Rapid global industrialization and urbanization has caused severe soil contamination problems. Among them, pollution due to heavy metals (HM) has been a serious concern due to its toxic effects on human health and the ecosystem, including the flora and fauna (Arul et al. 2016; Bind et al. 2019). Among all HM contaminants, Pb is considered as one of the most toxic pollutant because it tends to (bio)accumulate at high rates in soil environments (Kushwaha et al. 2017, Singh et al. 2019; Sathe et al. 2020). The common anthropogenic sources of Pb contamination in the environment include smelting of ores, burning of coal, mining, effluents from battery industries, automobile exhausts, metal plating, leather tanning, finishing operations, and the overuse of fertilizers and pesticides (Goswami et al. 2017, Kushwaha et al. 2020). The bioavailability of HM in soil is a serious aspect which has to be monitored in order to restore a contaminated site. Thus, environment researchers have given it a serious concern, thereby, ensuring human health (Bind et al. 2018; Goswami et al. 2020).
In this line of progressive research, phytoremediation is a novel, cost-effective, efficient, environment friendly and solar-driven remediation technology, which has proven to improve the soil quality (Kushwaha et al. 2015). Vetiveria zizanioides cv KS-1 (VZ) was selected in this study due to its ability to grow in extreme environmental conditions and demonstrate high biomass yield within short time durations. A native of India, it is also known as the ‘wonder grass’ and it is extensively used in majority of the tropical countries for the phytoremediation of various contaminants present in water bodies. Since, it grows rapidly, it is more effective for the restoration of contaminated sites in only 4–5 months as compared with trees and shrubs which usually require ~ 2–3 years (Sinha et al. 2009). Vetiveria zizanioides is known to tolerate high alkalinity and acidity (pH 3.0 to 10.5), salanity (EC 8 dSm− 1), high concentration of heavy metals (lead, mercury, copper, zinc, arsenic, cadmium, nickel, magnesium, manganese, aluminium and selenium), pesticides and herbicides. It has been reported that vetiver grass stabilized Pb(II) in root zone with moderate translocation) to above ground biomass (shoots). Thus, its shoots can be safely used as fodder or grazed by animals. Earlier literatures have reported the mechanisms involved in the accumulation and transportation of inorganic pollutants in V. zizanioides, but still some mechanisms remain rather unclear, especially the role of root exudates (REs).
REs plays a significant role in phytoremediation. The high density and diversity of rhizopsheric microorganisms is due to the effect of various RE components. The secondary metabolites released from plant roots during stressful environmental conditions could improve the nutrient uptake and help the plant to cope with those stresses (Luo et al. 2015). The chemical constituent of REs, root extracts (Ret) and their concentration depends upon various factors such as the plant type, plant species, soil, environmental and geographical conditions, biotic and abiotic stress and collection method (Rani and Juwarkar 2016; Dutta et al. 2019). REs is mainly classified into two types, i.e. low molecular weight (LMW) and high molecular weight (HMW) compounds. LMW REs is typically composed of organic acids, sugars, phenols and various amino acid and non-proteinaceous amino acids (phytosiderophore), while HMW is majorly composed of mucilage and proteins. Root secretions are known to stimulate the solubility and mobility of HM ions in soil thereby enhancing the phytoextraction efficiency of the plant (Rajkumar et al. 2012). Organic acids (citric and oxalic acid) released by the roots of Echinochloa crusgalli have been reported to increase the Cu(II), Cd(II) and Pb(II) translocation from below ground biomass to above ground biomass (Kim et al. 2010).
To the best of the authors knowledge, this is the first report that describes the exudation behaviour of VZ in presence of Pb(II) and its role as chemoattractant. Besides, very little data are available in the literatures concerning the effect of different level Pb(II) on the phytocomponents of vetiver grass and its chemotactic effects. Therefore, the present study will help to envisage the role of various phytocomponents of vetiver grass on the Pb(II) tolerance capacity. Thus, the specific objectives of this work can be stated as follows: (i) to perform chemical characterization of the REs and REt from the Pb hyperaccumulator VZ, and (ii) to determine the chemotaxis response of rhizospheric bacteria (Bacillus licheniformis, Bacillus subtilis and A. junii Pb1) towards VZ exudates.