K+ uptake systems in the yeast Hansenula polymorpha. Transcriptional and post-translational mechanisms involved in high-affinity K+ transporter regulation

Candida albicans Potassium Transporters

Potassium is basic for life. All living organisms require high amounts of intracellular potassium, which fulfils multiple functions. To reach efficient potassium homeostasis, eukaryotic cells have developed a complex and tightly regulated system of transporters present both in the plasma membrane and in the membranes of internal organelles that allow correct intracellular potassium content and distribution. We review the information available on the pathogenic yeast Candida albicans. While some of the plasma membrane potassium transporters are relatively well known and experimental data about their nature, function or regulation have been published, in the case of most of the transporters present in intracellular membranes, their existence and even function have just been deduced because of their homology with those present in other yeasts, such as Saccharomyces cerevisiae. Finally, we analyse the possible links between pathogenicity and potassium homeostasis. We comment on the possibility of using some of these transporters as tentative targets in the search for new antifungal drugs.

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.

Several Yeast Species Induce Iron Deficiency Responses in Cucumber Plants (Cucumis sativus L.)

Iron (Fe) deficiency is a first-order agronomic problem that causes a significant decrease in crop yield and quality. Paradoxically, Fe is very abundant in most soils, mainly in its oxidized form, but is poorly soluble and with low availability for plants. In order to alleviate this situation, plants develop different morphological and physiological Fe-deficiency responses, mainly in their roots, to facilitate Fe mobilization and acquisition. Even so, Fe fertilizers, mainly Fe chelates, are widely used in modern agriculture, causing environmental problems and increasing the costs of production, due to the high prices of these products. One of the most sustainable and promising alternatives to the use of agrochemicals is the better management of the rhizosphere and the beneficial microbial communities presented there. The main objective of this research has been to evaluate the ability of several yeast species, such as Debaryomyces hansenii, Saccharomyces cerevisiae and Hansenula polymorpha, to induce Fe-deficiency responses in cucumber plants. To date, there are no studies on the roles played by yeasts on the Fe nutrition of plants. Experiments were carried out with cucumber plants grown in a hydroponic growth system. The effects of the three yeast species on some of the most important Fe-deficiency responses developed by dicot (Strategy I) plants, such as enhanced ferric reductase activity and Fe²⁺ transport, acidification of the rhizosphere, and proliferation of subapical root hairs, were evaluated. The results obtained show the inductive character of the three yeast species, mainly of Debaryomyces hansenii and Hansenula polymorpha, on the Fe-deficiency responses evaluated in this study. This opens a promising line of study on the use of these microorganisms as Fe biofertilizers in a more sustainable and environmentally friendly agriculture.

Facultative symbiosis with a saprotrophic soil fungus promotes potassium uptake in American sweetgum trees

Several species of soil free-living saprotrophs can sometimes establish biotrophic symbiosis with plants, but the basic biology of this association remains largely unknown. Here, we investigate the symbiotic interaction between a common soil saprotroph, Clitopilus hobsonii (Agaricomycetes), and the American sweetgum (Liquidambar styraciflua). The colonized root cortical cells were found to contain numerous microsclerotia-like structures. Fungal colonization led to increased plant growth and facilitated potassium uptake, particularly under potassium limitation (0.05 mM K⁺ ). The expression of plant genes related to potassium uptake was not altered by the symbiosis, but colonized roots contained the transcripts of three fungal genes with homology to K⁺ transporters (ACU and HAK) and channel (SKC). Heterologously expressed ChACU and ChSKC restored the growth of a yeast K⁺ -uptake-defective mutant. Upregulation of ChACU transcript under low K⁺ conditions (0 and 0.05 mM K⁺ ) compared to control (5 mM K⁺ ) was demonstrated in planta and in vitro. Colonized plants displayed a larger accumulation of soluble sugars under 0.05 mM K⁺ than non-colonized plants. The present study suggests reciprocal benefits of this novel tree-fungus symbiosis under potassium limitation mainly through an exchange of additional carbon and potassium between both partners.

The Potassium Transporter Hak1 in Candida Albicans, Regulation and Physiological Effects at Limiting Potassium and under Acidic Conditions

The three families of yeast plasma membrane potassium influx transporters are represented in Candida albicans: Trk, Acu, and Hak proteins. Hak transporters work as K⁺-H⁺ symporters, and the genes coding for Hak proteins are transcriptionally activated under potassium limitation. This work shows that C. albicans mutant cells lacking CaHAK1 display a severe growth impairment at limiting potassium concentrations under acidic conditions. This is the consequence of a defective capacity to transport K⁺, as indicated by potassium absorption experiments and by the kinetics parameters of Rb⁺ (K⁺) transport. Moreover, hak1- cells are more sensitive to the toxic cation lithium. All these phenotypes became much less robust or even disappeared at alkaline growth conditions. Finally, transcriptional studies demonstrate that the hak1- mutant, in comparison with HAK1+ cells, activates the expression of the K⁺/Na⁺ ATPase coded by CaACU1 in the presence of Na⁺ or in the absence of K⁺.

