Genetic Engineering in Plants to Combat Acid Soil Toxicity

Genetic engineering enables us to displace genes among species. The newly incorporated foreign gene(s) in the host genome can create new trait(s) by regulating the expression of different genes and transcription factors.Recent advances in plant molecular biology provided superior genotypes are useful for enhanced production of food and other plant based valuable materials.Aluminumstress tolerant transgenic plants are also developed by overexpressing different genes. In particular, the malate synthase and citrate synthase genes displayed enhanced Aluminum tolerance by activating malate and citrate excretion. Notably, the transgenic plants modified for the expression of enzymes involved in malate and citrate synthesis showed increased organic anion concentration in tissues and Al3+ resistance.Molecular characterization of acid soil stress tolerance mechanism is critical to develop efficient breeding materials to make high yielding varieties under stress environments.  Number of genes and transcription factors which are liable for Aluminum tolerance have been isolated from various plant species.  Initially, genes encoding malate (ALMT1; Al-activated Malate Transporters) and citrate transporters (MATE; Multidrug and Toxic Compound Extrusion) were recognized as key regulators in the Al-exclusion mechanism (Sasaki et al., 2004; Magalhaes et al., 2007). ALS3 (Aluminum Sensitive 3), encoding a half type ATP-binding cassette transporter that may be concerned in the redistribution and sequestration process of Al from sensitive tissues in Arabidopsis
The toxic Al3+ exclusion mechanism which is facilitated by efflux of Al-chelating organic acids from the apex of the root into the rhizosphere. The toxic form of Al3+ is restricted into the cell by releasing organic acids, which form non-toxic complexes with Al3+ in the rhizosphere. The genes responsible for malate, Al-activated Malate Transporter (ALMT1) and citrate, Multidrug and Toxic Compound Extrusion (MATE) transporters are cloned and characterized well in several economically important crop plants like Wheat, Sorghum and Barley, Rice, Strawberry fruits, Soybean, Gossypium arboreum, Alfaalfa and Solanaceae members.
The ALMT transporters are associated with Al stress responses in plants. Typically, exposure to Al results in an upregulation of ALMT1 in Wheat, Arabidopsis and Brassica (TaALMT1, AtALMT1, and BnALMT1). These transporters are localized on the plasma membrane of root cells and control Al-regulated release of organic acids. Besides, MATE genes are associated to multidrug transporters family and belong to secondary transporters that use H+ or Na+ electrochemical gradients to drive substrate export. Besides, recent reports pointed out that the MATE transporters are mainly associated with Al3+ detoxification mechanisms. MATE transporters were firstly characterized in Sorghum (SbMATE) and Barley (HvAAC). Homologs of MATE genes were also isolated from different plant species.Furthermore, it was reported that the Nramp (Natural resistance-associated macrophage protein) and Nrat1 (Nramp Al transporter 1) are also involved in the Al sequestration.
Transgenic Nicotiana benthamiana plants harbouring a Magnesium transport protein (AtMGT1) exhibited tolerance to Mg2+ and Al3+. In particular, the transgenic plants showed significantly less reduction in root elongation under Al stress condtions (100 mM AlCl3) when compared with control plants. They also pointed out that the Al3+ tolerance of transgenic plants may be due to decreased callose deposition in comparison to wild-type plants. Furthermore, scientists investigated the role of MDAR (Monodehydroascorbate reductase) and DHAR (Dehydroascorbate reductase) in Ascorbic acid (AsA) regeneration during Al3+ stress using transgenic tobacco (Nicotiana tabacum) plants overexpressing Arabidopsiscytosolic MDAR or DHAR. DHAR-OX plants showed better root growth than wild-type and MDAR-OX plants. However, DHAR-OX plants showed lower hydrogen peroxide content, less lipid peroxidation and lower level of oxidative DNA damage than in control plants. Moreover, when compared with control plants, DHAR-OX plants consistently displayed a higher AsA level both with and without Al3+ exposure and higher Ascorbate peroxidase activity under Al3+ stress. In Arabidopsis, overexpression of blue copper-binding protein gene (AtBCB) was introduced and as a result, Al3+ resistance in plant was conferred. Similar to the above studies, overexpression of GDI-dissociation inhibitor gene (NtGDI1), glutathione S-transferase gene (parB) and peroxidase gene (NtPox) displayed Al tolerance in tobacco plants.
In addition to malate and citrate transporters, the oxalate transporters are another group of organic acid transporters which are playing critical role in Al3+ tolerance of the plants. During Al stress conditions, oxalate is produced from the root and it binds with Al3+ which is present in rhizosphere. The efflux of oxalate mainly takes place in the root apex region. Expression of oxalate is only found during the stress condition. Increased organic acids like malate, citrate and oxalate in the Al3+ toxic containing growth medium, the root growth improved significantly observed.Transgenic plant production is a challenging process and the transgenic plants provide a powerful tool for enhanced yield. Overexpression studies and silencing of desired genes provide functional importance of genes involved in plant growth and development under different biotic and abiotic stress conditions.

Dr. Atreyee Kundu, 
Professor, Department of Microbiology, 
Techno India University, West Bengal.
Senior Research Fellow, Presidency University(Ex)

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