Histidine promotes the loading of nickel and zinc, but not of cadmium, into the xylem in Noccaea caerulescens

Abstract

Histidine is known to be involved in Ni hyperaccumulation. Recently, histidine-dependent xylem loading of Ni and Zn has been demonstrated in the Zn/Ni/Cd hyperaccumulator, Noccaea caerulescens. Here we tested the hypothesis whether Cd xylem loading is histidine-dependent, too. In contrast to that of Ni and Zn, the xylem loading of Cd was not affected by exogenous histidine. Histidine accumulation in root cells appears to facilitate the radial transport of Ni and Zn, but not Cd, across the roots. This may be due to the relatively high preference of Cd for coordination with sulfur over coordination with nitrogen, in comparison with Ni and Zn.

Keywords: Noccaea caerulescens; cadmium; histidine; hyperaccumulation; xylem loading.

Figure 1. Cadmium concentration in root pressure exudates (A) and volume of root pressure exudates (B) of N. caerulescens plants (CMA and SLM accessions) treated with half-strength Hoagland’s nutrient solution supplied with 1 µM Cd overnight after 4-h pretreatment of their roots with 1 mM L-His, L-Ala, or MES/KOH-buffered demineralized water (pH 5.5) ( = unpretreated). Values are the means of 3 replicates with 6 plants per replicate ± SE. The experimental design was as described for Ni in Richau et al. and Zn in Kozhevnikova et al. The plants were grown on Cd-free half-strength Hoagland’s nutrient solution for 8 wk prior to the experiment. The absence of significant changes in the volume of exuded xylem sap after the exogenous amino acid supply (B) indicates that there were no changes in sap production which could influence the metal concentration in it.

Figure 2. Cd concentration (nmol ml⁻¹) in root pressure exudates (A) and Cd xylem loading, recalculated as a percentage of total root Cd burden (B) in CMA and SLM accessions of N. caerulescens treated overnight with nutrient solution without Cd (-Cd) or nutrient solution with 1 µM Cd (+Cd), after 4-h pretreatment of their roots with 1 mM L-His, or MES/KOH-buffered demineralized water (pH 5.5) ( = unpretreated). Before the experiment the plants were grown on a half-strength Hoagland’s nutrient solution containing 1 µM Cd for 8 wk. * = significantly different from corresponding control at P < 0.05. The experimental design was as described for Zn in Kozhevnikova et al.