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

CPK10 protein kinase regulates Arabidopsis tolerance to boron deficiency through phosphorylation and activation of BOR1 transporter

Boron (B) is crucial for plant growth and development. B deficiency can impair numerous physiological and metabolic processes, particularly in root development and pollen germination, seriously impeding crop growth and yield. However, the molecular mechanism underlying boron signal perception and signal transduction is rather limited. In this study, we discovered that CPK10, a calcium-dependent protein kinase in the CPK family, has the strongest interaction with the boron transporter BOR1. Mutations in CPK10 led to growth and root development defects under B-deficiency conditions, while constitutively active CPK10 enhanced plant tolerance to B deficiency. Furthermore, we found that CPK10 interacted with and phosphorylated BOR1 at the Ser689 residue. Through various biochemical analyses and complementation of B transport in yeast and plants, we revealed that Ser689 of BOR1 is important for its transport activity. In summary, these findings highlight the significance of the CPK10-BOR1 signaling pathway in maintaining B homeostasis in plants and provide targets for the genetic improvement of crop tolerance to B-deficiency stress.

Exploring silique number in Brassica napus L.: Genetic and molecular advances for improving yield

Silique number is a crucial yield-related trait for the genetic enhancement of rapeseed (Brassica napus L.). The intricate molecular process governing the regulation of silique number involves various factors. Despite advancements in understanding the mechanisms regulating silique number in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), the molecular processes involved in controlling silique number in rapeseed remain largely unexplored. In this review, we identify candidate genes and review the roles of genes and environmental factors in regulating rapeseed silique number. We use genetic regulatory networks for silique number in Arabidopsis and grain number in rice to uncover possible regulatory pathways and molecular mechanisms involved in regulating genes associated with rapeseed silique number. A better understanding of the genetic network regulating silique number in rapeseed will provide a theoretical basis for the genetic improvement of this trait and genetic resources for the molecular breeding of high-yielding rapeseed.

Golgi-localized APYRASE 1 is critical for Arabidopsis growth by affecting cell wall integrity under boron deficiency

Many nucleoside triphosphate-diphosphohydrolases (NTPDases/APYRASEs, APYs) play a key role in modulating extracellular nucleotide levels. However, the Golgi-localized APYs, which help control glycosylation, have rarely been studied. Here, we identified AtAPY1, a gene encoding an NTPDase in the Golgi apparatus, which is required for cell wall integrity and plant growth under boron (B) limited availability. Loss of function in AtAPY1 hindered cell elongation and division in root tips while increasing the number of cortical cell layers, leading to swelling of the root tip and abundant root hairs under low B stress. Further, expression pattern analysis revealed that B deficiency significantly induced AtAPY1, especially in the root meristem and stele. Fluorescent-labeled AtAPY1-GFP localized to the Golgi stack. Biochemical analysis showed that AtAPY1 exhibited a preference of UDP and GDP hydrolysis activities. Consequently, the loss of function in AtAPY1 might disturb the homoeostasis of NMP-driven NDP-sugar transport, which was closely related to the synthesis of cell wall polysaccharides. Further, cell wall-composition analysis showed that pectin content increased and borate-dimerized RG-II decreased in apy1 mutants, along with a decrease in cellulose content. Eventually, altered polysaccharide characteristics presumably cause growth defects in apy1 mutants under B deficiency. Altogether, these data strongly support a novel role for AtAPY1 in mediating responses to low B availability by regulating cell wall integrity.

Functional genomics of Brassica napus: Progresses, challenges, and perspectives

Brassica napus, commonly known as rapeseed or canola, is a major oil crop contributing over 13% to the stable supply of edible vegetable oil worldwide. Identification and understanding the gene functions in the B. napus genome is crucial for genomic breeding. A group of genes controlling agronomic traits have been successfully cloned through functional genomics studies in B. napus. In this review, we present an overview of the progress made in the functional genomics of B. napus, including the availability of germplasm resources, omics databases and cloned functional genes. Based on the current progress, we also highlight the main challenges and perspectives in this field. The advances in the functional genomics of B. napus contribute to a better understanding of the genetic basis underlying the complex agronomic traits in B. napus and will expedite the breeding of high quality, high resistance and high yield in B. napus varieties.

BnaA4.BOR2 contributes the tolerance of rapeseed to boron deficiency by improving the transport of boron from root to shoot

Boron (B) is essential for plant growth. However, the molecular mechanism of B transport in rapeseed (Brassica napus L.) is unknown well. Here, we report that B transporter BnaA4.BOR2 is involved in the transport of B from root to shoot and its distribution in shoot cell wall and flower in rapeseed. The results of GUS staining and in-situ PCR analysis showed that BnaA4.BOR2 is mainly expressed in cortex and endodermis of root tip meristem zone and endodermis of mature zone. BnaA4.BOR2 was mainly localized in plasma membrane and showed B transport activity in yeast. Overexpression of Bna4.BOR2 could rescue the phenotype of Arabidopsis mutant bor2-2 under low-B condition. Furthermore, knockout of BnaA4.BOR2 could significantly enhance the sensitivity of rapeseed mutants to B deficiency, including inhibition of root elongation and biomass decrease of roots and shoots. The B concentration in xylem sap of BnaA4.BOR2 mutants was significantly decreased under B deficiency, which resulted in significantly lower B concentrations in shoot cell wall at seedling stage and flower organ at reproductive stage compared to that of wild-type QY10. The growth of BnaA4.BOR2 mutants were severely inhibited, exhibiting a typical B-deficient phenotype of "flowering without seed setting", leading to a sharp decrease in seed yield in B deficient soil. Taken together, these results indicate that BnaA4.BOR2 is critical for rapeseed growth and seed yield production under low B level, which is mainly expressed in cortex and endodermis, and contributed to the transport of B from roots to shoots and its distribution in shoot.

