Boron-Dependent Translational Suppression of the Borate Exporter BOR1 Contributes to the Avoidance of Boron Toxicity

Rapid Vacuolar Sorting of the Borate Transporter BOR1 Requires the Adaptor Protein Complex AP-4 in Arabidopsis

Plants maintain nutrient homeostasis by controlling the activities and abundance of nutrient transporters. In Arabidopsis thaliana, the borate (B) transporter BOR1 plays a role in the efficient translocation of B under low-B conditions. BOR1 undergoes polyubiquitination in the presence of sufficient B and is then transported to the vacuole via multivesicular bodies (MVBs) to prevent B accumulation in tissues at a toxic level. A previous study indicated that BOR1 physically interacts with µ subunits of adaptor protein complexes AP-3 and AP-4, both involved in vacuolar sorting pathways. In this study, we investigated the roles of AP-3 and AP-4 subunits in BOR1 trafficking in Arabidopsis. The lack of AP-3 subunits did not affect either vacuolar sorting or polar localization of BOR1-GFP, whereas the absence of AP-4 subunits resulted in a delay in high-B-induced vacuolar sorting without affecting polar localization. Super-resolution microscopy revealed a rapid sorting of BOR1-GFP into AP-4-positive spots in the trans-Golgi network (TGN) upon high-B supply. These results indicate that AP-4 is involved in sequestration of ubiquitinated BOR1 into a TGN-specific subdomain 'vacuolar-trafficking zone', and is required for efficient sorting of MVB and vacuole. Our findings have thus helped elucidate the rapid vacuolar sorting process facilitated by AP-4 in plant nutrient transporters.

The Bor1 elevator transport cycle is subject to autoinhibition and activation

Boron, essential for plant growth, necessitates precise regulation due to its potential toxicity. This regulation is achieved by borate transporters (BORs), which are homologous to the SLC4 family. The Arabidopsis thaliana Bor1 (AtBor1) transporter from clade I undergoes slow regulation through degradation and translational suppression, but its potential for fast regulation via direct activity modulation was unclear. Here, we combine cryo-electron microscopy, mutagenesis, and functional characterization to study AtBor1, revealing high-resolution structures of the dimer in one inactive and three active states. Our findings show that AtBor1 is regulated by two distinct mechanisms: an autoinhibitory domain at the carboxyl terminus obstructs the substrate pathway via conserved salt bridges, and phosphorylation of Thr410 allows interaction with a positively charged pocket at the cytosolic face, essential for borate transport. These results elucidate the molecular basis of AtBor1's activity regulation and highlight its role in fast boron level regulation in plants.

Connecting calcium signaling with boron transport: the crucial role of CPK10 protein kinase

This article is a Commentary on Wang et al. (2024), 243: 1795–1809.

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.

Casparian strips prevent apoplastic diffusion of boric acid into root steles for excess B tolerance

Casparian strips are ring-like structures consisting of lignin, sealing the apoplastic space between endodermal cells. They are thought to have important functions in controlling radial transport of nutrients and toxic elements in roots. However, Arabidopsis mutants with a defective Casparian strip structure have been found to maintain nutrient homeostasis in ranges supportive of growth under standard laboratory conditions. In this study, we investigated the function of Casparian strips under excess boron (B) conditions using sgn3 and sgn4 mutants with defective Casparian strip development but which do not exhibit excessive deposition of suberin, another endodermal diffusion barrier. The growth of sgn3 and sgn4 mutants did not differ significantly from that of wild-type (WT) plants under different B conditions in plate cultures; however, they were highly sensitive to B excess in hydroponic culture, where transpiration drives the translocation of boric acid toward the shoot. In hydroponic culture with sufficient to excess boric acid, B accumulation in shoots of the sgn3 and sgn4 mutants was higher than that in the WT. A time-course tracer study using 10B-enriched boric acid at a sufficient or slightly excessive concentration showed higher translocation of B into shoots of the sgn3 and sgn4 mutants. Furthermore, a genetically encoded biosensor for boric acid expressed under a stele-specific promoter (proCIF2:NIP5;1 5'UTR : Eluc-PEST) visualized faster boric acid flux into the mutant steles. Collectively, our results demonstrate the importance of Casparian strips in preventing apoplastic diffusion of boric acid into the stele under excess supply.

