The underlying mechanisms by which boron mitigates copper toxicity in Citrus sinensis leaves revealed by integrated analysis of transcriptome, metabolome and physiology.

Both copper (Cu) excess and boron (B) deficiency are often observed in some citrus orchard soils. The molecular mechanisms by which B alleviates excessive Cu in citrus are poorly understood. Seedlings of sweet orange (Citrus sinensis (L.) Osbeck cv. Xuegan) were treated with 0.5 (Cu0.5) or 350 (Cu350 or Cu excess) µM CuCl2 and 2.5 (B2.5) or 25 (B25) µM HBO3 for 24 wk. Thereafter, this study examined the effects of Cu and B treatments on gene expression levels revealed by RNA-Seq, metabolite profiles revealed by a widely targeted metabolome, and related physiological parameters in leaves. Cu350 upregulated 564 genes and 170 metabolites, and downregulated 598 genes and 58 metabolites in leaves of 2.5 µM B-treated seedlings (LB2.5), but it only upregulated 281 genes and 100 metabolites, and downregulated 136 genes and 40 metabolites in leaves of 25 µM B-treated seedlings (LB25). Cu350 decreased the concentrations of sucrose and total soluble sugars and increased the concentrations of starch, glucose, fructose and total nonstructural carbohydrates in LB2.5, but it only increased the glucose concentration in LB25. Further analysis demonstrated that B addition reduced the oxidative damage and alterations in primary and secondary metabolisms caused by Cu350, and alleviated the impairment of Cu350 to photosynthesis and cell wall metabolism, thus improving leaf growth. LB2.5 exhibited some adaptive responses to Cu350 to meet the increasing need for the dissipation of excessive excitation energy (EEE) and the detoxification of reactive oxygen species (reactive aldehydes) and Cu. Cu350 increased photorespiration, xanthophyll cycle-dependent thermal dissipation, nonstructural carbohydrate accumulation, and secondary metabolite biosynthesis and abundances; and upregulated tryptophan metabolism and related metabolite abundances, some antioxidant-related gene expression, and some antioxidant abundances. Additionally, this study identified some metabolic pathways, metabolites and genes that might lead to Cu tolerance in leaves.

Circadian Rhythm and Nitrogen Metabolism Participate in the Response of Boron Deficiency in the Root of Brassica napus.

Boron (B) deficiency has been shown to inhibit root cell growth and division. However, the precise mechanism underlying B deficiency-mediated root tip growth inhibition remains unclear. In this study, we investigated the role of BnaA3.NIP5;1, a gene encoding a boric acid channel, in Brassica napus (B. napus). BnaA3.NIP5;1 is expressed in the lateral root cap and contributes to B acquisition in the root tip. Downregulation of BnaA3.NIP5;1 enhances B sensitivity in B. napus, resulting in reduced shoot biomass and impaired root tip development. Transcriptome analysis was conducted on root tips from wild-type B. napus (QY10) and BnaA3.NIP5;1 RNAi lines to assess the significance of B dynamics in meristematic cells during seedling growth. Differentially expressed genes (DEGs) were significantly enriched in plant circadian rhythm and nitrogen (N) metabolism pathways. Notably, the circadian-rhythm-related gene HY5 exhibited a similar B regulation pattern in Arabidopsis to that observed in B. napus. Furthermore, Arabidopsis mutants with disrupted circadian rhythm (hy5/cor27/toc1) displayed heightened sensitivity to low B compared to the wild type (Col-0). Consistent with expectations, B deficiency significantly disrupted N metabolism in B. napus roots, affecting nitrogen concentration, nitrate reductase enzyme activity, and glutamine synthesis. Interestingly, this disruption was exacerbated in BnaA3NIP5;1 RNAi lines. Overall, our findings highlight the critical role of B dynamics in root tip cells, impacting circadian rhythm and N metabolism, ultimately leading to retarded growth. This study provides novel insights into B regulation in root tip development and overall root growth in B. napus.

Arabidopsis transcription factor STOP1 directly activates expression of NOD26-LIKE MAJOR INTRINSIC PROTEIN5;1, and is involved in the regulation of tolerance to low-boron stress.

