Systems involved in K+ uptake from diluted solutions in pepper plants as revealed by the use of specific inhibitors

K+ Nutrition Exchange in the Serendipita-Arabidopsis Symbiosis: Study of the Fungal K+ Transporters Involved

There is mounting evidence that the root-colonizing endosymbiotic fungus Serendipita indica improves plant growth. The beneficial effects have been observed when plants are growing in optimal conditions or under nutritionally deficient soils (e.g., phosphate poor soil) or exposed to stressful environmental conditions such as drought or salinity. However, until now its role in the nutrition of other plant essential macronutrient, such as K+, has not been fully clarified. Here, we study the role of the fungus in the K+ nutrition of Arabidopsis thaliana plants, during growth under K+ limiting conditions. As a first step, we studied the high-affinity K+ uptake of the plant and fungus when growing separately and in symbiosis. In the search for putative fungal actors involved in K+ nutrition, we also have cloned and functionally characterized the K+ transporters of S. indica SiHAK1, SiTRK1, SiTRK2, and SiTOK1, among which it has been shown that SiHAK1 is the main transporter involved in the K+ uptake in the high affinity range of concentrations. In addition, a gene expression study of these transporters and other candidates that could participate in the K+ homeostasis of the fungus has been carried out. The results indicated that, contrary to what happens with P nutrition, S. indica seems not to improve neither the growth nor the plant K+ reserves during K+ starvation. Instead, this nutritionally restrictive condition favored fungal colonization, suggesting that the fungus obtains the greatest benefit in K+ supply during symbiosis.

Regulation of Cytosolic pH: The Contributions of Plant Plasma Membrane H+-ATPases and Multiple Transporters

Cytosolic pH homeostasis is a precondition for the normal growth and stress responses in plants, and H+ flux across the plasma membrane is essential for cytoplasmic pH control. Hence, this review focuses on seven types of proteins that possess direct H+ transport activity, namely, H+-ATPase, NHX, CHX, AMT, NRT, PHT, and KT/HAK/KUP, to summarize their plasma-membrane-located family members, the effect of corresponding gene knockout and/or overexpression on cytosolic pH, the H+ transport pathway, and their functional regulation by the extracellular/cytosolic pH. In general, H+-ATPases mediate H+ extrusion, whereas most members of other six proteins mediate H+ influx, thus contributing to cytosolic pH homeostasis by directly modulating H+ flux across the plasma membrane. The fact that some AMTs/NRTs mediate H+-coupled substrate influx, whereas other intra-family members facilitate H+-uncoupled substrate transport, demonstrates that not all plasma membrane transporters possess H+-coupled substrate transport mechanisms, and using the transport mechanism of a protein to represent the case of the entire family is not suitable. The transport activity of these proteins is regulated by extracellular and/or cytosolic pH, with different structural bases for H+ transfer among these seven types of proteins. Notably, intra-family members possess distinct pH regulatory characterization and underlying residues for H+ transfer. This review is anticipated to facilitate the understanding of the molecular basis for cytosolic pH homeostasis. Despite this progress, the strategy of their cooperation for cytosolic pH homeostasis needs further investigation.

Genome-Wide Association Study of Natural Variation in Arabidopsis Exposed to Acid Mine Drainage Toxicity and Validation of Associated Genes with Reverse Genetics

Acid mine drainage (AMD) is a huge environmental problem in mountain-top mining regions worldwide, including the Appalachian Mountains in the United States. This study applied a genome-wide association study (GWAS) to uncover genomic loci in Arabidopsis associated with tolerance to AMD toxicity. We characterized five major root phenotypes—cumulative root length, average root diameter, root surface area, root volume, and primary root length—in 180 Arabidopsis accessions in response to AMD-supplemented growth medium. GWAS of natural variation in the panel revealed genes associated with tolerance to an acidic environment. Most of these genes were transcription factors, anion/cation transporters, metal transporters, and unknown proteins. Two T-DNA insertion mutants, At1g63005 (miR399b) and At2g05635 (DEAD helicase RAD3), showed enhanced acidity tolerance. Our GWAS and the reverse genetic approach revealed genes involved in conferring tolerance to coal AMD. Our results indicated that proton resistance in hydroponic conditions could be an important index to improve plant growth in acidic soil, at least in acid-sensitive plant species.

