The polar-localized borate exporter BOR1 facilitates boron transport in tapetal cells to the developing pollen grains

Abstract

Boron is an essential micronutrient required for plant cell wall integrity, as it is necessary for crosslinking the pectic polysaccharide rhamnogalacturonan II. Reproductive organs require a greater amount of boron for development and growth compared to vegetative organs. However, the mechanism by which plants distribute boron to specific organs is not fully understood. Under boron-limited conditions, the borate exporter BOR1 plays a central role in transporting boron from the roots to the shoots in Arabidopsis (Arabidopsis thaliana). Here, we found that BOR1 is expressed in the tapetal cells of young anthers in unopened buds, showing polar localization toward the locule where microspores develop. Tapetum-localized BOR1 undergoes endocytosis and is subsequently degraded during anther development. BOR1 degradation occurs independently of the lysine residue at position 590 of BOR1, which is responsible for high boron-induced ubiquitination and degradation. Loss-of-function bor1 mutants exhibit disrupted pollen structure, causing reduced fertility under boron-sufficient conditions in the wild type. These phenotypes were rescued by supplementing with high boron concentrations. Furthermore, inflorescence stem grafting experiments suggested that BOR1-dependent boron transport in the flower is necessary for pollen development and subsequent fertilization under boron-sufficient conditions. Our findings suggest the borate exporter BOR1, together with the previously described boric acid channel NIP7;1, facilitates boron transport in tapetal cells toward the locule, thereby supporting pollen development in young anthers under boron-limited conditions.