AUTHOR=Lv Qiang , Wang Lei , Wang Jin-Zheng , Li Peng , Chen Yu-Li , Du Jing , He Yi-Kun , Bao Fang 

TITLE=SHB1/HY1 Alleviates Excess Boron Stress by Increasing BOR4 Expression Level and Maintaining Boron Homeostasis in Arabidopsis Roots

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.00790

DOI=10.3389/fpls.2017.00790

ISSN=1664-462X

ABSTRACT=<p>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, <italic>shb1</italic> (<italic>sensitive to high-level of boron 1</italic>), which exhibits hypersensitivity to excessive boron in roots. Positional cloning demonstrated that the <italic>shb1</italic> mutant bears a point mutation in a gene encoding a heme oxygenase 1 (HO1) corresponding to the <italic>HY1</italic> gene involved in photomorphogenesis. The transcription level of the <italic>SHB1/HY1</italic> gene in roots is up-regulated under excessive boron stimulation. Either overexpressing <italic>SHB1/HY1</italic> 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 <italic>shb1</italic>. Additionally, the mRNA level of <italic>BOR4</italic>, a boron efflux transporter, is reduced in <italic>shb1</italic> roots with high levels of boron supplementation, and hematin cannot relieve the boron toxicity-induced root inhibition in <italic>bor4</italic> 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 <italic>BOR4</italic> gene and therefore promoting the exclusion of excessive boron in root cells.</p>