Transcriptional regulation is a crucial component of plant adaptation to numerous different stresses; however, its role in how plants adapt to low-boron (B) stress remains unclear. In this study, we show that the C2H2-type transcription factor SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) in Arabidopsis is essential for improving plant growth under low-B conditions. STOP1 and the boric acid-channel protein NOD26-LIKE MAJOR INTRINSIC PROTEIN5;1 (NIP5;1) were found to co-localize in root epidermal cells, and STOP1 binds to the 5´-untranslated region of NIP5;1 to activate its expression and enhance B uptake by the roots. Overexpression of STOP1 increased tolerance to low-B stress by up-regulating NIP5;1 transcript levels. Further genetic analyses revealed that STOP1 and NIP5;1 function together in the same pathway to confer low-B tolerance. These results highlight the importance of the STOP1-NIP5;1 module in improving plant growth under low-B conditions.

Genome-wide transcriptome profiling of mulberry (Morus alba) response to boron deficiency and toxicity reveal candidate genes associated with boron tolerance in leaves.

Mulberry (Morus alba) is an essential plant with countless economic benefits; however, its growth and metabolic processes are hampered by boron (B) stresses. Very little research has been performed to elucidate boron tolerance and detoxification mechanisms in this species. The M. alba cultivar, Yu-711, was exposed to five different concentrations of boric acid (H3BO3), including deficient (T1; 0 mM) moderate B deficiency (T2; 0.02 mM), sufficient (CK; 0.1 mM) and toxic (T3 and T4; 0.5 and 1 mM) levels for 18 days of growth in pots experiment. Transcriptome analysis of B deficiency and toxicity treatments was performed on mulberry leaves. The transcriptome data reveal that a total of 6114 genes were differentially expressed (DEGs), of which 3830 were up-regulated and 2284 were down-regulated. A comparative analysis between treatment groups CK-vs-T1 (deficiency) and CK-vs-T4 (toxicity) indicates that 590 and 1383 genes were down-regulated in both deficiency and B toxicity, respectively. The results show that 206 genes were differentially expressed in all treatments. B deficiency and toxicity significantly altered the expression of the key aquaporins (PIP2-1, PIP2-7, PIP2-4 and NIP3-1) and high-affinity boron transporter genes (BOR1 and BOR7). In addition, boron stress also altered the expression of antioxidants and photosynthesis-related genes. B stresses were found to alter several transcription factors including ERF1B, which is associated with the regulation of boron uptake and the synthesis and signaling of phytohormones. Unravelling the mechanisms of B tolerance and detoxification is important and would give us further insight into how B stresses affect mulberry plants.

Single-cell transcriptomic analysis of pea shoot development and cell-type-specific responses to boron deficiency.

Understanding how nutrient stress impacts plant growth is fundamentally important to the development of approaches to improve crop production under nutrient limitation. Here we applied single-cell RNA sequencing to shoot apices of Pisum sativum grown under boron (B) deficiency. We identified up to 15 cell clusters based on the clustering of gene expression profiles and verified cell identity with cell-type-specific marker gene expression. Different cell types responded differently to B deficiency. Specifically, the expression of photosynthetic genes in mesophyll cells (MCs) was down-regulated by B deficiency, consistent with impaired photosynthetic rate. Furthermore, the down-regulation of stomatal development genes in guard cells, including homologs of MUTE and TOO MANY MOUTHS, correlated with a decrease in stomatal density under B deficiency. We also constructed the developmental trajectory of the shoot apical meristem (SAM) cells and a transcription factor interaction network. The developmental progression of SAM to MC was characterized by up-regulation of genes encoding histones and chromatin assembly and remodeling proteins including homologs of FASCIATA1 (FAS1) and SWITCH DEFECTIVE/SUCROSE NON-FERMENTABLE (SWI/SNF) complex. However, B deficiency suppressed their expression, which helps to explain impaired SAM development under B deficiency. These results represent a major advance over bulk-tissue RNA-seq analysis in which cell-type-specific responses are lost and hence important physiological responses to B deficiency are missed. The reported findings reveal strategies by which plants adapt to B deficiency thus offering breeders a set of specific targets for genetic improvement. The reported approach and resources have potential applications well beyond P. sativum species and could be applied to various legumes to improve their adaptability to multiple nutrient or abiotic stresses.

