Single-walled carbon nanohorns promote apple growth by regulating carbohydrate allocation and metabolism

Single-walled carbon nanohorns (SWCNHs) regulate plant growth and stress response. However, interaction mechanism between SWCNHs and plants remains largely unknown. This study aimed to evaluate if SWCNHsmediated plant growth can be correlated with carbohydrate allocation and metabolism. Our results showed that foliar spraying of 100 mg/L SWCNHs significantly promoted the growth of apple seedlings, as indicated by increased dry weight, fresh weight, total root length, root surface area, root volume, leaf photosynthesis rate and chlorophyll contents. The TEM results revealed that SWCNHs particles were absorbed by apple leaf cells. SWCNHs application enhanced A6PR activity and MdA6PR expression, accompanied by increased amount of photosynthetic assimilate-sorbitol in apple leaves. Activities of SDH, SUSY, FRK and HK, all of which are key enzymes related to carbohydrate catabolism, were higher in roots of SWCNHs-treated apple seedlings than that in control apple roots. Transcript levels of MdA6PR, MdSPS1, MdSDH2, MdSUSY3, MdFRK2, MdHK1, MdSOT1/2 and MdTMT1/3 were upregulated, whereas transcript levels of MdSPS6, MdUGP1/3 and MdPGM1/2 were downregualted in SWCNHs-treated plants. These findings indicate that foliar spraying SWCNHs increases the synthesis of photosynthate in apple leaves, and improves unloading and utilization capacity of carbohydrate in apple roots. These results establish the theoretical foundation for the application of SWCNHs in horticultural production and plant biotechnology.