Roots and Soil: an underground relationship

Plants, like us, need "supplements".  Micronutrients such as iron, manganese, and copper are critical components of enzymes that drive chemical reactions in living organisms. Without them, necessary metabolites cannot be produced causing deficiencies within the organism.  When we are faced with a nutrient deficiency, we can simply change our diet.  But since plants cannot, they must rely on the soil as a ready source of micronutrients.  This is the tenet of our research: if we can understand the genetic effects controlling micronutrient uptake in plants, we can make bigger strides in improving plant performance under marginal soil conditions.

We can think of soil as a very complex solution of chemicals.  There are inorganic and organic elements present in varying quantities, depending on how local climate and geology have affected soil development over time.  No two soils are therefore exactly alike: all are products of their environments.  Adding to the complexity is that most inorganic elements, such as micronutrients plants need, occur in different oxidation states within soils. That is micronutrients in the soil may be more likely to possess a certain electron configuration, which affects their bonding with other compounds.  For plants, micronutrients are only usable in certain oxidation states because of bonding properties present from specific electron configurations.  Thus we use the broad term micronutrient deficiency to describe a plant that is negatively affected by the lack of a certain micronutrient in the soil, or the inability of the plant to obtain the element from the soil.

Nutrient availability chart. https://www.pioneer.com

The availability of a micronutrient in a given oxidation state depends both on soil pH and composition.  As we can see, not every soil pH allows micronutrients to be available to plants in the same amounts.  Especially at high pH, very few micronutrients are readily available as they become insoluble in the soil and therefore unavailable to plants.  Take for example the rust stains in your shower or bathtub: this high pH "hard water" is often cloudy and leaves precipitates as elements are not soluble, notably rust.  At high pH, this rust is due to iron oxide that is not soluble and deposited on bathroom surfaces.  Plant roots can only transport iron which is reduced and soluble in both water and soil and not in the oxidized form, like you would find in hard water.
Luckily plants can modify surrounding soil pH from the roots to make certain micronutrients such as iron more available. We can see this below using a chemical to stain for reduced iron that is available to plants. The Rhododendron seedling roots on top slightly lowered the media pH, while the one on the bottom lowered media pH more drastically.  Note the greater saturation of the purple color on the top indicating a greater amount of reduced iron in the media.  Photo credit: Elsa Eshenaur.

Hopefully this puts the mechanisms of the project in perspective. It is our goal to see how different soils have driven root adaptation to enhance iron acquisition in our research species. All in an attempt to illuminate and improve the reactions that occur beneath our feet.