Functional Analysis of Bna-miR399c-PHO2 Regulatory Module Involved in Phosphorus Stress in Brassica napus

Phosphorus stress is one of the important factors restricting plant growth and development, and the microRNA (miRNA) family is involved in the regulation of the response to plant nutrient stress by repressing the expression of target genes at the post-transcriptional or translational level. miR399 is involved in the transportation of phosphate in multiple plants by improving tolerance to low Pi conditions. However, the effect of miR399 on the response of low Pi stress in rapeseed (Brassica napus L.) is unclear. The present study showed a significant increase in taproot length and lateral root number of plants overexpressing Bna-miR399c, while the biomass and Pi accumulation in shoots and roots increased, and the anthocyanin content decreased and chlorophyll content improved under low Pi stress. The results illustrate that Bna-miR399c could enhance the uptake and transportation of Pi in soil, thus making B. napus more tolerant to low Pi stress. Furthermore, we confirmed that BnPHO2 is one of the targets of Bna-miR399c, and the rejection of Pi in rapeseed seedlings increased due to the overexpression of BnPHO2. Hence, we suggest that miR399c-PHO2 module can effectively regulate the homeostasis of Pi in B. napus. Our study can also provide the theoretical basis for germplasm innovation and the design of intelligent crops with low nutrient input and high yield to achieve the dual objectives of income and yield increase and environmental protection in B. napus.