates. Known zinc transporters from Arabidopsis were over-expressed in Hordeum vulgare cv. Golden Promise by means of a ubiquitin promoter. Multiple transgenic lines were obtained, and the locus number and expression levels were verified. Transgenic lines were tested in long-term growth and short-term uptake experiments. Seeds from transgenic lines grown in soil had higher zinc and iron contents than controls. Short-term uptake rates were higher in the transgenic lines after zinc deprivation. Resupply of zinc after a period of deprivation resulted in the rapid decrease in zinc uptake even in transgenic lines in which a zinc transporter gene was constitutively expressed. Similar to processes in yeast and Arabidopsis, we hypothesize that this rapid decrease in zinc transport activity may be caused by the degradation of transporters in response to zinc-sufficient conditions. In the long-term growth experiments, there were no significant differences between transgenic and control lines in leaf zinc content or shoot biomass under zinc-sufficient or -deficient conditions. However, root-to-shoot ratios were higher in the transgenic plants grown under low-zinc conditions; this could impact zinc acquisition under field conditions. Increased seed zinc and iron content by over-expression of a zinc transporter provides a new strategy for increasing the micronutrient content of cereals.     

Overexpression of ZmIRT1 and ZmZIP3 Enhances Iron and Zinc Accumulation in Transgenic Arabidopsis

Iron and zinc are important micronutrients for both the growth and nutrient availability of crop plants, and their absorption is tightly controlled by a metal uptake system. Zinc-regulated transporters, iron-regulated transporter-like proteins (ZIP), is considered an essential metal transporter for the acquisition of Fe and Zn in graminaceous plants. Several ZIPs have been identified in maize, although their physiological function remains unclear. In this report, ZmIRT1 was shown to be specifically expressed in silk and embryo, whereas ZmZIP3 was a leaf-specific gene. Both ZmIRT1 and ZmZIP3 were shown to be localized to the plasma membrane and endoplasmic reticulum. In addition, transgenic Arabidopsis plants overexpressing ZmIRT1 or ZmZIP3 were generated, and the metal contents in various tissues of transgenic and wild-type plants were examined based on ICP-OES and Zinpyr-1 staining. The Fe and Zn concentration increased in roots and seeds of ZmIRT1-overexpressing plants, while the Fe content in shoots decreased. Overexpressing ZmZIP3 enhanced Zn accumulation in the roots of transgenic plants, while that in shoots was repressed. In addition, the transgenic plants showed altered tolerance to various Fe and Zn conditions compared with wild-type plants. Furthermore, the genes associated with metal uptake were stimulated in ZmIRT1 transgenic plants, while those involved in intra- and inter- cellular translocation were suppressed. In conclusion, ZmIRT1 and ZmZIP3 are functional metal transporters with different ion selectivities. Ectopic overexpression of ZmIRT1 may stimulate endogenous Fe uptake mechanisms, which may facilitate metal uptake and homeostasis. Our results increase our understanding of the functions of ZIP family transporters in maize.     

Tomato plants ectopically expressing Arabidopsis CBF1 show enhanced resistance to water deficit stress

A DNA cassette containing an Arabidopsis C repeat/dehydration-responsive element binding factor 1 (CBF1) cDNA and a nos terminator, driven by a cauliflower mosaic virus 35S promoter, was transformed into the tomato (Lycopersicon esculentum) genome. These transgenic tomato plants were more resistant to water deficit stress than the wild-type plants. The transgenic plants exhibited growth retardation by showing dwarf phenotype, and the fruit and seed numbers and fresh weight of the transgenic tomato plants were apparently less than those of the wild-type plants. Exogenous gibberellic acid treatment reversed the growth retardation and enhanced growth of transgenic tomato plants, but did not affect the level of water deficit resistance. The stomata of the transgenic CBF1 tomato plants closed more rapidly than the wild type after water deficit treatment with or without gibberellic acid pretreatment. The transgenic tomato plants contained higher levels of Pro than those of the wild-type plants under normal or water deficit conditions. Subtractive hybridization was used to isolate the responsive genes to heterologous CBF1 in transgenic tomato plants and the CAT1 (CATALASE1) was characterized. Catalase activity increased, and hydrogen peroxide concentration decreased in transgenic tomato plants compared with the wild-type plants with or without water deficit stress. These results indicated that the heterologous Arabidopsis CBF1 can confer water deficit resistance in transgenic tomato plants.     

Heterologous expression of the yeast arsenite efflux system ACR3 improves Arabidopsis thaliana tolerance to arsenic stress

? Arsenic contamination has a negative impact on crop cultivation and on human health. As yet, no proteins have been identified in plants that mediate the extrusion of arsenic. Here, we heterologously expressed the yeast (Saccharomyces cerevisiae) arsenite efflux transporter ACR3 into Arabidopsis to evaluate how this affects plant tolerance and tissue arsenic contents. ? ACR3 was cloned from yeast and transformed into wild-type and nip7;1 Arabidopsis. Arsenic tolerance was determined at the cellular level using vitality stains in protoplasts, in intact seedlings grown on agar plates and in mature plants grown hydroponically. Arsenic efflux was measured from protoplasts and from intact plants, and arsenic levels were measured in roots and shoots of plants exposed to arsenate. ? At the cellular level, all transgenic lines showed increased tolerance to arsenite and arsenate and a greater capacity for arsenate efflux. With intact plants, three of four stably transformed lines showed improved growth, whereas only transgenic lines in the wild-type background showed increased efflux of arsenite into the external medium. The presence of ACR3 hardly affected tissue arsenic levels, but increased arsenic translocation to the shoot. ? Heterologous expression of yeast ACR3 endows plants with greater arsenic resistance, but does not lower significantly arsenic tissue levels.