Unravelling the adaptive mechanisms of Alternanthera Tenella colla to heavy metal stress through physiological biochemical and molecular analyses

Abstract

Industrial development has resulted in widespread heavy metal contamination across ecosystems and pose serious risks to biodiversity, plant and human health. Despite this, the phytoremediation potential of riparian vegetation in industrial zones remains underexplored. The present study was conducted to identify a potential phytoremediator and to investigate its adaptive mechanisms in response to heavy metal stress. The study was conducted in three interconnected chapters, each addressing a specific aspect of heavy metal tolerance and detoxification. The first chapter focused on the phytosociological assessment of riparian vegetation in the industrial zone of Kanjikode along Walayar Riverbank to identify potential phytoremediators of heavy metal contamination. A total of 97 species belonging to 32 families were recorded, with Poaceae and Asteraceae being the most dominant. Therophytes were prevalent indicating their ability to survive in disturbed environments. Among the flora, invasive species such as Alternanthera tenella, Persicaria barbata and Croton bonplandianus showed high abundance. A. tenella emerged as dominant based on its Importance Value Index (IVI), wide distribution and high efficiency to accumulate and translocate heavy metals like chromium and zinc, demonstrating its strong phytoremediation potential. These characteristics make it a promising candidate for phytoremediation in contaminated riparian areas. The second chapter involved hydroponic studies to evaluate the morphological, anatomical, physiological and biochemical responses of A. tenella to cadmium (Cd), chromium (Cr) and lead (Pb) exposure. Rooted plants were treated with Cd (170 µM), Cr (240 µM) and Pb (680 µM) in Hoagland nutrient medium. Morphological assessments revealed gradual reductions in root and shoot growth, leaf area and biomass under heavy metal stress. Cr caused relatively higher inhibitory effects compared to Cd and Pb. Anatomical observations showed adaptive modifications such as thickened xylem walls, reduced xylem diameter and deposition of electron dense granules in cortical and vascular tissues. These changes indicate metal sequestration and restricted translocation of metals to aerial parts. Physiological analyses exhibited significant decline in photosynthetic pigments, stomatal conductance, and water content. Antioxidant enzymes and stress metabolites like proline and phenol were accumulated under heavy metal exposure, indicating activation of reactive oxygen species scavenging and osmotic adjustment. FTIR spectral shifts confirmed metal binding with functional groups such as hydroxyl, carboxyl, and amide groups in the plant tissues. A comparison of biomass, root- shoot distribution and metal accumulation potential indicated that A. tenella possesses high bioconcentration and bioaccumulation factors. This reflects the plant’s selective accumulation of Cd and Pb in roots (phytostabilization) and efficient Cr translocation to shoots (phytoextraction). Among the three heavy metals tested, chromium triggered the strongest transcriptional and metabolic changes in A. tenella. This led to a detailed transcriptomic and metabolomic profiling presented in chapter three. Metabolomic analysis showed significant accumulation of flavonoids, organic acids, amino acids, polyols, and fatty acids, which likely contribute to metal chelation, oxidative stress mitigation, and maintenance of redox balance. Transcriptomic profiling revealed the upregulation of genes associated with metal transport, chelation, antioxidant defence, and stress signalling and downregulation of photosynthetic and cell wall remodelling genes. RT-PCR validation of eight selected genes across leaves, stems, and roots supported these results. A strong induction of HIPP39, ABCC2, GST, YSL1, and PCS was observed in roots and leaves, while XTH, LAC6, and CASP were downregulated under prolonged Cr exposure. Together, these findings offer better understanding of the molecular and physiological basis of Cr tolerance in A. tenella and understanding of plant-metal interactions. The ability of A. tenella for chromium phytoextraction and its ecological dominance in polluted riparian habitats makes it a promising species for phytoremediation and ecosystem restoration.