Histochemical and gene expression changes in Cannabis sativa hypocotyls exposed to increasing concentrations of cadmium and zinc

• Higher concentrations of cadmium enhanced root development in hemp plantlets. • Roots showed the highest accumulation levels of cadmium and zinc. • Increased concentrations of cadmium and zinc led to histochemical changes in bast fibres. • Expression patterns of cell wall-related genes varied under cadmium and zinc exposure. Hemp ( Cannabis sativa L.) is a versatile crop that produces cellulosic bast fibres used in textiles and biocomposites. Is also finds use in phytoremediation, being a good candidate for the cultivation on marginal lands, such as those contaminated by heavy metals (HMs). HMs like cadmium (Cd) and zinc (Zn) are known to affect plant growth and impair the biosynthesis of cellulose and lignin at the cell wall level. Since cellulose is the major component in the gelatinous layer of bast fibres, HMs can impact the structure of hemp fibres and, consequently, their mechanical properties. This study investigates how varying concentrations of Cd and Zn in the soil affect the bast fibres of hemp plantlets. The chosen model is the hypocotyl, as it is ideal for studying bast fibre development: it exhibits a temporal separation between the elongation and thickening phases within a short period of approximately three weeks. C. sativa plantlets were grown for 20 days, and the hypocotyls sampled to perform histochemical observations, gene expression analysis, as well as to quantify biomass yield and Cd/Zn accumulation. Hemp plantlets grown in soils with the three highest Zn concentrations were smaller than the control group, whereas no decrease in size was observed under elevated Cd concentrations. However, at the highest Cd concentration, the root system exhibited enhanced development, accompanied by a significant increase in dry weight across all the concentrations tested. The quantification of Cd and Zn showed that the roots were the main organs accumulating HMs. Cd at the two highest concentrations decreased significantly the lumen area of bast fibres and increased their cell wall thickness. Zn decreased significantly the lumen area, but it did not impact the thickness of the cell wall at the highest concentration. Cd also increased the number of secondary fibres. Immunohistochemistry highlighted a different pattern of crystalline cellulose distribution with a signal that was less homogeneous in the presence of Cd and Zn. Gene expression analysis revealed changes in transcripts encoding cellulose synthases, fasciclin-like arabinogalactan proteins, class III peroxidases. The results obtained shed light on the molecular response and bast fibre histological changes occurring in young hemp plants exposed to Cd and Zn.