Exogenous Selenium Enhances Manganese Tolerance in Malus Robusta by Modulating Polyamine and Proline Metabolism

The accumulation of excess manganese (Mn) is toxic to plants and limits agricultural productivity. Although selenium (Se) is known to be a beneficial element that can alleviate heavy metal stress, its role in mitigating Mn-related stress remains insufficiently explored. This research explores the effects of Se (applied as sodium selenite at 0.5 μM) on 0.5 mM Mn toxicity in Malus robusta seedlings, focusing on Mn accumulation, physiological performance, polyamine metabolism, proline biosynthesis, and the enzymatic activity and expression levels of critical genes. Exogenous Se significantly reduced Mn accumulation and alleviated Mn toxicity, as evidenced by enhanced root growth, increased photosynthetic pigments, improved fluorescence parameters (Fv/fm and ΦPSII), and maintained antioxidant balance via a reduced production of reactive oxygen species (ROS) and an activation of the antioxidant system. Moreover, total putrescine (Put) and spermine (Spm) contents declined after Se application, whereas spermidine (Spd) levels showed no noticeable change. This led to an increased (Spd + Spm)/Put ratio, highlighting the pivotal role of Put reduction in Mn stress response. A decrease in Put corresponded with significant downregulation of ornithine decarboxylase (ODC; EC 4.1.1.17) and arginine decarboxylase (ADC; EC 4.1.1.19) activities and gene expressions. Furthermore, soluble conjugated and insoluble bound polyamines followed a similar trend, except for a notable increase in bound Spd. In addition, Se treatment decreased proline (Pro) content mainly through the suppression of ornithine aminotransferase (OAT; EC 2.6.1.13). It is observed that Se enhances the ability of M. robusta to withstand Mn stress by regulating polyamine and proline metabolism, thereby highlighting a possible mechanism for reducing Mn toxicity in plants.