Foliar Application of γ-Polyglutamic Acid Enhances Chilling Tolerance in Pepper Seedlings by Orchestrating Root-to-Shoot Defense Responses

Pepper (Capsicum annuum L.) is highly susceptible to chilling stress, which severely constrains its growth and productivity. Although the eco-friendly biostimulant γ-polyglutamic acid (γ-PGA) has shown promise in enhancing plant tolerance to abiotic stresses, its specific role and underlying mechanisms in alleviating chilling injury in pepper remain poorly understood. This study systematically investigated the physiological and molecular mechanisms by which foliar application of 100 mg·L−1 γ-PGA enhances chilling tolerance in pepper seedlings. Our results demonstrated that γ-PGA pretreatment significantly mitigated chilling-induced growth inhibition and promoted root development, evidenced by a 110.8% increase in the number of root tips. Analysis of photosynthetic performance revealed that γ-PGA effectively counteracted chilling-induced photosynthetic suppression, increasing the net photosynthetic rate (Pn) by 172.9% and the maximum photochemical efficiency of PSII (Fv/Fm) by 17.9%. Furthermore, γ-PGA treatment significantly reduced the accumulation of reactive oxygen species (O2·− and H2O2) and malondialdehyde (MDA) while promoting the synthesis of the osmoprotectant proline. This protective effect was associated with a strengthened antioxidant defense system; γ-PGA enhanced the activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). These changes were supported at the molecular level by the significant upregulation of their corresponding genes (CaSOD, CaCAT, and CaAPX), with CaAPX expression showing a striking 313.5% increase. In conclusion, foliar application of γ-PGA enhances chilling tolerance in pepper seedlings via a multi-faceted mechanism that includes improving root architecture, reinforcing the antioxidant defense system, facilitating osmotic adjustment, and protecting the photosynthetic apparatus. These findings provide a theoretical framework and practical support for using γ-PGA as an effective and sustainable biostimulant to improve pepper cultivation in environments prone to chilling stress.