B transport processes have traditionally been regarded as predominantly passive (Marschner, 1995). This perspective is largely due to the fact that boric acid, which is a principal form of B under physiological conditions, exists as an uncharged molecule that readily diffuses across the plasma membrane (Takano et al., 2008). However, several physiological experiments have identified active mechanisms for B transport. Dannel et al. (2000) demonstrated that B transport in sunflower (Helianthus annuus) occurs via carrier or channel-mediated processes. Major breakthroughs in understanding B transport mechanisms began with the identification of Arabidopsis BOR1 (AtBOR1) as the first known biological B transporter (Takano et al., 2002). Regarding the uptake and translocation of B in plants, this process is ensured by two transmembrane transporter protein families (Figure 1): (1) channel proteins from the NIPs (nodulin-26 like intrinsic proteins) family, which are boric acid channels that enable the passive transmembrane flow of uncharged boric acid, driven by concentration gradients; and (2) efflux transporters belonging to the BOR family, which mediate the efflux of borate ions (Miwa and Fujiwara, 2010).

The molecular mechanisms of B uptake and transport in plants have been most extensively studied in Arabidopsis (Miwa and Fujiwara, 2010; Onuh and Miwa, 2021). Major intrinsic proteins (MIPs) family have been identified as boric acid channels (Wallace et al., 2006). Plant MIPs are classified into four distinct groups: the tonoplast intrinsic proteins (TIPs), the plasma membrane intrinsic proteins (PIPs), the nodulin 26 (NOD26)-like intrinsic proteins (NIPs) and the small basic intrinsic proteins (SIPs) (Wallace et al., 2006; Maurel et al., 2015). NIPs are further classified into three subclasses (I-III) based on their pore structures, and the physiological function of NIP I proteins remains unclear, while NIP II and III are known to transport boric acid and silicic acid, respectively (Wallace and Roberts, 2004; Danielson and Johanson, 2010; Roberts and Routray, 2017). In Arabidopsis, the NIP subfamily consists of nine genes (Johanson et al., 2001), including three members belonging to the NIP II subgroup: AtNIP5;1, AtNIP6;1 and AtNIP7;1 (Wallace and Roberts, 2005).

AtNIP5;1, a major boric acid channel played a crucial role in B uptake from soil under B-limited conditions (Takano et al., 2006, 2010). AtNIP5;1 has been shown to be localized on plasma membrane of lateral root cap (LRC) and epidermal cells (Takano et al., 2010). A ThrProGly (TPG) repeat in the N-terminus of AtNIP5;1 was crucial for its polar localization and effective B transport in roots (Wang et al., 2017). Expression of the AtNIP5;1 was transcriptionally enhanced 10-fold in response to B limitation in roots (Takano et al., 2006). AtNIP6;1 was the most similar gene to AtNIP5;1 among the nine NIP genes in Arabidopsis and played a key role in the preferential translocation of B into young growing leaves (Wallace and Roberts, 2005; Tanaka et al., 2008). Limitation treatment and tracer experiments showed that B concentration were significantly reduced in young rosette leaves and shoot apices (reduced by 20% to 27%) under the conditions of B limitation in atnip6;1 mutants, suggesting that AtNIP6;1 was required for preferential distribution of B to sink tissues (e.g., young rosette leaves, shoot apices; Tanaka et al., 2008). Both AtNIP5;1 and AtNIP6;1 were the boric acid channels on plasma membrane, and AtNIP6;1 was completely impermeable to water and involved in xylem-phloem B transfer (Takano et al., 2006; Tanaka et al., 2008). Unlike AtNIP5;1, B limitation resulted in a slight transcriptional upregulation (1.4-fold) of AtNIP6;1 in stems, but no significant difference was observed in shoots (Tanaka et al., 2008). AtNIP7;1 was also identified as a boric acid channel expressed in floral anthers, functions as a water-tight boric acid permease and also transports glycerol at a lower rate (Li et al., 2011; Routray et al., 2018).

