In this issue, we to go to the heart of the matter – what is a plant nutrient? That was a question posed in a paper published last year in the journal Plant Soil. “What is a plant nutrient? Changing definitions to advance science and innovation in plant science” was authored by Professor Patrick Brown of UC Davis, U.S.; Fang-Jie Zhao of Nanjing Agricultural University, China; and Dr. Achim Dobermann, International Fertilizer Association, France. Professor Patrick Brown spoke to New AG International’s Editor-in-Chief Luke Hutson to explore the ideas within the paper, and why the question of essentiality has been key to the debate.
The benefits of certain substances to plant growth have been known for millennia, but it was really in the 19th century that a step-change in knowledge was made. Gradually, the definition of a plant nutrient started to take shape, with micronutrients being added to the established list of essential elements towards the end of the 20th century (see sidebar). This list of “essential” elements has become the basis for defining a plant nutrient ever since. But why is the definition so crucial?
“The definition of what’s a nutrient is important because it demarcates what can be registered as a fertilizer in many parts of the world,” says Brown. “If you look specifically at the U.S. Department of Agriculture's interpretation and International Standards Organization (ISO), it's quite explicitly stated that a fertilizer shall be one of the essential micro- or macronutrients.”
Standard bearer The current ISO fertilizers and soil conditioner standard from 2015 [8157:2015] describes a fertilizer as “a substance containing one or more recognized plant nutrient(s), which is used for its plant nutrient content and which is designed for use or claimed to have value in promoting plant growth.” Scroll down further, and it describes a plant nutrient as “chemical element, which is essential for plant growth.”
As an aside, Brown notes the difference when defining essential nutrients for humans and animals. “If you contrast this approach with what is stated for human biology or animal biology, they take the approach of a nutrient is an element or a substance that benefits the growth or productivity of the species.”
For Brown, this broader approach has broad implications as the human definition allows for complex substances, not just the element, to be considered a nutrient, and it does not require that the nutrient have a critical function for the support of life; merely being of benefit to health is sufficient to be considered a nutrient.
The ISO standard from 2015 does include a definition for beneficial substances. Under section 2.1.4, it defines a beneficial substance as a “substance or element other than primary, secondary, or micronutrient that can be demonstrated by scientific research to be beneficial or may be essential to one or more species of plants, when applied exogenously.” But because beneficial substances do not fall within the plant nutrient definition, a beneficial substance cannot be marketed as a fertilizer on its own. This is why the nutrient industry tends to circumvent this by combining beneficial substances with other fertilizer products.
Essential reading But what is meant by essential? The answer to this seems to have been set some decades ago, from work conducted in the late 1930s at the University of California, Berkeley by professors Daniel Arnon and Perry Robert Stout. Their work proposed a definition that a plant nutrient could be considered essential if “a deficiency of it makes it impossible for the plant to complete the vegetative or reproductive stage of its life cycle.”
Brown begins with the observation that many plants are not taken through their full life cycle. “If you're growing a number of vegetatively propagated species, you don't take them through the full life cycle so that's one of the issues with the current definition.”
Commercial consequences But in the business of fertilizers, what are the consequences of this definition for essentiality? In the U.S., Brown cites silicon as one of the clearest examples. “A lot of people working with a number of different crops have recognized that silicon does benefit the plant, such as the plant's ability to tolerate pests and diseases.”
In Brazil, silicon has been recognized as an essential nutrient, but not in the U.S. “In many states, you cannot buy a fertilizer that is sold explicitly for its silicon nutrient. What you can buy is potassium silicate, and use it for the silicon,” explains Brown.
Discussions over essentiality then lead to the difference between what is an essential nutrient and what is a beneficial one. The book Marschner's Mineral Nutrition of Higher Plants (2012) specifies three criteria for an element to be considered essential. These can be paraphrased as 1) A given plant cannot complete its life cycle in the absence of this element; 2) The function provided by the element cannot be substituted by another element; and 3) The element must be directly involved in plant metabolism, such as a component of an essential constituent, e.g., enzyme, or it must be required for a distinct enzyme reaction.
