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.