Nickel hyperaccumulators: A review of progress in plant uptake

• Current scientific knowledge on uptake mechanism for Ni-hyperaccumulators • Green, environmental approaches to enhance the Ni-accumulation potential of these plants. • Identification of research gaps and insights into future research. Increased anthropogenic activities have increased the nickel concentrations in the soil considerably (< 26,000 mg/kg) surpassing safety regulation thresholds (< 100 mg/kg). To address this, various strategies have been employed to reclaim heavy metal contaminated soil. Conventional methods (physiochemical) are undesirable due to their environmental damage, high cost and the potential for introducing secondary pollutants. Phytoremediation (bioremediation), a greener alternative, uses plants to extract heavy metals from the soil and store it in their above-ground tissues. Hyperaccumulating plants are particularly valuable in this application as they can accumulate high concentrations of heavy metals without suffering from any cytotoxic effects. Nickel hyperaccumulating plants constitute an ecologically significant group of plants that can accumulate exceptionally high concentrations of nickel, exceeding 1000 mg/kg of nickel in their above ground biomass. These plants have evolved unique physiological mechanisms to tolerate the high concentrations of nickel. These plants have applications in phytoremediation and phytomining, therefore, understanding the uptake pathways of these plants are crucial for effective implementation in land management strategies. In this review we present a comprehensive conceptual framework detailing the physiological and biochemical mechanisms of phytoextraction and evaluate their critical role in optimizing phytoremediation strategies. We examine how these key factors (microbial interactions, biochemical response (organic acids and amino acids), genetic profiles and different soil amendments) affect Ni uptake. Additionally, this review consolidates existing frameworks based on single focused aspects, highlighting research gaps and active areas of research and provides a new, mechanism-oriented overview.