The evaluation of T. qataranse growth parameters suggests that Pb has no adverse effect on the plant at concentrations of less than 100 mg/L Pb. However, at 100 mg/L, the metal disturbed healthy growth and, in particular, interfered with root development. Consistent with our findings, a similar study using Z. fabago reported that Pb negatively affects root development14. The root plays a vital role in plant health and development, influencing other tissues’ response to stress conditions. Despite it being one of the most critical parameters in the assessment of plant health, a significant reduction in total chlorophyll content was observed (Fig. 1c). However, Pb toxicity symptoms (e.g., leaf chlorosis and root darkening) were not apparent across any of the treatments. Typically, Pb accumulation in plants raises the level of chlorophyllase, an enzyme that negatively affects chlorophyll. An increased level of chlorophyllase slows down photosynthesis and, therefore, affects overall growth and development. Consequently, due to slow metabolic activities, cell division is adversely affected and healthy growth is inhibited15.
Pb accumulates differently in plant tissue parts, especially in the root22. Concerning T. qataranse Pb accumulation, overall data across all treatments indicates that T. qataranse preferentially concentrates Pb in the root (up to 2,784 mg/kg). Our result is consistent with the reports of many similar studies. For instance, known plant species, including Nerium oleander L. and Brassica juncea, accumulate higher Pb concentrations in their roots than other tissue parts16. Also, in a study involving different plants, Finster, et al.17 determined that the roots always accumulate more Pb than other plant parts, including the fruits, where only traces of the metal translocate the shoot. Our result is also in agreement with the work of Langley-Turnbaugh and Belanger18. Kumar, et al.19 and Pourrut, et al.20 conducted several critical reviews of Pb toxicity in plants and determined that several factors contribute to restricted metal translocation in plants. Of such, Casparian strip endodermis restriction is by far the most limiting for Pb. Notheless, the ability of T. qataranse to accumulate more than 1000 mg/kg Pb suggests that it is a Pb hyperaccumulator21.
Additionally, the root BCF across all treatments was higher than that of the shoot (Fig. 2b). The BCF indicates that, to some degree, T. qataranse sequestrate Pb from growth medium that contains up to 1600 mg/kg Pb. However, it was optimal in the 50 mg/kg treatment. At this concentration, the growth medium had up to 800 mg/kg Pb. The TF under all treatments was less than 1 (Fig. 2c), meaning that T. qataranse can not sufficiently transfer Pb to its aerial parts. In the current study, the restriction of Pb translocation finding is consistent with our previous report on T. qataranse where field samples were analyzed for various metals accumulation, including Pb22. Some of the factors that affect metal bioavailability and uptake include plant and metal types; metal form, concentration, and age in the soil; pH; and organic matter content. However, pH and total organic matter content are the most critical in terms of metal bioavailability and uptake of Pb. The pH significantly affects the behavior of Pb by dictating its chemical form. Metals, including Pb, are more soluble at low or near-neutral pH values. At pH > 8, metals tend to precipitate in the soil. Similarly, a high TOC limits the bioavailability of Pb11. In this work, the pH and TOC in the growth medium were 7.35 and 1.87%, respectively. Therefore, given the neutral pH and low TOC, their effects on Pb bioavailability and uptake by T. qataranse was insignificant and can be eliminated.
Various response mechanisms enable plants to withstand metal toxicity, of such, metal avoidance and uptake are the most common. Before compartmentalization, Pb is translocated to a degree that can be described by the TF23. Pb mainly precipitates on the root cell wall and only the free ions are transported to other parts via the xylem and phloem cells24. Previous works confirmed that Pb disrupts cellular homeostasis by replacing essential cations and altering metal-containing enzyme activity. In plants, the primary sources of ROS are chloroplasts, mitochondria, and peroxisomes. Pb toxicity interferes with electron transport chains in turn increasing ROS accumulation. Nearly every stage of the central dogma of plants (DNA, RNA, protein) is affected by Pb toxicity25.