The Brassica napus boron deficient inflorescence transcriptome resembles a wounding and infection response

Oilseed rape and other crops of Brassica napus have a high demand for boron (B). Boron deficiencies result in the inhibition of root growth, and eventually premature flower abortion. Understanding the genetic mechanisms underlying flower abortion in B-limiting conditions could provide the basis to enhance B-efficiency and prevent B-deficiency-related yield losses. In this study, we assessed transcriptomic responses to B-deficiency in diverse inflorescence tissues at multiple time points of soil-grown plants that were phenotypically unaffected by B-deficiency until early flowering. Whilst transcript levels of known B transporters were higher in B-deficient samples, these remained remarkably stable as the duration of B-deficiency increased. Meanwhile, GO-term enrichment analysis indicated a growing response resembling that of a pathogen or pest attack, escalating to a huge transcriptome response in shoot heads at mid-flowering. Grouping differentially expressed genes within this tissue into MapMan functional bins indicated enrichment of genes related to wounding, jasmonic acid and WRKY transcription factors. Individual candidate genes for controlling the "flowering-without-seed-setting" phenotype from within MapMan biotic stress bins include those of the metacaspase family, which have been implicated in orchestrating programmed cell death. Overall temporal expression patterns observed here imply a dynamic response to B-deficiency, first increasing expression of B transporters before recruiting various biotic stress-related pathways to coordinate targeted cell death, likely in response to as yet unidentified B-deficiency induced damage-associated molecular patterns (DAMPs). This response indicates new pathways to target and dissect to control B-deficiency-induced flower abortion and to develop more B-efficient crops.

A critical review of plant adaptation to environmental boron stress: Uptake, utilization, and interplay with other abiotic and biotic factors

Boron (B) is an indispensable mineral nutrient for plants and is primarily taken up by roots mainly in the form of boric acid (H3BO3). Recently, research shows that B has a significant impact on plant growth and productivity due to its narrow range between deficiency and toxicity. Fertilization and other procedures to address B stress (deficiency and toxicity) in soils are generally expensive and time-consuming. Over the past 20 years, substantial studies have been conducted to investigate the mechanisms underlying B acquisition and the molecular regulation of B stress in plants. In this review, we discuss the effects of B stress on plant growth, physiology, and biochemistry, and finding on enhancing plant tolerance from the perspective of plant B uptake, transport, and utilization. We also refer to recent results demonstrating the interactions among B and other biological and abiotic factors, including nitrogen, phosphorus, aluminum, and microorganisms. Finally, emerging trends in this field are discussed.

Multiomics reveals an essential role of long-distance translocation in regulating plant cadmium resistance and grain accumulation in allohexaploid wheat (Triticum aestivum)

Cadmium (Cd) is a highly toxic heavy metal that readily enters cereals, such as wheat, via the roots and is translocated to the shoots and grains, thereby posing high risks to human health. However, the vast and complex genome of allohexaploid wheat makes it challenging to understand Cd resistance and accumulation. In this study, a Cd-resistant cultivar of wheat, 'ZM1860', and a Cd-sensitive cultivar, 'ZM32', selected from a panel of 442 accessions, exhibited significantly different plant resistance and grain accumulation. We performed an integrated comparative analysis of the morpho-physiological traits, ionomic and phytohormone profiles, genomic variations, transcriptomic landscapes, and gene functionality in order to identify the mechanisms underlying these differences. Under Cd toxicity, 'ZM1860' outperformed 'ZM32', which showed more severe leaf chlorosis, poorer root architecture, higher accumulation of reactive oxygen species, and disordered phytohormone homeostasis. Ionomics showed that 'ZM32' had a higher root-to-shoot translocation coefficient of Cd and accumulated more Cd in the grains than 'ZM1860'. Whole-genome re-sequencing (WGS) and transcriptome sequencing identified numerous DNA variants and differentially expressed genes involved in abiotic stress responses and ion transport between the two genotypes. Combined ionomics, transcriptomics, and functional gene analysis identified the plasma membrane-localized heavy metal ATPase TaHMA2b-7A as a crucial Cd exporter regulating long-distance Cd translocation in wheat. WGS- and PCR-based analysis of sequence polymorphisms revealed a 25-bp InDel site in the promoter region of TaHMA2b-7A, and this was probably responsible for the differential expression. Our multiomics approach thus enabled the identification of a core transporter involved in long-distance Cd translocation in wheat, and it may provide an elite genetic resource for improving plant Cd resistance and reducing grain Cd accumulation in wheat and other cereal crops.

Molecular characterization of the genome-wide BOR transporter family and their responses to boron conditions in common wheat (Triticum aestivum L.)