To date, several AtNIP5;1 homologous genes have been identified in different crops species, such as rice (Oryza sativa), rapeseed (Brassica napus) and maize (Zea mays) (Wallace et al., 2006; Durbak et al., 2014; Hua et al., 2016; He et al., 2021a). Rice OsNIP3;1 exhibited the highest degree of similarity to AtNIP5;1 (Wallace et al., 2006), and was expressed in the vascular bundles of both leaf sheaths and blades, as well as in the root exodermis and stele (Figure 2) (Hanaoka et al., 2014). In the nodes, OsNIP3;1 was polarly localized at the xylem parenchyma cells of enlarged vascular bundles (EVBs), facing toward the xylem vessels (Shao et al., 2018). OsNIP3;1 RNAi plants showed disrupted B distribution between leaf blades and sheaths (Hanaoka et al., 2014). Subsequently, it was demonstrated that OsNIP3;1 mediated the unloading B from xylem of EVBs in the nodes, thus promoting its preferential distribution to developing tissues under B-limited conditions (Shao et al., 2018). TLS1/ZmNIP3;1 protein possessed the ability to transport both water and boric acid in Xenopus laevis oocytes, widely expressed across multiple tissue types, with highest levels in floral tissues and particularly in silks (Durbak et al., 2014; Leonard et al., 2014). Two orthologous AtNIP5;1 genes, BnaA2.NIP5;1 and BnaA3.NIP5;1, each with distinct functions, playing a crucial role in the growth of B. napus under B deficiency (He et al., 2021b). BnaA2.NIP5;1 and BnaA3.NIP5;1 functioned coordinately for efficient boron uptake. BnaA2.NIP5;1 was primarily expressed in root epidermal cells, mediated uptake, while BnaA3.NIP5;1 was polar-localized in the distal part of LRC cells and promoted root growth under deficiency to support translocation to the shoot (He et al., 2021a, 2021).

AtBOR1 was an efflux-type B transporter that expressed in pericycle cells of the root stele (Figure 2), functioned in xylem loading and essential for preventing B deficiency in shoots (Noguchi et al., 2000; Takano et al., 2002). Subsequently, six AtBOR1-homology genes were identified in Arabidopsis (Frommer and Wiren, 2002). AtBOR2, the most similar paralog of AtBOR1, functioned in root cell elongation under conditions of B limitation (Miwa et al., 2013). The concentrations of shoot B were lower in atbor2 mutant than that in wild-type plants, but a more significant reduction was observed in atbor1–3 under low-B conditions (Miwa et al., 2013). Thus, the role of AtBOR2 contributed to the root-to-shoot translocation was minor than that of AtBOR1 (Miwa et al., 2013). Additionally, AtBOR4 encoded an efflux-type B transporter localized to the plasma membrane of the distal side of epidermal cells in roots and mitigated toxic levels of B in roots (Miwa et al., 2007; Miwa and Fujiwara, 2011; Miwa et al., 2014).

To date, functional BOR genes have been identified from different plants such as rice (Oryza sativa), wheat (Triticum aestivum), barley (Hordeum vulgare) and maize (Zea mays) (Nakagawa et al., 2007; Reid, 2007; Sutton et al., 2007; Chatterjee et al., 2014). In rice, OsBOR1, a close paralog of AtBOR1, functioned as an efflux transporter for B and played a crucial role in both the xylem loading of B and its uptake into roots (Nakagawa et al., 2007). This function, unlike AtBOR1’s exclusive role in xylem loading, resembled the combined roles of AtBOR1 and AtNIP5;1 in Arabidopsis. A recent study showed that OsBOR1 was highly expressed in the nodes, where it mediated B efflux from cells toward diffuse vascular bundles (DVBs) for delivering B to developing tissues (Shao et al., 2021). OsBOR1 cooperated with OsNIP3;1 to establish a coordinated system for the preferential distribution of B to developing tissues (Shao et al., 2021). In wheat, three functional BORs (TaBOR1.1, TaBOR1.2 and TaBOR1.3) were reported to localize on the plasma membrane in Arabidopsis leaf cells, and exhibit B efflux activity in BY-2 cells (Leaungthitikanchana et al., 2013). In maize, the B efflux transporter ROTTEN EAR (RTE) functioned as a co-ortholog of AtBOR1 and was predominantly expressed in the cells surrounding the xylem within both vegetative and reproductive tissues (Chatterjee et al., 2014); RTE2 encoded a protein similar to its paralog RTE and could completely recover the deficiency of atbor1 mutant in Arabidopsis (Chatterjee et al., 2017). RTE and RTE2 were all predominantly localized on the plasma membrane (Chatterjee et al., 2017).

