Boron (B) deficiency is a highly prominent nutrient disorder. While B-efficient accessions have recently been identified in the highly B-demanding crop oilseed rape, it remained unclear which physiological processes underlie B efficiency and which signaling pathways trigger an efficient B-deficiency response. Here, we compared, under three different B supply conditions, two Brassica napus accessions with contrasting B efficiency. Shoot biomass formation, B distribution patterns and metabolic dynamics of different phytohormone species were studied using a combination of mass spectrometry-based analyses and physiological measurements. Our results show that the B-efficient accession CR2267 does not differ from the B-inefficient accession CR2262 in terms of B accumulation and subcellular B-partitioning, although it displays no morphological B-deficiency symptoms under severe B-deficient conditions. Investigating phytohormone metabolism revealed a strong accumulation of cytokinins in CR2267 at a developmental stage when striking B-dependent differences in biomass and organ formation emerge in the two B. napus accessions. In contrast, elevated levels of the stress hormone abscisic acid as well as bioactive auxins, representing functional antagonists of cytokinins in shoots, were detected only in CR2262.Our results indicate that superior B efficiency in CR2267 relies on a higher B utilization efficiency that builds on an earlier and higher cytokinin biosynthesis required for the maintenance of the shoot meristem activity and proper leaf development. We further conclude that an elevated abundance of cytokinins is not a consequence of better plant growth but rather a presumption for better plant growth under low-B conditions. | INTRODUCTIONBoron is an essential element for vascular plants (Marschner, 2012;Warington, 1923;Wimmer et al., 2020). The only well-established biochemical function of B with physiological relevance for plants is its role in maintaining the integrity of cell walls. Boron forms di-ester bonds with apiose residues of the polysaccharide rhamnogalacturonan II (RG-II), thereby crosslinking two RG-II monomers in the cell wall's pectin layer (O'Neill et al., 2004). These RG-II-B-RG-II complexes serve as space holders between cellulose fibers and add porosity and flexibility to the tight mesh formed by the remaining pectin This article is dedicated to Dr Kai Eggert, IPK Gatersleben, who significantly contributed to this study but deceased on August 13, 2018.