Deciphering boron-mediated responses in dicot plant growth : pH-stabilized nitrogen sources, drought, and salinity

Boron is essential for xylem integrity and cell wall pectin. Despite boron's importance for dicot plant growth, environmental cues such as drought and salinity, as well as NH₄⁺ sources, affect boron uptake, mobility, and translocation. This dissertation, for the first time, separates NH₄⁺ and  NO₃^- sources with pH-stabilized conditions and explores the boron uptake mechanism. pH-stabilized NH₄⁺-fed nutrition aligns better with rapeseed growth than NO₃^--fed nutrition for boron uptake. Facilitated diffusion of boron via BnaNIP5;1 enables distribution from root to shoot. BnaBOR1;2 mediates active uptake in the root under low-boron conditions. Water deficit conditions were assessed by tracking the mobility of foliar boron (¹⁰B) under boron conditions [sufficient vs deficient (¹¹B)] in sugar beet. Sufficient ¹¹B allowed foliar-applied ¹⁰B to translocate from source to sink tissues. This was influenced by increased sucrose accumulation and sink strength, by enhancing leaf antioxidant defenses (CAT and AsA-GSH cycle enzymes) against oxidative damage (ROS). Further, imbalanced plant water relations with higher Na⁺ accumulation interact with negatively charged pectin polymers. We revealed that NaCl stress affects arabinogalactan proteins (AGPs) and causes cross-linking defects in rhamnogalacturonan-II (RG-II), resulting in softening of the sugar beet plant’s cell wall. Adequate boron compensates for plant growth by reducing the Na^+/Ca²⁺ ratio in young leaves, with increasing RG-I content and dimeric pectin RG-II. This key cell wall component provides structural integrity. In addition, increased xylem translocation of exogenous boron (¹⁰B) facilitated the synthesis and transport of osmolytes (sucrose; BvSPS1 and BvSUT1) in the salt-grown leaf. Thus, water balance is facilitated by lowering the Na⁺/K⁺ ratio through increased expression of BvNHX8, BvSOS1, and BvHKT1;3 in the roots and BvPIP2;2, BvNIP5;1, and BvPM4 in the leaf. This aligns with the expression of cell wall-loosening and expansion candidates, expansins (BvEXPA12 and BvEXP26), and xyloglucan endotransglucosylases (BvXET8 and BvXET22). Altogether, this doctoral dissertation offers new insights into boron dynamics in dicotyledonous plants and evaluates water balance in relation to cell growth and leaf expansion.