**1. Introduction**

Plant growth and development often depend on various biotic and abiotic stressors. In particular, alterations in environmental conditions have a direct impact on the nutrient uptake and assimilation in plants [1]. In this context, a surplus and even more so a deficiency of essential macronutrients in soils represents one of the most common stress types, with nitrogen (N), potassium (K) and phosphorous (P) being the most relevant ones. These macronutrients are directly involved in multiple metabolic pathways and physiological responses, acting as structural constituents of vital metabolites, playing a key role in osmotic regulation and cellular permeability, being critical for proper growth and development [2]. Currently, it is often assumed that due to the existence of fast recycling mechanisms, macronutrient availability does not limit plant growth in natural, uncultivated systems. However, due to a widely spread overexploitation

of soils, it might be the case in the modern agricultural practice [3]. In particular, deficiency of nitrogen (N) and potassium (K) is quite common in developing and least developed countries, especially in rice and wheat production in Asia, Africa and Central and South America [4]. Such nutrient deficiency eventually leads to decrease of plant productivity and losses of crop yields. Visual manifestations of stress caused by a deficiency of individual macro- and microelements are well documented [5]. The underlying key physiological processes, affected by mineral deficiencies, are well characterized and include photosynthesis, protein synthesis, primary and secondary metabolism and carbohydrate distribution between source and sink tissues [6–8].

The methods of biochemistry and molecular biology proved to be efficient in disclosing the fine regulatory mechanisms behind ion homeostasis in plants [8]. Thus, the emergence of RNA microarray technology tremendously contributed to the investigation of rapid transcriptional changes associated with mineral imbalance [9–13]. Most of the studies addressing plant responses of ion-transporting systems to deficiencies of K<sup>+</sup> and NO3 <sup>−</sup> rely on *Arabidopsis thaliana* [9, 11–13]. The same is true for phosphorus [14, 15], not further detailed here. However, experiments with such crop plants as wheat [16], tomato [17], rice [18], barley, pepper as well as *Mesembryanthemum crystallinum* [19] revealed a pronounced increase in abundance of K<sup>+</sup> transporter transcripts in response to potassium starvation. Similarly, expression of nitrate transporters in wheat roots and leaves [20, 21], sorghum [22] and rice [23, 24] seedlings was enhanced in response to NO3 <sup>−</sup> starvation.

The fact, that all mineral nutrients enter the plant in ionic form and, along with involvement in metabolic processes, are crucial for maintaining the cell ion homeostasis seems to be underestimated. Thereby, the existence of cytosolic and vacuolar ion pools needs to be taken into account. These pools are maintained by numerous membrane ion transporters, representing an integrated part of a complex cellular regulatory network [25]. Hence, the specificity of plant responses to nutritional stress may imply a relevant adjustment of systems involved in absorption, transport, distribution, accumulation and remobilization of mineral ions.

Nitrogen and potassium are the most abundant mineral elements in plant nutrition. The principal features of their reception and distribution in plant organs and cells are well studied to date [2]. On the other hand, a lack of these nutrients in soils (especially in the form of NO3 <sup>−</sup> and K+ ) is quite a common phenomenon [8] that appears to be an appropriate argument for a more thorough analysis of plant responses to NO3 <sup>−</sup> and K+ deficiency given the role of ion homeostasis system in their development.
