Edited by: Hans-Henning Kunz, Washington State University, United States
Reviewed by: Hans-Henning Kunz, Washington State University, United States; Karen A. Sanguinet, Washington State University, United States
*Correspondence: Janin Riedelsberger
Michael R. Blatt
This article was submitted to Plant Physiology, a section of the journal Frontiers in Plant Science
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Most plant roots are found hidden underground. They can form immense root systems with lengths of several kilometers featuring an architecture with up to millions of branch roots. Their versatile functions range from anchoring plants in soil via storing photosynthetic products to the vital uptake of water and nutrients. Plants build the basis of a food chain that ends with more than seven billion people who have to be fed every day. Therefore, it is vital to secure and enhance crop production, especially in terms of prevailing agricultural threats and climate change. The challenge must be to optimize plant's nutrient acquisition and stress tolerance. Understanding plant nutrition, homeostasis, and stress responses is a first step to directed and efficient manipulations and adjustments to counterbalance stress and deficiency symptoms. This knowledge must be used to optimize the use of fertilizer and water to achieve well-balanced nutrition acquisition as well as the use of pesticides to treat biotic stress. Such optimization is desirable as the constant use of fertilizer, irrigation, and pesticides is associated with negative long-term effects. Knowledge about nutrient uptake and signaling as well as behavior in stress situations will be valuable if we are to design adequate treatments to prepare plants for environmental challenges.
Nutrient uptake is the first step in the production of plant mass. It starts in the roots, which represent the main entrance for macro- and micronutrients that are crucial for elementary cell processes. Deficiency or excess nutrient uptake can result in severe damage to the point of yield loss or even plant death. Therefore, a balanced nutrient supply and its uptake are essential for optimal plant growth and crop production. Nutrient distribution in the soil is heterogeneous in space and time and affected by many factors, including binding and sequestration, groundwater level, pH, and salinity.
The three primary plant macronutrients are potassium, nitrate, and phosphorus. Over the course of evolution, transport systems with differing affinities for nutrients evolved to cope with variations in nutrient availability.
Nitrate also acts as signaling molecule influencing lateral root primordia and root branching (
Micronutrients often occur abundantly in solid earth but mainly in an insoluble form that is not suitable for uptake, meaning that plants may suffer nutrient deficiency despite their presence in the soil. One example is iron. Plants exhibit a number of special uptake strategies for iron. Poaceae use a chelation strategy to uptake iron by secreting phytosiderophores or precursors of phytosiderophores into the rhizosphere. Then, the chelated iron(III)-phytosiderophore complex is taken up into the plant by special transport systems. Non-Poaceae use an acidification-reduction strategy, which acidifies the rhizosphere to reduce iron(III) to iron(II), which can be taken up by plant roots. Phenolic compounds like coumarins, especially catechol coumarins, have been reported to play a role in iron recruitment showing iron(III) reducing and iron(II) chelating activity (
Generally, acidic soils stimulate the uptake of micronutrients like iron. However, low pH also increases the solubility of toxic metals like aluminum, which directly affects the root system architecture (Rao et al.,
The most important plant-microbe interaction is the mycorrhizal symbiosis between plants and fungi. Plants benefit from enhanced nutrient uptake and stress resistance, while the fungi are provided with photosynthetical products. Schott and colleagues described a minimal network of transporters that is sufficient to describe realistically the bidirectional nutrient trade system (
Another important plant symbiotic relationship is that between legumes and rhizobia, a type of nitrogen-fixing bacteria. Here, the plant benefits from the biological nitrogen fixation that is carried out by the rhizobial symbiont, while bacteria receive photosynthetically fixed carbon. Establishing such interactions between plant root hairs and microorganisms is initiated by chemical communication. Bacterial symbionts respond to plant signaling compounds with nod factor secretion that induces broad changes in the plant root transport machinery (
Root exudates may also contain substances that function as allelochemicals for plant's defense. Infestation with pathogenic microorganisms can lead to substantial yield losses. It is thus essential to understand how plants are able to resist pathogen invasion if we are to enhance these defense mechanisms. A common signal on the part of bacteria is the bacterial protein flagellin flg22 that is recognized by the plant receptor kinase FLS2 and also induces ALMT transporter expression (
Generally, plants are exposed to several stresses simultaneously. Although much attention is drawn to the characteristics of the shoot, effects on roots are often ignored. Though, root plasticity is highly flexible and its development is guided by environmental conditions like nutrient availability.
One of the major agricultural challenges is the increasing salinization of agricultural land. The majority of crop plants are salt-sensitive, which is why articles of this research topic concentrate on glycophytes. Nieves-Cordones and colleagues reviewed present knowledge on sodium uptake systems in plant roots and their regulations (
For centuries the increase in crop yield raise and quality has been achieved by transferring the scion of one plant to the rootstock of another plant. This process, known as grafting, may lead to increased nutrient uptake and stress tolerance among other benefits. Despite long and still actively utilized practice of grafting, physiological, and molecular bases are not fully understood yet.
The root structure not only responds to external stimuli, but also to partitioning patterns of photoassimilates in roots.
Uptake and transport pathways of nutrients are well-organized for every mineral. These pathways intersect and result in complex networks that can be challenging to access experimentally. Modeling of transport processes, nutrient exchange and uptake processes have been subject to long-term investigations, which aim to predict and describe experimental data. Le Deunff and colleagues describe the advantage of the flow-force modeling approach over the enzyme-substrate approach by reviewing developments and advances carried out on modeling of nutrient uptake (
Modeling approaches go beyond the description of nutrient uptake and transport. Efforts have been made to simulate effects on whole root system architectures in response to nutrient availability (Postma et al.,
For future investigations it is crucial to combine studies of entire root systems and plants with modeling approaches. This ensures the assessment of systemic effects that result from changes in single traits. Advances in this field should be integrated into current experimental pipelines to refine and accelerate research. Further advances in the understanding of nutrient homeostasis are needed to enhance nutrient use efficiency and optimize the use of fertilizer as well as to improve and maintain soil quality and protect the environment from inefficient water and fertilizer use.
JR wrote the first draft of the manuscript. JR and MB revised and improved the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer KS and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.