Yeast to the rescue: Meyerozyma guilliermondii 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, Meyerozyma guilliermondii 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 M. guilliermondii or left untreated as controls, then divided into two groups, one infected with Fusarium oxysporum f. sp. radicis-lycopersici (FORL) and one not infected. Physiological, biochemical, and molecular responses were monitored after treatment and inoculation. In the absence of the pathogen, M. guilliermondii treatment significantly enhanced plant growth and chlorophyll content. Concurrently, the yeast elicited a priming effect, characterized by low-level upregulation of PR1, β-1,3-glucanase and chitinase genes, downregulation of the P69G gene, and activation of defense enzymes such as peroxidase, chitinase, and β-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 β-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, β-1,3-glucanase, and accumulation of phenolics. Upon FORL infection, treated plants exhibited strong upregulation of PR1, chitinase and β-1,3-glucanase genes mirrored by sustained increases in H₂O₂ and phenolic content and peroxidase, catalase, chitinase and β-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 M. guilliermondii 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.