Methionine application at low phosphate level promotes root-to-shoot cadmium translocation by mobilizing weakly bound apoplastic cadmium in valerian (Valeriana officinalis L.)

Abstract

Controlling the uptake and translocation of heavy metals in medicinal plants is crucial due to its strong impact on the safety and quality of derived medicinal plant products. This study investigated the effects of phosphate and methionine on the distribution and chemical forms of Cd in valerian (Valeriana officinalis L.) roots and shoots. Seedlings were grown for two weeks in a Cd-free nutrient solution containing either 400 or 1200 μM phosphate and then exposed for 10 days to 10 μM Cd either in combination with 400 μM or no methionine in the nutrient solution. Root Cd was partitioned into a symplastic and an apoplastic Cd fraction using a desorption procedure with EDTA. A sequential extraction method was applied to determine chemical forms in which Cd was retained in both roots and shoots. “Pectate- and protein-bound Cd” and HAc-soluble “Cd phosphates” were the dominant Cd forms in sequential extractions of both roots and shoots. Phosphate increased the HAc-soluble “Cd phosphate fraction” in the roots, but not in the shoots. Methionine did not affect this fraction, but strongly increased ethanol- and water-soluble Cd in roots and shoots. While it also increased the “pectate- and protein-bound Cd fraction” in the shoots, it decreased this fraction and the share of apoplastic Cd in the roots. The phosphate treatment reduced Zn accumulation in the roots but had no effect on Zn accumulation in the shoots, whereas Met increased shoot Zn. The results indicate that methionine induced root-to-shoot Cd translocation primarily by mobilizing weakly bound Cd to root cell walls at the low phosphate level. This could help limit Cd accumulation in valerian roots grown for medicinal use.