OsSPL9 promotes Cu uptake and translocation in rice grown in high-Fe red soil

Most rice varieties are able to grow in red high-Fe soil, but the underlying mechanism remains elusive. Through forward genetic screening, we identified a red soil-sensitive-1 (rss1) mutant that exhibited severely retarded growth when grown in red soil but showed no evident phenotype in cinnamon soil. Under the red soil/high-Fe conditions, rss1 exhibited increased Fe but decreased copper (Cu) concentrations in both roots and shoots, and the rss1 phenotype was partially rescued by Cu supplement. RSS1 encodes an OsSPL9 transcription factor that is expressed in pericycle cells and parenchyma cells surrounding xylem in roots. Under high-Fe conditions, OsSPL9 activated expression of Cu transporters, including OsYSL16, OsCOPT1, and OsCOPT5 by binding to their promoters, and OsYSL16 overexpression partially rescued rss1 defects. We thus propose that OsSPL9 overcomes high-Fe imposed Cu deficiency by activating the expressions of Cu transporter genes, allowing rice to adapt to red soil.