Iron is an essential nutrient for all organisms. Feruloyl-COA 6-hydroxylase 1 (F6′H1) plays a pivotal role in iron uptake in plant roots by catalyzing the biosynthesis of iron-mobilizing scopoletin, a secondary metabolite also possessing antimicrobial activity. However, it remains unclear whether F6′H1-mediated iron uptake affects plant resistance to foliar pathogens and how such a process might be regulated. Here, we show that enhanced expression of 2-oxoglutarate-dependent dioxygenases 2 (CmOGD2), a homolog of F6′H1 in Citron C-05 (Citrus medica L.), confers resistance to citrus canker caused by Xanthomonas citri subsp. citri (Xcc). CmOGD2-mediated pathogen resistance is achieved by promoting iron uptake and the accumulation of reactive oxygen species (ROS), which likely results in ferroptosis. Furthermore, CmOGD2 interacts with the enolase CmENO2 to destabilize CmZAT10.1, a transcriptional activator of CmOGD2, thereby forming a negative feedback loop that limits CmOGD2 expression. Notably, the Xcc effector pthA4 interferes with the CmOGD2–CmENO2 interaction, likely via a decoy mechanism, leading to CmZAT10.1 accumulation. These findings reveal complex regulatory mechanisms underlying the critical role of CmOGD2 in mediating Xcc resistance through iron- and ROS-dependent ferroptosis.

Seedless is a highly valued commercial characteristic in the citrus industry, both for fresh consumption and for processed markets. In this study, the ‘Succari Sweet Orange’ (WT) and its seedless mutant ‘Juxiangyuan’ (MT), which originated from a bud mutation, were selected to study the formation of a citrus seedless phenotype. Microscopic analysis of MT’s floral organs, including anther and ovary cross-sections, provides insights into its seedless phenotype compared to the original seeded cultivar. Additionally, pollen features, viability, and in vitro germination were examined to determine the cause of seedlessness. MT exhibited significant developmental deformities in both male and female gametes, with pollen grain analysis indicating a high rate of deformity (41.48%), low viability (9.59%), and minimal in vitro germination (9.56%). Hybridization experiments were conducted to assess male and female sterility and pollen incompatibility. Both WT and MT exhibited parthenocarpic development. Notably, MT fruit produced with an average of 3.51 seeds pollinated to WT, despite severe pollen abortion of MT. MT, however, produced seedless fruit through self-breeding or cross-breeding with other varieties, demonstrating stable female sterility. Despite reduced pollen quantity and viability in the seedless mutant ‘Juxiangyuan’, its seedlessness primarily stems from female sterility. This study contributed to a deeper understanding of seedless formation in ‘Juxiangyuan’ and provided valuable information relevant to its commercial cultivation.

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is commonly managed with copper-based formulations, yet the performance of individual products may differ between laboratory and plant assays. We compared 16 commercial copper-containing formulations in plate-diffusion and root-drench assays, and inhibition zones varied among products. Thiosen Copper, however, produced no visible inhibition zone on agar but suppressed Xcc growth after root application. In sweet orange, this suppression appeared earlier than with Bordeaux Mixture at the same nominal total Cu concentrations. Thiosen Copper raised the leaf chlorophyll index without a significant increase in total leaf Cu, whereas Bordeaux Mixture led to greater leaf Cu accumulation. In separate experiments with Poncirus trifoliata, the two formulations produced different growth and tissue Cu responses, and root treatment altered PtCOPT transcript levels. Transient overexpression of CsCOPT4 in sweet orange leaves was accompanied by lower Xcc populations. These results reveal formulation-specific responses and nominate CsCOPT4 as a candidate for further study.