Organic acid metabolism is of fundamental importance at the cellular and at the whole plant level. In recent years there has been increased attention in the role of organic acids in modulating adaptation to the environment, including organic acids participation in the detoxification of heavy metals. The basis of the phenomenon is the ability of acids such as citrate, malate, oxalate, malonate, aconitate and tartrate to form strong bonds with heavy metal ions through metal chelatation with carboxyl groups carrying the function of donor oxygen in metal-ligands. This review deals with aspects of extracellular and intracellular chelation of heavy metal ions with the involvement of organic acids. We consider the role of metal-induced secretion of malate, citrate and oxalate by roots of various plant species in extracellular complexation of heavy metals and in the reduction of their bioavailability for plants. We also review the possible mechanisms of stimulation of metals uptake by plants under the influence of exogenous application of organic acids in the soil. The efficiency of intracellular chelation of heavy metal ions with the participation of organic acids is considered due to the importance of this strategy in hyperaccumulators and non-hyperaccumulators to improve metal tolerance in plants.
Heavy metals, Zn and Cu, in high concentration (2 mM for Zn and 0.5 mM for Cu) have some inhibiting effect on the growth of Aspergillus niger and Penicillium citrinum. Toxic effects of these metals considerably depend on cultivation conditions including nitrogen sources, pH of nutrient media, and its consistency (presence or absence of agar). In general, nitrate media provides less inhibiting effect on fungal growth under heavy metal exposure than ammonium-containing media. Adding of Zn in nitrate media induces oxalic acid production by fungi. Importance of oxalic acid production in detoxification of heavy metals is confirmed by the formation of Zn-containing crystals in fungal cultures. Cu bringing to the cultural media had no stimulating effect on oxalic acid production as well as no copper-containing crystals were observed. But proceeding from essential increase in oxalic acid production during a long-term fungi adaptation to Cu, it may be proposed that oxalic acid plays some functional role in Cu tolerance of fungi as well. It may be concluded that the role of organic acids and oxalate, in particular, in fungi tolerance and adaptation to heavy metals can be determined by the nature of the metal and its ability to form stable complexes with an acid anion. Stimulating effect of metals on acid production is not universal for all species of fungi and largely depends on metal concentration, nitrogen form in a medium, and other cultivation conditions.
Tight regulation of intra-thallus metabolite distribution in Fucus vesiculosus in late summer reveals the complex biochemical processes complying with reproduction and the preparation to the dark season. We used inductively coupled plasma atomic emission spectroscopy to study the tissue-specific elemental composition, and gas chromatography coupled to mass spectrometry to study the distribution of small-molecular weight primary and secondary metabolites of the brown alga Fucus vesiculosus thalli in the reproductive phase. Beyond general physiological requirements, the observed distribution of the analysed nutrients was also found to depend on characteristics related to the season of harvesting, i.e., the reproductive period. However, a particular curious result was the high metabolic activity found in the stipe of the plant. In conclusion, our data not only provide valuable information for industrial use of fucoids targeting specific algal ingredients, but also give highly interesting insights in the multifaceted system of intra-thallus biochemical interactions during reproduction of the brown algae.
A crucial feature of plant performance is its strong dependence on the availability of essential mineral nutrients, affecting multiple vital functions. Indeed, mineral-nutrient deficiency is one of the major stress factors affecting plant growth and development. Thereby, nitrogen and potassium represent the most abundant mineral contributors, critical for plant survival. While studying plant responses to nutrient deficiency, one should keep in mind that mineral nutrients, along with their specific metabolic roles, are directly involved in maintaining cell ion homeostasis, which relies on a finely tuned equilibrium between cytosolic and vacuolar ion pools. Therefore, in this chapter we briefly summarize the role of the ion homeostasis system in cell responses to environmental deficiency of nitrate and potassium ions. Special attention is paid to the implementation of plant responses via NO 3 − and K + root transport and regulation of ion distribution in cell compartments. These responses are strongly dependent on plant species, as well as severity and duration of nutrient deficiency.Keywords: nutrient deficiency, ion homeostasis, nitrate, potassium, ion transport mechanisms, vacuolar and cytosolic ion pools Cell Growth 2 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][7][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][10][11][12][13]. Most of the studies addressing plant responses of ion-transporting systems to deficiencies of K + and NO 3 − rely on Arabidopsis thaliana [9,[11][12][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 + 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 NO 3 − starvation. The fact, that all mineral nutrients enter the plant in ionic form and, along with i...
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