Cadmium and Plant Development: An Agony from Seed to Seed

Abstract

Anthropogenic pollution of agricultural soils with cadmium (Cd) should receive adequate attention as Cd accumulation in crops endangers human health. When Cd is present in the soil, plants are exposed to it throughout their entire life cycle. As it is a non-essential element, no specific Cd uptake mechanisms are present. Therefore, Cd enters the plant through transporters for essential elements and consequently disturbs plant growth and development. In this review, we will focus on the effects of Cd on the most important events of a plant's life cycle covering seed germination, the vegetative phase and the reproduction phase. Within the vegetative phase, the disturbance of the cell cycle by Cd is highlighted with special emphasis on endoreduplication, DNA damage and its relation to cell death. Furthermore, we will discuss the cell wall as an important structure in retaining Cd and the ability of plants to actively modify the cell wall to increase Cd tolerance. As Cd is known to affect concentrations of reactive oxygen species (ROS) and phytohormones, special emphasis is put on the involvement of these compounds in plant developmental processes. Lastly, possible future research areas are put forward and a general conclusion is drawn, revealing that Cd is agonizing for all stages of plant development.

Keywords: cadmium; cell cycle; cell wall; germination; oxidative stress; plant growth and development; reproduction.

Figure 1

Possible interference mechanisms of cadmium on the process of seed germination. Cadmium (Cd) negatively affects metabolic reactivation by reducing levels of hydrolyzing enzymes, starch mobilization and seed imbibition. Furthermore, it can alter redox signaling via calcium (Ca), mitogen-activated protein kinases (MAPKs) and transcription factors (TFs) and the level of phytohormones such as abscisic acid (ABA), auxin (AUX), giberrellic acid (GA) and ethylene (ET). Both are of major importance in the seed germination process. One-way arrows: indicate a stimulating effect, whereas T-shaped arrows represent an inhibitory effect. Two-way arrows signify an interaction and dashed lines indicate effects which are still uncertain.

Figure 2

Schematic overview of important players affecting vegetative plant growth upon Cd exposure. Cadmium exposure is well known to affect concentrations of reactive oxygen species (ROS) and phytohormones, which are closely intertwined. Cadmium induces DNA damage, thereby activating the DNA damage response, which can either induce programmed cell death (PCD) or affect cell cycle progression, depending on the extent of DNA damage. In addition, Cd exposure induces cell wall modifications as a strategy to reduce Cd entry into cells. This in turn limits cell expansion, which is intertwined with the cell cycle and specifically endoreduplication. Cadmium-induced DNA damage and cell wall modifications could either result from its effects on ROS and phytohormone levels or arise through an alternative pathway. Together, Cd-induced PCD, cell cycle alterations and inhibition of cell expansion contribute to its negative effect on vegetative growth. One-way arrows indicate stimulating effects, whereas T-shaped arrows represent an inhibitory effect. Two-way arrows signify an interaction.