Integrative leaf transcriptomic and physiological insights into salt stress responses in Eucalyptus: A comparative analysis of two contrasting clones

Salinity severely limits forest productivity; however, the molecular basis of tolerance in Eucalyptus remains unclear. We investigated the physiological and transcriptomic responses of a salt-tolerant clone (B-112) and a salt-sensitive clone (W-12) subjected to 80 days of severe salinity (EC ≥ 20 dS/m). Both clones showed reductions in photosynthesis, stomatal conductance, internal CO₂ concentration, and transpiration; however, the extent of decline was considerably smaller in B-112, indicating superior physiological resilience. RNA-seq profiling revealed the targeted reprogramming of 1025 genes in B-112, compared with a broad, less coordinated response involving 14,994 genes in W-12. In B-112, enrichment analyses highlighted the strong activation of abscisic acid (ABA) and ethylene signaling, mitogen-activated protein kinases (MAPKs) cascades, carbohydrate metabolism, antioxidant defense, and protein folding pathways. Key salt-responsive genes included Dehydrins (DHN1), Ethylene Response Factors (ERFs), cupredoxins, and expansins, indicating coordinated osmotic adjustment, cell-wall modification, and stress signaling. Conversely, W-12 displayed extensive upregulation of ubiquitination and vesicle trafficking genes, alongside repression of photosynthesis and chloroplast-related processes, consistent with a damage-induced rather than adaptive program. qRTPCR validation confirmed high concordance with RNAseq results. Together, these findings reveal that B-112 sustains salt tolerance through proactive and focused transcriptomic reprogramming, while W-12 exhibits reactive stress-triggered responses, providing candidate targets for breeding and genetic improvement of Eucalyptus in saline environments.