Molecular and physiological strategies to increase aluminum resistance in plants

Abstract

Aluminum (Al) toxicity is a primary limitation to plant growth on acid soils. Root meristems are the first site for toxic Al accumulation, and therefore inhibition of root elongation is the most evident physiological manifestation of Al toxicity. Plants may resist Al toxicity by avoidance (Al exclusion) and/or tolerance mechanisms (detoxification of Al inside the cells). The Al exclusion involves the exudation of organic acid anions from the root apices, whereas tolerance mechanisms comprise internal Al detoxification by organic acid anions and enhanced scavenging of free oxygen radicals. One of the most important advances in understanding the molecular events associated with the Al exclusion mechanism was the identification of the ALMT1 gene (Al-activated malate transporter) in Triticum aestivum root cells, which codes for a plasma membrane anion channel that allows efflux of organic acid anions, such as malate, citrate or oxalate. On the other hand, the scavenging of free radicals is dependent on the expression of genes involved in antioxidant defenses, such as peroxidases (e.g. in Arabidopsis thaliana and Nicotiana tabacum), catalases (e.g. in Capsicum annuum), and the gene WMnSOD1 from T. aestivum. However, other recent findings show that reactive oxygen species (ROS) induced stress may be due to acidic (low pH) conditions rather than to Al stress. In this review, we summarize recent findings regarding molecular and physiological mechanisms of Al toxicity and resistance in higher plants. Advances have been made in understanding some of the underlying strategies that plants use to cope with Al toxicity. Furthermore, we discuss the physiological and molecular responses to Al toxicity, including genes involved in Al resistance that have been identified and characterized in several plant species. The better understanding of these strategies and mechanisms is essential for improving plant performance in acidic, Al-toxic soils.

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“…This was agreed with the findings of [1] on peanut and [12] and [23] on in vitro wheat plants. Accumulation of Al occurred at root meristems and as a result inhibition of root elongation was the first sign of Al toxicity [7,36,39]. It was reported that inhibition of root elongation of Leucaena leucocephala [28] and peanut [40] achieved at 30 µM and 100 µM Al respectively.…”

“…In acidic soil, Al could exist as Al 3+ , Al(OH) + and Al(OH) 2+ which are deteriorate to plants [1,4]. Many plant species are sensitive to Al even at micromolar concentration, thus, Al toxicity is a major factor restricting plant production on acidic soils [1,4,5,6,7], currently destroying more than 40% of agricultural land around the world [8,9,10].…”

“…The Al tolerance strategies of plants can be separated into those involved in exclusion of Al from the root apex and mechanisms that allow the plant to tolerate Al within cells [7]. Many Al tolerant plants exclude Al from roots by excretion of organic acids such as citrate, malate or oxalate from the roots that chelate Al [6,7,25,26,27,28], excretion of ion H + [17,29,30], excretion of phosphate that precipitate Al [31], excretion of mucilage [32,33] and protein [34] that bind Al.…”

“…Many Al tolerant plants exclude Al from roots by excretion of organic acids such as citrate, malate or oxalate from the roots that chelate Al [6,7,25,26,27,28], excretion of ion H + [17,29,30], excretion of phosphate that precipitate Al [31], excretion of mucilage [32,33] and protein [34] that bind Al. On the other hand, internal mechanisms of Al tolerance include reduction of accumulation of lipid peroxides through a higher proline and carbohydrate content related to osmoregulation and membrane stabilization [35], internal Al detoxification by organic acid anions and enhanced scavenging of free oxygen radicals, [7], protection of root apices by development of root border cells [36], acceleration of senescence of root tips [1], programed cell death under Al stress [1,37] immobilization of Al at cell wall [17], as well as selective permeability of the plasma membrane as a barrier to movement of Al into the cytosol [38].…”

Physiology and Biochemistry of Aluminum Toxicity and Tolerance in Crops

“…Among them, aluminum (Al) ion toxicity severely affects crop production in acid soil due to rapid inhibition of root growth and changes in other metabolic activities of plant cells. Consequently, Al toxicity directly affects the grain quality and plant yield (Kochian et al 2005;Inostroza-Blancheteau et al 2012). …”