Boron (B) deficiency is an agricultural problem that causes significant yield losses in many countries. B transporters (BORs) are responsible for B uptake and distribution and play important roles in yield formation. A comprehensive analysis of the BOR family members in common wheat is still lacking. In the present study, to clarify the molecular characterization and response to B status, genome-wide TaBOR genes and expression patterns were investigated. Fourteen TaBOR genes were identified in common wheat by a homology search. The corresponding phylogenetic tree indicated that 14 TaBOR genes were separately classified into subfamilies of TaBOR1, TaBOR3, and TaBOR4. All TaBOR genes had 12-14 extrons and 11-13 introns. Most TaBOR proteins contained 10 conserved motifs, and motifs 1, 2, 3, 4, and 6 constituted the conserved bicarbonate (HCO₃ ^-) domain. Fourteen TaBOR genes were mapped on 13 chromosomes mainly distributed in the first, third, fifth, and seventh homologous groups. The promoters of TaBOR genes consisted of phytohormones, light responses, and stress-related cis-elements. GO analysis indicated that TaBOR genes were enriched in terms of transmembrane transport and ion homeostasis. TaBOR genes showed diverse expression profiles in different tissues. The members of the TaBOR1 subfamily showed high expression in grains, leaves, roots, stems, and spikes, but members of the TaBOR4 subfamily were highly expressed only in spikes and grains. RT-qPCR indicated that TaBOR1-5A, TaBOR1-5B, and TaBOR1-5D were induced by low B concentrations and had much higher expression in roots than in shoots. TaBOR3-3A, TaBOR3-3B, TaBOR3-3D, TaBOR4-1A, TaBOR4-1B, TaBOR4-1D, and TaBOR3-4B were induced by low and high B concentrations and had high expression in roots and shoots. TaBOR3-4D and TaBOR3-7B were upregulated by low and high B concentrations, respectively, but had expression only in roots. Our results provide basic information on the TaBOR family, which is beneficial for elucidating the functions of TaBOR genes to overcome the problem of B deficiency.

Mechanistic insight on boron-mediated toxicity in plant vis-a-vis its mitigation strategies: a review

Boron (B) is an essential micronutrient, crucial for the growth and development of crop plants. However, the essential to a toxic range of B in the plant is exceptionally narrow, and symptoms develop with a slight change in its concentration in soil. The morphological and anatomical response, such as leaf chlorosis, stunted growth, and impairment in the xylem and phloem development occurs under B-toxicity. The transport of B in the plant occurs via transpiration stream with the involvement of B-channels and transporter in the roots. The higher accumulation of B in source and sink tissue tends to have lower photosynthetic, chlorophyll content, infertility, failure of pollen tube formation and germination, impairment of cell wall formation, and disruption of membrane systems. Excess B in the plant hinders the uptake of other micronutrients, hormone transport, and metabolite partitioning. B-mediated reactive oxygen species production leads to the synthesis of antioxidant enzymes which help to scavenge these molecules and prevent the plant from further oxidative damage. This review highlights morpho-anatomical, physiological, biochemical, and molecular responses of the plant under B toxicity and thereby might help the researchers to understand the related mechanism and design strategies to develop B tolerant cultivars.

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.

The Xyloglucan Endotransglucosylase/Hydrolase Gene XTH22/TCH4 Regulates Plant Growth by Disrupting the Cell Wall Homeostasis in Arabidopsis under Boron Deficiency

TCH4 is a xyloglucan endotransglucosylase/hydrolase (XTH) family member. Extensive studies have shown that XTHs are very important in cell wall homeostasis for plant growth and development. Boron (B), as an essential micronutrient for plants, plays an essential role in the cross-linking of cell wall pectin. However, the effect of B on cell wall organization is unclear. This study aimed to explore the mechanism of plant adaption to B stress by investigating the role of TCH4 in cell wall homeostasis. We conducted both plate and hydroponic cultures of wild-type Col-0 and overexpression and gene knockout lines of XTH22/TCH4 to analyze the phenotype, components, and characteristics of the cell wall using immunofluorescence, atomic force microscopy (AFM), and transmission electron microscopy (TEM). B deficiency induces the expression of TCH4. The overexpression lines of TCH4 presented more sensitivity to B deficiency than the wild-type Col-0, while the knockout lines of TCH4 were more resistant to low B stress. Up-regulation of TCH4 influenced the ratio of chelator-soluble pectin to alkali-soluble pectin and decreased the degree of methylesterification of pectin under B-deficient conditions. Moreover, we found that B deficiency disturbed the arrangement of cellulose, enlarged the gap between cellulose microfibrils, and decreased the mechanical strength of the cell wall, leading to the formation of a thickened and deformed triangular region of the cell wall. These symptoms were more profound in the TCH4 overexpression lines. Consistently, compared with Col-0, the O₂^- and MDA contents in the TCH4 overexpression lines increased under B-deficient conditions. This study identified the B-deficiency-induced TCH4 gene, which regulates cell wall homeostasis to influence plant growth under B-deficient conditions.

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.

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.

Specific and multiple-target gene silencing reveals function diversity of BnaA2.NIP5;1 and BnaA3.NIP5;1 in Brassica napus

Rapeseed (Brassica napus) is an economically important oilseed crop in the world, but its production is strongly dependent on boron (B) supplies. Major intrinsic protein NIP5;1 is essential for B uptake and plant development under B limitation. In this study, phylogenetic and expression analyses identified two NIP5;1 orthologue genes, BnaA2.NIP5;1 and BnaA3.NIP5;1, which are mainly expressed in roots of B. napus. Specific and multiple-target RNAi was used to suppress BnaA3.NIP5;1 or both BnaA2.NIP5;1 and BnaA3.NIP5;1 expression in B-efficient rapeseed Qingyou 10 (QY10), respectively, for revealing the roles of BnaA2.NIP5;1 and BnaA3.NIP5;1 in low-B tolerance in B. napus. We found that both BnaA2.NIP5;1 and BnaA3.NIP5;1 are important for B. napus normal growth under low-B conditions, while these two genes have distinct roles. BnaA2.NIP5;1 is mainly expressed in the epidermis cells, which is required for efficient B uptake into roots, hence for B translocation to the shoots. BnaA3.NIP5;1 is specifically localized in the distal part of lateral root cap cells to promoter root elongation under low-B conditions, which is important for seed production in the maturity stage of B. napus. Taken together, our specific and multiple-target RNAi strategy provides novel insights into the gene function diversification between BnaA2.NIP5;1 and BnaA3.NIP5;1, such an approach can be potentially applicable to other polyploid crops.