Cation Transporters of Candida albicans-New Targets to Fight Candidiasis?

Candidiasis is the wide-spread fungal infection caused by numerous strains of yeast, with the prevalence of Candida albicans. The current treatment of candidiasis is becoming rather ineffective and costly owing to the emergence of resistant strains; hence, the exploration of new possible drug targets is necessary. The most promising route is the development of novel antibiotics targeting this pathogen. In this review, we summarize such candidates found in C. albicans and those involved in the transport of (metal) cations, as the latter are essential for numerous processes within the cell; hence, disruption of their fluxes can be fatal for C. albicans.

Regulation and activity of CaTrk1, CaAcu1 and CaHak1, the three plasma membrane potassium transporters in Candida albicans

Wild-type cells of Candida albicans, the most common human fungal pathogen, are able to grow at very low micromolar concentrations of potassium in the external milieu. One of the reasons behind that behaviour is the existence of three different types of K⁺ transporters in their plasma membrane: Trk1, Acu1 and Hak1. This work shows that the transporters are very differently regulated at the transcriptional level upon exposure to saline stress, pH alterations or K⁺ starvation. We propose that different transporters take the lead in the diverse environmental conditions, Trk1 being the "house-keeping" one, and Acu1/Hak1 dominating upon K⁺ limiting conditions. Heterologous expression of the genes coding for the three transporters in a Saccharomyces cerevisiae strain lacking its endogenous potassium transporters showed that all of them mediated cation transport but with very different efficiencies. Moreover, expression of the transporters in S. cerevisiae also affected other physiological characteristics such as sodium and lithium tolerance, membrane potential or intracellular pH, being, in general, CaTrk1 the most effective in keeping these parameters close to the usual wild-type physiological levels.

The Role of Soil Fungi in K+ Plant Nutrition

K⁺ is an essential cation and the most abundant in plant cells. After N, its corresponding element, K, is the nutrient required in the largest amounts by plants. Despite the numerous roles of K in crop production, improvements in the uptake and efficiency of use of K have not been major focuses in conventional or transgenic breeding studies in the past. In research on the mineral nutrition of plants in general, and K in particular, this nutrient has been shown to be essential to soil-dwelling-microorganisms (fungi, bacteria, protozoa, nematodes, etc.) that form mutualistic associations and that can influence the availability of mineral nutrients for plants. Therefore, this article aims to provide an overview of the role of soil microorganisms in supplying K⁺ to plants, considering both the potassium-solubilizing microorganisms and the potassium-facilitating microorganisms that are in close contact with the roots of plants. These microorganisms can influence the active transporter-mediated transfer of K⁺. Regarding the latter group of microorganisms, special focus is placed on the role of endophytic fungus. This review also includes a discussion on productivity through sustainable agriculture.

A low K+ signal is required for functional high-affinity K+ uptake through HAK5 transporters

The high-affinity K(+) transporter HAK5 is a key system for root K(+) uptake and, under very low external K(+), the only one capable of supplying K(+) to the plant. Functional HAK5-mediated K(+) uptake should be tightly regulated for plant adaptation to different environmental conditions. Thus, it has been described that the gene encoding the transporter is transcriptionally regulated, being highly induced under K(+) limitation. Here we show that environmental conditions, such as the lack of K(+), NO(3)(-) or P, that induced a hyperpolarization of the plasma membrane of root cells, induce HAK5 transcription. However, only the deprivation of K(+) produces functional HAK5-mediated K(+) uptake in the root. These results suggest on the one hand the existence of a posttranscriptional regulation of HAK5 elicited by the low K(+) signal and on the other that HAK5 may be involved in yet-unknown functions related to NO(3)(-) and P deficiencies. These results have been obtained here with Solanum lycopersicum (cv. Micro-Tom) as well as Arabidopsis thaliana plants, suggesting that the posttranscriptional regulation of high-affinity HAK transporters take place in all plant species.

FgFim, a key protein regulating resistance to the fungicide JS399-19, asexual and sexual development, stress responses and virulence in Fusarium graminearum

Fimbrin is an actin-bundling protein found in intestinal microvilli, hair cell stereocilia and fibroblast filopodia. Its homologue Sac6p has been shown to play a critical role in endocytosis and diverse cellular processes in Saccharomyces cerevisiae. FgFim from the wheat scab pathogenic fungus Fusarium graminearum strain Y2021A, which is highly resistant to the fungicide JS399-19, was identified by screening a mutant library generated by HPH-HSV-tk cassette-mediated integration. The functions of FgFim were evaluated by constructing a deletion mutant of FgFim, designated ΔFgFim-15. The deletion mutant exhibited a reduced rate of mycelial growth, reduced conidiation, delayed conidium germination, irregularly shaped hyphae, a lack of sexual reproduction on autoclaved wheat kernels and a dramatic decrease in resistance to JS399-19. ΔFgFim-15 also exhibited increased sensitivity to diverse metal cations, to agents that induce osmotic stress and oxidative stress, and to agents that damage the cell membrane and cell wall. Pathogenicity assays showed that the virulence of the FgFim deletion mutant on flowering wheat heads was impaired, which was consistent with its reduced production of the toxin deoxynivalenol in host tissue. All of these defects were restored by genetic complementation of the mutant with the parental FgFim gene. Quantitative real-time polymerase chain reaction (PCR) assays showed that the basal expression of three Cyp51 genes, which encode sterol 14α-demethylase, was significantly lower in the mutant than in the parental strain. The results of this study indicate that FgFim plays a critical role in the regulation of resistance to JS399-19 and in various cellular processes in F. graminearum.

