Moderate salt stress enhances natural rubber yield by jointly increasing root biomass and rubber accumulation in Taraxacum kok-saghyz

Taraxacum kok-saghyz (TKS) is a promising temperate source of natural rubber, but the effects of NaCl intensity on root growth and rubber production remain unclear. Here, field observations from two native saline-alkali habitats were combined with a controlled 0–250 mM NaCl experiment, physiological and ion measurements, root transcriptome sequencing, and quantitative real-time PCR (qRT-PCR). Plants collected from the more saline Zhaosu habitat had a higher root rubber content than those from Tekes, whereas root dry weight did not differ significantly between the sites. Under controlled conditions, root dry weight, rubber content, and rubber yield per plant responded non-monotonically to NaCl and reached their highest values at 100 mM. Rubber yield per plant was 73% higher at 100 mM NaCl than in the control, but declined markedly at 200 mM and partially recovered at 250 mM. Compared with the high-NaCl treatments, 100 mM NaCl was associated with lower root Na⁺ accumulation and a lower Na⁺/K⁺ ratio. RNA sequencing identified 5365 and 8748 differentially expressed genes under 100 and 200 mM NaCl, respectively. Genes associated with isoprenoid precursor supply, farnesyl diphosphate (FPP) formation, cis-prenyl chain elongation, and rubber particle-related processes showed concentration-dependent expression patterns. qRT-PCR further confirmed stronger induction of eight selected rubber biosynthesis-related genes under 100 mM NaCl than under 200 mM NaCl. These results demonstrate that moderate NaCl treatment can enhance TKS rubber yield through the concurrent improvement of root biomass and rubber accumulation, and that this production advantage is associated with a more favorable root ionic status and coordinated transcriptional responses of rubber biosynthesis-related genes.