Exploring the gene pool of Brassica napus by genomics-based approaches

De novo allopolyploidization in Brassica provides a very successful model for reconstructing polyploid genomes using progenitor species and relatives to broaden crop gene pools and understand genome evolution after polyploidy, interspecific hybridization and exotic introgression. B. napus (AACC), the major cultivated rapeseed species and the third largest oilseed crop in the world, is a young Brassica species with a limited genetic base resulting from its short history of domestication, cultivation, and intensive selection during breeding for target economic traits. However, the gene pool of B. napus has been significantly enriched in recent decades that has been benefit from worldwide effects by the successful introduction of abundant subgenomic variation and novel genomic variation via intraspecific, interspecific and intergeneric crosses. An important question in this respect is how to utilize such variation to breed crops adapted to the changing global climate. Here, we review the genetic diversity, genome structure, and population-level differentiation of the B. napus gene pool in relation to known exotic introgressions from various species of the Brassicaceae, especially those elucidated by recent genome-sequencing projects. We also summarize progress in gene cloning, trait-marker associations, gene editing, molecular marker-assisted selection and genome-wide prediction, and describe the challenges and opportunities of these techniques as molecular platforms to exploit novel genomic variation and their value in the rapeseed gene pool. Future progress will accelerate the creation and manipulation of genetic diversity with genomic-based improvement, as well as provide novel insights into the neo-domestication of polyploid crops with novel genetic diversity from reconstructed genomes.

Physiological responses, tolerance, and remediation strategies in plants exposed to metalloids

Metalloids are a subset of particular concern to risk assessors and toxicologists because of their well-documented potential hazards to plant system. Most of the metalloids are major environmental contaminants which affect crop productivity when present in high concentrations in soil. Metalloids are coupled with carrier proteins of the plasma membrane and translocated to various organs causing changes in key metabolic processes, damages cell biomolecules, and finally inhibit its growth. Phytoremediation-based approaches help in understanding the molecular and biochemical mechanisms for prerequisite recombinant genetic approaches. Recent advancements in proteomics and plant genomics help in understanding the role of transcription factors, metabolites, and genes in plant system which confers metal tolerance. The present review summarizes our current status of knowledge in this direction related to various physiological responses, detoxification mechanisms, and remediation strategies of metalloids in crop plants in relation to plant-metalloid tolerance. Further, the role of various transcription factors and miRNAs in conferring metal tolerance is also briefed. Hence, the present review mainly focused on the alterations in the physiological activities of plants due to metalloid toxicity and the various mechanisms which get activated inside the plants to mitigate their toxic effects.

Genetic variation of BnaA3.NIP5;1 expressing in the lateral root cap contributes to boron deficiency tolerance in Brassica napus

Boron (B) is essential for vascular plants. Rapeseed (Brassica napus) is the second leading crop source for vegetable oil worldwide, but its production is critically dependent on B supplies. BnaA3.NIP5;1 was identified as a B-efficient candidate gene in B. napus in our previous QTL fine mapping. However, the molecular mechanism through which this gene improves low-B tolerance remains elusive. Here, we report genetic variation in BnaA3.NIP5;1 gene, which encodes a boric acid channel, is a key determinant of low-B tolerance in B. napus. Transgenic lines with increased BnaA3.NIP5;1 expression exhibited improved low-B tolerance in both the seedling and maturity stages. BnaA3.NIP5;1 is preferentially polar-localized in the distal plasma membrane of lateral root cap (LRC) cells and transports B into the root tips to promote root growth under B-deficiency conditions. Further analysis revealed that a CTTTC tandem repeat in the 5'UTR of BnaA3.NIP5;1 altered the expression level of the gene, which is tightly associated with plant growth and seed yield. Field tests with natural populations and near-isogenic lines (NILs) confirmed that the varieties carried BnaA3.NIP5;1Q allele significantly improved seed yield. Taken together, our results provide novel insights into the low-B tolerance of B. napus, and the elite allele of BnaA3.NIP5;1 could serve as a direct target for breeding low-B-tolerant cultivars.

Metalloid hazards: From plant molecular evolution to mitigation strategies

Metalloids such as boron and silicon are key elements for plant growth and crop productivity. However, toxic metalloids such as arsenic are increasing in the environment due to inputs from natural sources and human activities. These hazardous metalloids can cause serious health risks to humans and animals if they enter the food chain. Plants have developed highly regulated mechanisms to alleviate the toxicity of metalloids during their 500 million years of evolution. A better understanding the molecular mechanisms underlying the transport and detoxification of toxic metalloids in plants will shed light on developing mitigation strategies. Key transporters and regulatory proteins responsive to toxic metalloids have been identified through evolutionary and molecular analyses. Moreover, knowledge of the regulatory proteins and their pathways can be used in the breeding of crops with lower accumulation of metalloids. These findings can also assist phytoremediation by the exploration of plants such as fern species that hyperaccumulate metalloids from soils and water, and can be used to engineer plants with elevated uptake and storage capacity of toxic metalloids. In summary, there are solutions to remediate contamination due to toxic metalloids by combining the research advances and industrial technologies with agricultural and environmental practices.

