Insights into the Mechanisms Underlying Boron Homeostasis in Plants

Abstract

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.

Overexpression of X Intrinsic Protein 1;1 in Nicotiana tabacum and Arabidopsis reduces boron allocation to shoot sink tissues

“…In respect to B translocation from the soil to the shoot it was shown that both the AtNIP5;1 boric acid channel as well as the active xylem loading transporter AtBOR1 have to be localized polarly in specific root cell types to maintain the important directional nutrient transport of B (Takano, Miwa, Yuan, von Wirén, & Fujiwara, ; Wang et al., ). Recently it was speculated, that improper localization of AtNIP5;1 at the stele side of the cells can impair the B gradient from the cytosol to the apoplast generated by AtBOR1 (Yoshinari & Takano, ). Ubiquitous overexpression of the physiologically active B channel NtXIP1;1 in roots will accordingly disturb local B gradients, important for generating the directional flow of B towards the shoot.…”

Efficient and flexible uptake system for mineral elements in plants

“…Recently, this co‐operative uptake system has been reported for several elements. For example, boron (B) uptake in Arabidopsis is mediated by AtNIP5;1 (influx) and AtBOR1/2 (efflux) at the epidermal cells, and by AtNIP5;1 and AtBOR1 (efflux) at the endodermis of the mature root zone (Takano et al ., , ; Miwa et al ., ; Yoshinari & Takano, ). In rice, OsLsi1 (influx) and OsLsi2 (efflux) are responsible for Si uptake (Ma et al ., , ), and OsNramp5 (influx) and OsMTP9 (efflux) are required for efficient Mn uptake (Sasaki et al ., ; Ueno et al ., ).…”

Establishment of genetically encoded biosensors for cytosolic boric acid in plant cells

“…The B concentrations in shoots and roots showed an overall positive correlation with those in the medium, although the tissue B concentration was maintained at > 40 μmol/kg fresh weight in the presence of <30 μ m B (Figure (g)). This phenomenon is consistent with previous reports of accumulation of B via B transport proteins (Noguchi et al ., ; Takano et al ., ; Yoshinari and Takano, ). Importantly, a negative correlation was observed between the Venus protein level and the B concentration in the range ~100–1000 μmol/kg fresh weight (Figure (h)).…”

“…Therefore, passive diffusion of boric acid represents an important mode of B transport in plants. However, to maintain B at the appropriate level in tissues under both limited and excess B conditions, plants utilize several boric acid/borate transporters (Yoshinari and Takano, ). In roots of Arabidopsis thaliana , a boric acid channel, NIP5;1, and a borate exporter, BOR1, are required for efficient B uptake and subsequent translocation into the xylem under limited B conditions (Takano et al ., , ).…”