Nature and mechanisms of aluminium toxicity, tolerance and amelioration in symbiotic legumes and rhizobia

Abstract

Recent findings on the effect of aluminium (Al) on the functioning of legumes and their associated microsymbionts are reviewed here. Al represents 7% of solid matter in the Earth's crust and is an important abiotic factor that alters microbial and plant functioning at very early stages. The trivalent Al (Al 3+ ) dominates at pH < 5 in soils and becomes a constraint to legume productivity through its lethal effect on rhizobia, the host plant and their interaction. Al 3+ has lethal effects on many aspects of the rhizobia/legume symbiosis, which include a decrease in root elongation and root hair formation, lowered soil rhizobial population, and suppression of nitrogen metabolism involving nitrate reduction, nitrite reduction, nitrogenase activity and the functioning of uptake of hydrogenases (Hup), ultimately impairing the N 2 fixation process. At the molecular level, Al is known to suppress the expression of nodulation genes in symbiotic rhizobia, as well as the induction of genes for the formation of hexokinase, phosphodiesterase, phosphooxidase and acid/alkaline phosphatase. Al toxicity can also induce the accumulation of reactive oxygen species and callose, in addition to lipoperoxidation in the legume root elongation zone. Al tolerance in plants can be achieved through over-expression of citrate synthase gene in roots and/or the synthesis and release of organic acids that reverse Al-induced changes in proteins, as well as metabolic regulation by plant-secreted microRNAs. In contrast, Al tolerance in symbiotic rhizobia is attained via the production of exopolysaccharides, the synthesis of siderophores that reduce Al uptake, induction of efflux pumps resistant to heavy metals and the expression of metal-inducible (dmeRF) gene clusters in symbiotic Rhizobiaceae. In soils, Al toxicity is usually ameliorated through liming, organic matter supply and use of Al-tolerant species. Our current understanding of crop productivity in high Al soils suggests that a much greater future accumulation of Al is likely to occur in agricultural soils globally if crop irrigation is increased under a changing climate.

Genome-wide identification and transcriptional analyses of MATE transporter genes in root tips of wildCicerspp. under aluminium stress

“…On another note, legume plants including chickpea can characteristically form nodules in the root for N-fixation. Al in acidic soils may pose a constraint on nodule-formation due to its lethal effect on rhizobia 42 . Therefore, for the improvement in chickpea production in acidic soil, attention should also be given to the rhizobia acidity tolerance.…”

Importance of Mineral Nutrition for Mitigating Aluminum Toxicity in Plants on Acidic Soils: Current Status and Opportunities

“…Different forms of Al may exist in the soil such as inorganic, soluble, and/or organic forms. Inorganic forms of Al are exchangeable and are primarily bound to silicate clays, hydrous oxides, phosphates, and sulfates [ 60 ]. A significant correlation has been found between soil pH and phytotoxicity of Al species.…”

WITHDRAWN: Deep expression scrutiny provides genetic basis of aluminium tolerance in wild (Lens nigrican) and cultivated lentil (Lens culinaris Medik.)

“…Trimming of root biomass coaxed to impoverished uptake of water and nutrients. Also, Al toxicity can stimulate build-up of reactive oxygen species and callose, along with lipo-peroxidation in legume root elongation zone [3]. To scavenge and detoxify these physiochemical attributes, organic acids are induced to protect the plants under aluminium stress conditions.…”