Genome-Wide Analysis, Evolutionary History and Response of ALMT Family to Phosphate Starvation in Brassica napus

Low phosphorus (P) availability is one of the major constraints to plant growth, particularly in acidic soils. A possible mechanism for enhancing the use of sparsely soluble P forms is the secretion of malate in plants by the aluminum-activated malate transporter (ALMT) gene family. Despite its significance in plant biology, the identification of the ALMT gene family in oilseed rape (Brassica napus; B. napus), an allotetraploid crop, is unveiled. Herein, we performed genome-wide identification and characterization of ALMTs in B. napus, determined their gene expression in different tissues and monitored transcriptional regulation of BnaALMTs in the roots and leaves at both a sufficient and a deficient P supply. Thirty-nine BnaALMT genes were identified and were clustered into five branches in the phylogenetic tree based on protein sequences. Collinearity analysis revealed that most of the BnaALMT genes shared syntenic relationships among BnaALMT members in B. napus, which suggested that whole-genome duplication (polyploidy) played a major driving force for BnaALMTs evolution in addition to segmental duplication. RNA-seq analyses showed that most BnaALMT genes were preferentially expressed in root and leaf tissues. Among them, the expression of BnaC08g13520D, BnaC08g15170D, BnaC08g15180D, BnaC08g13490D, BnaC08g13500D, BnaA08g26960D, BnaC05g14120D, BnaA06g12560D, BnaC05g20630D, BnaA07g02630D, BnaA04g15700D were significantly up-regulated in B. napus roots and leaf at a P deficient supply. The current study analyzes the evolution and the expression of the ALMT family in B. napus, which will help in further research on their role in the enhancement of soil P availability by secretion of organic acids.

One-Time Foliar Application and Continuous Resupply via Roots Equally Improved the Growth and Physiological Response of B-Deficient Oilseed Rape

Oilseed rape (Brassica napus L.) is a high-boron (B)-demanding crop, and initially, normal growing plants might show B deficiency at advanced growth stages on soils with marginal B availability. Hence, we compared the effects of B resupply via roots and leaves on growth and physiological response, and relative expression of B transporters in B-deficient oilseed rape plants. Four-week-old plants initially grown with inadequate B (1 µM B for the first two weeks and 0.25 µM B for the next two weeks) were later grown either as such with 0.25 µM B, with 25 µM B in nutrient solution or foliar sprayed with 7 mL of 30, 60 and 150 mM B solution plant^-1 as boric acid. Plants grown with 25 µM B in the nutrient solution from the beginning were included as adequate B treatment. Results showed that B resupply to B-deficient plants via roots and leaves (60 mM B) equally improved root and shoot dry matter, but not to the level of plants grown with adequate B supply. Foliar-applied 150 mM B proved toxic, causing leaf burn but not affecting dry matter. Resupply of B via roots increased B concentration in roots and leaves, while leaf-applied B did so only in leaves. Net carbon assimilation had a positive relationship with dry matter accumulation. Except for the highest foliar B level, B resupply via roots and leaves increased the accumulation of glucose, fructose and sucrose in leaves. Boron-deficient plants showed significant upregulation of BnaNIP5;1 in leaves and roots and of BnaBOR1;2 in roots. Boron resupply via roots reversed the B-deficiency-induced upregulation of BnaNIP5;1 in roots, whereas the expression of BnaBOR1;2 was reversed by both root and foliar B resupply. In leaves, B resupply by both methods reversed the expression of BnaNIP5;1 to the level of B-adequate plants. It is concluded that B resupply to B-deficient plants via roots and leaves equally but partially corrected B deficiency in B. napus grown in hydroponics.

Genome-wide association mapping identifies HvNIP2;2/HvLsi6 accounting for efficient boron transport in barley

Boron (B) is an essential mineral element for plant growth, and the seed B pool of crops can be crucial when seedlings need to establish on low-B soils. To date, it is poorly understood how B accumulation in grain crops is genetically controlled. Here, we assessed the genotypic variation of the B concentration in grains of a spring barley (Hordeum vulgare L.) association panel that represents broad genetic diversity. We found a large genetic variation of the grain B concentration and detected in total 23 quantitative trait loci (QTLs) using genome-wide association mapping. HvNIP2;2/HvLsi6, encoding a potential B-transporting membrane protein, mapped closely to a major-effect QTL accounting for the largest proportion of grain B variation. Based on transport studies using heterologous expression systems and gene expression analysis, we demonstrate that HvNIP2;2/HvLsi6 represents a functional B channel and that expression variation in its transcript level associates with root and shoot B concentrations as well as with root dry mass formation under B-deficient conditions.

