title: Genetic and physiological analysis of tolerance to acute iron toxicity in rice.
abstract: BACKGROUND
  Fe toxicity occurs in lowland rice production due to excess ferrous iron (Fe(2+)) formation in reduced soils. To contribute to the breeding for tolerance to Fe toxicity in rice, we determined quantitative trait loci (QTL) by screening two different bi-parental mapping populations under iron pulse stresses (1,000 mg L(-1) = 17.9 mM Fe(2+) for 5 days) in hydroponic solution, followed by experiments with selected lines to determine whether QTLs were associated with iron exclusion (i.e. root based mechanisms), or iron inclusion (i.e. shoot-based mechanisms).
  
  
  RESULTS
  In an IR29/Pokkali F8 recombinant inbred population, 7 QTLs were detected for leaf bronzing score on chromosome 1, 2, 4, 7 and 12, respectively, individually explaining 9.2-18.7% of the phenotypic variation. Two tolerant recombinant inbred lines carrying putative QTLs were selected for further experiments. Based on Fe uptake into the shoot, the dominant tolerance mechanism of the tolerant line FL510 was determined to be exclusion with its root architecture being conducive to air transport and thus the ability to oxidize Fe(2+) in rhizosphere. In line FL483, the iron tolerance was related mainly to shoot-based mechanisms (tolerant inclusion mechanism). In a Nipponbare/Kasalath/Nipponbare backcross inbred population, 3 QTLs were mapped on chromosomes 1, 3 and 8, respectively. These QTLs explained 11.6-18.6% of the total phenotypic variation. The effect of QTLs on chromosome 1 and 3 were confirmed by using chromosome segment substitution lines (SL), carrying Kasalath introgressions in the genetic background on Nipponbare. The Fe uptake in shoots of substitution lines suggests that the effect of the QTL on chromosome 1 was associated with shoot tolerance while the QTL on chromosome 3 was associated with iron exclusion.
  
  
  CONCLUSIONS
  Tolerance of certain genotypes were classified into shoot- and root- based mechanisms. Comparing our findings with previously reported QTLs for iron toxicity tolerance, we identified co-localization for some QTLs in both pluse and chronic stresses, especially on chromosome 1.
authors.affiliation: Plant Nutrition, Institute for Crop Science and Resource Conservation (INRES), University of Bonn, Karlrobert-Kreiten-Straße 13, Bonn, 53115, Germany.
authors.forename: Lin-Bo
authors.initials: LB
authors.lastname: Wu
authors.affiliation: Plant Nutrition, Institute for Crop Science and Resource Conservation (INRES), University of Bonn, Karlrobert-Kreiten-Straße 13, Bonn, 53115, Germany.
authors.forename: Mohamad Yusser
authors.initials: MY
authors.lastname: Shhadi
authors.affiliation: Plant Breeding, Genetics and Biotechnology Division, International Rice Research Institute, Los Baños, Philippines.
authors.forename: Glenn
authors.initials: G
authors.lastname: Gregorio
authors.affiliation: Plant Nutrition, Institute for Crop Science and Resource Conservation (INRES), University of Bonn, Karlrobert-Kreiten-Straße 13, Bonn, 53115, Germany.
authors.forename: Elsa
authors.initials: E
authors.lastname: Matthus
authors.affiliation: Plant Nutrition, Institute for Crop Science and Resource Conservation (INRES), University of Bonn, Karlrobert-Kreiten-Straße 13, Bonn, 53115, Germany.
authors.forename: Mathias
authors.initials: M
authors.lastname: Becker
authors.affiliation: Plant Nutrition, Institute for Crop Science and Resource Conservation (INRES), University of Bonn, Karlrobert-Kreiten-Straße 13, Bonn, 53115, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Frei
keywords: Iron toxicity; Oryza sativa L.; Quantitative trait locus; Reactive oxygen species; Tolerance mechanism