Cyanobacterial priming enhances lignin biosynthesis for cell wall macromolecular reinforcement: A mechanism for mitigating In2O3 nanoparticle Stress in the legume alfalfa and the grass rye

Exposure to indium oxide nanoparticles (In₂O₃-NPs) presents a critical challenge as an emerging soil contaminant that severely impairs plant growth and metabolic health. In this study, In2O3-NP exposure reduced biomass by 58% in alfalfa, driven by excessive indium (In) accumulation, disrupted mineral (P and Fe) balance, and inhibited photosynthesis. This physiological decline led to a significant depletion of sugars and nitrogen metabolites, particularly in the more sensitive alfalfa model. To mitigate these toxic effects, we utilized cyanobacterial priming (CP) with Anabaena laxa to investigate how species-specific metabolic and physiological responses are shaped in alfalfa (legume) and rye (grass). CP treatment differentially mitigated toxicity by reducing In uptake, maintaining nutrient homeostasis, and restoring photosynthetic efficiency. This resulted in improved biomass, with alfalfa showing the most significant recovery. Mechanistically, CP enhanced sugar and nitrogen metabolism, providing the necessary precursors for the accumulation of protective secondary metabolites, such as phenolics and flavonoids, which reduced oxidative damage. Furthermore, CP induced a structural adaptation through the activation of the phenylpropanoid-lignin biosynthetic pathway. In alfalfa, elevated levels of cinnamic and coumaric acids were linked to increased activities of key enzymes, phenylalanine ammonia-lyase (PAL), caffeic acid O-methyltransferase (COMT), cinnamyl alcohol dehydrogenase (CAD), and cinnamoyl-CoA reductase (CCR). This resulted in a substantial 243.6% increase in lignin in alfalfa, compared to a 115.1% rise in rye. This study demonstrates that while rye relies on inherent physiological tolerance, alfalfa exhibits superior metabolic plasticity when primed with CP. These findings prove that alfalfa and rye utilize distinct survival strategies at the biochemical level; while rye leverages its natural resilience, alfalfa undergoes a complete metabolic reorganization to rescue its growth and survive nanoparticle stress.