Pharmacological and gene regulation properties point to the SlHAK5 K+ transporter as a system for high-affinity Cs+ uptake in tomato plants

Potassium (K⁺ ) and cesium (Cs⁺ ) are chemically similar but while K⁺ is an essential nutrient, Cs⁺ can be toxic for living organisms, plants included. Two different situations could lead to problems derived from the presence of Cs⁺ in agricultural systems: (1) presence of Cs⁺ at high concentrations that could produce toxic effects on plants, (2) presence of micromolar concentrations of radiocesium, which can be accumulated in the plant and affect animal and human health through the food chain. While K⁺ uptake has been well described in tomato plants, information on molecular mechanisms involved in Cs⁺ accumulation in this species is absent. Here, we show that in tomato plants, high concentrations of Cs⁺ produce deficiency of K⁺ but do not induce high-affinity K⁺ uptake or the gene encoding the high-affinity K⁺ transporter SlHAK5. At these concentrations, Cs⁺ uptake takes place through a Ca²⁺ -sensitive pathway, probably a non-selective cation channel. At micromolar concentrations, Cs⁺ is accumulated by a high-affinity uptake system upregulated in K⁺ -starved plants. This high-affinity Cs⁺ uptake shares features with high-affinity K⁺ uptake. It is sensitive to NH₄⁺ and insensitive to Ba²⁺ and Ca²⁺ and its presence parallels the pattern of SlHAK5 expression. Moreover, blockers of reactive oxygen species and ethylene action repress SlHAK5 and negatively regulate both high-affinity K⁺ and Cs⁺ uptake. Thus, we propose that SlHAK5 contributes to Cs⁺ uptake from micromolar concentrations in tomato plants and can constitute a pathway for radiocesium transfer from contaminated areas to the food chain.

A Ca(2+)-sensitive system mediates low-affinity K(+) uptake in the absence of AKT1 in Arabidopsis plants

K(+) acquisition by Arabidopsis roots is mainly mediated by the high-affinity K(+) transporter AtHAK5 and the inward-rectifier K(+) channel AtAKT1. This model is probably universal to plants. Mutant plants lacking these two systems (athak5,atakt1) take up K(+) and grow when the external K(+) concentration is above a certain level, indicating that an additional transport system may compensate for the absence of AtHAK5 and AtAKT1. Here we describe that this alternative system is essential for providing sufficient K(+) to sustain growth of athak5,atakt1 plants. This system is especially sensitive to Ca(2+), Mg(2+), Ba(2+) and La(3+), it transports Cs(+) and its activity is reduced by cyclic nucleotides. These results suggest that a Ca(2+)-permeable voltage-independent non-selective cation channel, probably belonging to the cyclic nucleotide gated channel (CNGC) family, may provide the pathway for K(+) uptake in athak5,atakt1 plants. The genes encoding the two members of the CNGC family that have been described as mediating root K(+) uptake, AtCNGC3 and AtCNGC10, are not up-regulated in athak5,atakt1 plants, excluding overexpression of these genes as a compensatory mechanism. On the other hand, an increased driving force for K(+) in athak5,atakt1 plants due to a hyperpolarization of the membrane potential of its root cells is also discarded. The identification of this unknown system may provide tools to improve plant K(+) nutrition in conditions where AtAKT1 functionality is reduced, such as under salinity. In addition, this system may constitute an important pathway for accumulation of toxic cations such as Cs(+) or radiocesium ((137)Cs(+)), and could play a role in phytoremediation.

Root K(+) acquisition in plants: the Arabidopsis thaliana model

K(+) is an essential macronutrient required by plants to complete their life cycle. It fulfills important functions and it is widely used as a fertilizer to increase crop production. Thus, the identification of the systems involved in K(+) acquisition by plants has always been a research goal as it may eventually produce molecular tools to enhance crop productivity further. This review is focused on the recent findings on the systems involved in K(+) acquisition. From Epstein's pioneering work >40 years ago, K(+) uptake was considered to consist of a high- and a low-affinity component. The subsequent molecular approaches identified genes encoding K(+) transport systems which could be involved in the first step of K(+) uptake at the plant root. Insights into the regulation of these genes and the proteins that they encode have also been gained in recent studies. A demonstration of the role of the two main K(+) uptake systems at the root, AtHKA5 and AKT1, has been possible with the study of Arabidopsis thaliana T-DNA insertion lines that knock out these genes. AtHAK5 was revealed as the only uptake system at external concentrations <10 μM. Between 10 and 200 μM both AtHAK5 and AKT1 contribute to K(+) acquisition. At external concentrations >500 μM, AtHAK5 is not relevant and AKT1's contribution to K(+) uptake becomes more important. At 10 mM K(+), unidentified systems may provide sufficient K(+) uptake for plant growth.