An extracellular cation coordination site influences ion conduction of OsHKT2;2

Structural insights into ion selectivity and transport mechanisms of Oryza sativa HKT2;1 and HKT2;2/1 transporters

Plant high-affinity K+ transporters (HKTs) play a pivotal role in maintaining the balance of Na+ and K+ ions in plants, thereby influencing plant growth under K+-depleted conditions and enhancing tolerance to salinity stress. Here we report the cryo-electron microscopy structures of Oryza sativa HKT2;1 and HKT2;2/1 at overall resolutions of 2.5 Å and 2.3 Å, respectively. Both transporters adopt a dimeric assembly, with each protomer enclosing an ion permeation pathway. Comparison between the selectivity filters of the two transporters reveals the critical roles of Ser88/Gly88 and Val243/Gly243 in determining ion selectivity. A constriction site along the ion permeation pathway is identified, consisting of Glu114, Asn273, Pro392, Pro393, Arg525, Lys517 and the carboxy-terminal Trp530 from the neighbouring protomer. The linker between domains II and III adopts a stable loop structure oriented towards the constriction site, potentially participating in the gating process. Electrophysiological recordings, yeast complementation assays and molecular dynamics simulations corroborate the functional importance of these structural features. Our findings provide crucial insights into the ion selectivity and transport mechanisms of plant HKTs, offering valuable structural templates for developing new salinity-tolerant cultivars and strategies to increase crop yields.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

Na improves the growth of K-deficient but not K-sufficient kale

The decrease in the use of K fertilizers may be relevant for developing countries that depend on imports, as well as for specific groups such as patients with chronic kidney disease, who have restricted K in their diets. However, the decrease in the use of K affects plant yield, requiring the study of alternatives to mitigate nutritional stress. Sodium is a beneficial element that can mitigate K deficiency, but studies on kale plants are lacking. We investigated the role of Na in kale grown with and without K in nutrient feed solution. Four treatments were used: abundant K, abundant K plus Na, deficient K, and deficient K plus Na. Low Na (2 mmol L^-1) attenuated the symptoms of K deficiency in kale by minimizing leaf water loss and increasing pigment content, leaf area, and plant dry mass. The synergism between K and Na negatively affected the growth of kale plants.

Nutrient cycling is an important mechanism for homeostasis in plant cells

Homeostasis in living cells refers to the steady state of internal, physical, and chemical conditions. It is sustained by self-regulation of the dynamic cellular system. To gain insight into the homeostatic mechanisms that maintain cytosolic nutrient concentrations in plant cells within a homeostatic range, we performed computational cell biology experiments. We mathematically modeled membrane transporter systems and simulated their dynamics. Detailed analyses of 'what-if' scenarios demonstrated that a single transporter type for a nutrient, irrespective of whether it is a channel or a cotransporter, is not sufficient to calibrate a desired cytosolic concentration. A cell cannot flexibly react to different external conditions. Rather, at least two different transporter types for the same nutrient, which are energized differently, are required. The gain of flexibility in adjusting a cytosolic concentration was accompanied by the establishment of energy-consuming cycles at the membrane, suggesting that these putatively "futile" cycles are not as futile as they appear. Accounting for the complex interplay of transporter networks at the cellular level may help design strategies for increasing nutrient use efficiency of crop plants.

Plant HKT Channels: An Updated View on Structure, Function and Gene Regulation

HKT channels are a plant protein family involved in sodium (Na⁺) and potassium (K⁺) uptake and Na⁺-K⁺ homeostasis. Some HKTs underlie salt tolerance responses in plants, while others provide a mechanism to cope with short-term K⁺ shortage by allowing increased Na⁺ uptake under K⁺ starvation conditions. HKT channels present a functionally versatile family divided into two classes, mainly based on a sequence polymorphism found in the sequences underlying the selectivity filter of the first pore loop. Physiologically, most class I members function as sodium uniporters, and class II members as Na⁺/K⁺ symporters. Nevertheless, even within these two classes, there is a high functional diversity that, to date, cannot be explained at the molecular level. The high complexity is also reflected at the regulatory level. HKT expression is modulated at the level of transcription, translation, and functionality of the protein. Here, we summarize and discuss the structure and conservation of the HKT channel family from algae to angiosperms. We also outline the latest findings on gene expression and the regulation of HKT channels.