Role of ABA in the adaptive response of Arabidopsis plants to long-term boron toxicity treatment

Boron (B) toxicity causes impairments in several plant metabolic and physiological processes. Under conditions of excessive B availability, this micronutrient is passively transported through the transpiration stream and accumulates in leaves, causing the development of necrotic regions in leaf tips. Some plants have developed adaptive mechanisms to minimize the toxic effects of excessive B accumulation in their tissues. Thus, for instance, in Arabidopsis it has been described an ABA-dependent decrease in the transpiration rate that would restrict B accumulation in aerial plant tissues in response to short-term B toxicity, this effect being mediated by AtNCED3 (which encodes a key enzyme for ABA biosynthesis). The present work aimed to study the possible involvement of ABA in the adjustment of plant water balance and B homeostasis during the adaptive response of Arabidopsis to prolonged B toxicity. For this purpose, Arabidopsis wild-type and the ABA-deficient nced3-2 mutant plants were subjected to B toxicity for 7 days. We show that ABA-dependent stomatal closure is determinant for the adjustment of plant water relations under conditions of prolonged B toxicity. Results suggest that, in addition to the AtNCED3 gene, the AtNCED5 gene could also be involved in this ABA-dependent stomatal closure. Finally, our results also indicate the possible role of endogenous root ABA content in the mechanism of active efflux of B via BOR4 (efflux-type B transporter) from the root to the external environment under excess B conditions.

Improving Boron and Molybdenum Use Efficiencies in Contrasting Cultivars of Subirrigated Greenhouse-Grown Pot Chrysanthemums

Fertilizer boron (B) and molybdenum (Mo) were provided to contrasting cultivars of subirrigated pot chrysanthemums at approximately 6-100% of current industry standards in an otherwise balanced nutrient solution during vegetative growth, and then all nutrients were removed during reproductive growth. Two experiments were conducted for each nutrient in a naturally lit greenhouse using a randomized complete block split-plot design. Boron (0.313-5.00 µmol L^-1) or Mo (0.031-0.500 µmol L^-1) was the main plot, and cultivar was the sub-plot. Petal quilling was observed with leaf-B of 11.3-19.4 mg kg^-1 dry mass (DM), whereas Mo deficiency was not observed with leaf-Mo of 1.0-3.7 mg kg^-1 DM. Optimized supplies resulted in leaf tissue levels of 48.8-72.5 mg B kg^-1 DM and 1.9-4.8 mg Mo kg^-1 DM. Boron uptake efficiency was more important than B utilization efficiency in sustaining plant/inflorescence growth with decreasing B supply, whereas Mo uptake and utilization efficiencies appeared to have similar importance in sustaining plant/inflorescence growth with decreasing Mo supply. This research contributes to the development of a sustainable low-input nutrient delivery strategy for floricultural operations, wherein nutrient supply is interrupted during reproductive growth and optimized during vegetative growth.

Involvement of NGATHA-Like 1 Transcription Factor in Boron Transport under Low and High Boron Conditions

NGATHA-Like 1 (NGAL1) transcription factor has been identified as a gene regulated through AUG-stop-mediated boron (B)-dependent translation stall; however, its function in B response remains unknown. Here, we show that NGAL1 plays an important role in the maintenance of B transport under both low- and high-B conditions in Arabidopsis thaliana. NGAL1 mRNA is accumulated predominantly in shoots in response to B stress. Independent ngal1 mutants carrying transferred DNA (T-DNA) and Ds-transposon insertions exhibit reduced B concentrations in aerial tissues and produce shortened and reduced number of siliques when B supply is limited. Furthermore, the expression of B transporter genes including nodulin 26-like intrinsic protein 6; 1 (NIP6;1), NIP5;1, NIP7;1 and borate exporter 1 (BOR1) is significantly decreased in ngal1 mutants under low-B condition, suggesting that NGAL1 is required for the transcript accumulation of B transporter genes to facilitate B transport and distribution under B limitation. Under high-B condition, ngal1 mutants exhibit reduced growth and increased B concentration in their shoots. The accumulation of BOR4 mRNA, a B transporter required for B efflux to soil, is significantly reduced in roots of ngal1 plants under high-B condition, suggesting that NGAL1 is involved in the upregulation of BOR4 in response to excess B. Together, our results indicate that NGAL1 is involved in the transcriptional regulation of B transporter genes to facilitate B transport and distribution under both low- and high-B conditions.

