Dalton D A, Russell S A, Evans H J
Department of Biology, Reed College, Portland, OR 97202.
Biofactors. 1988 Jan;1(1):11-6.
The detrimental effects of excessive Ni on plant growth have been well known for many years. More recent evidence indicates that Ni is required in small amounts for normal plant growth and development. Ni is an essential component of urease in plants and microorganisms. A deficiency of Ni in plants is reported to result in necrotic lesions in leaves in response to toxic accumulations of urea. Urease plays an essential role in mobilization of nitrogenous compounds in plants, a process that is especially important during seed germination and fruit formation when protein reserves are degraded into amino acids. Arginine, an abundant amino acid in plants, when degraded produces urea as a product and urease is needed for urea utilization. Theories of urea formation during allantoin degradation in Glycine max have been recently refuted. In G. max ureides apparently are metabolized via an amidohydrolase reaction with subsequent degradation of ureidoglycine, yielding glyoxylate, NH+4 and CO2. No evidence is available for the formation of urea in this pathway. Nitrogen-fixing symbionts, such as Rhizobium and Bradyrhizobium, contain two known Ni enzymes: urease and hydrogenase. Optimum growth of nodulated legumes and actinorhizal plants may depend on an adequate supply of Ni to meet the requirements of the Ni-requiring enzymes in host plants and endophytes. The seeds of severely Ni-deficient Hordeum are completely inviable, thus providing conclusive evidence for the essentiality of Ni for this species. The evidence indicates that Ni must be added to the list of micronutrient elements generally required by plants.
多年来，过量镍对植物生长的有害影响已广为人知。最近的证据表明，正常的植物生长发育需要少量的镍。镍是植物和微生物中脲酶的重要组成部分。据报道，植物中镍缺乏会导致叶片出现坏死斑，这是对尿素毒性积累的反应。脲酶在植物中含氮化合物的调动过程中起着至关重要的作用，这一过程在种子萌发和果实形成期间尤其重要，因为此时蛋白质储备会降解为氨基酸。精氨酸是植物中含量丰富的氨基酸，降解时会产生尿素作为产物，而尿素的利用需要脲酶。最近，关于大豆中尿囊素降解过程中尿素形成的理论已被推翻。在大豆中，脲类化合物显然是通过酰胺水解酶反应进行代谢的，随后脲基甘氨酸降解，产生乙醛酸、NH⁺₄和CO₂。在这条途径中没有形成尿素的证据。固氮共生体，如根瘤菌和慢生根瘤菌，含有两种已知的镍酶：脲酶和氢化酶。结瘤豆科植物和放线菌根植物的最佳生长可能取决于充足的镍供应，以满足宿主植物和内生菌中需要镍的酶的需求。严重缺镍的大麦种子完全无法存活，从而为镍对该物种的必要性提供了确凿证据。证据表明，必须将镍添加到植物普遍所需的微量营养元素清单中。
