Physiological and Transcriptomic Analysis of Bread Wheat MicroRNAs in Response to Zinc Availability

Zinc (Zn) is a mineral micronutrient that is essential for plant growth and development. Soil Zn deficiency or excess severely impacts plant health and crop yields. MicroRNAs (miRNAs) play crucial roles in plant responses to abiotic stress, but their roles in Zn homeostasis in important crop bread wheat (Triticum aestivum L.) remain unknown. This study investigated miRNA expression profiles in wheat roots under different Zn supply conditions using high-throughput sequencing. Phenotypic and physiological analyses revealed that high Zn promoted wheat plant growth, while low and excess Zn resulted in wheat plant growth inhibition and oxidative stress. A total of 798 miRNAs (including 70 known and 728 novel miRNAs) were identified; among them, 10 known and 122 novel miRNAs were differentially expressed. Many key miRNAs, such as miR397-5p, miR398, 4D_25791, and 5A_27668, are up-regulated under low Zn but down-regulated under high Zn and excess Zn. Target gene prediction and enrichment analysis revealed that the regulated genes of these miRNAs focused on "zinc ion transmembrane transporter activity", "divalent inorganic cation transmembrane transporter activity", and "cellular detoxification" processes in the low Zn vs. CK group. However, "glutathione metabolism" and "ABC transporter" pathways were obviously enriched in high Zn vs. excess Zn conditions, implying their potential functions in alleviating the oxidative damage and Zn efflux caused by Zn toxicity. Together, this study identified key miRNAs that respond to both Zn deficiency and excess Zn in bread wheat, revealing distinct regulatory patterns of the target genes in different Zn supply conditions. These findings provide a new field and valuable candidate miRNAs for molecular breeding aimed at improving zinc's utilization efficiency in wheat.