Cadmium exposure is associated with increased transcript abundance of multiple heavy metal associated transporter genes in roots of hemp (Cannabis sativa L.)

Abstract

Industrial hemp (Cannabis sativa L.) has demonstrated promise for phytoremediation due to an extensive root system, large biomass, and ability to survive under relatively high levels of heavy metals. However, little research has been conducted to determine the impact of heavy metal uptake in hemp grown for medicinal use. This study evaluated the potential for cadmium (Cd) uptake and its impact on growth, physiological responses, and transcript expression of metal transporter genes in a hemp variety grown for flower production. The cultivar ‘Purple Tiger’ was exposed to 0, 2.5, 10, and 25 mg·L-1 Cd in a greenhouse hydroponic study in two independent experiments. Plants exposed to 25 mg·L-1 Cd displayed stunted plant growth characteristics, reduced photochemical efficiency, and premature senescence suggesting Cd toxicity. At the two lower concentrations of Cd (2.5 and 10 mg·L-1 Cd), plant height, biomass, and photochemical efficiency were not affected, with chlorophyll content index (CCI) being slightly lower at 10 mg·L-1 Cd, compared to 2.5 mg·L-1 Cd. There were no consistent differences between the two experiments in total cannabidiol (CDB) and tetrahydrocannabinol (THC) concentrations in flower tissues at 2.5 and 10 mg·L-1 Cd, compared to the control treatment. Root tissue accumulated the highest amount of Cd compared to other tissues for all the Cd treatments, suggesting preferential root sequestration of this heavy metal in hemp. Transcript abundance analysis of heavy metal-associated (HMA) transporter genes suggested that all seven members of this gene family are expressed in hemp, albeit with higher expression in the roots than in the leaves. In roots, CsHMA3 was up-regulated at 45 and 68 d after treatment (DAT), and CsHMA1, CsHMA4, and CsHMA5 were upregulated only under long term Cd stress at 68 DAT, at 10 mg·L-1 Cd. Results suggest that expression of multiple HMA transporter genes in the root tissue may be upregulated in hemp exposed to 10 mg·L-1 Cd in a nutrient solution. These transporters could be involved in Cd uptake in the roots via regulating its transport and sequestration, and xylem loading for long distance transport of Cd to shoot, leaf, and flower tissues.

Cadmium Bioconcentration and Translocation Potential in Day Neutral and Photoperiod Sensitive Hemp Grown Hydroponically for the Medicinal Market

“…In contrast, the DLS variety, T1 had similar Cd concentrations in roots, leaf, and stem tissue upon exposure to Cd. These results are consistent with previous literature, which reported that roots were the preferred tissue for Cd accumulation in hemp [27,34,35,40,41] and that Cd accumulation increased with increasing Cd concentrations in the growing media [39]. For instance, Cd concentrations in the roots of C. sativa fiber variety Santhica 27 exposed to 20 µM Cd (2.25 mg•L −1 Cd) for one week averaged 2687 mg•kg −1 dw, while Cd concentrations in stems and leaves averaged 1243 mg•kg −1 dw and 717 mg•kg −1 dw, respectively [34].…”

“…This suggests that the DLS varieties used in the present study may be more tolerant to Cd exposure at 2.5 mg•L −1 than the DN varieties evaluated. These results agree with previous studies that have reported a decrease in shoot biomass of hemp plants exposed to Cd [26,34,35,39].…”

“…Plants were grown for 3 weeks in the base nutrient solution after which nutrient solutions were replaced completely and Cd treatments were added using CdSO 4 •8H 2 O, to achieve 0 (control) and 2.5 mg•L −1 Cd. Cadmium concentrations were chosen based on the results of previous studies [34,35], which evaluated hemp exposure to Cd in hydroponic systems. The experimental treatments were arranged in a randomized complete block design with four hemp varieties exposed to two levels of Cd with four replicates each.…”

“…For instance, Cd concentrations in the roots of C. sativa fiber variety Santhica 27 exposed to 20 µM Cd (2.25 mg•L −1 Cd) for one week averaged 2687 mg•kg −1 dw, while Cd concentrations in stems and leaves averaged 1243 mg•kg −1 dw and 717 mg•kg −1 dw, respectively [34]. Additionally, the C. sativa medicinal variety Purple Tiger exposed to 2.5 mg•L −1 Cd for 68 days had average Cd concentrations of 1982.6 mg•kg −1 dw in roots, 13.2 mg•kg −1 dw in leaves, 5.1 mg•kg −1 dw in stems, and 7.6 mg•kg −1 dw in flowers [35]. All varieties had increased concentrations of Cd in all plant tissues exposed to Cd in the nutrient solution, except in the floral tissues of T1.…”

“…Few studies have evaluated heavy metal accumulation in hemp flowers, with most research utilizing fiber hemp varieties to determine heavy metal uptake for phytoremediation purposes [16,17,19,34]. Due to the harmful effects of Cd and other heavy metals on human health, the U.S. hemp industry has attempted to implement standards regarding maximum allowable levels of metals in C. sativa consumer products, which vary by state [35]. As hemp flowers are increasingly grown for the medicinal market, determining Cd distribution among plant organs, as well as bioconcentration and root-to-shoot translocation factors, are of importance.…”