Coordinated Regulation of Iron Homeostasis, Antioxidant Defenses and Hormone Signaling Underlies Pepper Responses to Iron Deficiency and Excess

Iron availability strongly influences crop performance, yet the mechanisms in non-model horticultural species remain unclear. We combined morphological, physiological, and root transcriptome analyses to dissect the response of Capsicum annuum L. cultivar CA#8 to iron deficiency, low iron, and excess iron. Seedlings grown hydroponically under four iron supplies (0, 5, 25, and 150 μM EDTA-Fe) showed distinct shoot and root phenotypes: iron deficiency caused interveinal chlorosis and strong inhibition of root growth, excess iron induced leaf wrinkling with white spots and reduced root development, whereas low iron had milder effects. Under deficiency, rhizosphere pH decreased and ferric chelate reductase activity increased sharply, supporting activation of a Strategy I-type acquisition system. Shoot and root iron contents declined in deficient and low-iron plants but increased under excess iron, while manganese accumulated mainly in deficient shoots and zinc remained largely unchanged. Iron stress altered multiple hormones and triggered transient increases in superoxide dismutase, peroxidase, and catalase activities, consistent with regulated adjustments of the reactive oxygen species homeostasis. Root RNA-seq at two days identified 1822, 436, and 1211 differentially expressed genes under deficiency, low iron, and excess iron, respectively. Enrichment and co-expression analyses highlighted a coordinated induction or repression of iron transporters and regulators, including IRT- and NRAMP-like uptake components, YSL- and VTL-like transporters, and transcription factors and hormone-related genes linked to iron and oxidative signaling. Research indicates that pepper plants maintain iron homeostasis by integrating proton extrusion, reduction capacity, metal transport, vacuolar sequestration, and antioxidant defense mechanisms to limit damage under both iron deficiency and excess.