The primary cell wall is a fundamental plant constituent that is flexible but sufficiently rigid to support the plant cell shape. Although many studies have demonstrated that reactive oxygen species (ROS) serve as important signaling messengers to modify the cell wall structure and affect cellular growth, the regulatory mechanism underlying the spatial-temporal regulation of ROS activity for cell wall maintenance remains largely unclear. Here, we demonstrate the role of the Arabidopsis (Arabidopsis thaliana) multicopper oxidase-like protein skewed 5 (SKU5) and its homolog SKU5-similar 1 (SKS1) in root cell wall formation through modulating ROS homeostasis. Loss of SKU5 and SKS1 function resulted in aberrant division planes, protruding cell walls, ectopic deposition of iron, and reduced nicotinamide adeninedinucleotide phosphate (NADPH) oxidase-dependent ROS overproduction in the root epidermis–cortex and cortex–endodermis junctions. A decrease in ROS level or inhibition of NADPH oxidase activity rescued the cell wall defects of sku5 sks1 double mutants. SKU5 and SKS1 proteins were activated by iron treatment, and iron over-accumulated in the walls between the root epidermis and cortex cell layers of sku5 sks1. The glycosylphosphatidylinositol-anchored motif was crucial for membrane association and functionality of SKU5 and SKS1. Overall, our results identified SKU5 and SKS1 as regulators of ROS at the cell surface for regulation of cell wall structure and root cell growth.

Establishing the symbiosis between legumes and nitrogen-fixing rhizobia requires the precise modulation of auxin levels. However, our understanding of auxin’s regulatory roles, particularly rhizobia-derived auxins, remains limited. Our study reveals that the auxin biosynthesis gene YUC2a is essential for the spatiotemporal control of nodule development in soybean (Glycine max). This process is orchestrated by three transcription factors: Nuclear Factor-YA9 (NF-YA9), Lateral Organ Boundaries Domain 41 (LBD41), and Nodule Inception 1a (NIN1a). In the early stages of nodulation, rhizobial auxin stimulates NF-YA9 expression, NF-YA9 then activates YUC2a expression in the cortical cell layer, establishing optimal auxin levels for nodule initiation. In the middle stages, rhizobial auxin elevates LBD41 expression, and LBD41 suppresses YUC2a to control auxin levels, ensuring proper rhizobia colonization. In the late stages, rhizobial auxin inhibits NIN1a expression, which increases YUC2a expression in nitrogen-fixing symbiosomes, fine-tuning optimal auxin levels for nodule maturation. Disruption of YUC2a and its homologs impairs cell division in nodule primordia, reducing nodule density and nitrogen fixation capacity. Conversely, cortex-specific overexpression of YUC2a promotes nodule formation but inhibits rhizobia colonization. This dynamic auxin regulation optimizes nodule development in soybean, revealing rhizobia-derived auxin's critical role in nitrogen-fixing symbiosis.