Insights into physiological, biochemical and molecular mechanisms of abiotic stress tolerance in Persian walnut (Juglans regia L.).

Walnut (Juglans regia) is an economically and nutritionally valuable tree species that often encounters diverse abiotic stresses such as drought, salinity, cold, heat and heavy metal toxicity. These stresses adversely affect its growth, productivity and survival by altering physiological functions, disturbing cellular homeostasis and triggering oxidative damage. In response, J. regia deploy a multifaceted adaptive system comprising morphological changes, biochemical adjustments and intricate molecular signaling pathways. The review aims to analyze current knowledge on the physiological, morphological changes observed under stress with biochemical defense mechanism. These include the crucial antioxidant defense system (increase in Superoxide Dismutase (SOD), Peroxidase (POD) and Catalase (CAT) activity), the accumulation of protective soluble solutes and amino acids and the biosynthesis of secondary metabolites through the Methylerythritol Phosphate (MEP) pathway involved in mitigating oxidative stress caused by Reactive Oxygen Species (ROS). Crucially, we synthesize the understanding of molecular regulation that underpins stress adaptation. This encompasses stress-responsive gene expression including Jr (Juglans regia) VHAG1 (V-ATPase H+ transporting ATPase subunit G1), JrDREB (Dehydration-Responsive Element Binding protein), JrRD29 (Responsive to Dehydration 29), transcriptional regulation by myeloblastosis (MYB), Dehydration-responsive element-binding proteins (DREB) and WRKY-TF families and their interaction with hormonal (abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA) and ethylene) which play a pivotal role in signal transduction and stress tolerance. These mechanisms employed by walnut under abiotic stresses, highlighting key genetic and hormonal pathways that can be targeted for the development of stress-resilient cultivars and ensuring sustainable production under changing climatic conditions. Recent advancements in genomics, transcriptomics and proteomics got attention that provides new insights into the regulatory networks and candidate genes conferring stress resilience. Furthermore, it explores biotechnological approaches for improving stress tolerance, highlighting the prospects of latest high-throughput techniques, including molecular breeding, genetic engineering, Next-Generation Sequencing (NGS), microRNA (miRNA)-based regulation and Clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9 (CRISPR/Cas) gene editing. This integrated review connects multi-level stress response mechanisms, highlighting potential molecular markers and biotechnological interventions for accelerating walnut improvement and ensuring sustainable production under changing climatic conditions.