The role of iron competition in the antagonistic action of Metschnikowia pulcherrima against three major pathogenic fungi on postharvest citrus fruit

This study systematically investigated how Metschnikowia pulcherrima XX04 competes with Penicillium digitatum, Penicillium italicum, and Geotrichum citri-aurantii for iron ions to control these major postharvest citrus pathogens. Exogenous Fe³⁺ addition significantly reduced the inhibitory activity of the pigment-producing yeast against these pathogens on both agar medium and fruit. Pearson correlation analysis revealed a strong positive relationship between pigment production and antagonistic effectiveness of M. pulcherrima XX04. Comparative analysis of iron-capturing proficiency between M. pulcherrima XX04 and three pathogenic fungi on PDA medium and citrus wounds indicated that M. pulcherrima XX04 exhibited superior iron sequestration capabilities. Colony growth viability assay performed on Pulcherrimin-containing PDA (Pul-PDA) solid medium showed M. pulcherrima XX04 could utilize pulcherrimin-bound iron for growth and metabolism, and further evidenced by the significant upregulation of PUL3, a putative transporter involved in the utilization of extracellular pulcherrimin-iron complexes, whereas the pathogens exhibited limited or no ability to assimilate iron from this source. Growth assay of three pathogens under iron-limited, iron-rich, and iron-replenish conditions showed that P. digitatum was the most sensitive to iron deficiency, followed by P. italicum, while G. citri-aurantii displayed the highest tolerance to low-iron environments. This observation was further validated by iron chelation sensitivity assays using the agar well diffusion method with two specialized chelators, 2,2′-dipyridyl and tropolone. Overall, the pigment-producing yeast M. pulcherrima XX04 may have a greater capacity than the three tested pathogens to compete for and acquire iron in an iron-limited coexistence environment. This capability facilitates the induction of iron deprivation by M. pulcherrima against various fungal pathogens and represents a key determinant of its biocontrol efficacy. Our findings provide new insights into elucidating the mechanism of competitive iron consumption mediated by antagonistic yeasts. Furthermore, strategies based on inducing iron starvation could offer a promising alternative for managing postharvest diseases in citrus fruits.