Top 4 Types of Mineral Nutrient Transport | Plants

The following points highlight the top four types of mineral nutrient transport. The types are: 1. Potassium (K⁺ ) 2. Phosphorus (P) 3. Iron (Fe) 4. Zinc (Zn).

Mineral Nutrient Transport: Type # 1.

Potassium (K⁺ ) Transport:

K⁺ is absorbed into the root symplasm through transport across the root cell plasma membrane. From the root symplasm it travels to the xylem parenchyma wherefrom it is unloaded into the xylem vessels for long distance transport to the leaves.

K⁺ is reabsorbed from the xylem into the leaf cells. From there it can be loaded into phloem for translocation to actively growing sink tissues, where it can be unloaded through symplastic or apoplastic pathways. Potassium may also be transported into the vacuole through the tonoplast for storage both in root and shoot cells.

Early physiological and biochemical studies indicated the existence of multiple K⁺ transporters. Biophysical investigations provide a mechanistic basis for high- and low-affinity K⁺ transport. The high-affinity K⁺ transport is an active process, whereas low-affinity transport involves passive K⁺ uptake through K⁺ channels.

The high-affinity K⁺ uptake could be selectively inhibited by N-ethylmaleimide (NEM) without impairing the transport through low-affinity K⁺ channel. Again, the application of K⁺ channel blocker like tetraethyl-ammonium (TEA) ion selectively inhibits the low-affinity transport component.

From the electrophysiological experiments it is evident that high-affinity K⁺ uptake is electro genic. In this uptake K⁺ is absorbed against the concentration gradient.

Recent biophysical investigations of root K⁺ transport have provided evidence for a high-affinity K⁺ transport mechanism that is coupled to proton gradient. Patch clamp studies on root protoplasts have shown that one major component of low-affinity K⁺ uptake involves inward-rectifying (K⁺_in) channels.

These channels open on hyperpolarization of E_m (membrane potential) and facilitate K⁺ uptake. The outward-rectifying channels, on the other hand, open on depolarization of E_m and transport K⁺ out of the cell.

The outward-rectifying channels are involved in (1) osmotic adjustment, (2) stomatal function, (3) regulation of E_m, and (4) the unloading of salts from xylem parenchyma into xylem vessels for long distance transport to the shoot. K⁺_in channels in the root plasma membrane correspond to the low-affinity K⁺ transporters.

In roots K⁺ absorption also takes place by high-affinity K⁺ transporters, which act as K⁺-H⁺ symporters energized by H⁺-ATPase. These transporters function in parallel with one or more types of K⁺_in channels that mediate low-affinity uptake and are important for K⁺ acquisition at higher concentrations of soil K⁺.

Molecular investigations have identified many plant genes that encode K⁺ transporters. The first low-affinity K⁺ transport genes to be cloned in plants are KATI and AKTI. Cloning has been done by complementation of yeast mutants defective in K⁺ uptake.

The expression of a heterologous gene restores function to a known mutant of yeast. In this way a diverse range of plant transporter genes encoding transporters of sugars, amino acids, NH₄⁺, SO₄²⁺, etc. have been identified and cloned.

By the same technique high-affinity K⁺ transporter gene, HKTI, has been identified and cloned. The product HKTI protein has a mass of 59 kDa and 12 trans membrane domains. Recent investigations suggest that HKTI is a Na⁺ – K⁺ co-transporter. Charophytic algae, which grow in brackish waters containing high Na⁺ possess a Na⁺ coupled high-affinity K⁺ transport system.

The same co-transport system operates in the plasma membrane of leaf cells of the aquatic plant, Egeria.

But there is no evidence for the existence of such a system in the roots of well-studied plant materials like wheat, barley and Arabidopsis. HKTI mRNA has been found to be localized in inner root cortex, stele and the vasculature of leaves and stems, but not in the root epidermis and outer cortex — the primary absorption zone for high-affinity uptake of K⁺ from the soil.

The hypothesis that a Na⁺ – K⁺ co-transporter mediates high affinity K⁺ transport in roots has been disputed for several reasons as follows:

1. It is difficult to conceive a transport system in roots through which absorption of an essential element (K⁺) is energized by the entrance of a toxic cation (Na⁺).

2. Most soils contain low concentration of Na⁺.

3. It is not essential for the growth of most plants.

Members of another family of genes encoding high-affinity K⁺ transporters cloned in Arabidopsis are AtKUP1 and AtKUP2. The encoding protein has a mass of 79kDa and 12 trans membrane domains. These transporters appear to be expressed in roots, leaves, and flowers. Reverse genetics strategies helped to analyse plant ion transporters.

The T-DNA (transferred DNA) of Agrobacterium tumefaciens is used to generate insertional mutagens in the plant genome. T-DNA after insertion knocks out the function of a specific gene and at the same time tags the gene for subsequent cloning.

As the T-DNA sequence is known, PCR (polymerase chain reaction) primers for T-DNA and for identified plant transport genes of interest can be designed to detect the presence of T-DNA with a particular gene. Thus, 9100 T-DNA-transformed Arabidopsis lines for mutations in genes involved in signal transduction and ion transport have been screened.

It has been observed that high-affinity K⁺ uptake is inhibited by NH₄⁺. The possibility may be that the transporter mediates both K⁺ and NH₄⁺ uptake.

