Wax on, wax off: What do plant roots and karate have in common?

To cite the famous 80’s movie ‘Karate Kid’, plant roots can literally wax on and wax off. Yes, you read that correctly.

The roots of all plants have a layer of wax called suberin. This waxy layer surrounds large parts of the inside of the root, known as the vascular bundle, which is responsible for the transport of water and nutrients from the soil to the leaves.

Scientists have always assumed that suberin hindered these uptake processes, since the wax is not very permeable. Therefore, this phenomenon is thought to be a defense mechanism that stops the plant from taking up harmful substances from the soil.

Our new study suggests something rather different and reveals new processes that allow plants to balance their nutrient uptake in order to stay healthy.

These newly discovered processes truly challenge the way we think about plant growth and how roots are able to adapt to changing conditions.

Suberin helps plants survive tough conditions

Previous research shows that this waxy layer can appear either close to or far away from the root tip and continues upwards, typically covering 30-60 percent of the whole root. Under certain stressful circumstances, suberin can appear even closer to the root tip and as much as 80 percent of the root may be suberized.

This can help plants to better tolerate less than ideal growing conditions, such as high salt levels or heavy metals, infections, and drought.

In the plant Arabidopsis thaliana, which is a common test plant in many scientific labs, it was recently shown that nutrient deficiency also has an effect on the suberin layer, however we still know very little about how this actually affects the plant.

In our study, we looked at how roots respond to poor growth conditions in which manganese (Mn) levels, a key nutrient for plant growth, become too low.

Mn deficiency is a very common agricultural problem in Northern Europe, and so studying it can tell us more about how roots adapt to nutrient-poor growing conditions.

Investigating how plant roots respond to various degrees of nutrient deficiency

For our study, we grew barley plants in buckets filled with nutrient solutions that allowed us to expose the plants to different degrees of Mn deficiency.

All nutrients (other than Mn) that the plants needed were supplied in the exact same amounts. While comparing roots that are mildly or strongly deficient in Mn, we found some quite remarkable differences.

Initially, when Mn levels start to fall (mild Mn deficiency), the roots produce less of the waxy suberin layer compared to a healthy control plant, allowing more Mn to seep in to the root. We call this drop in suberin “wax off.”

Later, during more severe Mn deficiency and much to our surprise, the roots rapidly become more suberized than the control plants, which we call “wax on.”

When the same plants are re-supplied with Mn, suberin levels return to normal, which tells us that the only factor in our cultivation system affecting the amount of suberin is the degree of Mn deficiency.

The “wax on” and “wax off” responses represent two different root uptake strategies:

1. Less suberized roots: Under mild Mn deficiency, plants produce less suberin, which increases the permeability to Mn, which allows more Mn in to the root.

2. More suberized roots: If the plants become increasingly stressed due to a severe lack of Mn, they switch strategy and produce more suberin. In this scenario, nutrient uptake becomes focused at the root tip, where there is no suberin and where nutrients can easily enter the root. This is the part of the root that grows into new, hopefully Mn-containing, soil, in a desperate attempt to find some Mn. The acquired Mn will then be transported up to the shoot much more quickly and efficiently, since a strongly suberized root guarantees that no Mn is leaking out of the vascular bundle during transport upwards to the shoot.

Suberin levels affect more than just a lack of nutrients

While measuring all the nutrients in the plants, we see that the root “wax on” or “wax off” states, also have a strong effect on the transport efficiency of other important nutrients, like calcium (Ca), and potassium (K) – nutrients that we would normally assume to be unaffected by Mn deficiency.

The most striking observation is that suberin seems to have opposite effects on the various nutrients. In fact, one of the most important nutrients for plant production, potassium (K), increase significantly during “wax on”, which is not at all what we expected to see.

With less suberin in the roots, we see the opposite results (i.e. poorer transport of K). Calcium (Ca), another important plant nutrient, decreases during “wax on” but increases during “wax off”, hence it behaves in the opposite way to K.

These findings show that the degree of Mn deficiency determines if a root will have more or less suberin than unstressed plants (“wax on” or “wax off”) and that both of these responses has a strong effect on the overall nutrient balance of the plant.

These facts truly challenge the way we think about plant growth and root adaptation.

Improving plant production while saving the environment

Our new observations show that roots adapt quickly to changes in nutrient availability and that these changes have a strong effect on the overall plant health.

These are unknown and potent root processes that are completely overlooked in modern agriculture – a business that faces a lot of negative press for their over-use of chemical fertilizers. So, both for environmental and economic reasons, modern agriculture needs new strategies in order to maximize plant production while at the same time using a minimum of chemicals.

As roots adapt to the environment via suberin, their nutrient requirements are also affected. With more research, we hope to further understand these processes and ultimately help farmers to reduce their use of fertilizer and to use them in a smarter way by compensating for these effects.

So, just like karate, suberin root adaptations are not just about defense. Karate also encompasses elements of philosophy, balance, and inner peace.

Similarly, root adaptations also encompass a pursuit for nutritional balance and health.

And this, Daniel-san, we are only just beginning to understand.

Translated by: Stephanie Lammers-Clark

Read the Danish version of this article at videnskab.dk

References:

'The intensity of manganese deficiency strongly affects root endodermal suberization and ion homeostasis'. Plant Physiology (2019). DOI: 10.1104/pp.19.00507