Introgression of genes and mechanisms conferring tolerance to iron toxicity from wild rice species Oryza meridionalis into domesticated rice Oryza sativa

Iron toxicity is one of the most common mineral disorders affecting Oryza sativa production in flooded lowland fields. Efforts have been made to develop new rice varieties tolerant to Fe toxicity (+Fe). Oryza meridionalis is an endemic from Northern Australia and grows in regions with Fe rich soils, which may provide Fe tolerance genes and mechanisms that can be used for adaptive breeding. Aiming to understand tolerance mechanisms in rice, we screened a population of interspecific introgression lines (IL) from a cross between O. sativa and O. meridionalis for the identification of QTLs contributing to Fe excess tolerance. Six putative QTLs were identified. A line carrying one introgression from O. meridionalis on chromosome 9 associated with one QTL for leaf bronzing score was identified as tolerant in terms of lipid peroxidation and electrolyte leakage despite presenting very high shoot Fe concentrations. Further physiological, biochemical, ionomic and transcriptomic analyses showed that the IL tolerance could be partly explained by Fe partitioning between the leaf sheath and culm. After the in silico construction of an interspecific hybrid genome to map the sequences from transcriptomic analysis, we identified 47 and 27 genes from O. meridionalis up and down-regulated, respectively, by Fe treatment on the tolerant IL. Among possible genes associated with shoot-based tolerance, we identified metallothionein-like proteins, genes from glutathione S-transferase family and transporters from ABC and Major Facilitator Superfamily. This is the first work to demonstrate that introgressions of O. meridionalis in O. sativa genome confer increased tolerance to +Fe Overall design: We analyzed the shoot transcriptomic response to Fe excess in two species from the Oryza genus: Oryza sativa (cv. Curinga) and Oryza meridionalis (WG 2112), and in one introgression line from a cross between these two species, CM23. Plants were grown in greenhouse conditions under control and Fe toxicity treatments for three weeks and then exposed to treatment. Three biological replicates were collected after seven days of treatment. Total RNA was extracted from shoots and libraries were generated and sequenced at BGI Genomics Co. using HiSeq Illumina 4000.