Aluminum, a Friend or Foe of Higher Plants in Acid Soils

Abstract

Aluminum (Al) is the most abundant metal in the earth’s crust, but its availability depends on soil pH. Despite this abundance, Al is not considered an essential element and so far no experimental evidence has been put forward for a biological role. In plants and other organisms, Al can have a beneficial or toxic effect, depending on factors such as, metal concentration, the chemical form of Al, growth conditions and plant species. Here we review recent advances in the study of Al in plants at physiological, biochemical and molecular levels, focusing mainly on the beneficial effect of Al in plants (stimulation of root growth, increased nutrient uptake, the increase in enzyme activity, and others). In addition, we discuss the possible mechanisms involved in improving the growth of plants cultivated in soils with acid pH, as well as mechanisms of tolerance to the toxic effect of Al.

Breeding Maize for Tolerance to Acidic Soils: A Review

“…The exclusion of Al in the root apoplast as well as intracellular tolerance to Al toxicity [79,80] have been suggested as important mechanisms for Al tolerance in maize [36][37][38]. Aluminum exclusion is related to the ability of Al-tolerant plants to excrete organic acids (predominantly citric acid and oxalate in maize) and phenolics from the root apex [37,81,82].…”

“…The secreted organic anions (OA) bind with Al to form a complex (Al-OA) which protects the root apex, thus allowing it to continue growing [38,80,81]. The exudation of other organic compounds may be important for the chelation of Al in the root apex [38], even though little is known about their mechanisms of action [79]. Other ligands released in the root apex include phenolic compounds, flavonoids, succinates, phosphates, uridine diphosphate-glucose, and polysaccharides in the form of mucilages [38,[79][80][81]83,84].…”

“…Other ligands released in the root apex include phenolic compounds, flavonoids, succinates, phosphates, uridine diphosphate-glucose, and polysaccharides in the form of mucilages [38,[79][80][81]83,84]. Mucilage is a gelatinous material made of high molecular weight polysaccharides exuded from the most external layers of the root apex and is an important mechanism of resistance to metals [79,85].…”

“…Physiological, molecular, and biochemical studies have shown that the modification of the cell wall composition imparts resistance to Al toxicity to some genotypes [79,[84][85][86]. The most important internal tolerance mechanisms are Al-binding proteins, the chelation of Al using organic acids (such as citric acid) and other organic ligands in the cytosol, the compartmentalization or sequestration of Al in the vacuoles, the evolution of Al-tolerant enzymes (such as Nicotinamide adenine dinucleotide phosphate-specific isocitrate dehydrogenase), elevated enzyme activity [37,86,87], and the Al-induced synthesis of callose (such as 1, 3-glucan) [78,88].…”

“…The most important internal tolerance mechanisms are Al-binding proteins, the chelation of Al using organic acids (such as citric acid) and other organic ligands in the cytosol, the compartmentalization or sequestration of Al in the vacuoles, the evolution of Al-tolerant enzymes (such as Nicotinamide adenine dinucleotide phosphate-specific isocitrate dehydrogenase), elevated enzyme activity [37,86,87], and the Al-induced synthesis of callose (such as 1, 3-glucan) [78,88]. The phospholipid composition of the plasma membrane plays an important role in Al toxicity since it creates a negative charge on the surface of the membrane and increases the sensitivity to Al [79,89].…”