Physiological and molecular mechanisms of Acacia melanoxylon stem in response to boron deficiency.

Boron is an essential micronutrient for plant growth as it participates in cell wall integrity. The growth and development of Acacia melanoxylon stem can be adversely affected by a lack of boron. To explore the mechanism of boron deficiency in A. melanoxylon stem, the changes in morphological attributes, physiological, endogenous hormone levels, and the cell structure and component contents were examined. In addition, the molecular mechanism of shortened internodes resulting from boron deficiency was elucidated through transcriptome analysis. The results showed that boron deficiency resulted in decreased height, shortened internodes, and reduced root length and surface area, corresponding with decreased boron content in the roots, stems, and leaves of A. melanoxylon. In shortened internodes of stems, oxidative damage, and disordered hormone homeostasis were induced, the cell wall was thickened, hemicellulose and water-soluble pectin contents decreased, while the cellulose content increased under boron deficiency. Furthermore, plenty of genes associated with cell wall metabolism and structural components, including GAUTs, CESAs, IRXs, EXPs, TBLs, and XTHs were downregulated under boron deficiency. Alterations of gene expression in hormone signaling pathways comprising IAA, GA, CTK, ET, ABA, and JA were observed under boron deficiency. TFs, homologous to HD1s, NAC10, NAC73, MYB46s, MYB58, and ERF92s were found to interact with genes related to cell wall metabolism, and the structural components were identified. We established a regulatory mechanism network of boron deficiency-induced shortened internodes in A. melanoxylon based on the above results. This research provides a theoretical basis for understanding the response mechanism of woody plants to boron deficiency.

Physiological and metabolic changes in response to Boron levels are mediated by ethylene affecting tomato fruit yield.

Boron (B) is an essential nutrient for the plant, and its stress (both deficiency and toxicity) are major problems that affect crop production. Ethylene metabolism (both signaling and production) is important to plants' differently responding to nutrient availability. To better understand the connections between B and ethylene, here we investigate the function of ethylene in the responses of tomato (Solanum lycopersicum) plants to B stress (deficiency, 0 µM and toxicity, 640 µM), using ethylene related mutants, namely nonripening (nor), ripening-inhibitor (rin), never ripe (Nr), and epinastic (Epi). Our results show that B stress does not necessarily inhibit plant growth, but both B stress and ethylene signaling severely affected physiological parameters, such as photosynthesis, stomatal conductance, and chlorophyll a fluorescence. Under B toxicity, visible symptoms of toxicity appeared in the roots and margins of the older leaves through necrosis, caused by the accumulation of B which stimulated ethylene biosynthesis in the shoots. Both nor and rin (ethylene signaling) mutants presented similar responses, being these genotypes more sensitive and displaying several morphophysiological alterations, including fruit productivity reductions, in response to the B toxicity conditions. Therefore, our results suggest that physiological and metabolic changes in response to B fluctuations are likely mediated by ethylene signaling.

The wall-associated kinase gene family in pea (Pisum sativum) and its function in response to B deficiency and Al toxicity.

Plant cell walls are embedded in a pectin matrix which is physically linked with the wall-associated kinases (WAKs), a subfamily of receptor-like kinases that participate in the cell wall integrity (CWI) sensing. Since cell walls are also the main binding sites for boron (B) and aluminum (Al), WAK may be potentially associated with the regulation of plant responses to Al toxicity and B deficiency. Using pea as a model species, we have identified a total of 28 WAK genes in the genome and named them according to its chromosomal location. All the PsWAKs were phylogenetically grouped into three clades. Phylogenetic relationship and synteny analysis showed that the PsWAKs in pea and Glycine max or Medicago truncatula shared a relatively conserved evolutionary history. Protein domain, motif, and transmembrane analysis indicated that all PsWAK proteins were predicted to be localized to the plasma membrane, and most PsWAKs shared a similar structure to their homologs. The RNA-seq data showed that the expression pattern of WAK genes in response to B deficiency was similar to that of Al toxicity, with most of PsWAKs being up-regulated. The qRT-PCR results further confirmed that PsWAK5, PsWAK9 and PsWAK14 were more specific for both B-deficiency and Al toxicity, and the expression levels of PsWAK5, PsWAK9 and PsWAK14 were significantly higher in the Al-sensitive cultivar Hyogo than in the Al-resistant cultivar Alaska under Al toxicity. This study provided an important basis for the functional and evolutionary analysis of PsWAKs and linked them to responses to cell wall damage induced by B-deficiency and Al toxicity, suggesting that PsWAKs may play a key role in the perception of cell wall integrity under Al toxicity or B-deficiency, as well as in the regulation of Al tolerance in pea.

