Rhizosphere regulation with cerium oxide nanomaterials promoted carrot taproot thickening†

Nanomaterials (NMs) provide great potential for sustainable development by regulating rhizosphere processes to improve crop productivity. The edible parts of rhizome crops have direct contact with the rhizosphere, which may lead to a more direct positive effect of NMs and rhizosphere interaction on the growth and development of rhizome crops. Here, 50 mg kg⁻¹ cerium oxide (CeO₂) NMs had the greatest promotion on carrot growth, and 5 days was the initial time for promoting taproot thickening. The application of CeO₂ NMs to soil first stimulated the secretion of organic acids, amino acids, fatty acids, phenols, and carbohydrates (such as citric acid, asparagine, and alpha-linolenic acid), as well as growth regulators indole-3-acetic acid (IAA) and jasmonic acid, from the roots to the rhizosphere. This resulted in a significant increase in the relative abundances of Altererythrobacter, Gemmatimonas, Pseudomonas, Sphingomonas and Chryseolinea by 16.2–61.4%, thereby enhancing nutrient accumulation and elevating IAA levels in taproots. Meanwhile, CeO₂ NMs were effectively absorbed by the seedling roots and transferred to the leaves. The internalized CeO₂ NMs induced cell division in the taproot vascular cambium by increasing levels of IAA and cytokinins by 22.2% and 33.7%, respectively. The responsive differentially expressed genes were mainly involved in the cell wall and cell division, carbohydrate metabolism, and phytohormone signal transduction pathways. Furthermore, photosynthesis was enhanced, leading to a significant increase in sucrose and starch content by 55.3% and 71.7%, respectively. The integration of rhizobacteria, phytohormones, and gene regulations synergistically promoted carrot taproot thickening. This study contributes to our understanding of rhizosphere regulation in nano-enabled agriculture.