Aluminum (Al) stress is one of the serious limiting factors in plant productivity in acidic soils, which constitute about 50 % of the world's potentially arable lands and causes anywhere between 25 and 80 % of yield losses depending upon the species. The mechanism of Al toxicity and tolerance has been examined in plants, which is vital for crop improvement and enhanced food production in the future. Two mechanisms that facilitate Al tolerance in plants are Al exclusion from the roots and the ability to tolerate Al in the symplast or both. Although efforts have been made to unravel Al-resistant factors, many aspects remain unclear. Certain gene families such as MATE, ALMT, ASR, and ABC transporters have been implicated in some plants for resistance to Al which would enhance the opportunities for creating crop plants suitable to grow in acidic soils. Though QTLs have been identified related to Al-tolerance, no crop plant that is tolerant to Al has been evolved so far using breeding or molecular approaches. The remarkable changes that plants experience at the physiological, biochemical and molecular level under Al stress, the vast array of genes involved in Al toxicity-tolerance, the underlying signaling events and the holistic image of the molecular regulation, and the possibility of creating transgenics for Al tolerance are discussed in this review.

The aim of this work was to evaluate the alleviating efficacy of salicylic acid (100, 250 and 500 μM) against the toxic effects of aluminum (Al) on two tomato cultivars (GOWRI and SIRI) differing in their sensitivity to Al stress. Al treatment (500 µM) caused 40% -80% drop in plant growth, relative water content (RWC) and cell viability and a reduction of 1 -2.5 fold and 0.5 -2 fold in glutathione and proline content respectively, when compared to their control plants grown in Al free medium. Al treatment also resulted in 2 -5 fold raise in malondialdehyde (MDA) levels, 2 -3 fold higher Al uptake and 55% -80% more electrolyte leakage and caused severe DNA damage. Al stress enhanced (1 -2 fold) the activities of superoxide dismutase (SOD) and peroxidase (POD), but decreased catalase (CAT) activity over their respective controls. Exogenously-applied Salicylic acid (SA) significantly (p < 0.05) and dose-dependently alleviated Al-induced toxicity in tomato seedlings as marked by much improved plant growth, retention of higher RWC, cell viability, glutathione and proline content. SA also caused 0.6 -1.5 fold reduction in Al uptake, 50% -80% less electrolyte leakage, 40% -80% drop in lipid peroxidation and considerable protection against DNA damage. Also, supplementation of SA could considerably reverse the Al-induced changes in the activities of SOD, POD and CAT. Together, our findings demonstrate that, SA is an efficient growth regulator with diversified roles that contribute to its potential alleviating effect against Al induced toxicity and SIRI is a relatively Al-resistant cultivar compared to GOWRI. * Corresponding author. V. Surapu et al.778