Cell Walls in Salt stories from the Roots in Salt team

During the last years, our lab has been busy understanding the role of one of the very complex features of the plant cell, the wall, in plant responses to salt. One of the key questions we aim to answer in our research is how plants sense salinity of the soil. Recently, it is becoming clear that part of the answer lies in the cell wall. Salt ions affect the integrity of the wall, very similar to the damage done by pathogens, and root cells are able to detect this damage.

When Nora joined the team, we started digging into the role of the cell wall polymers and we discovered that modifications of pectin (one of the most abundant cell wall components) occur in response to salt stress in our model plant Arabidopsis. We have recently published these exciting results in the journal Development, where we show that salt alters the methyl esterification degree of pectin. In fact, pectin, which is known for its central importance in the food field and the “jelly” industry, in plants controls the mechanical properties of the cell wall, likely altering the cell structural integrity. We have discovered that salt directly activates enzymes involved in pectin remodeling. These enzymes known as Pectin methylesterases (PMEs), often depicted as “Pacman” (see figure below), can be blocked by the application of known inhibitors. Interestingly, PME inhibition alleviates some salt stress responses likely pointing to a connection between pectin remodeling and salt stress activation.

This figure shows in 4 panels how the cell wall pectin structure is altered by salt. The first panel forms the legend for the different symbols used in the figure. The second panel shows a matrix of pectin and Methyl-esters on top of the cell membrane. Through the membrane, touching the cell wall a Cell wall sensor is drawn.
The thrid panel shows how the cell wall matrix is broken up by the application of salt, changing the shape of the cell wall sensor. The fourth panel shows a abstract schematic of the this reaction.
Figure: Our results show that salt application directly modifies the activity of pectin methyl-esterase triggering pectin de-methyl esterification. Detection of these cell wall modifications seems to be responsible, at least in part, for the activation of salt stress responses. Moreover chemicals that can inhibit PME activity can also alleviate the responses to salt stress. For more info and results please check out the original manuscript.

Yet, this doesn’t seem to be all!

From another unexpected side, we also found a key role for the cell wall in a directional response of roots to salt; its inhibition of gravitropism. In our recent preprint, we describe that modification of other structural cell wall components, the cell wall glycoproteins (named extensins) are required here.

When plants are plants exposed to salt stress, their normal gravitropic response of the root is inhibited. In our manuscript, we present a novel method to exploit and analyze natural variation in this root bending response, to find new genetic loci that contribute to root responses to salinity. The assay allows the dynamic monitoring of the effect of salt on roots over time in a time-lapse setup. In a major group effort, combining expertise from several group members, as well as other experts within Wageningen and abroad, we discovered that modification of cell wall extensins by arabinosylation is induced by salt and is required for the directional response of roots to salt. Mutants that lack this activity display increased cell wall thickness when exposed to salt and fail to change root direction in response to salt.

Together, our results highlight and point to the complexity and the involvement of the cell wall in the regulation of salt stress responses.

So, stay tuned for more exciting results!

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