An Aluminum-Inducible IREG Gene is Required for Internal Detoxification of Aluminum in Buckwheat

Buckwheat (Fagopyrum esculentum Moench) is able to detoxify aluminum (Al) both externally and internally, but the molecular mechanisms underlying its high Al tolerance are not understood. We functionally characterized a gene (FeIREG1) belonging to IRON REGULATED/ferroportin in buckwheat, which showed high expression in our previous genome-wide transcriptome analysis. FeIREG1 was mainly expressed in the roots, and its expression was up-regulated by Al, but not by other metals and low pH. Furthermore, in contrast to AtIREG1 and AtIREG2 in Arabidopsis, the expression of FeIREG1 was not induced by Fe deficiency. Spatial expression analysis showed that the Al-induced expression of FeIREG1 was found in the root tips and higher expression was detected in the outer layers of this part. Immunostaining also showed that FeIREG1 was localized at the outer cell layers in the root tip. A FeIREG1-green fluorescent protein (GFP) fusion protein was localized to the tonoplast when transiently expressed in onion epidermal cells. Overexpression of FeIREG1 in Arabidopsis resulted in increased Al tolerance, but did not alter the tolerance to Cd, Co and Fe. The tolerance to Ni was slightly enhanced in the overexpression lines. Mineral analysis showed that the accumulation of total root Al and other essential mineral elements was hardly altered in the overexpression lines. Taken together, our results suggest that FeIREG1 localized at the tonoplast plays an important role in internal Al detoxification by sequestering Al into the root vacuoles in buckwheat.