The impact of boron nutrient supply in mulberry (Morus alba) response to metabolomics, enzyme activities, and physiological parameters.

Boron (B) is essential for normal and healthy plant growth. Therefore, Boron stress is a common abiotic stress that limits plant growth and productivity. However, how mulberry copes with boron stress remains unclear. In this study, seedlings of the Morus alba cultivar, Yu-711, were treated with five different concentrations of boric acid (H3BO3), including deficient (0 and 0.02 mM), sufficient (0.1 mM) and toxic (0.5 and 1 mM) levels. Physiological parameters, enzymatic activities and non-targeted liquid chromatography-mass spectrometry (LC-MS) technique were employed to evaluate the effects of boron stress on the net photosynthetic rate (Pn), chlorophyll content, stomatal conductance (Gs), transpiration rate (Tr), intercellular CO2 concentration (Ci) and metabolome signatures. Physiological analysis revealed that Boron deficiency and toxicity induced a decline in Pn, Ci, Gs, Tr, and chlorophyll content. Also, enzymatic activities, including catalase (CAT) and superoxide dismutase (SOD), decreased, while POD activity increased in response to Boron stress. Osmotic substances such as soluble sugars, soluble proteins, and proline (PRO) presented elevated levels under all Boron concentrations. Metabolome analysis indicated that differential metabolites, including amino acids, secondary metabolites, carbohydrates, and lipids, played a key role in Yu-711's response to Boron stress. These metabolites were mainly involved in amino acid metabolism, biosynthesis of other secondary metabolites, lipid metabolism, metabolism of cofactors and vitamins, and metabolism of other amino acids pathways. Our findings reveal the various metabolites pathways in mulberry response to boron nutrient supply and may serve as fundamental knowledge in breeding resistance mulberry plants, so that it can cope with climate changes.

Transcriptome Analysis in Pyrus betulaefolia Roots in Response to Short-Term Boron Deficiency.

Boron (B) deficiency stress is frequently observed in pear orchards and causes a considerable loss of productivity and fruit quality. Pyrus betulaefolia is one of the most important rootstocks that has been widely used in pear production. The present study confirmed that the boron form of different tissues showed various changes, and the free boron content was significantly decreased under the short-term B deficiency condition. Moreover, the ABA and JA content also significantly accumulated in the root after short-term B deficiency treatment. A comprehensive transcriptome analysis of 24 h B deficiency treatment P. betulaefolia root was performed in this study. Transcriptome results revealed a total of 1230 up-regulated and 642 down-regulated differentially expressed genes (DEGs), respectively. B deficiency significantly increased the expression of the key aquaporin gene NIP5-1. In addition, B deficiency also increased the expression of ABA (ZEP and NCED) and JA (LOX, AOS and OPR) synthesis genes. Several MYB, WRKY, bHLH and ERF transcription factors were induced by B deficiency stress, which may relate to the regulation of B uptake and plant hormone synthesis. Overall, these findings suggested that P. betulaefolia root had adaptive responses to short-term B deficiency stress by improved boron absorption ability and hormone (JA and ABA) synthesis. The transcriptome analysis provided further information for understanding the mechanism of the pear rootstock responses to B deficiency stress.

Insights into physiological and molecular mechanisms underlying efficient utilization of boron in different boron efficient Beta vulgaris L. varieties.

