Alleviates Excess Boron Stress by Increasing Expression Level and Maintaining Boron Homeostasis in Arabidopsis Roots.

Boron is an essential mineral nutrient for higher plant growth and development. However, excessive amounts of boron can be toxic. Here, we report on the characterization of an Arabidopsis mutant, (), which exhibits hypersensitivity to excessive boron in roots. Positional cloning demonstrated that the mutant bears a point mutation in a gene encoding a heme oxygenase 1 (HO1) corresponding to the gene involved in photomorphogenesis. The transcription level of the gene in roots is up-regulated under excessive boron stimulation. Either overexpressing or applying the HO1 inducer hematin reduces boron accumulation in roots and confers high boron tolerance. Furthermore, carbon monoxide and bilirubin, catalytic products of HO1, partially rescue the boron toxicity-induced inhibition of primary root growth in . Additionally, the mRNA level of , a boron efflux transporter, is reduced in roots with high levels of boron supplementation, and hematin cannot relieve the boron toxicity-induced root inhibition in mutants. Taken together, our study reveals that HO1 acts via its catalytic by-products to promote tolerance of excessive boron by up-regulating the transcription of the gene and therefore promoting the exclusion of excessive boron in root cells.