Towards the discovery of novel genetic component involved in stress resistance in Arabidopsis thaliana.

The exposure of plants to high concentrations of trace metallic elements such as copper involves a remodeling of the root system, characterized by a primary root growth inhibition and an increase in the lateral root density. These characteristics constitute easy and suitable markers for screening mutants altered in their response to copper excess. A forward genetic approach was undertaken in order to discover novel genetic factors involved in the response to copper excess. A Cu(2+) -sensitive mutant named copper modified resistance1 (cmr1) was isolated and a causative mutation in the CMR1 gene was identified by using positional cloning and next-generation sequencing. CMR1 encodes a plant-specific protein of unknown function. The analysis of the cmr1 mutant indicates that the CMR1 protein is required for optimal growth under normal conditions and has an essential role in the stress response. Impairment of the CMR1 activity alters root growth through aberrant activity of the root meristem, and modifies potassium concentration and hormonal balance (ethylene production and auxin accumulation). Our data support a putative role for CMR1 in cell division regulation and meristem maintenance. Research on the role of CMR1 will contribute to the understanding of the plasticity of plants in response to changing environments.

Combined transcriptomic and physiological approaches reveal strong differences between short- and long-term response of rice (Oryza sativa) to iron toxicity.

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) FeSO(4) 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.

Response to copper excess in Arabidopsis thaliana: Impact on the root system architecture, hormone distribution, lignin accumulation and mineral profile.

Growth, in particular reorganization of the root system architecture, mineral homeostasis and root hormone distribution were studied in Arabidopsis thaliana upon copper excess. Five-week-old Arabidopsis plants growing in hydroponics were exposed to different Cu(2+) concentrations (up to 5 muM). Root biomass was more severely inhibited than shoot biomass and Cu was mainly retained in roots. Cu(2+) excess also induced important changes in the ionome. In roots, Mg, Ca, Fe and Zn concentrations increased, whereas K and S decreased. Shoot K, Ca, P, and Mn concentrations decreased upon Cu(2+) exposure. Further, experiments with seedlings vertically grown on agar were carried out to investigate the root architecture changes. Increasing Cu(2+) concentrations (up to 50 muM) reduced the primary root growth and increased the density of short lateral roots. Experiment of split-root system emphasized a local toxicity of Cu(2+) on the root system. Observations of GUS reporter lines suggested changes in auxin and cytokinin accumulations and in mitotic activity within the primary and secondary root tips treated with Cu(2+). At toxic Cu(2+) concentrations (50 muM), these responses were accompanied by higher root apical meristem death. Contrary to previous reports, growth on high Cu(2+) did not induce an ethylene production. Finally lignin deposition was detected in Cu(2+)-treated roots, probably impacting on the translocation of nutrients. The effects on mineral profile, hormonal status, mitotic activity, cell viability and lignin deposition changes on the Cu(2+)-induced reorganization of the root system architecture are discussed.