The Role of Iron in Enhancing Rice Blast Resistance Induced by Endophytic Streptomyces hyaluromycini OsiPR-1

Iron (Fe) is widely involved in critical life processes such as plant growth, development, and immune responses. Endophytes colonizing plant tissues have evolved efficient mechanisms for Fe uptake and homeostasis regulation, thereby positively influencing host disease resistance. This study aims to explore rice endophytic resources and investigate the impact of Fe nutrition on their ability to induce host resistance, and to provide a theoretical basis for green management of plant diseases.

Endophytic bacteria were isolated from rice leaves using surface disinfection combined with the streak plate method. Target strains with antagonistic activity against Magnaporthe oryzae were screened using the dual culture method. The selected strain was identified through morphological observation and phylogenetic analysis based on 16S rRNA and multi-gene sequences. Its siderophore-producing ability was analyzed using CAS assay medium and by amplifying genes related to siderophore biosynthesis. Three culture conditions (normal Fe, Fe-deficient, and high Fe) were established to assess the reactive oxygen species (ROS) burst, Fe accumulation, and expression levels of Fe metabolism-related genes in rice leaves after spraying with the target strain. The efficacy of the endophyte in inducing rice resistance to rice blast under different Fe regimes was evaluated.

An endophytic actinomycete strain, OsiPR-1, with stable antagonistic activity against M. oryzae, was isolated from rice and identified as Streptomyces hyaluromycini. This strain exhibited a strong ability to produce siderophores, with peak production (60.2%) observed on the 6th day under Fe-deficient conditions, and three siderophore biosynthesis gene clusters were detected in its genome. Reinoculation tests showed that the strain’s colonization and its induction of blast resistance were closely related to environmental Fe levels. Under normal Fe conditions, OsiPR-1 induced early accumulation of Fe and ROS in rice stomata to trigger immune responses, and subsequently regulated Fe homeostasis restoration via secreted siderophores to establish a stable symbiotic relationship, significantly reducing rice blast lesion length by 74.8%. Under high Fe conditions, Fe and ROS accumulation induced by OsiPR-1 appeared transiently in leaf veins only at 4 h post-treatment and then rapidly disappeared. Although the strain proliferated substantially, the symbiotic relationship was unstable, resulting in a lesion length reduction of only 64.5%. Under Fe-deficient conditions, Fe and ROS accumulation in stomata was weak. However, genes involved in Fe transport (OsIRT1), Fe homeostasis (OsFRO2), and lipoxygenase catalysis (OsLOX2) were significantly up-regulated (7.7-10.3 folds). The drastic fluctuation in Fe homeostasis and the subsequent intense immune response inhibited endophyte colonization, hindering the establishment of symbiosis and leading to a sharp decline in disease control efficacy.

Normal Fe supply is a critical prerequisite for S. hyaluromycini OsiPR-1 to enhance rice resistance against blast through a siderophore-mediated immune induction mechanism. This study provides an important theoretical foundation for developing efficient biocontrol agents against rice blast based on precise regulation of Fe nutrition and offers new insights into the role of Fe nutrition in plant-endophyte interactions.