Ferrous iron toxicity is a mineral disorder frequently occurring under waterlogged soils where rice is cultivated. To decipher the main metabolic pathways involved in rice response to iron excess, seedlings have been exposed to 125 mg L -1 FeSO4 for 3 weeks. A combined transcriptomic, biochemical and physiological study has been performed after short-term (3 d) or long-term (3 weeks) exposure to iron in order to elucidate the strategy of stress adaptation with time. Our results showed that short-and long-term exposure involved a very different response in gene expression regarding both the number and function. A larger number of genes were up-or down-regulated after 3 d than after 3 weeks of iron treatment; these changes also occurred in shoot even though no significant difference in iron concentration was recorded. Those modifications in gene expression after 3 d affected not only genes involved in hormonal signalling but also genes involved in C-compound and carbohydrate metabolism, oxygen and electron transfer, oxidative stress, and iron homeostasis and transport. Modification in some gene expression can be followed by modification in corresponding metabolic products and physiological properties, or differed in time for some others, underlying the importance of an integrated study.

Lowland rice is often subject to iron toxicity which may lead to yield reduction. In order to cope with this nutrient disorder, plants have developed resistance strategies. The aim of this research was to assess morphological and physiological parameters linked to iron toxicity resistance mechanisms and to identify quantitative trait loci (QTLs) involved in their genetic determinism. A segregating population consisting of 164 recombinant inbred lines (RILs) derived from a cross between Azucena and IR64 was tested twice in hydroponics at the vegetative stage at 0 and 250 mg Fe 2? l -1 . Morphological traits were measured on all 164 RILs. Physiological traits, which were too time-consuming to allow their measurement on all the population, were measured on the two parents and extreme individuals only, selected on the basis of their leaf bronzing index and shoot dry weight. A total of 24 putative QTLs was identified on chromosomes 1, 2, 3, 4, 7 and 11 for leaf bronzing index, shoot water content, shoot and root dry weight, relative variation of shoot and root dry weight, shoot iron concentration, stomatal resistance and chlorophyll content index. Several QTLs were detected in overlapping regions for different parameters. The pertinence of phenotyping extreme RILs only for a QTL analysis is discussed in this study. The QTL analysis allowed to better understand the physiological response of rice in the presence of an excess of ferrous iron, inclusive the relations existing between the stomata closure, the shoot water content reduction and the oxidative stress linked to these growth conditions.

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Lowland rice {Oryza sativa L.) is often affected by iron toxicity, which may iead to yield losses. One important constraint in the study of the inheritance of resistance strategies to this stress is the inconsistency of gene expression across different environments. This study aimed to determine the stability of quantitative trait loci (QTL) across several environments. Quantitative trait loci mapping for traits related to resistance mechanisms had been previously performed using 164 recombinant inbred lines derived from 'Azucena' and 'IR64' screened in hydroponics in a phytotron. In the present study, this population was tested under excessive ferrous iron conditions in three additional environments: in hydroponics in a greenhouse, on washed sand, and in the field. Altogether, 44 putative QTL were identified in the four single QTL analyses for morphological, physiological, and agronomic traits. From these 44 QTL, 20 were found in overlapping regions for the same or related traits in different environments, identifying six genomic regions of great interest for the inheritance of resistance to iron toxicity. Quantitative trait loci stability across environments was also checked by performing a joint QTL analysis, which confirmed the position of nine QTL previously found in the same or adjacent regions by at least one single analysis. Combining both single and joint analyses helps in separating QTL specific to a particular environment from generally expressed ones thus is more suitable for marker-assisted selection.

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