Boron (B) deficiency and consequent limitation of plant yield and quality, particularly of sugar beet (Beta vulgaris L.) has emerged as a maior problem,which is exacerbating due to cultivar dependent variability in B deficiency tolerance. Pertinently, the current study was designed to elucidate the physiological and molecular mechanisms of B deficiency tolerance of sugar beet varieties KWS1197 (B-efficient variety) and KWS0143 (B-inefficient variety). A hydroponic experiment was conducted employing two B levels B0.1 (0.1 µM L-1 H3BO3, deficiency) and B50 (50 µM L-1 H3BO3, adequacy). Boron deficiency greatly inhibited root elongation and dry matter accumulation; however, formation of lateral roots stimulated and average root diameter was increased. Results exhibited that by up-regulating the expression of NIP5-1, NIP6-1, and BOR2, and suppressing the expression of BOR4, cultivar KWS1197, in contrast to KWS0143, managed to transfer sufficient amount of B to the aboveground plant parts, facilitating its effective absorption and utilization. Accumulation of malondialdehyde (MDA) and reactive oxygen species (ROS) was also mellowed in KWS1197, as well as the oxidative damage to root cells via preservation of the antioxidant enzyme system. Additionally, the expression of essential enzymes for biosynthesis of phytohormone (PYR/PYL) and lignin (COMT, POX, and CCoAOMT) were found to be highly up-regulated in KWS1197. Deductively, through effective B absorption and transportation, balanced nutrient accumulation, and an activated antioxidant enzyme system, B-efficient cultivars may cope with B deficiency while retaining a superior cellular structure to enable root development.

What Can Boron Deficiency Symptoms Tell Us about Its Function and Regulation?

On the eve of the 100th anniversary of Dr. Warington's discovery of boron (B) as a nutrient essential for higher plants, "boronists" have struggled to demonstrate a role beyond its structural function in cell walls dimerizing pectin molecules of rhamnogalacturonan II (RGII). In this regard, B deficiency has been associated with a plethora of symptoms in plants that include macroscopic symptoms like growth arrest and cell death and biochemical or molecular symptoms that include changes in cell wall pore size, apoplast acidification, or a steep ROS production that leads to an oxidative burst. Aiming to shed light on B functions in plant biology, we proposed here a unifying model integrating the current knowledge about B function(s) in plants to explain why B deficiency can cause such remarkable effects on plant growth and development, impacting crop productivity. In addition, based on recent experimental evidence that suggests the existence of different B ligands other than RGII in plant cells, namely glycolipids, and glycoproteins, we proposed an experimental pipeline to identify putative missing ligands and to determine how they would integrate into the above-mentioned model.

Exogenous citrate restores the leaf metabolic profiles of navel orange plants under boron deficiency.

Boron (B) is an essential micronutrient for higher plants, and its deficiency causes a change in the citrate concentration in leaves of young navel orange plants. Although citrate has been implicated in the regulation of gene expression for many transcripts, it is unclear whether citrate can affect metabolic profiling under B deficiency and if so, how many metabolites are affected. In this study, GC-TOF-MS-based untargeted metabolite profiling was used to identify the physiological effects of exogenous citrate on recovery of metabolites in B-deficient orange plants. There were 31 increased and 24 decreased metabolites in the boron-deficient (BD) group leaves relative to those of the boron-adequate (BA) group. Boron deficiency-induced changes in many metabolites were restored to the level of BA (control) group leaves or showed a recovery tendency at 1 week after citrate supply (foliar application of citrate, BDFC), including 11 organic acids, 9 sugars and polyols, 10 amino acids, and 4 other compounds. To compare with the metabolic recovery effects of exogenous citrate on B deficiency, exogenous application of B (borate) was also performed under same conditions (BDFB), and similar effects on the regulation of metabolic homeostasis under B deficiency were observed. Both the results of principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that BA, BDFC, and BDFB were relatively similar and clustered close to each other. There are different responsive and regulatory mechanisms to the additions of exogenous citrate in navel orange leaves under B adequate and deficient conditions. Based on these results, we suggest that citrate is an important component of the B deficiency stress response, and exogenous application of citrate generally restores the leaf metabolic profiles of navel orange plants under boron deficiency, which might play a positive role in this stress tolerance.

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.

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.

Boron decreases cadmium influx into root cells of Capsicum annuum by altering cell wall components and plasmalemma permeability.