Another possibility is that when K⁺ concentrations are low, high concentrations of NH₄⁺ inhibit K⁺ transport pathway of the plant. This situation makes the plant dependent on AKT1-mediated K⁺ transport. NH₄⁺ probably blocks K⁺ uptake through AtKUP1. So, it appears that K⁺ transport in plants is very complicated.

A summary of the cloned K⁺ transporter plant genes is given in the following table:

Mineral Nutrient Transport: Type # 2.

Phosphorus (P) Transport:

Physiological investigations indicate that phosphate is transported into roots by an active, high- affinity mechanism. H₂PO₄^– is the primary form of P transported into root cells. Usually, 25 to 40 kl is required to transport 1 mol of P_i into root cells, which is equivalent to the amount of energy derived from the hydrolysis of 1 mol of ATP.

Thus, the transport mechanism for active P_i influx into root cells must either be an ATPase, or a secondary active transporter indirectly coupled to the trans membrane electrochemical H⁺ gradient generated by the plasma membrane H ⁺ -ATPase.

The following observations indicate that P, absorption by roots is mediated by H ⁺-coupled co-transporter:

1. 2-4 protons are absorbed by roots along with the entry of every P_i, accompanied by an alkalinization of the external medium.

2. Electrophysiological measurements indicate that the process is accompanied by a transient depolarization of E_m, followed by a repolarization of E_m. It results in the acidification of the cytoplasm, which is detected by pH microelectrode and pH sensitive fluorescent dyes.

3. Protonophores such as CCCP (carbonyl cyanide m-chlorophenylhydrazone), that can dissipate proton gradient, abolish root P_i absorption.

Studies with physiologically relevant concentrations of P_i (low micro molar range) supports the existence of high-affinity P_i-transporter. Several high-affinity p, transporter genes have been cloned and characterized. These are PHO84 from yeast, AtPT1 and AtPT2 from Arabidopsis, and similar genes from Neurospora and Glomus versiforme (a mycorrhizal fungus).

All the product P_i-transporters possess some important structural similarities. They are characterized by six N-terminal trans membrane domains and six C-terminal trans membrane domains separated by a central hydrophilic region. These P_i-transporters are encoded by a small gene family comprising 2 or 3 members. The signal regulating the transcription of these genes comes from the P status of the shoot.

Mineral Nutrient Transport: Type # 3.

Iron (Fe) Transport:

Fe absorption from the soil is a two-step process. In the first step the plasma membrane Fe reductase reduces extracellular Fe(III) chelates, releasing free Fe²⁺ ions. In the second step, Fe²⁺ is absorbed by way of a specific Fe^{2 +} transporter or a less specific divalent cation transport system.

In addition, induction of the plasma membrane H ⁺ -ATPase increases Fe availability by acidifying the rhizosphere.

Two genes encoding plasma membrane ferric reductases, FRE1 and FRE2, have been cloned in yeast. These genes are transcriptionally up-regulated by Fe deficiency. From the same organism high- and low-affinity Fe²⁺ transporter genes have also been cloned. The high-affinity Fe²⁺ transporter is encoded by the FTR1 gene, whereas the low-affinity Fe²⁺ transporter is encoded by the FET4 gene.

The high-affinity transporter requires another gene product, FET3, an oxidase that contains several Cu ions. Thus, Cu is required for normal Fe absorption. Expression of ferric reductase and the high- and low-affinity Fe²⁺ transporters is induced by Fe deficiency. This response and the Fe reduction mechanism (= increased H ⁺ excretion and reductase activity) is ordinarily observed in dicots.

When grasses and dicots are grown in calcareous soils containing low amounts of available Fe, grasses often are more effective than dicots in resisting Fe-deficiency chlorosis. It indicates that grasses acquire Fe from the soil by a different mechanism.

In response to Fe deficiency rice and oat roots release Fe^{3 +}-chelating compounds. These Fe chelating compounds are non-protein amino acids such as mugineic acid and avenic acid. As they are effective ferric chelators, they are known as phytosiderophores (= specific low-molecular mass organic compounds with a high affinity for Fe^{3 +}, which solubilize ferric ions and make them available for absorption).

There is an Fe transporter in the root cell plasma membrane in grasses that recognizes specific Fe(III) – phytosiderophore complexes and transport the entire complex across the plasma membrane.

Mineral Nutrient Transport: Type # 4.

Zinc (Zn) Transport:

Zn plays an important role in key structural motifs in transcriptional regulatory proteins, including Zn finger, Zn cluster, and RING finger domains, which are also found in a large number of different proteins. Zinc deficiency in soils and its uptake has been recognized recently as an important worldwide problem.

In Saccharomyces cerevisiae two Zn²⁺ transporter genes have been cloned. ZRTI gene encodes a high-affinity Zn²⁺ transporter and ZRT2 gene encodes a low-affinity Zn²⁺ transporter. Both of them are transcriptionally up-regulated by low concentrations of cellular Zn.

Recently, a number of Zn transporter genes identified in different plant species are given in the following table:

IRT1 and RIT1 transporters can transport Fe along with Zn. In general multiple members of each mineral ion transport family encode similar transporters that are expressed in different tissues or cell types in response to specific environmental stresses.