Characterization of two Peruvian maize landraces differing in boron toxicity tolerance

Boron (B) toxicity is a major agricultural problem that causes a considerable decrease in crop yield and quality. The soil in arid and semi-arid areas is often subjected to excessive B content. Southwestern Perú (department of Tacna) is characterized by high B levels in its agricultural land and irrigation water. This work analyzes the response of two local maize (Zea mays) landraces (Pachía and Sama) from Tacna to B toxicity. Both landraces were, therefore, grown in hydroponic media under control and B toxicity conditions, and after 10 days, seedlings were harvested and B content, B-transporter gene expressions, and several morphological and physiological parameters were determined. The leaf and root soluble B content was lower in Sama than in Pachía when both landraces were subjected to high B concentrations, which could be explained by its higher expression levels of B-efflux transporters. The capacity of Sama to maintain reduced levels of soluble B in its leaves and roots led to decreased leaf damage and higher photosynthetic and growth parameters under B toxicity conditions. These results support the proposal that Sama would perform better than Pachía under excessive B conditions, thus making it a more suitable landrace to be used in soils with toxic levels of B.

Biogeochemical cycling, tolerance mechanism and phytoremediation strategies of boron in plants: A critical review

Boron (B) is an inimitable plant micronutrient, predominantly distinguished by super-narrow range between its deficiency and toxicity concentrations, which depends upon boron speciation in the growth media and form of living organisms. Moreover, the significant variations in its mobility amid different species, and distinctive inter and intra-species responses to extreme concentrations. Therefore, it is necessary to screen the bioavailability, speciation, biogeochemical cycling and exposure pathways of boron in the environment. The genes involved in the tolerance mechanism of boron in different plants were discussed in the current review paper. The functions related to scarcity and excess of boron concentration are assessed. The diverse plants implicated in phytoremediation of boron contaminated sites that restrict boron accumulation in food crops and health risks associated with soils containing deficit or surplus boron concentration were appraised. Ultimately, a summary of numerous strategies involved in the recovery and repair of boron-contaminated soils have been reviewed.

Translational Landscape of a C4 Plant, Sorghum bicolor, Under Normal and Sulfur-Deficient Conditions

Recent accumulation of genomic and transcriptomic information has facilitated genetic studies. Increasing evidence has demonstrated that translation is an important regulatory step, and the transcriptome does not necessarily reflect the profile of functional protein production. Deep sequencing of ribosome-protected mRNA fragments (ribosome profiling or Ribo-seq) has enabled genome-wide analysis of translation. Sorghum is a C4 cereal important not only as food but also as forage and a bioenergy resource. Its resistance to harsh environments has made it an agriculturally important research subject. Yet genome-wide translational profiles in sorghum are still missing. In this study, we took advantage of Ribo-seq and identified actively translated reading frames throughout the genome. We detected translation of 4,843 main open reading frames (ORFs) annotated in the sorghum reference genome version 3.1 and revealed a number of unannotated translational events. A comparison of the transcriptome and translatome between sorghums grown under normal and sulfur-deficient conditions revealed that gene expression is modulated independently at transcript and translation levels. Our study revealed the translational landscape of sorghum's response to sulfur and provides datasets that could serve as a fundamental resource to extend genetic research on sorghum, including studies on translational regulation.

Boron uptake in rice is regulated post-translationally via a clathrin-independent pathway

Uptake of boron (B) in rice (Oryza sativa) is mediated by the Low silicon rice 1 (OsLsi1) channel, belonging to the NOD26-like intrinsic protein III subgroup, and the efflux transporter B transporter 1 (OsBOR1). However, it is unknown how these transporters cooperate for B uptake and how they are regulated in response to B fluctuations. Here, we examined the response of these two transporters to environmental B changes at the transcriptional and posttranslational level. OsBOR1 showed polar localization at the proximal side of both the exodermis and endodermis of mature root region, forming an efficient uptake system with OsLsi1 polarly localized at the distal side of the same cell layers. Expression of OsBOR1 and OsLsi1 was unaffected by B deficiency and excess. However, although OsLsi1 protein did not respond to high B at the protein level, OsBOR1 was degraded in response to high B within hours, which was accompanied with a significant decrease of total B uptake. The high B-induced degradation of OsBOR1 was inhibited in the presence of MG-132, a proteasome inhibitor, without disturbance of the polar localization. In contrast, neither the high B-induced degradation of OsBOR1 nor its polarity was affected by induced expression of dominant-negative mutated dynamin-related protein 1A (OsDRP1AK47A) or knockout of the mu subunit (AP2M) of adaptor protein-2 complex, suggesting that clathrin-mediated endocytosis is not involved in OsBOR1 degradation and polar localization. These results indicate that, in contrast to Arabidopsis thaliana, rice has a distinct regulatory mechanism for B uptake through clathrin-independent degradation of OsBOR1 in response to high B.

