Responses to Cadmium in Early-Diverging Streptophytes (Charophytes and Bryophytes): Current Views and Potential Applications

Abstract

Several transition metals are essential for plant growth and development, as they are involved in various fundamental metabolic functions. By contrast, cadmium (Cd) is a metal that can prove extremely toxic for plants and other organisms in a dose-dependent manner. Charophytes and bryophytes are early-diverging streptophytes widely employed for biomonitoring purposes, as they are able to cope with high concentrations of toxic metal(loid)s without showing any apparent heavy damage. In this review, we will deal with different mechanisms that charophytes and bryophytes have evolved to respond to Cd at a cellular level. Particular attention will be addressed to strategies involving Cd vacuolar sequestration and cell wall immobilization, focusing on specific mechanisms that help achieve detoxification. Understanding the effects of metal(loid) pollution and accumulation on the morpho-physiological traits of charophytes and bryophytes can be in fact fundamental for optimizing their use as phytomonitors and/or phytoremediators.

Keywords: biomonitoring; bryophytes; charophytes; glutathione; heavy metals; phytochelatins; phytoremediation.

Figure 1

Cd transport and detoxification mechanisms in early-diverging streptophytes. Blue circles represent Cd. Red arrow shows a transporter-mediated Cd extrusion mechanism via transporters such as CDF (Cation Diffusion Facilitators) or PDR (ATP Binding Cassettes), whereas black arrows depict Cd internalization via different transporters such as NRAMP (Natural Resistance-Associated Macrophage Protein) or IRT (Iron-Regulated Transporter, ZIP family). Tonoplast transporters in blue represent systems responsible for Cd uptake into vacuole, such as HMA (Heavy Metal-Associated) or MRP (ATP Binding Cassettes). Tonoplast transporters in gray represent NRAMPs, which are responsible for Cd efflux out of the vacuole. GSH: reduced glutathione; GS-Cd: glutathione-bound Cd; PCn: phytochelatins; PCS: phytochelatin synthase; PCn-Cd: phytochelatin-bound Cd; Vacuolar organic acids-Cd complex: complex formed by Cd binding to Krebs acids and derivatives; LMW and HMW: Low and High Molecular Weight. Created with BioRender.com.