Yeast to the rescue: <i>Meyerozyma guilliermondii</i> primes tomato vigor and resistance to Fusarium crown and root rot

The use of biological control agents offers a sustainable alternative to chemical pesticides for managing soil-borne plant diseases. This study investigates the biocontrol potential of a newly isolated yeast strain, <i>Meyerozyma guilliermondii</i> INAT-MT731365, as a biotic elicitor to enhance growth and Fusarium crown and root rot (FCRR) resistance in hydroponically grown tomato plants. Tomato plants were treated with <i>M. guilliermondii</i> or left untreated as controls, then divided into two groups, one infected with <i>Fusarium oxysporum</i> f. sp. <i>radicis-lycopersici</i> (FORL) and one not infected. Physiological, biochemical, and molecular responses were monitored after treatment and inoculation. In the absence of the pathogen, <i>M. guilliermondii</i> treatment significantly enhanced plant growth and chlorophyll content. Concurrently, the yeast elicited a priming effect, characterized by low-level upregulation of PR1, <i>β</i>-1,3-glucanase and chitinase genes, downregulation of the P69G gene, and activation of defense enzymes such as peroxidase, chitinase, and <i>β</i>-1,3-glucanase, along with increased phenolic content and hydrogen peroxide accumulation, indicative of both SA- and JA/ET-mediated signalling induced systemic resistance (ISR). In control plants, FORL impaired plant defense with an early downregulation of <i>β</i>-1,3-glucanase and chitinase genes and stability in PR1 gene expression, followed by transient activation of peroxidase and chitinase and low activation of catalase, <i>β</i>-1,3-glucanase, and accumulation of phenolics. Upon FORL infection, treated plants exhibited strong upregulation of PR1, chitinase and <i>β</i>-1,3-glucanase genes mirrored by sustained increases in H₂O₂ and phenolic content and peroxidase, catalase, chitinase and <i>β</i>-1,3-glucanase activity. The simultaneous activation of both SA- and JA/ET-mediated signalling ISR resulted in a 61.8% reduction in FCRR severity and improved growth and photosynthetic traits. These findings highlight <i>M. guilliermondii</i> as a promising biocontrol agent that primes tomato plants for faster, stronger responses to soilborne pathogens while promoting growth under both healthy and stress conditions.