AtBOR homologous genes have also been isolated from dicotyledonous species, such as grapevine (Vitis vinifera), citrus (Citrus macrophylla) and rapeseed (Brassica napus) (Pérez-Castro et al., 2012; Sun et al., 2012; Cañon et al., 2013). CmBOR1 from C. macrophylla and VvBOR1 from V. vinifera were both homologous to AtBOR1 (Cañon et al., 2013; Pérez-Castro et al., 2012). Functional assays in yeast showed that CmBOR1 mediated B efflux (Cañon et al., 2013), while VvBOR1 localized to the proximal plasma membrane of root pericycle cells and restored the wild-type phenotype in Arabidopsis atbor1–3 mutants under B deficiency (Pérez-Castro et al., 2012). In B. napus, BnaC4.BOR1;1c was widely expressed in shoot nodes and localized to the plasma membrane, displaying characteristics generally similar to AtBOR1 (Zhang et al., 2017); Nevertheless, BnaC4.BOR1;1c showed distinctive features compared with AtBOR1, it was also extensively expressed in immature floral buds, and preferential distribution of B to the reproductive organs (Zhang et al., 2017). Two homologous genes of AtBOR2 were identified in B. napus: BnaC4.BOR2 and BnaA4.BOR2, both of which were mainly localized to the plasma membrane and showed B transport activity in yeast (Liu et al., 2024a, 2024). BnaC4.BOR2, expressed in lateral root caps and steles, was involved in B absorption in roots and its translocation to shoots (Liu et al., 2024b). BnaA4.BOR2 was primarily expressed in the cortex and endodermis of the root tip meristem zone, as well as in the mature endodermis, it facilitated the transport of B from roots to shoots and its distribution within shoots (Liu et al., 2024a).

Conversely, while boric acid is an essential plant micronutrient, excess B inhibits plant growth, impairing various cellular functions and often causes necrosis of tissues (CamachoCristóbal et al., 2008; Landi et al., 2019; Wu et al., 2019). AtBOR4 mediated B efflux and was located on the distal surface of epidermal cells, where it reduced B concentrations in roots and shoots, thereby protecting plants from B accumulation and toxicity (Miwa et al., 2007; Takeda and Matsuoka, 2008). Unlike AtBOR4, OsBOR4 in rice exhibited distinct functional characteristics, showed an anther-specific expression pattern, and was involved in maintaining boron homeostasis during fertilization (Tanaka et al., 2013). Moreover, in barley, borate exporters Bot1/HvBOR2 was responsible for the high B tolerance and protected plants from B accumulation and subsequent toxicity (Miwa et al., 2007; Sutton et al., 2007).

The mRNA levels of AtBOR1 remained largely stable across the tested B conditions, and B translocation from roots to shoots increased under low B and decreased rapidly under high B treatment, suggesting there was post-transcriptional control of AtBOR1 (Takano et al., 2005). The trafficking of AtBOR1 shifted from PM-endosome recycling under B deficiency to endocytosis and vacuolar degradation under high B conditions, thereby regulating B homeostasis (Takano et al., 2010; Kasai et al., 2011). A series of forward studies demonstrated that DYNAMIN-RELATED PROTEIN 1A (DRP1A) and the clathrin adaptor protein ADAPTOR PROTEIN 2 (AP2)-mediated endocytosis maintained the polar localization of BOR1, thereby supporting plant growth under low-B conditions (Yoshinari et al., 2016, 2019). In contrast, boron-induced vacuolar sorting of BOR1 was DRP1-dependent but occurred through an AP2-independent endocytic pathway (Yoshinari et al., 2016, 2019). Additionally, K63-linked polyubiquitination of BOR1 at lysine 590 proved essential for its high B-induced endocytosis and degradation (Yoshinari et al., 2021a). GNOM, a guanine-nucleotide exchange factor (ARF-GEF), mediated endocytosis that contributed to maintaining BOR1 polar localization under boron-limited conditions (Yoshinari et al., 2021b). Similarly, AtBOR2, which was degraded under high B conditions, exhibited cycling behavior between the plasma membrane and endosomes under low B conditions, mirroring the dynamics of AtBOR1 (Miwa et al., 2013). In addition, OsBOR1 underwent gradual degradation in response to high B, however, its degradation pathway differs from that of AtBOR1 (Shao et al., 2021).

