Physiological Effects and Mechanisms of Chlorella vulgaris as a Biostimulant on the Growth and Drought Tolerance of Arabidopsis thaliana

Microalgae have demonstrated biostimulant potential owing to their ability to produce various plant growth-promoting substances, such as amino acids, phytohormones, polysaccharides, and vitamins. Most previous studies have primarily focused on the effects of microalgal biostimulants on plant growth. While biomass extracts are commonly used as biostimulants, research on the use of culture supernatant, a byproduct of microalgal culture, is scarce. In this study, we aimed to evaluate the potential of <i>Chlorella vulgaris</i> culture as a biostimulant and assess its effects on the growth and drought tolerance of <i>Arabidopsis thaliana</i>, addressing the gap in current knowledge. Our results demonstrated that the <i>Chlorella</i> cell-free supernatant (CFS) significantly enhanced root growth and shoot development in both seedlings and mature <i>Arabidopsis</i> plants, suggesting the presence of specific growth-promoting compounds in CFS. Notably, CFS appeared to improve drought tolerance in <i>Arabidopsis</i> plants by increasing glucosinolate biosynthesis, inducing stomatal closure, and reducing water loss. Gene expression analysis revealed considerable changes in the expression of drought-responsive genes, such as <i>IAA5</i>, which is involved in auxin signaling, as well as glucosinolate biosynthetic genes, including <i>WRKY63</i>, <i>MYB28</i>, and <i>MYB29</i>. Overall, <i>C. vulgaris</i> culture-derived CFS could serve as a biostimulant alternative to chemical products, enhancing plant growth and drought tolerance.