Potassium: A key modulator for cell homeostasis
Introduction
Potassium (K) plays a significant role in various vital processes within the plants. K is one of the 17 chemical elements required for plant growth and development (Ajewole et al., 2018). K is found in four forms in the soil: Ionic form (0.1−0.2%), exchangeable form (1–2 %), non-exchangeable form (1–10 %) and unavailable form (90–98 %). The only first two forms are the one that can be utilized by the plants and available for the immediate requirement by plants, while the latter two are the non-labile one and they maintain the supply for longer-term (Volf et al., 2018; Grzebisz et al., 2012; Askegaard et al., 2003). The availability of the K from non-labile source depends on the time of deposition, the intensity of weathering and the type as well as the proportion of clay materials (Huang, 2005) but the release of K from exchangeable source is very low than the uptake rate in plants; thus K+ concentration becomes a limiting factor for plants (Zorb et al., 2014).
Although K is the seventh most abundant element, only 1–2 % is available for the plant (Talibudeen et al., 1978). Importantly tobacco stems, wool waste, sugar beet factory waste, and flue dust contain K. However, their usage as a fertilizer is limited. K is symbolized as K2O on the fertilizer labels and occupying the third numbers on the labelled like 'N: P: K'. In most of the places, 95 % of all K based fertilizers come in the form of muriate of potash, which is also known as potassium chloride (Zahoor et al., 2017). Samuel William Jackson, a botanist in Connecticut, burned plants and analysed the ash in 1868 for crosschecking the K content in plants where Jackson found consisting traces of K as along with other minerals (Zhu, 2001a, 2001b).
More than 60 enzymes within the plant system depend on K for its activation, in which K act as a key regulator. K helps plants to resist several abiotic and biotic stresses in the environment (Borrelli et al., 2018). K is the most profuse cation in the plant, which forms up to 10 % on a dry weight basis (Leigh and Storey, 1993). It is present in cytosol and chloroplast at higher concentration. It activates many enzymes by stabilizing the pH in the range of 7–8, with the help of changes made in enzymatic conformation and its attachment to the enzyme surface (Naeem et al., 2018; Godsey et al., 2007). It plays a significant role in induced cell elongation and maintenance of osmoregulation within the plants (Huang et al., 2017; Zahoor et al., 2017). K assimilation is an essential pathway for equalizing the heavy metal toxicity in plants by the formation of soluble protein, carbohydrate and soluble nitrogen compounds in the cell sap of the plants. It also provides the tolerance of a plant to diseases (Ahmad et al., 2016a, b; Ma et al., 2016). Additionally, K controls the opening and closing of stomata which controls efficient use of the water required by plants for their growth and development. K also aids plants in the production of starch as well as regulate root-growth. Interestingly K+ requirement is high in the starchy plants (Jansson, 1980). K further promotes the strength and length of cotton fibers as well as enhances the shelf life of fruits and overall positive changes in qualitative and quantitative parameters of plants (Yasin et al., 2018).
Although K does not display a part of the chemical structure of plant proteins, it does play many regulatory roles in the overall growth of plants (Guo et al., 2017). The concentration of K present in the cell determines how many enzymes can be activated, and this determines the rate at which the chemical reaction takes place (Huang et al., 2017). Stomata become inactive in low K availability, and that results in a slower response (Nieves-Cordones et al., 2016). Consequently, the closure of the stomata takes several hours rather than a few minutes; thus, conflicts of water stress arises. K plays a significant role in the electrical charge balance at the site of the ATP production (Al-Younis et al., 2018; Zhang et al., 2018). Less availability of K reduces ATP production and the rate of photosynthesis in plants (Adams et al., 2018; Borrelli et al., 2018; Pramanik et al., 2018). Accordingly, all the processes which are dependent on the ATP are affected directly or indirectly on the level of K (Cuin et al., 2010; Xiang et al., 2018). Thus, K has been recognized for its various roles in plants.
