Boron (B) is an essential micronutrient for plant growth and development. Its major physiological function is to maintain the structure of the cell wall by crosslinking pectic polysaccharides through borate-diol bonding of two rhamnogalacturonan II molecules. B is immobile in most plant species. Therefore, a continuous supply of B is required to maintain growth of newly developing tissues and deficiency of B will result in the cessation of root elongation, reduced leaf expansion, and loss of fertility. On the other hand, B also shows toxicity to plants when present in excess. Both B deficiency and toxicity cause crop losses in many areas of the world. B is preferentially delivered to developing tissues in many plant species under conditions of low or no transpiration but the molecular mechanisms underlying the preferential distribution of B to these developing tissues are poorly understood. Here, Shao et al. (10.1104/pp.17.01054) present evidence that a member of nodulin 26-like intrinsic protein (NIP), OsNIP3;1, is involved in this preferential distribution in rice. OsNIP3;1 was highly expressed in the nodes and its expression was up-regulated by B deficiency, but down-regulated by high B. OsNIP3;1 was localized at the xylem parenchyma cells of enlarged vascular bundles of nodes facing toward the xylem vessels. Furthermore, this protein was rapidly degraded within a few hours in response to high B. Knockout of this gene scarcely affected the uptake and root-to-shoot translocation of B, but altered B distribution in different organs in the shoot. These results indicate that OsNIP3;1 located in the nodes is involved in the preferential distribution of B to the developing tissues by unloading B from the xylem in rice and that it is regulated at both the transcriptional and protein level in response to external B level.
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