Molecular mechanisms of metal-specific root growth responses in Arabidopsis thaliana

Abstract

Background: Understanding more about root development under stress conditions is essential for future crop improvement, which may be aimed at safe biomass production or clean up of polluted soils. Sublethal stress-exposed roots exhibit stress-induced morphogenic responses (SIMRs). However, for the metals cadmium (Cd), copper (Cu) and Zinc (Zc) in Arabidopsis thaliana seedlings, metalspecific growth responses were found instead of the general SIMR. It is hypothesised that LOX-genes are involved in these metal-specific growth adaptations. Objectives: The aim of this study is to participate in unravelling the molecular mechanims underlying these metal-specific growth responses in Arabidopsis thaliana and to understand more about local and systemic effects on root development in a more relevant context of heterogeneous exposure. Methodology: The phenotypic change under metal-stress conditions of lox-mutants (lox1-1, lox3A, lox5-1) compared to wild-type was investigated in split-root systems. Furthermore, the expression of genes involved in plant hormone synthesis or metabolism was examined in metal-exposed wild-type roots using quantitative reverse transcription PCR. Results: Phenotypic comparison of lox-mutants with wild-type after growth in split-root vertical agar plates revealed that Cd, Cu and Zn in the lower zone stimulate lateral root elongation and number in the upper control zone, which can be due to the elevation of the inhibition on lateral root elongation by LOX3 and LOX5. An incomplete primary root avoidance was observed in the Cd- and Zn-containing lower zone. The incomplete avoidance under Cd-exposure may be due to 9-LOX-signalling. LOX-genes did not seem to be involved in primary root growth under Cu-exposure, which may be due to the fact that Cu can induce reactive oxygen species directly. The systemic Zn-effect in wild-types is not present in lox3. The gene expression data revealed an significant increase in AAO1-expression under Cu- and Zn-exposure. Conclusion: LOX-genes are involved in the metal-specific growth responses and have a preference for the metal-free zone under heterogeneous exposure.