10 plus 10 does not make 20 : the stress response of Arabidopsis thaliana seedlings to Fe and Al metals

Seedling roots, post-germination, need to grow quickly to anchor the growing plant in the soil. However, the growth is regulated by ascertaining that the environment is satisfactory for the plant. When encountering a stress in the soil, roots have a very limited number of options : it can stop growing, it can develop secondary roots, it can change direction. Often, all these options are combined. Plant roots grow by cell division and cell elongation. One or both processes can be altered during stress and a plant root may or may not continue to grow. The relationship between growth and growth arrest lacks a clear understanding at the molecular level.

In a previous work, the Pellequer team (IBS/Methods and Electron Microscopy Group) has shown that root extension arrest due to the presence of iron was concomitant with a stiffening of the external primary cell wall of 4-day-old Arabidopsis thaliana seedlings. However, in this work, serendipity led us not only to evaluate the stiffening of plant roots under Fe, but also under Al stress, as well as their combining effect.

To perform precise mechanical measurements of such stiffening on living seedlings, the researchers recently developed an improved robust protocol (https://doi.org/10.1016/j.xpro.2023.102265). In this current work, they have shown that using a stress of 10 µM FeCl2 or 10 µM AlCl3 did not modify the stiffness of plant roots nor modified the root growth phenotype. However, by combining 10 µM FeCl2 with 10 µM AlCl3, they obtained a total root extension arrest and a significant increase in external plant cell wall stiffness. They decided to stress Arabidopsis thaliana with 20 µM of FeCl2 or 20 µM of AlCl3. It was a surprise to notice that although no root extension arrest was observed (plants continue to grow at least for several days), a significant increase in the stiffness of the cell wall was measured (similarly to previous mixed metal stress).
To summarize, they understand that the external primary cell wall stiffening is a good detector of plant stress. They found that cell wall stiffening was not a trigger for root extension arrest. We proposed a mechanism that involves the binding of metals to negatively charged pectin for increasing the cell wall stiffness. They also conclude that in the presence of Al and Fe, the malate acts as a hyper-accumulator of Fe in the cell wall apoplast, which triggers a reactive oxygen species response leading to the growth arrest.

Correlation between plant cell wall stiffening and root extension arrest phenotype in the combined abiotic stress of Fe and Al. Kaur H, Teulon J-M, Godon C, Desnos T, Chen S-wW and Pellequer J-L. Plant Cell Environ. in press. DOI:10.1111/pce.14744

Contact : Jean-Luc Pellequer (IBS/Methods and Electron Microscopy Group)