Metalloid transporters and their regulation in plants

Transport of metalloids including B, Si, and As is mediated by a combination of channels and efflux transporters in plants, which are strictly regulated in response to environmental changes.

Involvement of nitric oxide (NO) in plant responses to metalloids

Plants respond to the limited or excess supply of metalloids, boron (B), silicon (Si), selenium (Se), arsenic (As), and antimony (Sb) via complex signaling pathways that are mainly regulated by nitric oxide (NO). The absorption of metalloids from the soil is facilitated by pathways that involve aquaporins, aquaglyceroporins, phosphate, and sulfate transporters; however, their regulation by NO is poorly understood. Using in silico software, we predicted the S-nitrosation of known metalloid transporters, proposing NO-dependent regulation of metalloid transport systems at the posttranslational level. NO intensifies the stress-mitigating effect of Si, whereas in the case of Se, As, and Sb, the accumulation of NO or reactive nitrogen species contributes to toxicity. NO promotes the beneficial effect of low Se concentrations and mitigates the damage caused by B deficiency. In addition, the exogenous application of NO donor, sodium nitroprusside, reduces B, Se, and As toxicity. The primary role of NO in metalloid stress response is to mitigate oxidative stress by activating antioxidant defense at the level of protein activity and gene expression. This review discusses the role of NO in plant responses to metalloids and suggests future research directions.

Regulation, Diversity and Evolution of Boron Transporters in Plants

Boron (B) is an essential trace element in plants, and borate cross-linking of pectic polysaccharide rhamnogalacturonan-II (RG-II) in cell walls is required for normal cell growth. High concentrations of B are toxic to cells. Therefore, plants need to control B transport to respond to B conditions in the environment. Over the past two decades, genetic analyses of Arabidopsis thaliana have revealed that B transport is governed by two types of membrane transport molecules: NIPs (nodulin-26-like intrinsic proteins), which facilitate boric acid permeation, and BORs, which export borate from cells. In this article, we review recent findings on the (i) regulation at the cell level, (ii) diversity among plant species and (iii) evolution of these B transporters in plants. We first describe the systems regulating these B transporters at the cell level, focusing on the molecular mechanisms underlying the polar localization of proteins and B-dependent expression, as well as their physiological significance in A. thaliana. Then, we examine the presence of homologous genes and characterize the functions of NIPs and BORs in B homeostasis, in a wide range of plant species, including Brassica napus, Oryza sativa and Zea mays. Finally, we discuss the evolutionary aspects of NIPs and BORs as B transporters, and the possible relationship between the diversification of B transport and the occurrence of RG-II in plants. This review considers the sophisticated systems of B transport that are conserved among various plant species, which were established to meet mineral nutrient requirements.

Global analysis of boron-induced ribosome stalling reveals its effects on translation termination and unique regulation by AUG-stops in Arabidopsis shoots

We previously reported that ribosome stalling at AUG-stop sequences in the 5'-untranslated region plays a critical role in regulating the expression of Arabidopsis thaliana NIP5;1, which encodes a boron uptake transporter, in response to boron conditions in media. This ribosome stalling is triggered specifically by boric acid, but the mechanisms are unknown. Although upstream open reading frames (uORFs) are known in many cases to regulate translation through peptides encoded by the uORF, AUG-stop stalling does not involve any peptide synthesis. The unique feature of AUG-stops - that termination follows immediately after initiation - suggests a possible effect of boron on the translational process itself. However, the generality of AUG-stop-mediated translational regulation and the effect of boron on translation at the genome scale are not clear. Here, we conducted a ribosome profiling analysis to reveal the genome-wide regulation of translation in response to boron conditions in A. thaliana shoots. We identified hundreds of translationally regulated genes that function in various biological processes. Under high-boron conditions, transcripts with reduced translation efficiency were rich in uORFs, highlighting the importance of uORF-mediated translational regulation. We found 673 uORFs that had more frequent ribosome association. Moreover, transcripts that were translationally downregulated under high-boron conditions were rich in minimum uORFs (AUG-stops), suggesting that AUG-stops play a global role in the boron response. Metagene analysis revealed that boron increased the ribosome occupancy of stop codons, indicating that this element is involved in global translational termination processes.