In contrast, a beneficial element can be said to stimulate growth, but is not essential according to the three criteria in Marschner, or is essential only for certain plant species, or under specific conditions. The reference to specific conditions leads to a question about universality.
Universal question Embedded in that beneficial definition was a clause that the substance was only essential for certain plant species. But as defined in current law, an element can only be a plant nutrient if it is needed universally – namely, true for all plants, in all conditions and geographies.
Brown acknowledges this is one of the issues and says this is largely because of the manner in which that definition was written. “It has always been assumed that an element should be essential for all species in all environments, which means if you cannot demonstrate an element is essential for a species in its environment, then it doesn't count.”
For Brown, a good illustration is aluminium. “In tea plants, it's clearly beneficial and perhaps even essential, but you can't apply it as a fertilizer.”
Biochemical approach One approach to refining the definition of a plant nutrient has been to go to the biochemical level.
In 1955, Arnon proposed identifying an essential cellular constituent or biochemical reaction in which the element plays a role. You can probably already spot the subtle change – it is the cellular constituent or the biochemical reaction that is essential.
This approach is not without its challenges. It requires the function of the nutrient or at least its direct effect on the metabolism of the plant to be identified. It opens a further set of questions in terms of the experiments needed to establish unequivocal evidence and their acceptance.
Professor Emanuel Epstein, UC Davis, entered the discussion in
1965 and remains to this day an active researcher at UC Davis, all while approaching his 105th birthday. Epstein was trying to reduce the essentiality criteria to two, namely: 1) failure to grow normally and to complete the life cycle in a medium purged of the element; and 2) the element is a constituent of a molecule which is known to be an essential metabolite.
Epstein, to whom the Brown, Zhao, and Dobermann paper is dedicated, noted that his second criteria moved the test of essentiality from the element itself to the metabolite of which it is a part. This transfer could also bring further difficulties since it too uses the term essential and overlooks beneficial functions.
Proposed definition At the heart of the Brown, Zhao, and Dobermann paper is the desire for a broader definition.
“What do we mean by essential or beneficial,” asks Brown. “We have historically only considered specific functions in an enzyme or a molecule of the plant, but I believe it should also be broadened that an element or a substance could be essential for the normal microbial populations and microbiome functioning that supports the plant.”
Essentially this is a holistic approach, taking into account the research area of microbiome-plant interactions, which is still in its relative infancy. Brown describes a world of reactions going on in the soil that are microbial driven and that together with the plant can result in optimal productivity.
The corollary is then this – what elements do those microbes need and are there any that are not currently considered a plant nutrient? Brown holds up vanadium as an example, where some azotobacter are dependent on vanadium for their nitrogen-fixation properties. “If you are a plant in an environment that relies on bacteria to provide nitrogen for you, and the bacteria need vanadium, then there's a linkage.”
Research impact As well as the commercial aspects of a revised definition of plant nutrient, Brown also feels it could
have a positive impact on the research landscape. But does a definition really impede research?
“It has stymied innovation to a certain extent,” posits Brown. “If a nutrient is defined by its global ‘essential’ requirement and its absolute requirement for life cycle completion, then there's not much interest in pursuing the elements and substances that have more narrower effects … which can compromise the depth and breadth of scientific endeavour.”
Flood gates? On another level, in what might be seen as an academic debate, could there be drawbacks in the expansion of the definition of a “plant nutrient” in the sense that growers could become overwhelmed with the ever-growing number of plant nutrients?
Brown notes he’s had similar comments from colleagues. “One of the concerns I hear from fellow professors is that this might open the door for everybody selling spurious elements and unknown magical things, and that's the last thing we need as we're trying to get sensible biostimulant regulations implemented. To overcome this, we must simultaneously build a robust testing and validation platform.”
