"Iodine should be considered a plant nutrient." That's the main conclusion of a scientific paper published in September 2020 by a group of researchers from Italy. Iodine is recognized as an essential element for the health of humans and farm animals and now it has been discovered that plants also need iodine, in a micro molar dose. in the soil solution. The natural occurrence of iodinated proteins in higher plants is described for the first time and 82 iodinated proteins have been identified. Based on phenotypic, genomic and proteomic studies, it was shown that plants need iodine for leaf and root growth, efficient photosynthesis (the process of converting sunlight into chemical energy in the leaf), timely flowering, increased seed production and activation of an early warning system that defends the plant from abiotic and biotic stress damage. In areas where soil and water are naturally low in iodine, iodine deficiency can result in crop yield loss and decreased fruit quality. To provide this element easily and safely through the correct source, dosage and timing, there are commercial products that combine iodine and potassium nitrate in a single specialized fertilizer, since plant iodine demand is well timed with typical applications of potassium nitrate as a source of potassium in soil solutions.

Iodine is a mineral nutrient necessary for plants.

The elements currently considered essential plant nutrients are C, H, O, N, P, K (primary nutrients), Ca, Mg, S (secondary nutrients), and Fe, Zn, Cu, Mn, B, Cl, Mo, Co and Ni (micronutrients). This list of known plant nutrients can now be expanded with the element iodine, the first micronutrient added since the incorporation of Ni in 1987. In Italy, Prof. Pierdomenico Perata, Dr. Claudia Kiferle, his team, Dr. Claudia Kiferle, their team at the Sant'Anna Scuola Universitaria Superiore in Pisa, and scientists linked to the Consiglio Nazionale delle Ricerche, Naples, have now published this important new discovery: plants link iodine with 82 different proteins with functions in important biological processes, such as the Rubisco protein for efficient photosynthesis in leaves, or the peroxidase enzymes that defend the plant from abiotic and biotic stress and the ATPase enzyme, necessary to supply energy for plant growth and nutrient transport. An iodine deficiency in the plant can result in yield loss, similar to what can occur if the plant is deficient in any other micronutrient. For optimum crop production, an adequate dose of iodine should be applied.

What is iodine

Iodine is an element of the periodic table with symbol I and atomic number 53. Iodine is a halogen, an element classified in the same group as chlorine (Cl) and bromine (Br). Halogens react easily with metals such as sodium or potassium. Examples are sodium chloride (table salt, NaCl) or potassium iodide (KI) which is added to salt, producing iodized table salt for human health.

Where iodine occurs naturally

Iodine is present everywhere, but only in small amounts. The highest amount of iodine is found in the oceans, with an average concentration of 0.5 micro moles of iodine per liter of seawater. In contrast, rain, soil solution and irrigation water contain lower concentrations(less than 0.2 micro moles per liter). In addition, typically, less than 10% of the total iodine in soil is available for plant uptake. In humans and farm animals disorders such as goiter and hypothyroidism are caused by iodine deficiency, which alters thyroid function.

Endemic goiter and cretinism in highland regions and Amazon jungle has been recognized in Peruvian history, due to the permanent natural deficiency of iodine in these soils and plants.

Research initiated in the 1960's proves the persistence of severe iodine deficiency and also that this deficiency is a cause of preventable brain damage. A program to control iodine deficiency disorders for the Peruvian population was created in 1983 and was fully implemented in 1986, with 90% of households reported to have access to iodized salt in 1998.

Plants can absorb and accumulate iodine.

It has long been known that plants can take up iodine with their roots and store iodine in their leaves and fruits. In many previous studies the benefit of supplementing small amounts of iodine for plant growth and stress resilience was observed. Researchers in Italy (Scuola Superiore Sant'Anna in Pisa) have come to the same conclusion after reviewing all previously published evidence: plants can accumulate iodine because it is beneficial for their growth, for nitrogen metabolism, resistance to salinity stress in the root solution and, particularly, for the production of antioxidants by the plant. As with other micronutrients, supplying the correct dose (not too little, not too much) of the nutrient is very important.

It is also important to supply the correct form of iodine. For example, iodine present in disinfectants (free iodine, _I2 and iodide ^I-) can have harmful effects at a lower dose compared to other forms of iodine.

Why iodine is necessary for plants

Despite the published benefits of iodine applied at the correct dosage, the role of iodine as a plant nutrient has not received the attention it deserves from the scientific community. Until now. A recently released publication describes a series of experiments. These experiments were conducted by a group of scientists in Pisa, Italy, and demonstrate how plants need iodine.

For these experiments, Arabidopsis thaliana was used as a model plant. This plant is fast-growing in the laboratory (only six weeks from seed to seed) and all the knowledge about genetics and metabolism is shared online by scientists around the world.