BnaA02.NIP6;1a encodes a boron transporter required for plant development under boron deficiency in Brassica napus

Boron (B) is an essential micronutrient for the plant normal growth. In Arabidopsis, NIP6;1 is a boric acid channel required for the proper distribution of boric acid, especially in the nodal regions of shoots. BnaA02.NIP6;1a, a homologous gene of AtNIP6;1 in Brassica napus, was reported to play a key role in B transport activity. However, little is known about the other functions of BnaA02.NIP6;1a in Brassica napus. In this study, we found that BnaA02.NIP6; 1a was localized in both plasma membrane and cytoplasm, which was different from that in Arabidopsis. The transgenic Arabidopsis plant containing a BnaA02.NIP6;1a promoter driven GUS reporter gene displayed strong GUS activity in roots, stems, leaves, especially in buds and open flowers, which are different from the expression pattern from its homologous gene in Arabidopsis. Silencing BnaA02.NIP6;1a repressed vegetative growth under B-deficient condition in Brassica napus. More importantly, knockdown of BnaA02.NIP6;1a in rapeseed resulted in the reduction of boron accumulation in the flower under boron deficiency and lead to severe sterility, which has not yet been reported before. Furthermore, nip6;1 mutant in Arabidopsis only showed the loss of apical dominance phenotype under boron deficiency at reproductive stage, whereas BnaA02.NIP6;1 RNAi lines exhibited large amounts of abnormal development of the inflorescence as compared with the wild type under boron limitation. Taken together, our results demonstrate that BnaA02.NIP6;1a encodes a boron transporter required for plant development under boron deficiency in Brassica napus, which shows its novel and diverse function in rapeseed compared with model plant Arabidopsis.

Evolutionary Understanding of Aquaporin Transport System in the Basal Eudicot Model Species Aquilegia coerulea

Aquaporins (AQPs) play a pivotal role in the cellular transport of water and many other small solutes, influencing many physiological and developmental processes in plants. In the present study, extensive bioinformatics analysis of AQPs was performed in Aquilegia coerulea L., a model species belonging to basal eudicots, with a particular focus on understanding the AQPs role in the developing petal nectar spur. A total of 29 AQPs were identified in Aquilegia, and their phylogenetic analysis performed with previously reported AQPs from rice, poplar and Arabidopsis depicted five distinct subfamilies of AQPs. Interestingly, comparative analysis revealed the loss of an uncharacterized intrinsic protein II (XIP-II) group in Aquilegia. The absence of the entire XIP subfamily has been reported in several previous studies, however, the loss of a single clade within the XIP family has not been characterized. Furthermore, protein structure analysis of AQPs was performed to understand pore diversity, which is helpful for the prediction of solute specificity. Similarly, an AQP AqcNIP2-1 was identified in Aquilegia, predicted as a silicon influx transporter based on the presence of features such as the G-S-G-R aromatic arginine selectivity filter, the spacing between asparagine-proline-alanine (NPA) motifs and pore morphology. RNA-seq analysis showed a high expression of tonoplast intrinsic proteins (TIPs) and plasma membrane intrinsic proteins (PIPs) in the developing petal spur. The results presented here will be helpful in understanding the AQP evolution in Aquilegia and their expression regulation, particularly during floral development.

Transcription factor BnaA9.WRKY47 contributes to the adaptation of Brassica napus to low boron stress by up-regulating the boric acid channel gene BnaA3.NIP5;1

Boron (B) deficiency is one of the major causes of growth inhibition and yield reduction in Brassica napus (B. napus). However, the molecular mechanisms of low B adaptation in B. napus are largely unknown. Here, fifty-one BnaWRKY transcription factors were identified as responsive to B deficiency in B. napus, in which BnaAn.WRKY26, BnaA9.WRKY47, BnaA1.WKRY53 and BnaCn.WRKY57 were tested in yeast one-hybrid assays and showed strong binding activity with conserved sequences containing a W box in the promoters of the B transport-related genes BnaNIP5;1s and BnaBOR1s. Green fluorescent protein fused to the target protein demonstrated the nuclear localization of BnaA9.WRKY47. CRISPR/Cas9-mediated knockout lines of BnaA9.WRKY47 in B. napus had increased sensitivity to low B and lower contents of B than wild-type plants. In contrast, overexpression of BnaA9.WRKY47 enhanced the adaptation to low B with higher B contents in tissues than in wild-type plants. Consistent with the phenotypic response and B accumulation in these transgenic lines, the transcription activity of BnaA3.NIP5;1, a B efficiency candidate gene, was decreased in the knockout lines but was significantly increased in the overexpressing lines under low B conditions. Electrophoretic mobility shift assays, transient expression experiments in tobacco and in situ hybridizations showed that BnaA9.WRKY47 directly activated BnaA3.NIP5;1 expression through binding to the specific cis-element. Taken together, our findings support BnaWRKYs as new participants in response to low B, and BnaA9.WRKY47 contributes to the adaptation of B. napus to B deficiency through up-regulating BnaA3.NIP5;1 expression to facilitate efficient B uptake.

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.

With or Without You: Altered Plant Response to Boron-Deficiency in Hydroponically Grown Grapevines Infected by Grapevine Pinot Gris Virus Suggests a Relation Between Grapevine Leaf Mottling and Deformation Symptom Occurrence and Boron Plant Availability

Despite the increasing spread of Grapevine Leaf Mottling and Deformation (GLMD) worldwide, little is known about its etiology. After identification of grapevine Pinot gris virus (GPGV) as the presumptive causal agent of the disease in 2015, various publications have evaluated GPGV involvement in GLMD. Nevertheless, there are only partial clues to explain the presence of GPGV in both symptomatic and asymptomatic grapevines and the mechanisms that trigger symptom development, and so a consideration of new factors is required. Given the similarities between GLMD and boron (B)-deficiency symptoms in grapevine plants, we posited that GPGV interferes in B homeostasis. By using a hydroponic system to control B availability, we investigated the effects of different B supplies on grapevine phenotype and those of GPGV infection on B acquisition and translocation machinery, by means of microscopy, ionomic and gene expression analyses in both roots and leaves. The transcription of the genes regulating B homeostasis was unaffected by the presence of GPGV alone, but was severely altered in plants exposed to both GPGV infection and B-deficiency, allowing us to speculate that the capricious and patchy occurrence of GLMD symptoms in the field may not be related solely to GPGV, but to GPGV interference in plant responses to different B availabilities. This hypothesis found preliminary positive confirmations in analyses on field-grown plants.

