Cellular Responses of Maize Roots to Long-Term Cadmium Exposure: Adjustments of Class III Peroxidases, Plasma Membrane and Tonoplast Sub-Proteomes.

Abstract

Background: Crop plants have to deal with long-term cadmium exposure to farmlands contaminated by intensive use of fertilizers and pesticides. For uptake and sequestration, Cd²⁺ has to pass the plasma membrane and tonoplast. Class III peroxidases, plasma membrane, and tonoplast sub-proteomes were studied.

Methods: Control and Cd²⁺-treated maize (Zea mays L.) were grown in hydroponics for 18 days. Soluble peroxidases were partially purified by chromatofocusing and characterized by substrate specificity. Membrane-bound peroxidases were analyzed spectrophotometrically and by non-reducing SDS-PAGE. Soluble and plasma membrane-bound peroxidases were identified by mass spectrometry. Shotgun proteomics was used to identify membrane proteins of differential abundance.

Results: Guaiacol peroxidase activities increased in soluble fractions of Cd²⁺ samples. A Cd²⁺-specific soluble peroxidase (ZmPrx101) was identified, and ZmPrx85 abundance increased significantly in the plasma membrane. Substrate specificity of peroxidases revealed a preference for ferulic acid and esculetin, which was confirmed by docking analyses. Primary active transporters increased auxin efflux (brachytic2, ABCB9, and ABCB21), Cd²⁺ exclusion (ABCG34), and sequestration into the vacuole (HMA2, ABCB27). Evaluation of sub-proteome fractions demonstrated significant changes for proteins involved in disease resistance responses and cell wall modification.

Conclusions: Molecular adjustments of maize root proteome to long-term Cd²⁺ exposure revealed relevance of low-abundant proteins for Cd²⁺ tolerance and putative stress markers.