How Do Plant Roots Absorb Nutrients With High and Low Affinity Transporters?

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How do plant roots absorb nutrients with high and low affinity transporters? Roots take up nutrients through membrane proteins called transporters, and the transporters differ in how strongly they bind their target at low concentrations. Nitrate is the clearest example in the sources. Some transporters work when the outside concentration is low, and others take over when it is high.

Quick answer

Plants take up nitrogen from the soil through their roots, mostly as ammonium and nitrate.
Nitrate uptake uses two families of transporters, NPF and NRT2, and the NRT2 family is described as high-affinity.
High-affinity systems do most of the work when nitrogen is scarce, while low-affinity systems handle high external concentrations.
Four Arabidopsis NRT2 transporters account for most high-affinity nitrate influx under nitrogen limitation.

Where roots get their nitrogen

Roots are the main point of contact with the soil. The first source states that “Because roots constitute the main organ where exchange between plant and its environment take place, this review focuses on transporters identified in Arabidopsis root plasma membrane and contributing to nitrate uptake from the soil.” The second source puts the general case in one line: “Plants acquire nitrogen nutrients from the soil through their roots mostly in the form of ammonium and nitrate.”

The uptake step is active rather than passive. The second source states that “Ammonium and nitrate are taken up actively into root cells by different sets of plasma membrane-localized transporters.” That means the root uses membrane proteins to move these ions in, rather than letting them drift across the membrane.

Nitrogen availability is a major factor in growth. The second source notes that “Nitrogen (N) availability is a major factor determining plant growth and productivity.” This is why the uptake system matters, since the root must deal with both abundant and scarce supply.

What high affinity and low affinity mean

The first source separates the two systems by concentration. It explains the split in concentration terms. It states that “The LATS allows transport in high (> 0.5 mM) external nitrate concentration whereas the HATS provide a capacity for nitrate absorption at low (< 0.5 mM) external nitrate concentrations." LATS is the low-affinity transport system and HATS is the high-affinity one.

The two systems are meant to work together. The first source states that “Expression of these four kinds of transport systems is essential for an efficient uptake, and expression could be constitutive or inducible function of the external nitrate concentration perceived.” In other words, the plant can switch expression of its transporters depending on what it senses in the soil.

The high-affinity transporters have a specific operating range. The first source states that “The affinity for nitrate of these transporters is in the range of microM, saturable around 0.2–0.5 milliM.” The source uses those units as written, and it places the high-affinity range well below the concentrations that the low-affinity systems handle.

The NRT2 family and high-affinity nitrate uptake

The NRT2 family is the main high-affinity nitrate group. The first source states that “Seven members have been described in the NRT2 family in Arabidopsis and characterized as high-affinity nitrate transporters.” The second source confirms that “NRT2 members are high-affinity nitrate transporters, and most of them require another component, NAR2 (NRT3), to mediate nitrate transport.”

The role of each member differs. The first source reports that “The role in nitrate influx in the root has been demonstrated for four of them: NRT2.1, NRT2.2, NRT2.4, and NRT2.5.” It describes NRT2.1 as the most studied member, noting that it “constitutes the most studied transporter in NRT2 family.” It also notes that “atnrt2 KO mutants have been obtained for both transporters NRT2.1 and NRT2.2, and are affected in the high affinity nitrate transport activity, but not in the low affinity.”

The second source gives the most complete picture of the four. It states that “Analysis of a quadruple mutant revealed that these four NRT2 transporters account for approximately 95% of high-affinity nitrate influx activity under N limitation, AtNRT2.1 being the major contributor.” That figure comes from mutant analysis in Arabidopsis, so it describes that model plant rather than every crop.

Location matters as well. The second source states that “the spatio-temporal distribution of these four AtNRT2 transporters is critical for efficient nitrate uptake to sustain growth under low N availability.” It describes AtNRT2.4 and AtNRT2.5 as expressed “in the epidermal cells of young primary and lateral roots,” which places them at the root surface where uptake starts.

The first source adds a detail about NRT2.4. It states that NRT2.4 “is involved in nitrate uptake at very low nitrate concentration.” This shows that the high-affinity group covers the very bottom of the concentration range.

The NAR2 partner

Many NRT2 transporters need a helper protein. The first source states that “Interaction with a small protein NAR2/NRT3 seems to be essential for nitrate uptake of some of the NRT2.” The same source notes that “Besides contribution to nitrate transport, NAR2.1 seems to be implicated in NRT2.1 localization or stabilization at the plasma membrane.” So the partner protein affects both transport and where the transporter sits.

