A CCT protein GmCIC5 activates GmP5CS to regulate proline accumulation and cadmium tolerance in vegetable soybean

Cadmium (Cd) toxicity poses a severe environmental threat, leading to substantial declines in crop growth and productivity worldwide. Accumulation of proline is recognized as a critical adaptive response in plants facing heavy metal stress. However, the precise mechanisms regulating Cd-induced proline accumulation remain elusive. Here, we discovered that a Cd-induced CCT5 gene in vegetable soybean, designated as GmCIC5, plays a pivotal role in regulating proline biosynthesis by activating the transcription of the proline biosynthesis gene, GmP5CS. Under greenhouse conditions, silencing GmCIC5 plants exhibited heightened sensitivity to Cd stress and suppressed root growth after 15 days of 30 mg/L CdCl₂ treatment. Furthermore, qRT-PCR and ChIP-qPCR revealed that GmCIC5 directly targeted GmP5CS. Consequently, the GmCIC5 knockdown plants showed drastically reduced levels of GmP5CS transcripts and proline accumulation. Overexpression of GmP5CS largely suppressed the Cd hypersensitivity phenotype of GmCIC5 knockdown plants, suggesting that GmCIC5-mediated Cd tolerance was dependent on proline. Taken together, our results indicated that GmCIC5 is a novel regulator of Cd stress tolerance through proline biosynthesis, which may provide new insights into the molecular mechanisms of plant responses to Cd stress and offer a theoretical foundation for breeding Cd-resistant crops.