Does Folcisteine (NATCA) Play a Role in Facilitating Seed Germination, Root Development, and Elevating Root AsA-GSH Cycle Efficiency Under Combined Copper–Cadmium Stress in Maize?

Copper (Cu) and cadmium (Cd) are common co-occurring environmental pollutants inducing combined stress, which severely harms maize growth. Previous studies have confirmed the involvement of the ascorbate-glutathione (AsA-GSH) cycle in heavy metal stress tolerance, but the regulatory effect of NATCA on this cycle under Cu-Cd combined stress-especially during maize seed germination and root development-remains unelucidated. Exogenous folcisteine (NATCA, 3-acetylthiazolidine-4-carboxylic acid) can enhance plant tolerance to abiotic stress. This study investigated the role of NATCA, a novel plant growth regulator with antioxidant potential, in alleviating Cu-Cd combined stress in maize. Two maize (<i>Zea mays</i> L.) varieties-Jiuyuan 15 (Cu/Cd-tolerant) and Longfuyu 6 (Cu/Cd-intolerant)-were exposed to combined stress (80 mg·L^-1 CuSO₄ + 100 mg·L^-1 CdCl₂) with/without 20 mg·L^-1 NATCA. Germination and hydroponic experiments were conducted to investigate NATCA's effects on seed germination, growth, root traits, photosynthetic characteristics, reactive oxygen species (ROS) metabolism, AsA-GSH cycle (ascorbate-glutathione cycle), and endogenous hormones under stress. The results showed that combined Cu/Cd stress inhibited seed germination (reduced vigor, rate, index), while NATCA significantly reversed these declines, increased tolerance index, lowered relative damage rate, and improved seed activity-with more pronounced effects on Longfuyu 6. Stress stunted seedling growth (reduced dry/fresh weight, water content; increased water deficit), whereas NATCA promoted growth (taller plants, less leaf chlorosis, more fibrous roots), enhanced dry matter accumulation, and improved water metabolism. Stress impaired root morphology (shorter length, smaller surface area/volume, fewer tips) and absorption capacity; NATCA improved root traits, stress tolerance, and vitality. Stress weakened ROS scavenging, but NATCA elevated antioxidant enzyme activity and non-enzymatic antioxidant content, strengthened AsA-GSH cycle-mediated ROS clearance, mitigated stress damage, and maintained intracellular ROS balance in maize seedling root. These findings not only reveal a new regulatory role of NATCA in enhancing heavy metal stress tolerance via the AsA-GSH cycle but also provide a potential eco-friendly strategy for improving maize production in heavy metal-contaminated soils.