Expression level of Na+ homeostasis-related genes and salt-tolerant abilities in backcross introgression lines of rice crop under salt stress at reproductive stage

Salt stress in the rice field is one of the most common abiotic stresses, reducing crop productivity, especially at reproductive stage, which is very sensitive to salt stress. The aim of this investigation was to study mRNA-related Na⁺ uptake/translocation and Na⁺ enrichment in the cellular level, leading to physiological changes, growth characteristics, and yield attributes in FL530 [salt-tolerant genotype; carrying SKC1 (in relation to high-affinity potassium transporters controlling Na⁺ and K⁺ translocation) and qSt1b (linking to salt injury score) QTLs] and KDML105 (salt-sensitive cultivar; lacking both QTLs) parental lines and 221-48 (carrying SKC1 and qSt1b QTLs) derived from BILs (backcross introgression lines) at 50% flowering of rice, under 150-mM NaCl until harvesting process. The upregulation of OsHKT1;5 (mediating Na⁺ exclusion into xylem parenchyma cells) and OsNHX1 (Na⁺/H⁺ exchanger to secrete Na⁺ into vacuole) and downregulation of OsHKT2;1 and OsHKT2;2 (mediating Na⁺ restriction in the roots, leaf sheath and older leaves) in cvs. FL530 and 221-48 (+ SKC1; + qSt1b) under salt stress were observed. It restricted Na⁺ level in flag leaf, thereby preventing salt toxicity, as indicated by maintenance of photon yield of PSII (Φ_PSII), net photosynthetic rate (P_n), transpiration rate (E) and overall growth performances. In contrast, Na⁺ enrichment in flag leaf of cv. KDML105 (-SKC1;-qSt1b) caused the reduction in Φ_PSII by 30.5% over the control, leading to the reduction in P_n by 62.3%, in seed sterility by 88.2%, and yield loss by 85.1%. Moreover, the negative relationships between Na⁺ enrichment in flag leaf, physiological changes, and yield traits in rice crop grown under salt stress were demonstrated. Based on this investigation, rice genotype 221-48 was found to possess salt-tolerant traits at reproductive stage and thus could prove to be a potential candidate for future breeding programs.

The grapevine NaE sodium exclusion locus encodes sodium transporters with diverse transport properties and localisation

Grapevine (Vitis vinifera L.) is a valuable crop for human consumption and wine production, and is prone to suffering from salinity stress in arid regions or when exposed to low quality irrigation water. A previous study identified a quantitative trait locus (QTL) NaE, containing six High-affinity Potassium Transporter 1 genes, that was associated with shoot Na⁺ exclusion in grapevine. While HKT1;1 was predicted to be the most likely gene within this QTL to encode for this important salinity tolerance sub-trait, four other HKTs within the QTL remained uncharacterised; VviHKT1;2 encodes a truncated transcript unlikely to form a functional transporter. In this study, two allelic variants for each of VviHKT1;6, VviHKT1;7 and VviHKT1;8 from the heterozygous grapevine variety Cabernet Sauvignon were functionally characterised. Using the Xenopus laevis oocyte heterologous expression system, as well as transient expression in tobacco leaves, we found that the VviHKT1;6 and VviHKT1;7 alleles encoded plasma membrane localised proteins that facilitated significant non-rectifying Na⁺ transport. Conversely, proteins encoded by the VviHKT1;8 alleles were inwardly-rectifying, weak Na⁺ transporters that localised to intracellular organelles. Mining of previous RNA-seq gene expression data suggested that VviHKT1;6-8 are weakly expressed in grapevine roots, flower buds, and seeds under normal conditions and different nutrient regimes. We propose that VviHKT1;6 and VviHKT1;7 are likely to have a less significant role in grapevine leaf Na⁺ exclusion than VviHKT1;1, and that VviHKT1;8 is involved in endomembrane Na⁺ transport.