Roles of the OsbHLH156-OsIRO2 transcription factor complex in manganese,
copper and zinc transport in rice
Abstract
Iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn) are essential
micronutrients that are necessary for plant growth and development, but
can be toxic at supra-optimal levels. Plants have evolved a complex
homeostasis network that includes uptake, transport, and storage of
these metals. Previously, we reported that the OsbHLH156-OsIRO2
transcription factor (TF) complex is activated under Fe deficient
condition and acts as a central regulator on Strategy II Fe acquisition.
In this study, the role of the TF complex on Mn, Cu and Zn uptake was
evaluated. Fe deficiency caused significant increased Mn, Cu and Zn
concentrations in shoot. The increase of these divalent metal
concentrations was significantly suppressed in osbhlh156 and
osiro2 mutants, suggesting that the TF complex plays roles on Mn,
Cu and Zn uptake and transport. RNA-sequencing assay showed that the
genes associated with Mn, Cu, and Zn uptake and transport were
significantly suppressed in the osbhlh156 and osiro2
mutants. Transcriptional activation assays demonstrated that the TF
complex could directly bind to the promoters of OsIRT1,
OsYSL15, OsNRAMP6, OsHMA2, OsCOPT1/7 and
OsZIP5/9/10 and activate their expression. In addition, the TF
complex is required to activate the expression of NA and 2’deoxymugineic
acid (DMA) synthesis genes, which in turn facilitate the uptake and
transport of Mn, Cu and Zn. Furthermore, OsbHLH156 and OsIRO2 promote Cu
accumulation to partially restore the Fe-deficiency symptom. Taken
together, the OsbHLH156-OsIRO2 TF complex functions as a core regulator
not only in Fe homeostasis, but also in Mn, Cu and Zn accumulation.