In eukaryotes, short open reading frames (ORFs) in the 5’-untranslated region (5’-UTR), known as upstream ORFs (uORFs), are often affected the translation of the downstream ORF (Jackson et al., 2010; Hellens et al., 2016). The 5’-UTR mediated B-dependent AtNIP5;1 mRNA degradation for plant acclimation to high-B conditions (Tanaka et al., 2011). AtNIP5;1 had two minimum ORFs (AUG-stops) in its 5’-UTR, and ribosome stalling at these AUG-stops, which was enhanced under high-B conditions and led to suppressed translation and mRNA degradation, depended on a well-conserved region 12 to 19 nucleotides upstream that acted in enhancing mRNA degradation but not in ribosome stalling (Tanaka et al., 2016). The 5’-UTRs was highly conserved between OsNIP3;1 and AtNIP5;1 (Tanaka et al., 2011). In rice protoplasts, the luciferase activity driven by the 5’UTR of DTE1/OsNIP3;1 exhibited a dual B-dependent response, increasing at 1 μM B but decreasing at 100 μM B, indicating the 5’UTR’s essential role in B-responsive regulation and suggesting an AtNIP5;1-like mRNA control mechanism to prevent excessive B accumulation under high-B conditions (Liu et al., 2015).

In contrast, AtBOR1 protein abundance was regulated through two distinct mechanisms: protein endocytic degradation and B-dependent mRNA level regulation. When the B supply was sufficient (100 μM), AtBOR1 level was down-regulated by endocytic protein degradation (Takano et al., 2005; Kasai et al., 2011). However, at higher B concentrations, AtBOR1 level was decreased further by both translational suppression and protein degradation to avoidance of B toxicity in plants (Aibara et al., 2018). Furthermore, a ribosome profiling analysis revealed that transcripts with reduced translation efficiency under high-B conditions were rich in uORFs, and B played a general role in termination of translation by high B induced global ribosome stalling at the stop codon of main open reading frame (mORFs) (Sotta et al., 2021).

The abundance of B transporters in diverse plant species is coordinately controlled through B-responsive mRNA regulation. The OsBOR1 promoter exhibited a progressive shift in its cell-specific activity between the stele and exodermis under varying B conditions, which reflected its functional adaptation to B availability (Nakagawa et al., 2007). The CTTTC tandem repeats in the BnaA3.NIP5;1 5’UTR negatively regulated its expression, and their deletion enhanced BnaA3.NIP5;1 expression, which promoted root growth and increased seed yield under B limitation (He et al., 2021a). In roots, CmBOR1 expression remained unchanged under both B deficiency and excess conditions, whereas in shoots, its expression was upregulated under B deficiency but unaffected by excess B (Cañon et al., 2013). RT-qPCR analysis of TaBOR1s revealed that the accumulation of TaBOR1.1 and TaBOR1.3 mRNA was up-regulated under B limitation, whereas TaBOR1.2 mRNA accumulation increased under excess B conditions compared with low or normal B conditions in roots (Leaungthitikanchana et al., 2013). In contrast, TaBORs and CmBOR1 exhibited distinct regulation, implying functional diversification among BOR1 genes (Cañon et al., 2013; Leaungthitikanchana et al., 2013). This divergence may reflect species-specific adaptations, particularly in plants with complex genomes, where different BOR1 paralogs could fulfill varied physiological roles.