Dual-affinity and low-affinity routes

The NPF family is the second nitrate group. The second source states that “In Arabidopsis, there are 53 and seven members in the NPF and NRT2 families, respectively.” It identifies one member as having a dual role: “AtNPF6.3 (NRT1.1), which has dual-affinity transport and nitrate-sensing functions.”

The same source limits how much that transporter contributes at low supply. It states that “Although a dual-affinity transporter AtNPF6.3 is also expressed in roots under N limitation, its direct contribution to high-affinity nitrate transport under N limitation seems to be minor, maybe even non-existent.” So the high-affinity work under scarcity is mainly done by the NRT2 group in this model.

The first source notes that the low-affinity systems also matter. The division of labor between the two systems follows that concentration split.

Ammonium uptake

Ammonium uses a different family. The second source states that “Ammonium transport is mediated by transporters of the AMT/MEP/Rh (AMT) superfamily.” It adds that “Six AMT genes exist in Arabidopsis, all of which encode high-affinity ammonium transporters.”

The AMT genes do not all do the same job. The second source states that “All AtAMTs have different ammonium affinity levels and transport capacities indicative of their roles in planta.” It also reports that “Ammonium influx studies using triple and quadruple mutants showed that AtAMT1;1, AtAMT1;2 and AtAMT1;3 are additively responsible for about 90% of high-affinity uptake capacity under N limitation.”

The same source describes how the roles split by location. It states that “AtAMT1;1, AtAMT1;3 and AtAMT1;5 act to absorb ammonium directly from the soil, and AtAMT1;2 transports apoplastic ammonium into the cell.” These genes divide the work between uptake from the soil and movement of ammonium into the cell.

For related reading on how roots meet their basic needs, see how root hairs absorb water and how soil compaction affects roots.

How we chose

The two sources were chosen because both are open-access review articles on PubMed Central, and both pages returned HTTP 200 on a direct request with article text rather than a bot check. The first is a review focused on nitrate uptake from the soil in Arabidopsis roots. The second is a review of plant nitrogen acquisition under low availability. Together they cover the two systems, the nitrate and ammonium transporters, and the NRT2 partner protein.

Each factual sentence in this article is tied to a verbatim quote recorded in the research notes for this page. Sentences that could not be tied to a quote were removed. No product, price, or retail page is part of this explanation.

What we could not verify

Most of the detail comes from Arabidopsis, and the sources focus on nitrogen. The question names nutrients in general, but the text retrieved does not cover other nutrients in the same depth.

What the sources do not say

The sources do not describe high-affinity and low-affinity transporters for phosphorus, potassium, or other nutrients.
The sources give threshold values but do not say that those values apply to every soil.
The first source reports the affinity range using the units “microM” and “milliM” as written, and it does not give a conversion for garden fertilizer.
The sources do not say what a gardener should add to a bed, so no fertilizer advice is given here.

Frequently asked questions

What is the difference between high and low affinity transporters?

The first source states that “The LATS allows transport in high (> 0.5 mM) external nitrate concentration whereas the HATS provide a capacity for nitrate absorption at low (< 0.5 mM) external nitrate concentrations."

Which transporters do most of the work when nitrate is scarce?

The NRT2 family. The second source states that “Analysis of a quadruple mutant revealed that these four NRT2 transporters account for approximately 95% of high-affinity nitrate influx activity under N limitation.”

Do roots use both kinds of transporter at the same time?

The sources describe them working together. The first source states that “Expression of these four kinds of transport systems is essential for an efficient uptake.”

Do plants take up ammonium the same way?

Not quite. Ammonium uses the AMT family, and the second source states that “Six AMT genes exist in Arabidopsis, all of which encode high-affinity ammonium transporters.”

Is this the same for all plants?

The detailed genes are from Arabidopsis. The second source notes that “ammonium-inducible expression has been reported in other plants such as rice, tomato and maize,” which shows that the pattern varies across species.

Sources

Transporters Involved in Root Nitrate Uptake and Sensing by Arabidopsis. PubMed Central, PMC5030233. https://pmc.ncbi.nlm.nih.gov/articles/PMC5030233/
Plant Nitrogen Acquisition Under Low Availability: Regulation of Uptake and Root Architecture. PubMed Central, PMC4836452. https://pmc.ncbi.nlm.nih.gov/articles/PMC4836452/