Plants always contain some iodine as it can be found in small concentrations in air and water. In fact, iodine is added to a known culture medium to study plant physiology in Arabidopsis. To study the effect of iodine deficiency in Arabidopsis, the water to prepare the soil solution was demineralized by reverse osmosis and ultrapure chemicals were used to make the nutrient solution.

Without the deliberate application of iodine, plant growth and flowering were much slower compared to plants given 0.2 or 10 micro moles of iodine per liter. Application of iodine at micro molar concentrations increased root and shoot growth, seed production, and earlier flowering.

To find out why plant growth is compromised with the use of iodine-free soil solutions, the genetic response of the plant to the presence or absence of iodine in the soil solution was investigated. Iodine treatments specifically regulated the expression of several genes involved in photosynthesis, the salicylic acid (SA) stress response pathway, plant hormone response, ^Ca2+ signaling, and plant defense to pathogen attack. The combination of these processes confirms previously published observations that iodine helps plants prevent damage from biotic and abiotic stresses.

To test whether the response in plant growth and gene expression was unique to the addition of iodine to an iodine-deficient soil solution, the same experiment was conducted using the halogen that most closely resembles the atomic structure of iodine: bromine. In contrast to iodine, neither gene expression nor plant growth responded to bromine supplementation. This proves that the plant response to iodine is unique and cannot be replaced by another element.

Finally, it was discovered that iodine was incorporated into plant proteins providing plants with iodine radiolabeled isotopes recovered in proteins. These proteins can be enzymes or components of structural complexes that are necessary for all cellular functions and for collaboration and communication with other cells in and between plant organs.

Iodinated proteins were not only discovered in Arabidopsis, but also in tomato, maize, wheat and lettuce. Finding iodinated proteins in unrelated plant families demonstrates that these iodine-containing proteins exist widely in the plant kingdom.

A total of 82 iodinated proteins were identified for Arabidopsis thaliana using bioinformatics approaches in independent proteomic databases containing all plant proteins studied worldwide. In shoots, iodinated proteins are mainly associated with chloroplasts and are functionally involved with photosynthetic processes, whereas those in roots are mainly diverse peroxidase enzymes, important for stress signaling, or related to peroxidase activity. Some of these proteins are essential for root growth. Iodinated proteins with a crucial role in nitrogen metabolism, phytohormone regulation and energy production were also found in both root and leaf cells.

These discoveries open a new perspective on new emerging aspects of plant physiology, especially in the fields of proteomics and enzymology. Iodinated enzymes were found to have fundamental roles in basic conserved evolutionary functions, and this discovery is the first step that has generated academic interest in iodine as a factor of importance in crop production.

Providing sufficient iodine to crops helps prevent yield loss and maintain fruit quality.

A correct iodine supply is of direct benefit to growers by improving crop yields and preventing crop losses in areas where there is insufficient iodine available for optimal plant growth. Iodine deficiency for crops in Peru can be predicted from iodine concentration in water and soil. In well water samples in Ica and Lima, levels between 0.05 and 0.6 micro moles of iodine per liter have been found. It is also known that iodine availability is generally low in the type of soil that predominates in the area (sandy soils, with little organic matter).

To make it easy and safe for growers to supply the correct dose of iodine, iodine can best be applied with potassium nitrate rather than just as a micronutrient. Plant iodine demand is well timed with typical applications of potassium nitrate as a source of nitrate and potassium in soil solutions. Potassium nitrate is the optimal source of potassium, as it provides potassium (K) combined with the preferred source of nitrogen: nitrate (_N-NO3). The uptake of nitrate nitrogen promotes the uptake of soil cations such as potassium, calcium and magnesium.

Iodine is the natural complement to nitrate for calcium transport in the plant, which will be hindered by iodine deficiency. Iodinated proteins in roots are involved in energy metabolism and response to oxidative stress, predicting lower calcium transport to fruit under iodine deficiency. Iodine is also necessary for the plant to maintain photosynthesis and leaf transpiration. It has been observed in cherry tomato cultivation in Almeria, Spain, that by adequately supplying iodine with potassium nitrate, the improved redox balance and energy metabolism favored calcium transport from the roots to the fruit, resulting in a higher calcium concentration in the fruit. This is associated with a lower probability of quality issues related to calcium deficiency, such as apical rot, and a longer shelf life during transport. Therefore, ensuring an adequate level of iodine in the soil solution helps growers achieve optimal crop yields with high quality produce, especially under adverse weather conditions.

For additional information on iodine: https://www.worldiodineassociation.com/elemental_chemistry

Main source:

Kiferle, C., Martinelli^, M., Salzano, A.M., Gonzali, S., Beltrami, S., Salvadori, P.A., Hora, K., Holwerda, H.T., Scaloni, A., Perata, P. (2020) Evidences for a nutritional role of iodine in plants. B ioRxiv https://doi.org/10.1101/2020.09.16.300079.