Brown is referring to a new ISO classification of fertilizers being drafted. This new standard would define “fertilizer” as a “substance containing one or more recognized plant nutrient(s), which is used for the purpose of providing the plants or mushrooms with nutrients and designed for use or claimed to have value in promoting their growth”, while defining “plant nutrient” as a
“substance, which is essential for plant growth” (ISO 2021b).
In order to counter concerns of opening the flood gates, Brown stresses that the requirement to demonstrate and verify benefit was hugely important in any definition. To highlight the point, he notes they were careful in the Brown, Zhao, and Dobermann paper to include in the proposed definition (see sidebar) that for a plant nutrient to be considered beneficial, it had to be shown to benefit plant growth and development, and with a notable addition, could benefit the quality attributes of a plant or its harvested products.
Next step Brown is looking to continue the discussion at the 19th International Plant Nutrition Colloquium (IPNC),
22-27 August 2022, where there is a roundtable on the definition of a plant nutrient. He is confident there is already widespread acceptance within the scientific community for a broader definition of plant nutrient, and he cites the proposed revised ISO definition as a step in that direction. ●
Established list of 17 elements commonly classified as “essential”Carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca), magnesium (Mg), chlorine (Cl), boron (B), zinc (Zn), manganese (Mn) iron ( Fe), copper (Cu), molybdenum (Mo), nickel (Ni).
Other elements are considered beneficial but not essential, such as sodium (Na), silicon (Si), selenium (Se), aluminum (Al), cobalt (Co), iodine (i). From a legal point of view, these elements are not able to be marketed in many countries as a plant nutrient.
Proposed new definition of “plant nutrient” from “What is a plant nutrient? Changing definitions to advance science and innovation in plant science” was authored by Professor Patrick Brown of UC Davis, U.S., Fang-Jie Zhao of Nanjing Agricultural University, China, and Dr. Achim Dobermann, International Fertilizer Association, France.
“A mineral plant nutrient is an element which is essential or beneficial for plant growth and development or for the quality attributes of the plant or harvested product, of a given plant species, grown in its natural or cultivated environment. A plant nutrient may be considered essential if the life cycle of a diversity of plant species cannot be complete in the absence of the element. A plant nutrient may be considered beneficial if it does not meet the criteria of essentiality, but can be shown to benefit plant growth and development or the quality attributes of a plant or its harvested product.”
The definition of what’s a nutrient is important because it demarcates what can be registered as a fertilizer in many parts of the world.
At the heart of the Brown, Zhao, and Dobermann paper is the desire for a broader definition.
Some 22,000 new seed samples from 10 gene banks in Australia, New Zealand, Africa, the Middle East and Europe were deposited in the Svalbard Global Seed Vault in Norway on 14 February 2022
The vault safeguards more than 1.1 million seed samples from nearly 6,000 plant species of importance to food and agriculture worldwide. A total of 89 gene banks around the world have sent samples to the Svalbard Global Seed Vault, the world’s largest and most diverse collection of seeds.
The seed collection in Svalbard is growing steadily. This latest deposit will add 150 new species, most of them fodder plants from the Australian Pastures Genebank. Another 50 species not yet in the vault are being deposited from the Leibniz Institute of Plant Genetics and Crop Plant Research (IPK) in Germany.
The International Center for Agricultural Research in the Dry Areas (ICARDA) deposited roughly 6,000 bags of seed. ICARDA has retrieved seeds in 2015, 2017 and 2019 to reconstruct a seed collection lost in Syria. Reconstruction efforts are taking place in Lebanon and Morocco among other places. The current shipment for deposit brings ICARDA back up to about 100,000 seeds, the amount it had stored prior to withdrawing seeds as a result of the Syrian civil war.
Also deposited were seed samples originating from Nordic countries. Among these is svedjerug or finnrug, a variety of rye which has been successfully saved from extinction. This variety was brought from the east by slash-and-burn farmers to woodlands in the eastern part of southern Norway a couple of hundred years ago. Also, seeds of fjelltimotei (Phleum alpinum), a rare bunchgrass species from Norway’s Hardangervidda plateau and an important relative of Timothy grass, and the seeds of the Welsh onion (Allium fistulosum), which occurs naturally on grass roofs in a few locations in and around Gudbrandsdal valley, were deposited.