Section snippets
Potassium availability in the soil
The exponentially increasing world population needs additional food supply to cater to the need of masses. Modern climate models predict an increase in nutrient deficiency, heat and salt stress worldwide, which effectively decreasing the crop yield, thus disturbing food security. Therefore, the focus should be to look for alternative remedial steps that can lead to increased crop yield by mitigating abiotic and biotic stress conditions (Raza et al., 2019). In this context, K is one of the vital
Potassium and salt stress
The accumulation of high salt concentrations in the soil exerts dual stress on the crop (Munns and Tester, 2008). Firstly, it causes osmotic stress by making the soil harder for plant root to take up the water due to which soil water potential decreases with the accumulation of salts in the plant cell wall. Secondly, salinity exerts ionic stress on the plant due to phytotoxicity of the ions present in high concentrations in the soil, which may be fatal to the plants. The fatality may be in the
Potassium and growth hormones: Responses to K-ion deficiency
The importance of K in plant hormone signaling is an important aspect. K deficiency is a significant problem in the agriculture field (Römheld and Kirkby, 2010). This issue was identified in the early stage of plant physiological responses related to K deficiency symptoms at the physiological level (Prajapati and Modi, 2012). Its starvation leads to [a] growth arrest, [b] impaired nitrogen and carbon ratios, photosynthesis process and translocation, [c] probably increased susceptibility to
Potassium crosstalk with other related element
Under nutritional stress conditions, signaling in plant roots for ROS production is initiated. In response to various nutritional stresses, crosstalk occurs in several signaling pathways. Under both transcriptional and post-transcriptional stages, nutrition through nitrogen and K+ ion signaling has a close association (Tsay et al., 2011). K+ transporter genes expression has been stimulated due to deficiency of K+ ions in Arabidopsis, whereas alteration in the transcription of NO3− transporter
Regulation of ROS production under K-ion deficiency
ROS is a free radical molecule which impairs physiological processes under various types of abiotic and biotic stresses (Das and Roychoudhury, 2014). ROS interrupts mechanism of ion transport through oxidation of sulphydryl groups, lipid peroxidation and, or inhibiting membrane-bound regulatory enzymes leading to disrupted oxidative phosphorylation cycle as well as ATP levels (Kourie, 1998). But these radicals are scavenged either by antioxidant machinery or compatible solutes (Hasegawa et al.,
Potassium ion channels
Previous studies reported that soil has low K+ concentration (0.1–1.0 mM) as compared to its requirement in plants i.e. high K+ (∼100 mM) (Schroeder et al., 1994; Wyn Jones and Pollard, 1983). The plants are unable to avoid this stress and has to invest energy for the uptake of K+ and its distribution throughout the plant. Plant roots are the primary organ for balancing such type of pressures (Borrelli et al., 2018). Therefore, for adopting such kind of nutrient acquisition, plants have
Effect of excess potassium in plants
Nutrients are essential for the healthy growth of plants. The quantities of nutrients may vary for example the quantity of macronutrients requirement such as N, P and K is 50–150 lbs/acre (McCauley et al., 2009). Although K is not excessively absorbed by plants and hence, it is not toxic. During the compensation of charge, K acts as a dominant cation for counterbalancing the immobile anions in the chloroplast, cytoplasm, vacuoles, phloem and xylem (Hasanuzzaman et al., 2018a,b). To balance the
Biotechnological aspects and application of potassium for Molecular Breeding in Crops
The redistribution and uptake of K in various organs and cellular K+ homeostasis is facilitated by a number of genes which encode different channels and K+ transporters in plants. K+ transporters of plants are derived from CHX, NHX, HKT, K+ channel proteins, KT/HAK/KUP families which are encoded by genes from K+ inward rectifier (KIR), tandem-pore K+ (TPK) and Shaker families (Wang and Wu, 2013). All these transporters have shown great potential in stress tolerance and improving K use
Conclusion
K acts as a cross-talking molecule and plays a significant role in various vital processes within the plants. Being an extraordinary nutrient in plant growth and developments, K holds high importance for further research on its molecular dynamics in plant biology. Despite its vast presence in soil it is not freely available to plants and its deficiency cause several diseases in plants. The mobilization and availability of K in plants depend on K+-solubilizing microbes that help in releasing K
CRediT authorship contribution statement
Prasann Kumar: Conceptualization, Data curation, Investigation, Methodology, Validation, Writing - original draft. Tapan Kumar: Data curation, Investigation, Methodology, Validation, Writing - original draft. Simranjeet Singh: Data curation, Investigation, Methodology, Writing - review & editing. Narendra Tuteja: Supervision, Validation, Writing - review & editing. Ram Prasad: Methodology, Supervision, Validation, Writing - review & editing. Joginder Singh: Methodology, Supervision, Validation,
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgements
PK, TK, SS, JS gratefully acknowledge the support provided by Lovely Professional University and RP acknowledge the support of MGCUB, India.
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