Transcription factors play pivotal roles in multiple biological processes by activating or repressing the transcription of target genes (Levine and Davidson, 2005). Accumulating evidence has highlighted the importance of transcription factors in responding to nutrient conditions in plants. AtWRKY6 was the first transcription factor reported to involve in the response to B deficiency, with its promoter activity and transcription being induced by low B conditions (Kasajima et al., 2010). BnaA9.WRKY47 positively regulated low-B tolerance through up-regulating BnaA3.NIP5;1 expression to facilitate efficient B uptake (Feng et al., 2020). The Arabidopsis homolog AtWRKY47 acted as a negative regulator that involved in boron homeostasis (Feng et al., 2021). NGATHA-Like 1 (NGAL1, also known as ABNORMAL SHOOT 2, ABS2) was a B-responsive gene regulated in a B-dependent manner through AUG-Stop, similar to AtNIP5;1 (Tanaka et al., 2016). NGAL1 positively regulated the expression of AtBOR1, AtNIP5;1, AtNIP6;1 and AtNIP7;1 in response to low B, and up-regulated AtBOR4 in response to high B to enhance B transport and distribution in both conditions (Tsednee et al., 2022).

Overexpression of AtBOR1 enhanced root-to-shoot translocation of B, and improved shoot growth and fertility under B-deficient conditions but not root growth (Miwa et al., 2006). This was attributed to the degradation of AtBOR1 under high-B supply, and enhanced the translocation of B from root-to-shoot under low-B conditions (Takano et al., 2005; Miwa et al., 2006). Furthermore, tomato (Solanum lycopersicum) plants overexpressing AtBOR1 maintained normal leaf development under B deficiency, and elevated B accumulation in shoots and fruits (Uraguchi et al., 2014). In addition, overexpression of CmBOR1 in Arabidopsis resulted in enhanced shoot growth with limited B supply, as did overexpression of AtBOR1 (Cañon et al., 2013). Moreover, Overexpression of BnaC4.BOR1;1c in the B-inefficient B. napus cultivar W10 alleviated shoot B-deficiency symptoms by improving boron distribution from roots to shoots (Chen et al., 2018).

AtNIP5;1 was a major boric acid channel required for efficient import of B into roots (Takano et al., 2006). Arabidopsis plants with AtNIP5;1 activated by a T-DNA insertion with a enhancer improved root growth under B limitation, but did not improved shoot growth (Kato et al., 2009). Furthermore, introduction of Pro_(35S+NIP5;1):NIP5;1 into the AtBOR1 over expressor improved root elongation, fertility and short-term B uptake under low-B supply (Kato et al., 2009). Elevated BnaA3.NIP5;1 expression improved low-B tolerance in transgenic lines at both seedling and mature stages, and field trials demonstrated that the BnaA3.NIP5;1^Q allele significantly increased seed yield under B deficiency conditions (He et al., 2021a).

Overexpression of AtBOR4 improved growth under conditions of B toxicity through AtBOR4-mediated B efflux that decreased B concentrations in roots and shoots (Miwa et al., 2007). AtBOR4-overexpressing transgenic plants were more capable of expanding leaves and accumulating chlorophyll in shoot tissues under high-B concentration, suggesting overexpressed AtBOR4 alters B distribution in leaves by exporting B from cytoplasm into apoplasm for enhancing high-B tolerance in shoots (Miwa and Fujiwara, 2011). Arabidopsis SHB1/HY1 gene, encoded HO1 (heme oxygenase 1), was up-regulated under excessive B stimulation, and the shb1 seedlings exhibited root inhibition under excessive B treatments (Lv et al., 2017). However, overexpressing SHB1/HY1 or applying the HO1 catalytic products could induced BOR4 transcription, reduced B accumulation in roots and restored primary root growth that confers high B tolerance (Lv et al., 2017).

Moreover, in a B-stress tolerant cultivar ‘Sahara’ of barley, unlike intolerant genotypes, which had four tandem copies of the Bot1 gene with higher transcript levels, and Bot1 expression levels directly correlating with tolerance across various landraces (Hayes and Reid, 2004; Reid, 2007; Sutton et al., 2007; Mickelbart et al., 2015). Similarly, TaBOR2 and HvBOR2 reduced root B concentrations in the tolerant cultivars, and their expression levels showed positive correlations with tolerance (Reid, 2007; Sutton et al., 2007).