The antioxidant system is one mechanism used by plants for protection against metal toxicity. In this study, the result of the SOD, CAT, APX, GPX, and GR assay show increased activity of all five enzymes. SOD activity was the highest, up to ten times higher than the control (0 mg/kg Pb), particularly in the root (Fig. 3a), suggesting the critical role of SOD in T. qataranse antioxidative defense. Having the highest enzymatic activity be in the root changes root organic constituents due to Pb complexation. This indicates that, as suggested in our previous work, as an uptake mechanism, Pb ions bind to T. qataranse root by complexation through cationic exchange with hydroxyl and carboxyl functional groups7. This is a well-established complexation mechanism of transition metals, including Pb1. In comparison, GR demonstrated the least activity (Fig. 3e). Such differences can be attributed to the specific roles each enzyme has in ameliorating Pb stress. Many other studies reported a similar increase in the activities of one or all the enzymes analyzed in this work following plants’ exposure to Pb; examples are increases in the activity of CAT, SOD, APX, and GPX in Ceratophyllum demersum L.26 and the cotton plant27; SOD, APX, GPX, and GR in Oryza sativa L.28; and APX, CAT, and GR in Triticum aestivum9. Changes in enzymatic activity account for the elimination of ROS and the improvement of stress conditions in plants. Therefore, the enhanced activities of all enzymes suggest that their role is crucial in ameliorating Pb toxicity in T. qataranse. Other studies support this conclusion, including Ferrer, et al.16 who attribute enhanced CAT and APX activities to the efficient ROS scavenging capability of Z. fabago exposed to Pb. In addition, Nikalje and Suprasanna30 reviewed several other similar studies involving halophytes . However, to the best of our knowledge, our work is the first on T. qataranse. In addition, primarily due to GR activity suggesting the utilization of glutathione, we can conclude that Pb detoxification in T. qataranse may partly involve glutathione metabolism30. Glutathione, which exists in either the reduced (GSH) or oxidized form (GSSG), acts as an antioxidant and chelating bioligand majorly accountable for metals detoxification. Enzymes involved in glutathione metabolism mediates detoxification Glutathione-S-transferases (GSTs) are a major phase II GSH-dependent ROS scavenging enzymes. They play significant roles in GSH conjugation with exogenous and endogenous species found during oxidative stress, including H2O2 and lipid peroxides9,10. Consistent with our findings, a more recent review by Kumar and Prasad14 discussed several other studies, some of which include the use of model species, Arabidopsis thaliana, and Oryza sativa, all of which support our findings.
It is worth mentioning that part of the discussion presented in this work is limited to the perspective of deciphering Pb tolerance and uptake mechanisms from metal translocation and plant antioxidative systems. However, both molecular and biochemical mechanisms play significant roles in toxic metals detoxification, including Pb. For instance, glutathione metabolism is known to regulates the biosynthesis of phytochelatins (PC), which bind Pb and transports it to vacuoles where detoxification can occur. Additionally, genes, such as glutamate cysteine ligae 1 (GSH1), glutamate cysteine ligae 1 (GSH2), phytochelatins synthase 1 (PCS1), and phytochelatins synthase 2 (PCS2), are actively involved in GSH-dependent PCs synthesis. Other primary and secondary metabolites that act as antioxidants, such as Tocols, flavonoids, anthocyanidins, and ascorbic acid, are essential to protecting plants against oxidative damage as well. The functions of these metabolites are well documented8,9,20. Further, we recognize that proteins regulate ROS signaling and the expression of such proteins changes due to Pb exposure11. Due to metal stress in plants, increased protein synthesis is essential in cellular metabolic processes. Mitogen-activated protein (MAP) kinase pathways regulate such processes, serving as a signaling system against oxidative stress. Signaling occurs through multiple stages of the reaction, modifying gene expression and, ultimately, protein synthesis4. Therefore, our future work will focus on the differential expression of proteins, particularly those that are related to stress responses, such as the heat shock protein family, due to Pb exposure in T. qataranse.
Source: Ecology - nature.com