Plant transporters involved in combating boron toxicity: beyond 3D structures

Membrane transporters control the movement and distribution of solutes, including the disposal or compartmentation of toxic substances that accumulate in plants under adverse environmental conditions. In this minireview, in the light of the approaching 100th anniversary of unveiling the significance of boron to plants (K. Warington, 1923; Ann. Bot.37, 629) we discuss the current state of the knowledge on boron transport systems that plants utilise to combat boron toxicity. These transport proteins include: (i) nodulin-26-like intrinsic protein-types of aquaporins, and (ii) anionic efflux (borate) solute carriers. We describe the recent progress made on the structure–function relationships of these transport proteins and point out that this progress is integral to quantitative considerations of the transporter's roles in tissue boron homeostasis. Newly acquired knowledge at the molecular level has informed on the transport mechanics and conformational states of boron transport systems that can explain their impact on cell biology and whole plant physiology. We expect that this information will form the basis for engineering transporters with optimised features to alleviate boron toxicity tolerance in plants exposed to suboptimal soil conditions for sustained food production.

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.

JASMONATE RESISTANT 1 negatively regulates root growth under boron deficiency in Arabidopsis

Boron (B) is an essential micronutrient for plant growth and development. Jasmonic acid (JA) plays pivotal roles in plant growth, but the underlying molecular mechanism of JA involvement in B-deficiency-induced root growth inhibition is yet to be explored. In this study, we investigated the response of JA to B deficiency and the mechanism of JAR1-dependent JA signaling in root growth inhibition under B deficiency in Arabidopsis. B deficiency enhanced JA signaling in roots, and root growth inhibition was partially restored by JA biosynthesis inhibition. The jar1-1 (jasmonate-resistant 1, JAR1) mutant, and mutants of coronatine-insensitive 1 (coi1-2) and myc2 defective in JA signaling showed insensitivity to B deficiency. The ethylene-overproduction mutant eto1 and ethylene-insensitive mutant etr1 showed sensitivity and insensitivity to B deficiency, respectively, suggesting that ethylene is involved in the inhibition of primary root growth under B deficiency. Furthermore, after a decline in levels of EIN3, which may contribute to root growth, ethylene signaling was weakened in the jar1-1 mutant root under B deficiency. Under B deficiency, B concentrations were increased in the roots and shoots of the jar1-1 mutant, owing to the large root system and its activity. Therefore, our findings revealed that JA, which is involved in the inhibition of root growth under B deficiency, is regulated by JAR1-activated JA and ethylene signaling pathways.

Fine regulation system for distribution of boron to different tissues in rice

Boron (B) is essential for growth and development, with the B requirement differing depending on the particular organs and tissues, but the molecular mechanisms underlying the preferential distribution of B to different tissues are poorly understood. We investigated the role of a rice gene (OsBOR1) encoding a B efflux transporter in the distribution of B to different tissues under different B supplies. OsBOR1 was highly expressed in the nodes at all growth stages. The OsBOR1 protein shows polar localization at the distal side of bundle sheath cells in nodes and xylem parenchyma cells of elongating leaf sheath, but in the mature leaf sheath and blade at the proximal side of bundle sheath cells. Furthermore, the expression of OsBOR1 was not affected by external B fluctuations, but the OsBOR1 protein was gradually degraded in response to high B. Knockout of this gene altered B distribution, decreasing the distribution of B to new leaves and panicles but increasing B distribution to old leaves. These results indicate that OsBOR1 expressed in nodes and leaf sheath is involved in the preferential distribution of B to different tissues in rice. Furthermore, the OsBOR1 undergoes degradation in response to high B for fine regulation of B distribution to different tissues.

Involvement of boron transporter BOR1 in growth under low boron and high nitrate conditions in Arabidopsis thaliana

BOR1 is an efflux transporter of boron (B), responsible for loading B into the xylem. It has been reported that nitrate (NO₃ ^- ) concentrations significantly influence B concentrations in leaves and BOR1 mRNA accumulation in roots. Here, to unravel the interactive effects of B and NO₃ ^- on plant growth and the function of BOR1 under the combination of B and NO₃ ^- , seedling growth was analyzed in Col-0 and bor1 mutants. The growth of bor1 mutants was negatively affected by high NO₃ ^- but neither by potassium chloride (KCl) nor ammonium (NH₄ ⁺ ) under low B conditions, suggesting the involvement of BOR1 in growth under high NO₃ ^- . Mutants of bor2 and bor4 did not exhibit such growth responses, suggesting that this effect was specific to BOR1 among the BORs tested. Under low B conditions, loss of the BOR1 function led to a more significant decrease in B concentrations in the presence of high NO₃ ^- compared to normal NO₃ ^- . Additionally, grafting experiments demonstrated that these effects of NO₃ ^- occurred when BOR1 is absent in roots. High NO₃ ^- treatment elevated BOR1 mRNA accumulation while the BOR1 protein accumulation was downregulated. These apparent opposite responses indicated that the transcriptional and (post-)translational regulations follow different patterns. Our work provides evidence of a novel regulation of BOR1 and another B transport system by both B and NO₃ ^- in an interactive manner.