Boron demanding tissues of Brassica napus express specific sets of functional Nodulin26-like Intrinsic Proteins and BOR1 transporters

The sophisticated uptake and translocation regulation of the essential element boron (B) in plants is ensured by two transmembrane transporter families: the Nodulin26-like Intrinsic Protein (NIP) and BOR transporter family. Though the agriculturally important crop Brassica napus is highly sensitive to B deficiency, and NIPs and BORs have been suggested to be responsible for B efficiency in this species, functional information of these transporter subfamilies is extremely rare. Here, we molecularly characterized the NIP and BOR1 transporter family in the European winter-type cv. Darmor-PBY018. Our transport assays in the heterologous oocyte and yeast expression systems as well as in growth complementation assays in planta demonstrated B transport activity of NIP5, NIP6, NIP7 and BOR1 isoforms. Moreover, we provided functional and quantitative evidence that also members of the NIP2, NIP3 and NIP4 groups facilitate the transport of B. A detailed B- and tissue-dependent B-transporter expression map was generated by quantitative polymerase chain reaction. We showed that NIP5 isoforms are highly upregulated under B-deficient conditions in roots, but also in shoot tissues. Moreover, we detected transcripts of several B-permeable NIPs from various groups in floral tissues that contribute to the B distribution within the highly B deficiency-sensitive flowers.

Differential Alternative Splicing Genes in Response to Boron Deficiency in Brassica napus

Alternative splicing (AS) can increase transcriptome diversity, protein diversity and protein yield, and is an important mechanism to regulate plant responses to stress. Oilseed rape (Brassica napus L.), one of the main oil crops in China, shows higher sensitivity to boron (B) deficiency than other species. Here, we demonstrated AS changes that largely increased the diversity of the mRNA expressed in response to B deficiency in B. napus. Each gene had two or more transcripts on average. A total of 33.3% genes in both Qingyou10 (QY10, B-efficient cultivar) and Westar10 (W10, B-inefficient cultivar) showed AS in both B conditions. The types of AS events were mainly intron retention, 3′ alternative splice site, 5′ alternative splice site and exon skipping. The tolerance ability of QY10 was higher than that of W10, possibly because there were far more differential alternative splicing (DAS) genes identified in QY10 at low B conditions than in W10. The number of genes with both DAS and differentially expressed (DE) was far lower than that of the genes that were either with DAS or DE in QY10 and W10, suggesting that the DAS and DE genes were independent. Four Serine/Arginine-rich (SR) splicing factors, BnaC06g14780D, BnaA01g14750D, BnaA06g15930D and BnaC01g41640D, underwent differentially alternative splicing in both cultivars. There existed gene–gene interactions between BnaC06g14780D and the genes associated with the function of B in oilseed rape at low B supply. This suggests that oilseed rape could regulate the alterative pre-mRNA splicing of SR protein related genes to increase the plant tolerance to B deficiency.

BvCOLD1: A novel aquaporin from sugar beet (Beta vulgaris L.) involved in boron homeostasis and abiotic stress

Beta vulgaris (sugar beet) is one of the most important industrial crops. Screening of a cDNA library for sugar beet genes able to confer cold tolerance upon overexpression in yeast identified a novel aquaporin, which we named BvCOLD1. The amino acid sequence of BvCOLD1 indicated that an acidic protein (pI 5.18) is similar to tonoplast intrinsic protein aquaporins. RNA expression analysis indicated that BvCOLD1 is expressed in all sugar beet organs. Confocal microscopy of a green fluorescent protein-tagged version localized BvCOLD1 in the endoplasmic reticulum in yeast and in plant cells. Experiments in yeast showed that BvCOLD1 has an important role in transporting several molecules, among them is boron, one of the most limiting micronutrients for sugar beet cultivation. Transgenic Arabidopsis thaliana plants overexpressing BvCOLD1 showed enhanced tolerance to cold, to different abiotic stresses, and to boron deficiency at different developmental stages. Searches in databases only retrieved BvCOLD1 orthologues in genomes from the Chenopodioideae, a subfamily of the Amaranthaceae family that includes the closely related crop Spinacea oleracea and halotolerant plants such as Salicornia herbacea or Suaeda glauca. Orthologues share a conserved sequence in the carboxy terminus, not present in other aquaporins, which is required for the functionality of the protein.

Molecular characterization of the genome-wide BOR transporter gene family and genetic analysis of BnaC04.BOR1;1c in Brassica napus

BACKGROUND

Boron (B) deficiency is an agricultural problem that causes significant losses of crop yield in many areas of the world. However, systematic analysis of BOR family genes for B transport in rapeseed is still lacking. We aimed to identify and characterize BOR transporters in Brassica napus and the potential role of these transporters in B homeostasis in response to B deficiency.

RESULTS

Here, we identified 20 BOR transporters from the Brassica napus genome, which were classified into six distinct groups that represent clear orthologous relationships to their family members in Arabidopsis. qRT-PCR revealed distinct expression profiles for BnBORs in different tissues and in response to external B levels. The B-efficient cultivar QY10 accumulated more B in shoots than the B-inefficient cultivar W10, and overexpression of BnaBOR1;1c could alleviate shoot B-deficiency symptoms in W10 by distributing more B from roots to shoots. Additionally, BnBOR1;1c expression was up-regulated by B deficiency, and the induction of BnBOR1;1c was more intense in QY10. Moreover, two conserved InDels were found in the promoter regions of BnBOR1;1c within different B-efficient genotypes.

