Plants and their interactions with their environment are always in the middle of a physiological quandary. When it comes to their roots, they need to be permeable so they can absorb water and other nutrients from the surrounding soil, but also impermeable to anything that could be harmful to take up, such as toxic heavy metals and even an excess of potentially toxic nutrients like salt. The process of keeping a balance between these two needs is of paramount importance to plant development and there is a substance they use that helps accomplish that goal, suberin. 

The Protection of Suberin

This biopolymer, which is one of the primary molecular components that makes up cork, is used by the plants as a layer of division between the inner endodermal cells that surround the water and nutrient vascular tubing in the center of the roots from the outer cell layer. The process of laying down this layer is appropriately termed suberization. It allows for nutrients to be selectively let through, while keeping out unwanted molecules while still allowing for them to go through the outer cell layers first, as the plant would be unable to determine what is being potentially absorbed otherwise. 

The actual mechanisms of how the suberin layer conducts nutrient transport is still poorly understood, as is how and when the suberin layer is produced in the roots. That is because it is not always utilized and there is evidence that there is a high nutrient cost for producing it, so it must be used sparingly and only in dangerous soil environments that the roots need protection from. But, again, how that environmental response is regulated remains a mystery. 

Controlling Suberin

An aspect that has been identified in the past few years are several transcription factors within the model organism Arabidopsis thaliana’s genome that are expressed when under high salt stress or under stresses that result in the activation of the phytohormone abscisic acid (ABA) in order to protect the plant. So it appears that these factors have some connection to both suberin and the stress response system, but direct testing through knockouts and overexpression had not been done. Until now, that is. 

Researchers from the universities of Geneva, Lausanne, and Nottingham joined forces to test suberin expression in two ways, trying to induce suberin covering in all of the roots at the same time and trying to remove all suberin coverings. To do the former, a growth medium including large amounts of ABA was made that induced overexpression of the four transcription factors and succeeded in extending the suberin coverings once the suberization process was started. This did appear to improve resistance to stressful conditions in the soil, but also highly stressed the plants due to both the increased production state and the nutrient costs of remaining in such a state. 

Conversely, the scientists used CRISPR-Cas9 to create a quadruple mutant deficient in the four factors, which proved to severely reduce suberin production even in unstressed conditions. Such a trait has rarely been observed in other research and indicates that these four factors are the core components of the biosynthesis of suberin or, at least, the activation of that system. While these mutants had greater nutrient uptake speed, they were clearly more susceptible to harm at the same time. 

Another Step

While this research in and of itself won’t produce plants with either better nutrient capabilities or environmental stress resistance capabilities, understanding how plants use this process and the genetic pieces that are involved give us new options and steps to take to modify plants in a way that will be able to retain the benefits and lose the deficits. If we can make plants that are able to deal with environmental stresses in the soil in general without having a major nutrient cost, all of agriculture and even other plant fields would benefit. And this is one of the key insights that will lead to that possibility. 

Study link

Press release

Photo CCs: Müürlooga (Arabidopsis thaliana) lehekarv (trihhoom) 311 1004.JPG from Wikimedia Commons

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