Phytohormones in plant responses to boron deficiency and toxicity

Boron (B) is an essential element for plant growth. Many agricultural soils around the globe have either insufficient or excessive amounts of available B, with major implications for crop production. Understanding major limitations imposed by B nutritional disorders may allow breeding crops for improved B use efficiency as well as make them more resilient to excessive B, thus reducing yield penalties. It has become apparent that B-related physiological disorders are mediated in large part by their impact on plant hormone production and signaling. The aim of this review is to summarize current knowledge of the roles of hormones in plant responses to B and their impact on plant growth and development. The most significant effect of B deficiency is the inhibition of root elongation. Boron deficiency promotes the redistribution of auxin in the root elongation zone. Together with cytokinin signals and ethylene, this redistribution and modulation of auxin content triggers inhibition of the root cell elongation. Under B deficiency, root development is also regulated by brassinosteroids and jasmonic acid. Excess B can induce the production of reactive oxygen species (ROS). Abscisic acid and salicylic acid are both produced in response to B toxicity, and both can induce the antioxidant defense system to detoxify ROS. Another adaptation to B toxicity involves changes in the expression levels and activity of aquaporins in roots, thus reducing the uptake of water and delivery of B into the transpiration stream. In addition, abscisic acid mediates stomatal closure to further limit transpiration and the consequent accumulation of B in leaves.