Green fluorescent carbon dots facilitate the growth and augment the salt tolerance of rapeseed

Carbon dots (CDs) are a novel class of fluorescent nanomaterials with great potential in agriculture. However, CDs’ dual mechanisms of promoting plant growth through direct regulation of plant physiology and indirect remodeling of the microbiome remain to be further elucidated. In this study, a complementary experimental design was employed to systematically investigate the regulatory effects of green fluorescent CDs on rapeseed and to distinguish between their direct and indirect modes of action. In a sterile system using 1/2 MS medium, application of 90 mg/L CDs significantly alleviated growth inhibition induced by salt stress, increasing the fresh weight of shoots and roots by 35.80% and 49.13%, respectively. CDs alleviated salt damage by regulating ion transport, reducing Na⁺ translocation from roots to shoots, and promoting K⁺ uptake, thereby increasing the K⁺/Na⁺ ratio and maintaining ion homeostasis. Transcriptome analysis further indicated that key ion transporter genes, including BnaC09.HKT1 and BnaA04.AKT1 , are central players in this direct regulatory process. In a soil-based pot system under non-stress conditions, root irrigation with 45 mg/L CDs increased shoot fresh weight by 22.11%. Amplicon sequencing analysis revealed that CDs significantly altered the diversity of microbial communities associated with multiple ecological niches, particularly bulk soil and root endophytes, with notable enrichment of beneficial microbial taxa such as Rhizobium , Flavobacterium , and Peziza . These findings suggest that CDs can indirectly promote plant growth through modulation of the beneficial microbiome. Collectively, this study elucidates a dual-mode mechanism by which CDs enhance rapeseed performance that involves directly regulating ion homeostasis to improve salt tolerance under stress conditions and indirectly promoting growth by enriching beneficial microbes under non-stress conditions. These insights provide a theoretical basis for the diversified application of CDs in sustainable agriculture. • CDs treatment enhanced the biomass and salt tolerance of rapeseed seedlings. • CDs improved Na + /K + balance by limiting Na + translocation, enhanced K + uptake, increased K + /Na + ratio, and mitigated salt stress. • CDs modulate ion transporter gene expression, with Na + and K + transporters BnaC09.HKT1 and BnaA04.AKT1 playing pivotal roles. • CDs promote plant growth by enhancing the abundance of beneficial microorganisms in the soil.