From outside to inside: mechanisms modulating plant responses to boron stress

Almost a century ago the essentiality of boron (B) was established and its functions in several physiological processes have been elucidated. B is heterogeneously distributed through different soils, leading to intriguing problems associated with its deficiency and toxicity in agricultural systems. The narrow range between deficiency and toxicity of B coupled with the differential crop requirement for this element, suggest that enhancing B use efficiency is a promising strategy for mitigating the consequences of B availability on crop yields. Several disorders in plant anatomy (e.g., changes in vascular cells structure, the thickness of the mesophyll tissues and cuticle), and physiology (e.g., changes in sugar metabolism, cell division, hormone metabolism, protein breakdown, and antioxidant activity) are caused by B stress. Plants have a diverse set of strategies to avoid or tolerate B deficiency and toxicity, retaining nutritional homeostasis with normal growth. Here, we discuss these physiological mechanisms and adaptive strategies used by plants to cope with B stress conditions. These adaptive strategies involve complex morphoanatomical and physiological changes orchestrated by a fine-tuning of metabolic and hormonal pathways associated with molecular networks regulation. Mechanisms that allow the control of uptake efficiency, which include root architecture modifications are associated with the regulation of B transporter proteins. Changes in the structure of the cell walls and membranes, the accumulation of actin and tubulin, signaling of reactive oxygen species (ROS), changes in redox status and sugar-related metabolism are remarkable in the response to B stress and will be discussed. Here, we highlight the current state of research on the role of gene expression, metabolism and signaling in response to B stress as adaptive responses. The further identification and characterization of transcription factors involved in the control of the B homeostasis and responses offer an interesting opportunity to modulate the expression of B-responsive genes, with the ultimate aim of understanding the entire signaling networks that underpin B stress responses. Furthermore, we highlight how and to which extent the understanding of B stress responses contribute to optimum growth and crop production with biotechnological applications allowing for the development of transgenic plants.