K+ uptake in plant roots. The systems involved, their regulation and parallels in other organisms

Potassium (K(+)) is an essential macronutrient for plants. It is taken into the plant by the transport systems present in the plasma membranes of root epidermal and cortical cells. The identity of these systems and their regulation is beginning to be understood and the systems of K(+) transport in the model species Arabidopsis thaliana remain far better characterized than in any other plant species. Roots can activate different K(+) uptake systems to adapt to their environment, important to a sessile organism that needs to cope with a highly variable environment. The mechanisms of K(+) acquisition in the model species A. thaliana are the best characterized at the molecular level so far. According to the current model, non-selective channels are probably the main pathways for K(+) uptake at high concentrations (>10mM), while at intermediate concentrations (1mM), the inward rectifying channel AKT1 dominates K(+) uptake. Under lower concentrations of external K(+) (100μM), AKT1 channels, together with the high-affinity K(+) uptake system HAK5 contribute to K(+) acquisition, and at extremely low concentrations (<10μM) the only system capable of taking up K(+) is HAK5. Depending on the species the high-affinity system has been named HAK5 or HAK1, but in all cases it fulfills the same functions. The activation of these systems as a function of the K(+) availability is achieved by different mechanisms that include phosphorylation of AKT1 or induction of HAK5 transcription. Some of the characteristics of the systems for root K(+) uptake are shared by other organisms, whilst others are specific to plants. This indicates that some crucial properties of the ancestral of K(+) transport systems have been conserved through evolution while others have diverged among different kingdoms.

Coordination of K+ transporters in neurospora: TRK1 is scarce and constitutive, while HAK1 is abundant and highly regulated

Fungi, plants, and bacteria accumulate potassium via two distinct molecular machines not directly coupled to ATP hydrolysis. The first, designated TRK, HKT, or KTR, has eight transmembrane helices and is folded like known potassium channels, while the second, designated HAK, KT, or KUP, has 12 transmembrane helices and resembles MFS class proteins. One of each type functions in the model organism Neurospora crassa, where both are readily accessible for biochemical, genetic, and electrophysiological characterization. We have now determined the operating balance between Trk1p and Hak1p under several important conditions, including potassium limitation and carbon starvation. Growth measurements, epitope tagging, and quantitative Western blotting have shown the gene HAK1 to be much more highly regulated than is TRK1. This conclusion follows from three experimental results: (i) Trk1p is expressed constitutively but at low levels, and it is barely sensitive to extracellular [K(+)] and/or the coexpression of HAK1; (ii) Hak1p is abundant but is markedly depressed by elevated extracellular concentrations of K(+) and by coexpression of TRK1; and (iii) Carbon starvation slowly enhances Hak1p expression and depresses Trk1p expression, yielding steady-state Hak1p:Trk1p ratios of ∼500:1, viz., 10- to 50-fold larger than that in K(+)- and carbon-replete cells. Additionally, it appears that both potassium transporters can adjust kinetically to sustained low-K(+) stress by means of progressively increasing transporter affinity for extracellular K(+). The underlying observations are (iv) that K(+) influx via Trk1p remains nearly constant at ∼9 mM/h when extracellular K(+) is progressively depleted below 0.05 mM and (v) that K(+) influx via Hak1p remains at ∼3 mM/h when extracellular K(+) is depleted below 0.1 mM.

Nitrogen-dependent calcineurin activation in the yeast Hansenula polymorpha

Non-preferred nitrogen sources, unlike preferred ones, raised total cell Ca(2+) content and expression of ENA1, a very well-known calcineurin-regulated gene. This indicates calcineurin activation is regulated by nitrogen source. Nitrogen catabolite repression (NCR) and nitrate induction mechanisms, both regulating nitrate assimilation in Hansenula polymorpha, are controlled by calcineurin. Concerning NCR, lack of calcineurin (cnb1 mutant) decreased nitrate-assimilation gene expression, levels of the transcription factor Gat1 and growth in several nitrogen sources. We found that the role of calcineurin in NCR was mediated by Crz1 via Gat1. Regarding nitrate induction, calcineurin also affects the levels of transcription factors Gat2 and Yna2 involved in this process. We conclude that Ca(2+) and calcineurin play a central role in nitrogen signalling and assimilation. Thus, the nitrogen source modulates Ca(2+) content and calcineurin activation. Calcineurin in turn regulates nitrogen assimilation genes.