Correlation between plant cell wall stiffening and root extension arrest phenotype in the combined abiotic stress of Fe and Al

The plasticity and growth of plant cell walls (CWs) remain poorly understood at the molecular level. In this work, we used atomic force microscopy (AFM) to observe elastic responses of the root transition zone of 4-day-old Arabidopsis thaliana wild type and almt1 mutant seedlings grown under Fe or Al stresses. The elastic parameters were deduced from force-distance measurements by AFM using the trimechanic-3PCS framework. In all metal stresses tested, the presence of single metal species Fe2+ or Al3+ at 10 {micro}M exerts no noticeable effect on the root growth compared with the control conditions. On the contrary, a mix of both the metal ions produced a strong root extension arrest concomitant with significant increase of CW stiffness. This was not found for the almt1 mutant which substantially abolishes the ability to exude malate. By raising the concentration of either Fe2+ or Al3+ to 20 {micro}M, no root extension arrest was observed; nevertheless, a rise of root stiffness occurred. Our results indicate that the combination of Fe2+ and Al3+ with exuded malate is crucial for both CW stiffening and root extension arrest. However, stiffness increase induced by single Fe or Al metal is not sufficient for arresting root growth. Summary statementWe record the change in stiffness of the external primary cell wall of living Arabidopsis thaliana seedlings in presence of metallic stress using atomic force microscopy. Results reveals for the first time the uncoupling between mechanical response (CW stiffening) and root extension arrest.