Table_1.XLSX

Cadmium (Cd) pollution renders many soils across the world unsuited or unsafe for food- or feed-orientated agriculture. The main mechanism of Cd phytotoxicity is the induction of oxidative stress, amongst others through the depletion of glutathione. Oxidative stress can damage lipids, proteins, and nucleic acids, leading to growth inhibition or even cell death. The plant cell has a variety of tools to defend itself against Cd stress. First and foremost, cell walls might prevent Cd from entering and damaging the protoplast. Both the primary and secondary cell wall have an array of defensive mechanisms that can be adapted to cope with Cd. Pectin, which contains most of the negative charges within the primary cell wall, can sequester Cd very effectively. In the secondary cell wall, lignification can serve to immobilize Cd and create a tougher barrier for entry. Changes in cell wall composition are, however, dependent on nutrients and conversely might affect their uptake. Additionally, the role of ascorbate (AsA) as most important apoplastic antioxidant is of considerable interest, due to the fact that oxidative stress is a major mechanism underlying Cd toxicity, and that AsA biosynthesis shares several links with cell wall construction. In this review, modifications of the plant cell wall in response to Cd exposure are discussed. Focus lies on pectin in the primary cell wall, lignification in the secondary cell wall and the importance of AsA in the apoplast. Regarding lignification, we attempt to answer the question whether increased lignification is merely a consequence of Cd toxicity, or rather an elicited defense response. We propose a model for lignification as defense response, with a central role for hydrogen peroxide as substrate and signaling molecule.

镉（Cd）污染使得全球众多土壤不适宜或存在食品安全风险，用于食品或饲料导向的农业。镉的植物毒性主要机制之一是诱导氧化应激，其中包括通过耗竭谷胱甘肽。氧化应激能够损害脂质、蛋白质和核酸，导致生长抑制甚至细胞死亡。植物细胞拥有多种工具来抵御镉应激。首先，细胞壁可能阻止镉进入并损害原生质体。初级和次级细胞壁均配备了一系列防御机制，能够适应应对镉的挑战。果胶，作为初级细胞壁中大部分负电荷的载体，能够非常有效地隔离镉。在次级细胞壁中，木质化作用能够固定镉并形成更坚固的进入屏障。然而，细胞壁组成的改变取决于营养物质的供应，反之亦然，可能会影响其吸收。此外，抗坏血酸（AsA）作为最重要的外质抗氧化剂的作用引起了广泛关注，因为氧化应激是镉毒性的主要机制之一，并且抗坏血酸的生物合成与细胞壁构建存在多个关联。在本综述中，讨论了植物细胞壁对镉暴露的响应性改变。重点在于初级细胞壁中的果胶、次级细胞壁中的木质化以及抗坏血酸在外质中的重要性。至于木质化，我们试图回答的是，增加的木质化仅仅是镉毒性的后果，还是一种被诱导的防御反应。我们提出了一种木质化作为防御反应的模型，其中过氧化氢作为底物和信号分子发挥核心作用。