To examine the potential roles of melatonin in Cd uptake, accumulation and detoxification in Malus plants, we exposed two different apple rootstocks varying greatly in Cd uptake and accumulation to either 0 or 30 μM Cd together with 0 or 100 μM melatonin. Cd stress stimulated endogenous melatonin production to a greater extent in the Cd-tolerant M. baccata than in the Cd-susceptible M. micromalus ‘qingzhoulinqin’. Melatonin application attenuated Cd-induced reductions in growth, photosynthesis, and enzyme activity, as well as ROS and MDA accumulation. Melatonin treatment more effectively restored photosynthesis, photosynthetic pigments, and biomass in Cd-challenged M. micromalus ‘qingzhoulinqin’ than in Cd-stressed M. baccata. Exogenous melatonin lowered root Cd²⁺ uptake, reduced leaf Cd accumulation, decreased Cd translocation factors (Tfs), and increased root, stem, and leaf melatonin contents in both Cd-exposed rootstocks. Melatonin application increased both antioxidant concentrations and enzyme activities to scavenge Cd-induced ROS. Exogenous melatonin treatment altered the mRNA levels of several genes regulating Cd uptake, transport, and detoxification including HA7, NRAMP1, NRAMP3, HMA4, PCR2, NAS1, MT2, ABCC1, and MHX. Taken together, these results suggest that exogenous melatonin reduced aerial parts Cd accumulation and mitigated Cd toxicity in Malus plants, probably due to the melatonin-mediated Cd allocation in tissues, and induction of antioxidant defense system and transcriptionally regulated key genes involved in detoxification.

中文翻译：

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外源褪黑素可减轻苹果砧木中镉的吸收和毒性。

为了检查褪黑素在苹果属植物中对Cd吸收，积累和排毒的潜在作用，我们将两种在Cd吸收和积累方面差异很大的苹果砧木暴露于0或30μMCd和0或100μM褪黑素。与耐镉的微小苹果青霉“青州林琴”相比，耐镉的baccata镉胁迫对内源性褪黑激素产生的刺激程度更大。褪黑素的应用减弱了镉诱导的生长，光合作用和酶活性以及ROS和MDA积累的减少。褪黑素处理比Cd胁迫的M. baccata更有效地恢复了Cd挑战的苹果小苹果'qingzhoulinqin'的光合作用，光合色素和生物量。外源褪黑激素降低了根中的Cd ²⁺两种镉暴露的砧木的吸收，降低的叶片Cd积累，降低的Cd转运因子（Tfs）以及增加的根，茎和叶片褪黑激素含量。褪黑素的使用增加了抗氧化剂的浓度和酶的活性，以清除Cd诱导的ROS。外源性褪黑素治疗改变了几个调节Cd吸收，转运和排毒的基因的mRNA水平，包括HA7，NRAMP1，NRAMP3，HMA4，PCR2，NAS1，MT2，ABCC1和MHX。综上所述，这些结果表明，外源褪黑素减少了苹果属植物中的空中部分Cd积累并减轻了Cd毒性，这可能是由于褪黑素介导的Cd在组织中的分配以及抗氧化防御系统的诱导和参与排毒的转录调节关键基因的诱导。