Cadmium distribution and transformation in leaf cells involved in detoxification and tolerance in barley

Barley is a diagnostic plant that often used in the research of soil pollution by heavy metals, our research explored the detoxification and tolerance mechanism of cadmium(Cd) in barley through pot experiment. We investigated subcellular distribution, chemical forms and oxidative damage of Cd in barley leaves, combing with the transmission electron microscopy and Fourier-transform infrared spectroscopy(FT-IR) to further understand the translocation, transformation characteristics and toxic effect of Cd in cells. The results showed that, the bioaccumulation factors in roots and shoots of barley were ranged of 4.03-7.48 and 0.51-1.30, respectively. Barley reduces the toxic effects by storing Cd in the roots and reducing its transport to the shoots. Compared to the control treatment (0 mg/kg), the percentage of Cd in the cell wall fractions of leaves in 300 mg/kg Cd treatment increased from 34.74 % to 38.41 %; the percentage of the organelle fractions increased from 24.47 % to 56.02 %; and the percentage of soluble fraction decreased from 40.80 % to 5.57 %. We found that 69.13 % of the highly toxic inorganic Cd and water-soluble Cd were converted to less toxic pectates and protein-integrated Cd (50.20 %) and undissolved Cd phosphates (18.93 %). This conversion of Cd was mainly due to its combination with -OH, -NH, -CN, -C-O-C, and -C-O-P groups. Excessive Cd induced a significant (P < 0.05) increase in the levels of peroxidase, malondialdehyde, and cell membrane permeability, which damaged the cell membrane and allowed Cd to enter the organelles. The chloroplasts and mitochondria were destroyed, and eventually the metabolism of intracellular substances was affected, resulting in symptoms of toxicity. Our research provides cellular-scale insight into the mechanisms of Cd tolerance in barley.