Analysis of Endocytosis and Intracellular Trafficking of Boric Acid/Borate Transport Proteins in Arabidopsis

Plants take up inorganic nutrients from the soil by transport proteins located in the plasma membrane of root cells. Boron (B) is an essential element for plant growth; it taken up and translocated by boric acid channels such as NIP5;1 and borate exporters such as BOR1 in Arabidopsis. NIP5;1 and BOR1 are localized to the plasma membrane of various root cells in polar manners toward soil- and stele-side, respectively, for efficient transport of B. In response to elevated B concentration, BOR1 undergoes vacuolar sorting for degradation to avoid accumulation of B to a toxic level in tissues. The polar localization and vacuolar sorting of the transport proteins are regulated through differential mechanisms of endocytosis and intracellular trafficking. In this chapter, we describe methods for quantitative live-cell imaging of GFP-NIP5;1 and BOR1-GFP as markers for the polar and vacuolar trafficking.

Boron-dependent regulation of translation through AUGUAA sequence in yeast

Under high boron (B) conditions, nodulin 26-like intrinsic protein 5;1 (NIP5;1) mRNA, a boric acid channel, is destabilized to avoid excess B entry into roots of Arabidopsis thaliana. In this regulation, the minimum upstream open reading frame (uORF), AUGUAA, in its 5'-untranslated region (5'-UTR) is essential, and high B enhances ribosome stalling at AUGUAA and leads to suppression of translation and mRNA degradation. This B-dependent AUGUAA-mediated regulation occurs also in animal transient expression and reticulocyte lysate translation systems. Thus, uncovering the ubiquitousness of B-dependent unique regulation is important to reveal the evolution of translational regulation. In the present study, we examined the regulation in Saccharomyces cerevisiae. Reporter assay showed that in yeast, carrying ATGTAA in 5'-UTR of NIP5;1 upstream of the reporter gene, the relative reporter activities were reduced significantly under high B conditions compared with control, whereas deletion of ATGTAA abolished such responses. This suggests that AUGUAA mediates B-dependent regulation of translation in Saccharomyces cerevisiae. Moreover, the deletion of ATGTAA resulted in up to 10-fold increase in general reporter activities indicating the suppression effect of AUGUAA on translation of the main ORF. Interestingly, mRNA level of the reporter gene was not affected by B in both yeast cells with and without AUGUAA. This finding reveals that in yeast, unlike the case in plants, mRNA degradation is not associated with AUGUAA regulation. Together, results suggest that B-dependent AUGUAA-mediated translational regulation is common among eukaryotes.

Translational gene regulation in plants: A green new deal

The molecular machinery for protein synthesis is profoundly similar between plants and other eukaryotes. Mechanisms of translational gene regulation are embedded into the broader network of RNA-level processes including RNA quality control and RNA turnover. However, over eons of their separate history, plants acquired new components, dropped others, and generally evolved an alternate way of making the parts list of protein synthesis work. Research over the past 5 years has unveiled how plants utilize translational control to defend themselves against viruses, regulate translation in response to metabolites, and reversibly adjust translation to a wide variety of environmental parameters. Moreover, during seed and pollen development plants make use of RNA granules and other translational controls to underpin developmental transitions between quiescent and metabolically active stages. The economics of resource allocation over the daily light-dark cycle also include controls over cellular protein synthesis. Important new insights into translational control on cytosolic ribosomes continue to emerge from studies of translational control mechanisms in viruses. Finally, sketches of coherent signaling pathways that connect external stimuli with a translational response are emerging, anchored in part around TOR and GCN2 kinase signaling networks. These again reveal some mechanisms that are familiar and others that are different from other eukaryotes, motivating deeper studies on translational control in plants. This article is categorized under: Translation > Translation Regulation RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Modulation of Shoot Phosphate Level and Growth by PHOSPHATE1 Upstream Open Reading Frame