CONCLUSIONS

Overall, the molecular characterization of the BnBOR genes of two B-efficient cultivars and their responses to B deficiency highlights the diversity of the family members in B. napus, and BnaC4.BOR1;1c has potential as a candidate gene for improving B nutrition.

Identification of Rapeseed (Brassica napus) Cultivars With a High Tolerance to Boron-Deficient Conditions

Boron (B) is an essential micronutrient for seed plants. Information on B-efficiency mechanisms and B-efficient crop and model plant genotypes is very scarce. Studies evaluating the basis and consequences of B-deficiency and B-efficiency are limited by the facts that B occurs as a trace contaminant essentially everywhere, its bioavailability is difficult to control and soil-based B-deficiency growth systems allowing a high-throughput screening of plant populations have hitherto been lacking. The crop plant Brassica napus shows a very high sensitivity toward B-deficient conditions. To reduce B-deficiency-caused yield losses in a sustainable manner, the identification of B-efficient B. napus genotypes is indispensable. We developed a soil substrate-based cultivation system which is suitable to study plant growth in automated high-throughput phenotyping facilities under defined and repeatable soil B conditions. In a comprehensive screening, using this system with soil B concentrations below 0.1 mg B (kg soil)-1, we identified three highly B-deficiency tolerant B. napus cultivars (CR2267, CR2280, and CR2285) among a genetically diverse collection comprising 590 accessions from all over the world. The B-efficiency classification of cultivars was based on a detailed assessment of various physical and high-throughput imaging-based shoot and root growth parameters in soil substrate or in in vitro conditions, respectively. We identified cultivar-specific patterns of B-deficiency-responsive growth dynamics. Elemental analysis revealed striking differences only in B contents between contrasting genotypes when grown under B-deficient but not under standard conditions. Results indicate that B-deficiency tolerant cultivars can grow with a very limited amount of B which is clearly below previously described critical B-tissue concentration values. These results suggest a higher B utilization efficiency of CR2267, CR2280, and CR2285 which would represent a unique trait among so far identified B-efficient B. napus cultivars which are characterized by a higher B-uptake capacity. Testing various other nutrient deficiency treatments, we demonstrated that the tolerance is specific for B-deficient conditions and is not conferred by a general growth vigor at the seedling stage. The identified B-deficiency tolerant cultivars will serve as genetic and physiological "tools" to further understand the mechanisms regulating the B nutritional status in rapeseed and to develop B-efficient elite genotypes.

Identification and Characterization of an Arabidopsis thaliana Mutant lbt With High Tolerance to Boron Deficiency

Boron (B) is an essential micronutrient of plants. In the present study, we characterized an Arabidopsis mutant lbt with significant low-boron tolerance that was identified based on our previous mapping of QTL for B efficiency in Arabidopsis. Multiple nutrient-deficiency analyses point out that lbt mutant is insensitive to only B-limitation stress. Compared with wild-type Col-0, the fresh weight, leaf area, root length and root elongation rate of lbt mutant were significantly improved under B deficiency during vegetative growth. lbt mutant also showed the improvements in plant height, branches and inflorescences compared with Col-0 during the reproductive stage under B limitation. Ultrastructure analysis of the leaves showed that starch accumulation in lbt mutant was significantly diminished compared with Col-0. Furthermore, there were no significant differences in the expression of transporter-related genes and B concentrations between Col-0 and lbt mutant under both normal B and low-B conditions. These results suggest that lbt mutant has a lower B demand than Col-0. Genetic analysis suggests that the low-B tolerant phenotype of lbt mutant is under the control of a monogenic recessive gene. Based on the high-density SNP linkage genetic map, only one QTL for low-B tolerance was mapped on chromosome 4 between 10.4 and 14.8 Mb. No any reported B-relative genes exist in the QTL interval, suggesting that a gene with unknown function controls the tolerance of lbt to B limitation. Taken together, lbt is a low-B tolerant mutant that does not depend on the uptake or transport of B and is controlled by a monogenic recessive gene mapped on chromosome 4, and cloning and functional analysis of LBT gene are expected to reveal novel mechanisms for plant resistance to B deficiency.

Insights into the Mechanisms Underlying Boron Homeostasis in Plants

Boron is an essential element for plants but is toxic in excess. Therefore, plants must adapt to both limiting and excess boron conditions for normal growth. Boron transport in plants is primarily based on three transport mechanisms across the plasma membrane: passive diffusion of boric acid, facilitated diffusion of boric acid via channels, and export of borate anion via transporters. Under boron -limiting conditions, boric acid channels and borate exporters function in the uptake and translocation of boron to support growth of various plant species. In Arabidopsis thaliana, NIP5;1 and BOR1 are located in the plasma membrane and polarized toward soil and stele, respectively, in various root cells, for efficient transport of boron from the soil to the stele. Importantly, sufficient levels of boron induce downregulation of NIP5;1 and BOR1 through mRNA degradation and proteolysis through endocytosis, respectively. In addition, borate exporters, such as Arabidopsis BOR4 and barley Bot1, function in boron exclusion from tissues and cells under conditions of excess boron. Thus, plants actively regulate intracellular localization and abundance of transport proteins to maintain boron homeostasis. In this review, the physiological roles and regulatory mechanisms of intracellular localization and abundance of boron transport proteins are discussed.