The Svalbard Global Seed Vault was established by and is owned by Norway. Its operation is managed in a partnership between the Ministry of Agriculture and Food, Crop Trust and the Nordic Genetic Resource Centre (NordGen). ●
Svalbard Global Seed Vault. Photo: Crop Trust
Arsenic, uranium and other trace elements naturally occur in topsoil across the U.S. Corn Belt. Crops grown in soils containing elevated levels of those trace elements can absorb them through roots, potentially curbing
growth and threatening the health of those who regularly consume them.
Researchers with Nebraska Water Center, University of Nebraska–Lincoln, Arindam Malakar, Chittaranjan Ray and colleagues, were curious about whether ferrihydrite — a nanoscopic mineral sometimes found in soils but also used to treat groundwater and drinking water — might help address the issue.
As part of a greenhouse experiment using soil from the university’s Panhandle Research and Extension Center, the team planted corn in three soils: one with no ferrihydrite, another with 0.05 percent ferrihydrite, and a third with 0.10 percent of the mineral. After irrigating the soils with arsenic- and uranium-fortified water, the researchers tracked the growth of the corn plants and monitored concentrations of the trace elements in water surrounding the plant roots.
The team found that ferrihydrite-enriched soils lowered the concentrations of arsenic and uranium by about 20 percent. They also appeared to reduce the loss of nitrate, which is essential to plant growth but can cause health issues when leaching into groundwater, by roughly 30-50 percent. Water retention, meanwhile, rose from about 13 percent with no ferrihydrite to roughly 17 percent with it.
Crucially, the corn likewise seemed to benefit: Plants in the ferrihydrite-enriched soils grew taller, produced 12-15 percent more living tissue, synthesized more chlorophyll and yielded kernels containing nearly twice as much iron.
Conducting similar experiments in actual field conditions, rather than a greenhouse, will be necessary to validate the study’s results, the researchers said. But the initial findings suggest that adding even small doses of ferrihydrite to irrigation could limit concentrations of toxic elements while boosting crop growth and nutrient uptake. ●
Cover crops are often planted following cash crop harvest to reduce erosion and help the soil retain nutrients, among other benefits. While cover crops can ultimately improve the yields of cash crops through improved soil health, new research suggests that they might also protect them from disease.
Pseudomonas syringae is a common bacterial pathogen that affects an array of important agricultural crops. Infections start on the leaf surface and spread through openings such as exposed wounds and pores. Farmers typically treat diseased plants with copper solutions, but some studies suggest that recruiting beneficial microbes may prevent P. syringae infection. Thus, creating reservoirs of helpful microbes in agricultural fields could be an important strategy for preventing disease.
In a paper recently published in the Phytobiomes Journal, Rémi Maglione, Marie Ciotola, Mélanie Cadieux, Vicky Toussaint, Martin Laforest and Steven Kembel explored cover cropping as a potential tool for cultivating a healthier, disease-suppressive “phyllosphere,” or aboveground plant microbiome. To do so, they grew P. syringae-inoculated squash in fields that were over-wintered under four different conditions: winter rye cover crop, chemically-terminated winter rye cover crop, plastic cover, and bare soil. They compared the pathogen loads on the squash plants by culturing P. syringae from their leaves. The team also characterized the microbiomes of over 2,200 leaf samples to examine how cover cropping affects phyllosphere assembly. They found that cover cropping reduced populations of P. syringae and increased the abundance of genera such as Sphingomonas and Methylobacterium, which have been used as biocontrol agents against pathogens.
"To our knowledge, our study is the first to explore the importance of the phyllosphere microbiome in the context of cover cropping practices," said Laforest. "Our results suggest that cover cropping treatments can be used to manipulate biological interactions to protect plants against pathogens."
