Involvement of the ethylene-signalling pathway in sugar-induced tolerance to the herbicide atrazine in Arabidopsis thaliana seedlings

Soluble sugars can induce tolerance to otherwise lethal concentrations of the herbicide atrazine in Arabidopsis thaliana seedlings. This sugar-induced tolerance involves modifications of gene expression which are likely to be related to sugar and xenobiotic signal transduction. Since it has been suggested that ethylene- and sugar-signalling pathways may interact, the effects of glucose (Glc) and sucrose (Suc) on seedling growth and tolerance to atrazine were analysed in etr1-1, ein2-1, ein4, and sis1/ctr1-12 ethylene-signalling mutant backgrounds, where key steps of ethylene signal transduction are affected. Both ethylene-insensitive and ethylene-constitutive types of mutants were found to be affected in sugar-induced chlorophyll accumulation and root growth and in sugar-induced tolerance to atrazine. Interactions between ethylene and sugars were thus shown to take place during enhancement of seedling growth by low-to-moderate (up to 80 mM) sugar concentrations. The strong impairment of sugar-induced atrazine tolerance in etr1-1, ein2-1, and ein4 mutants demonstrated that this tolerance required active signalling pathways and could not be ascribed to mere metabolic effects nor to mere growth enhancement. Sugar-induced atrazine tolerance thus seemed to involve activation by sugar and atrazine of hexokinase-independent sugar signalling pathways and of ethylene signalling pathways, resulting in derepression of hexokinase-mediated Glc repression and in induction of protection mechanisms against atrazine injury.