Inorganic orthophosphate (Pi) is an essential nutrient for plant growth, and its availability strongly impacts crop yield. PHOSPHATE1 (PHO1) transfers Pi from root to shoot via Pi export into root xylem vessels. In this work, we demonstrate that an upstream open reading frame (uORF) present in the 5' untranslated region of the Arabidopsis (Arabidopsis thaliana) PHO1 inhibits its translation and influences Pi homeostasis. The presence of the uORF strongly inhibited the translation of a PHO1 5'UTR-luciferase construct in protoplasts. A point mutation removing the PHO1 uORF (ΔuORF) in transgenic Arabidopsis resulted in increased association of its mRNA with polysomes and led to higher PHO1 protein levels, independent of Pi availability. Interestingly, deletion of the uORF led to higher shoot Pi content and was associated with improved shoot growth under low external Pi supply and no deleterious effects under Pi-sufficient conditions. We further show that natural accessions lacking the PHO1 uORF exhibit higher PHO1 protein levels and shoot Pi content. Increased shoot Pi content was linked to the absence of the PHO1 uORF in a population of F2 segregants. We identified the PHO1 uORF in genomes of crops such as rice (Oryza sativa), maize (Zea mays), barley (Hordeum vulgare), and wheat (Triticum aesativum), and we verified the inhibitory effect of the rice PHO1 uORF on translation in protoplasts. Our work suggests that regulation of PHO1 expression via its uORF might be a genetic resource useful-both in natural populations and in the context of genome editing-toward improving plant growth under Pi-deficient conditions.

From element to development: the power of the essential micronutrient boron to shape morphological processes in plants

Deficiency of the essential nutrient boron (B) in the soil is one of the most widespread micronutrient deficiencies worldwide, leading to developmental defects in root and shoot tissues of plants, and severe yield reductions in many crops. Despite this agricultural importance, the underlying mechanisms of how B shapes plant developmental and morphological processes are still not unequivocally understood in detail. This review evaluates experimental approaches that address our current understanding of how B influences plant morphological processes by focusing on developmental defects observed under B deficiency. We assess what is known about mechanisms that control B homeostasis and specifically highlight: (i) limitations in the methodology that is used to induce B deficiency; (ii) differences between mutant phenotypes and normal plants grown under B deficiency; and (iii) recent research on analyzing interactions between B and phytohormones. Our analysis highlights the need for standardized methodology to evaluate the roles of B in the cell wall versus other parts of the cell.

Upstream Open Reading Frame and Phosphate-Regulated Expression of Rice OsNLA1 Controls Phosphate Transport and Reproduction

Rice (Oryza sativa) OsNLA1 has been proposed to play a crucial role in regulating phosphate (Pi) acquisition in roots, similar to that of Arabidopsis (Arabidopsis thaliana) AtNLA. However, unlike AtNLA, OsNLA1 is not a target of miR827, a Pi starvation-induced microRNA. It is, therefore, of interest to know whether the expression of OsNLA1 depends on Pi supply and how it is regulated. In this study, we provide evidence that OsNLA1 controls Pi acquisition by directing the degradation of several OsPHT1 Pi transporters (i.e. OsPT1/2/4/7/8/12). We further show that OsNLA1 has an additional function in reproduction and uncover the mechanism of its expression regulation. Analysis of mRNA levels, promoter-GUS activity, and protoplast transient expression showed that the expression of OsNLA1.1, the most abundant transcript variant, is up-regulated in response to increasing Pi supply. The OsNLA1 promoter region was found to contain an upstream open reading frame that is required for Pi-responsive expression regulation. OsNLA1 promoter activity was observed in roots, ligules, leaves, sheaths, pollen grains, and surrounding the vascular tissues of anthers, suggesting that OsNLA1 is important throughout the development of rice. Disruption of OsNLA1 resulted in increased Pi uptake from roots as well as impaired pollen development and reduced grain production. In summary, our study reveals that Pi-induced OsNLA1 expression regulated by a unique mechanism functions in Pi acquisition, Pi translocation, and reproductive success.

Metabolite Control of Translation by Conserved Peptide uORFs: The Ribosome as a Metabolite Multisensor

Ribosomes translate the mRNA code into protein, and this process can be controlled by metabolites that bind to the translating ribosome in interaction with the nascent protein.