Cover crops might not only promote a healthy microbiome by providing a reservoir of helpful microbes but could also minimize the colonization of soil-dwelling pathogens by creating a physical barrier. They could also affect microbial colonization by altering local soil conditions (e.g., soil moisture and temperature). ●
Scientists are one step closer to giving more plants the ability to harness nitrogen-fixing bacteria, which would reduce the need for fertilizer, and in turn, lower costs for farmers and mitigate environmental impacts.
Plants can only absorb nitrogen in some of its chemical forms. Some of these forms of nitrogen are naturally found in soils, but usually not in quantities needed to reach adequate crop yields. Nitrogen is plentiful in the air, but in a form plants can’t use.
“Some bacteria that live in the soil are able to convert atmospheric nitrogen into one of the forms plants can use — this is called nitrogen fixation. A few plant species, mainly in the legume family, have evolved root nodules that attract and host these bacteria. These nodules allow the plant absorb the nitrogen the bacteria fix, and in exchange, the bacteria get sugars from the plant,” said Matias Kirst, a professor of plant genomics at the University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) School of Forest, Fisheries and Geomatic Sciences and a member of the UF Genetics Institute. “The big questions we have are, can we teach other plants to make nodules, and will these nodules attract nitrogen fixing bacteria?”
Before these questions can be answered, scientists need a better understanding of how legumes, the original nitrogen-fixing plants, form their own nodules. Uncovering this complex process could allow scientists to replicate it in other plants, said Kirst, senior author of a new study that unpacks that process.
“When legumes come in contact with nitrogen-fixing microbes, we know there is a big surge in a plant hormone called cytokinin and that this leads to nodule formation. In this study, we wanted to get a real-time picture of when that surge happens and where the activity is happening in plants that forms nodules,” Kirst said.
To observe that play-by-play, the research team used a technique that causes fluorescence in the presence of cytokinin — the area glows in the dark — allowing researchers to see the hormone’s every move. They found that cytokinin activity happens in two stages. In the first stage, cytokinin is produced in the outer layer of the root and moves inward. In the second stage, that inner part of the root pushes outward like a balloon, forming the nodule.
The study also found that this second stage of cytokinin activity is controlled by a gene called IPT3. They confirmed this both through the fluorescence technique and by observing plants that were missing the IPT3 gene. In the plants missing the gene, nodule formation didn’t occur, which tells the researchers this gene plays a key role in the process.
All plants have cytokinin and the IPT3 gene, Kirst said.
“Biologically speaking, every plant has the ingredients for making a nodule, but it’s a matter of expressing the right gene at the right time and place. This is what we’re learning from this research. We hope to apply this understanding to developing plants that can generate nodules,” Kirst explained.
After that, the next big question is whether nitrogen-fixing bacteria will move into those nodules.
The study, which is published in the journal Plant Physiology, was authored by an interdisciplinary team of researchers from UF/IFAS, University of Wisconsin-Madison and the Noble Research Institute. The study was supported by a grant from the U.S. Department of Energy. ●
A technique that causes fluorescence allows researchers to see how the hormone cytokinin is involved in forming root nodules.
Photo: UF/IFAS
Kula Bio has closed a $50 million Series A round led by Lowercarbon Capital and includes Collaborative Fund, Grantham Environmental Trust’s Neglected Climate Opportunities Fund and iSelect Fund.
Kula Bio produces Kula-N, a sustainable nitrogen biofertilizer that helps farmers maintain yield, improve soil quality and reduce carbon emissions and other environmental impacts.
Founded in 2018, the company’s technology stores energy in a naturally occurring microbe that fixes nitrogen from the air to the soil. It is patent-protected and derived from peer-reviewed research performed at Harvard University.
The funding will be used to increase production capacity as well as further technology innovation of Kula Bio’s technology. Other investors in the round include Pillar VC, Embark Ventures and BOPU. ●
Kula Bio’s energized bacteria live in the soil and communicate with plants to produce nitrogen on-demand. Image: Kula Bio