Context-dependent responses of tomato (Solanum lycopersicum) to a Chlamydomonas reinhardtii biostimulant extract under saline and non-saline conditions

Disentangling the complex interplay between biostimulants and environmental stress is a key frontier in sustainable agriculture. In particular, robustly distinguishing between a biostimulant’s intrinsic bioactivity and its specific stress-mitigating properties remains a challenge. We used a fully factorial design and high-throughput RNA-sequencing to examine the molecular interaction between an aqueous extract from the microalga Chlamydomonas reinhardtii (MA) and NaCl stress (75 mM) in tomato ( Solanum lycopersicum ). We assessed vegetative growth, leaf ion content, and performed transcriptomic analysis of leaf tissue. The application of MA significantly improved vegetative growth, increasing leaf area by 16% and leaf hydration (dry matter decreased from 13.49% to 11.47%) regardless of salinity. Factorial transcriptomic analysis revealed that MA’s molecular effects depend on the plant’s stress status, with 138 genes showing a significant Salt × MA interaction. Under salinity, MA suppressed typical osmotic and oxidative stress-response genes, suggesting it reduces stress perception and costly defenses. In non-saline conditions, MA triggered a “priming” effect, upregulating temperature-response genes while downregulating genes involved in energy-heavy ribosome biogenesis, highlighting an anticipatory mechanism that prepares the plant for future challenges while conserving resources. This study provides a conceptual framework for developing next-generation tools to enhance crop resilience through context-aware biostimulant application.