Large areas of soil are boron (B) deficient and contaminated with cadmium (Cd) in southern China. The aim of this study was to select the optimal B supply level and elucidate the underlying physiological and biochemical mechanisms to understand how B reduces Cd influx into root cells of hot pepper (Capsicum annuum). An experiment was conducted to investigate the changes in Cd accumulation with B supply. Hot pepper seedlings were grown in two nutrient solutions containing 0.05- and 0.2-mg Cd L-1 and supplied with six different B concentrations for 2 weeks. The other experiment was conducted to determine the Cd2+ flux into cells, cell wall components, antioxidative ability, and plasmalemma permeability of root tips of hot pepper exposed to 0.1-mg Cd L-1 in the presence and absence of B. The results showed that the optimal B concentration to promote plant growth and reduce Cd accumulation was 0.25 mg L-1. Moreover, B application significantly decreased Cd2+ influx into cells, increased the contents of lignin and pectin, enhanced the activities of antioxidant enzymes, reduced the production of reactive oxygen species, and decreased membrane peroxidation and permeability. Overall, boron in moderation can promote plant growth, maintain the normal structures and functions of the cell wall and membrane, and thus decrease Cd2+ influx into root cells and subsequently Cd translocation to shoots. Consequently, B is a reliable inhibitor of Cd uptake, and the functional and structural integrity of cell walls and membranes may have some relevance to reduced Cd uptake after B application.

Boron deficiency-induced root growth inhibition is mediated by brassinosteroid signalling regulation in Arabidopsis.

Brassinosteroids (BRs) are pivotal phytohormones involved in the control of root development. Boron (B) is an essential micronutrient for plants, and root growth is rapidly inhibited under B deficiency conditions. However, the mechanisms underlying this inhibition are still unclear. Here, we identified BR-related processes underlying B deficiency at the physiological, genetic, molecular/cell biological and transcriptomic levels and found strong evidence that B deficiency can affect BR biosynthesis and signalling, thereby altering root growth. RNA sequencing analysis revealed strong co-regulation between BR-regulated genes and B deficiency-responsive genes. We found that the BR receptor mutants bri1-119 and bri1-301 were more insensitive to decreased B supply, and the gain-of-function mutants bes1-D and pBZR1-bzr1-D exhibited insensitivity to low-B stress. Under B deficiency conditions, exogenous 24-epibrassinolide rescued the inhibition of root growth, and application of the BR biosynthesis inhibitor brassinazole exacerbated this inhibitory effect. The nuclear-localised signal of BES1 was reduced under low-B conditions compared with B sufficiency conditions. We further found that B deficiency hindered the accumulation of brassinolide to downregulate BR signalling and modulate root elongation, which may occur through a reduction in BR6ox1 and BR6ox2 mRNA levels. Taken together, our results reveal a role of BR signalling in root elongation under B deficiency.

Physiological and proteomic analysis reveals the impact of boron deficiency and surplus on alfalfa (Medicago sativa L.) reproductive organs.

Boron (B), an essential element for increasing seed yield and germinability in alfalfa (Medicago sativa L.), plays a vital role in its reproductive processes. However, effects of B stress on physiological and proteomic changes in reproductive organs related to alfalfa seed yield and germinability are poorly understood. In order to gain a better insight into B response or tolerance mechanisms, field trials were designed for B deficiency (0 mg B L-1), B sufficiency (800 mg B L-1), and B surplus (1600 mg B L-1) application during alfalfa flowering to analyze the proteomics and physiological responses of alfalfa 'Aohan' reproductive organs. Results showed that B deficiency weakened the stress-responsive ability in these organs, while B surplus reduced the sugar utilization of 'Aohan' flowers and caused lipid membrane peroxidation in 'Aohan' seeds. In addition, four upregulated stress responsive proteins (ADF-like protein, IMFP, NAD(P)-binding Rossmann-fold protein and NAD-dependent ALDHs) might play pivotal roles in the response of 'Aohan' reproductive organs to conditions of B deficiency and B surplus. All of the above results would be helpful to understand the tolerance mechanisms of alfalfa reproductive organs to both B deficiency and B surplus conditions, and also to give insight into the regulatory role of B in improving seed yield and germinability in alfalfa seed production. In summary, B likely plays a structural and regulatory role in relation to lipid metabolism, carbohydrate metabolism, amino acid metabolism, and signal transduction, thus regulates alfalfa reproductive processes eventually affecting the seed yield and germinability of alfalfa seeds.

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.