Boron toxicity in higher plants: an update

In this review, emphasis is given to the most recent updates about morpho-anatomical, physiological, biochemical and molecular responses adopted by plants to cope with B excess. Boron (B) is a unique micronutrient for plants given that the range of B concentration from its essentiality to toxicity is extremely narrow, and also because it occurs as an uncharged molecule (boric acid) which can pass lipid bilayers without any degree of controls, as occurs for other ionic nutrients. Boron frequently exceeds the plant's requirement in arid and semiarid environments due to poor drainage, and in agricultural soils close to coastal areas due to the intrusion of B-rich seawater in fresh aquifer or because of dispersion of seawater aerosol. Global releases of elemental B through weathering, volcanic and geothermal processes are also relevant in enriching B concentration in some areas. Considerable progress has been made in understanding how plants react to B toxicity and relevant efforts have been made to investigate: (I) B uptake and in planta partitioning, (II) physiological, biochemical, and molecular changes induced by B excess, with particular emphasis to the effects on the photosynthetic process, the B-triggered oxidative stress and responses of the antioxidant apparatus to B toxicity, and finally (III) mechanisms of B tolerance. Recent findings addressing the effects of B toxicity are reviewed here, intending to clarify the effect of B excess and to propose new perspectives aimed at driving future researches on the topic.

Combined Boron Toxicity and Salinity Stress-An Insight into Its Interaction in Plants

The continuously changing environment has intensified the occurrence of abiotic stress conditions. Individually, boron (B) toxicity and salinity stress are well recognized as severe stress conditions for plants. However, their coexistence in arid and semi-arid agricultural regions has shown ambiguous effects on plant growth and development. Few studies have reported that combined boron toxicity and high salinity stress have more damaging effects on plant growth than individual B and salt stress, while other studies have highlighted less damaging effects of the combined stress. Hence, it is interesting to understand the positive interaction of this combined stress so that it can be effectively employed for the improvement of crops that generally show the negative effects of this combined stress. In this review, we discussed the possible processes that occur in plants in response to this combined stress condition. We highly suggest that the combined B and salinity stress condition should be considered as a novel stress condition by researchers; hence, we recommend the name "BorSal" for this combined boron toxicity and high salinity state in the soil. Membrane-bound activities, mobility of ions, water transport, pH changes, transpiration, photosynthesis, antioxidant activities, and different molecular transporters are involved in the effects of BorSal interaction in plants. The discussed mechanisms indicate that the BorSal stress state should be studied in light of the involved physiological and molecular processes that occur after B and salt interaction in plants.

Polar Localization of the Borate Exporter BOR1 Requires AP2-Dependent Endocytosis

Boron (B) is an essential element in plants but is toxic when it accumulates to high levels. In root cells of Arabidopsis (Arabidopsis thaliana), the borate exporter BOR1 is polarly localized in the plasma membrane toward the stele side for directional transport of B. Upon high-B supply, BOR1 is rapidly internalized and degraded in the vacuole. The polar localization and B-induced vacuolar sorting of BOR1 are mediated by endocytosis from the plasma membrane. To dissect the endocytic pathways mediating the polar localization and vacuolar sorting, we investigated the contribution of the clathrin adaptor protein, ADAPTOR PROTEIN2 (AP2) complex, to BOR1 trafficking. In the mutants lacking µ- or σ-subunits of the AP2 complex, the polar localization and constitutive endocytosis of BOR1 under low-B conditions were dramatically disturbed. A coimmunoprecipitation assay showed association of the AP2 complex with BOR1, while it was independent of YxxΦ sorting motifs, which are in a cytosolic loop of BOR1. A yeast two-hybrid assay supported the interaction of the AP2 complex µ-subunit with the C-terminal tail but not with the YxxΦ motifs in the cytosolic loop of BOR1. Intriguingly, lack of the AP2 subunit did not affect the B-induced rapid internalization/vacuolar sorting of BOR1. Consistent with defects in the polar localization, the AP2 complex mutants showed hypersensitivity to B deficiency. Our results indicate that AP2-dependent endocytosis maintains the polar localization of BOR1 to support plant growth under low-B conditions, whereas the B-induced vacuolar sorting of BOR1 is mediated through an AP2-independent endocytic pathway.

A Missing Link in Radial Ion Transport: Ion Transporters in the Endodermis

In higher plants, roots have important functions, such as the acquisition of water and ions, as well as transportation into the aerial parts of the plant via the xylem vessels. Radial ion transport in the root is strongly regulated in the endodermis, which is characterized by the presence of the Casparian strip (CS) and suberin lamellae. Although tremendous progress has been made with regard to the ion transporters and endodermal cells, little is known about the relationship between the ion transporters in the endodermis and ion homeostasis in aboveground tissues. This review summarizes the current knowledge about the location of the ion transporters (or channels) in the endodermis. We mainly discuss the effects of mutants related to the CS and/or suberin formation on the role of endodermal ion transporters in ion homeostasis. We also wish to emphasize the critical role of endodermal ion transporters in the regulation of radial ion transport in the root.