Cadmium (Cd) is a nonessential and highly toxic element causing agricultural problems. However, little information is available about the variation in Cd tolerance among apple rootstocks and its underlying physiological regulation mechanisms. This study investigated Cd accumulation, subcellular distribution and chemical forms as well as physiological changes among four apple rootstocks exposed to either 0 or 300 μM CdCl2. The results showed that variations in Cd tolerance existed among these rootstocks. Cd exposure caused decline in photosynthesis, chlorophyll and biomass in four apple rootstocks, which was less pronounced in M. baccata, indicating its higher Cd tolerance. This finding was corroborated with higher Cd tolerance indexes (TIs) of the whole plant in M. baccata than those in the other three apple rootstocks. Among the four apple rootstocks, M. baccata displayed the lowest Cd concentrations in roots, wood and leaves, the smallest total Cd amounts as well as the lowest BCF. In apple rootstocks, it was found that to immobilize Cd in cell wall and soluble fraction (most likely in vacuole) and to convert it into pectate- or protein- integrated forms and undissolved Cd phosphate forms may be the primary strategies to reduce Cd mobility and toxicity. The physiological changes including ROS, carbohydrates and antioxidants were in line with the variations of Cd tolerance among four apple rootstocks. In comparison with the other three apple rootstocks, M. baccata had lower concentrations of ROS in roots and bark, H2O2 in roots and leaves and MDA in roots, wood and bark, but higher concentrations of soluble sugars in bark and starch in roots and leaves, and enhanced antioxidants. These results indicate that M. baccata are more tolerant to Cd stress than the other three apple rootstocks under the current experiment conditions, which is probably related to Cd accumulation, subcellular partitioning and chemical forms of Cd and well-coordinated antioxidant defense mechanisms.

To examine the potential roles of melatonin in cadmium (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. Cadmium stress stimulated endogenous melatonin production to a greater extent in the Cd-tolerant Malus baccata Borkh. than in the Cd-susceptible Malus micromalus ‘qingzhoulinqin’. Melatonin application attenuated Cd-induced reductions in growth, photosynthesis and enzyme activity, as well as reactive oxygen species (ROS) and malondialdehyde 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 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.

Arbuscular mycorrhizal fungi (AMF) affect cadmium (Cd) accumulation and tolerance in host plants. However, the effects of AMF on Cd accumulation and phytotoxicity and their underlying mechanism in apples remain uncharacterized. In this study, the comprehensive physiological and molecular responses of uninoculated and Rhizophagus intraradices-inoculated Malus hupehensis Rehd. rootstocks exposed to 0 or 300 μM Cd were investigated. AMF inoculation mitigated Cd-induced growth and photosynthesis inhibition and nutrient ion disorders. It also lowered the concentrations of Cd in all tissues and reduced Cd transport to the shoots. Compared to uninoculated apple plants, those inoculated with mycorrhizal fungi reduced the mobility and toxicity of Cd by altering its form and binding it to the cell walls of the roots and leaves. AMF inoculation ameliorated Cd stress by altering endogenous phytohormone levels and triggering enzymatic and non-enzymatic antioxidant systems. Transcriptome analysis revealed that the differentially expressed genes (DEGs) associated with AMF under Cd stress regulated carbohydrate and amino acid biosynthesis and metabolism, as well as phytohormone biosynthesis and signal transduction. Furthermore, AMF inoculation downregulated certain genes involved in Cd uptake and transport while upregulating other genes involved in detoxification. These results suggest that AMF alleviate Cd phytotoxicity by orchestrated physiological and transcriptomic regulation in M. hupehensis Rehd., providing valuable insights into the efficacy of AMF inoculation in improving the heavy metal resistance of fruit trees.

Soil salinization, caused by the extensive use of inorganic fertilizers, has restricted the development of the sweet cherry industry. Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria that colonize the area around the roots, playing a crucial role in promoting plant growth, alleviating salt stress, and enhancing crop productivity. However, few studies have investigated whether PGPR can alleviate salt stress in sweet cherry. This study aimed to investigate whether co-inoculation (MIX) of Pantoea ananatis D1-28 and Bacillus aryabhattai F isolated from the rhizosphere of Zea mays L. could improve salt tolerance in seedlings of the common cherry (Prunus avium) rootstock Gisela 6. The results indicated that MIX inoculation alleviated salt stress and enhanced plant height and biomass. Under 100 mM salt stress, the MIX strain inoculant treatment significantly increased the net photosynthetic rate (14.38%), transpiration rate (22.61%), stomatal conductance (15.63%), total soluble sugar content (18.72%), free proline content (23.79%), superoxide dismutase activity (19.53%), and peroxidase activity (21.96%) but reduced the O₂•^- (16.99%) and H₂O₂ contents (17.59%) in Gisela 6 compared to CK without PGPR inoculation. After the MIX treatment, multiple indices showed strong correlations and improved salt stress resistance. In addition, principal component analysis showed that under 100 mM salt concentration, the MIX strain exhibited the largest separation from CK and had the strongest alleviating effect on plant salt stress. These results suggest that co-inoculation with the strains P. ananatis D1-28 and B. aryabhattai F effectively alleviated salt stress, improved plant photosynthetic capacity, and increased plant biomass by regulating antioxidant defense system and osmotic adjustment substances.