Comparative physiological, hormonal, and transcriptional analyses reveal the difference between diploid and autotetraploid rice under cadmium stress.

key messageElevated cadmium (Cd) stress tolerance in autotetraploid rice at the seedling stage is correlated with coordinated remodeling of ROS homeostasis, hormone signaling and transcriptional programs. Polyploid plants commonly exhibit robust tolerance to various abiotic stresses, while the underlying regulatory mechanisms remain largely elusive. To elucidate the intrinsic mechanisms underlying enhanced Cd stress tolerance in autotetraploid rice, two pairs of diploid (2X) rice and their corresponding autotetraploid (4X) lines were used as experimental materials for Cd stress treatment and systematic phenotypic, physiological, and molecular evaluation at the seedling stage. Phenotypic observations confirmed that autotetraploid rice displayed markedly stronger Cd stress tolerance than diploid rice, as reflected by approximately 1.5-fold greater plant height alongside elevated biomass and chlorophyll content. Physiological and biochemical assays revealed that autotetraploids exhibited markedly lower Cd content, higher levels of soluble sugars and proline, distinctly lower levels of ROS-related indicators (MDA and H₂O₂), and markedly elevated activities of antioxidant enzymes (catalase, CAT; peroxidase, POD). Comparative phytohormone quantification demonstrated that autotetraploid rice had significantly higher levels of tryptamine (TAM), indole-3-acetic acid (IAA), indole-3-acetyl-L-aspartic acid (IAA-Asp), jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-Ile), and gibberellins A₁ (GA₁) and A₇ (GA₇) but significantly lower levels of cis- (cZ) and trans-zeatin (tZ), with about 2.6, 1.3, 2.3, 1.4, 2.0, 2.6, 2.1, 0.5, 0.6 folds of diploid plants, respectively. Transcriptome profiling revealed that under control conditions, the number of differentially expressed genes (DEGs) between autotetraploid and diploid was merely 939 in roots and 313 in shoots. Under Cd stress, however, the counts of DEGs markedly increased to 4974 in roots and 1023 in shoots. Notably, genes involved in heavy metal transport, and hormone and ROS metabolism were significantly enriched in autotetraploids. Furthermore, qRT-PCR validated consistent expression trends of core metal transporters, hormone biosynthesis genes, and antioxidant genes between the RNA-seq data and independent samples. Collectively, this study suggests that the distinct regulations of physiological and biochemical processes, hormone metabolism, and transcription in autotetraploids under Cd stress are correlated with their enhanced Cd stress tolerance.