Effect of cadmium on resumption of respiration in cotyledons of germinating pea seeds

Cadmium effects on embryo growth of pea seeds during germination: Investigation of the mechanisms of interference of the heavy metal with protein mobilization-related factors

This work aims to give more insight into mechanisms of action of cadmium (Cd) on germinating pea seeds (Pisum sativum L. var. douce province), specifically the different ways by which Cd cations may interfere with the principal factors involved during germination process, notably storage proteins mobilization, amino acids freeing and proteolytic activities. Obtained results revealed that the process of hydrolysis of main storage proteins showed a significant disruption, which resulted in the decrease of the release of free amino acids, thus imposing a lack in nitrogen supply of essential nutrients to growing embryo under Cd stress. This hypothesis was evidenced by Cd-induced changes occurring in main purified protein fractions; Albumins, Legumins and Vicilins, during their breakdown. Besides, at enzymatic level, the activities of main proteases responsible for this hydrolysis were altered. Indeed, assays using synthetic substrates and specific protease inhibitors followed by protease activity measurements demonstrated that Cd inhibited drastically the total azocaseinolytic activity (ACA) and activities of different proteolytic classes: cysteine-, aspartic-, serine- and metallo-endopeptidases (EP), leucine- and proline-aminopeptidases (LAP and PAP, respectively), and glycine-carboxypeptidases (Gly-CP). The data here presented may suggest that the vulnerability of the embryonic axes towards Cd toxicity could be explained as a result of eventual disruption of metabolic pathways that affect mobilization of reserves and availability of nutrients. In vitro studies suggest that Cd cations may act either directly on the catalytic sites of the proteolytic enzymes, which may cause their deactivation, or indirectly via the generation of oxidative stress and overproduction of free radicals that can interact with enzymes, by altering their activity and structure.

Copper excess impairs mobilization of storage proteins in bean cotyledons

Germination represents a limiting stage of plant life cycle. One of the underlying metabolic activities following imbibition of seed is the reserve mobilization. Seeds of bean (Phaseolus vulgaris L. var. soisson nain hatif) were germinated by soaking in distilled water or 200 μM CuCl(2). Storage proteins breakdown and amino acids freeing from reserve tissues were investigated. Compared to the control, Cu caused a reduction in germination rate, embryo growth, and in mobilization of cotyledonary biomass. The failure in albumin and globulin hydrolysis after the exposure to the pollutant was argued by (1) higher contents of remaining proteins than control ones, (2) persistence of some polypeptide bands resolved by polyacrylamide gel electrophoresis of albumin and globulin-rich fractions, and (3) decrease in the availability of amino acids. Nitrogen starvation in embryonic axis should be associated with the Cu-imposed delay in growth.

Oxidative damage and redox change in pea seeds treated with cadmium

Pea seeds (Pisum sativum L.) were germinated by soaking in distilled water or 5mM CdCl2 for 5 days. The relationships among Cd treatment, germination rate, embryonic axis growth, NAD(P)H levels and NAD(P)H oxidase activities in mitochondrial and peroxisomal fractions of cotyledons and embryonic axis were investigated. Heavy metal stress provoked a diminution in germination percent and embryonic axis growth, as compared to the control. A drastic disorder in reducing power was imposed after exposure to cadmium. Heavy metal caused a significant increase in the redox ratio of coenzymes. NADPH oxidase is considered to be oxidative stress-related enzymes. The NAD(P)H oxidase activities were strongly stimulated after Cd exposure. The changes in redox and oxidative properties are discussed in relation to the delay in seed germination and embryonic axis growth.