Roles of brassinosteroid in regulating nitrogen uptake and metabolism of rapeseed (Brassica napus L.) in response to nitrogen availability

Brassinosteroids (BRs) are pivotal phytohormones that function in plant development as well as responses to nutrient levels. However, the mechanisms of BR in regulation of nitrogen uptake and metabolism in rapeseed are not fully elucidated. Here, we investigated the effects of BR on rapeseed growth, nitrogen uptake, nitrogen metabolism, and transcript levels of nitrate transporter genes NRT） under varying nitrogen availability in rapeseed seedlings. Low nitrate (LN) significantly promoted the accumulation of brassinolide and castasterone to upregulate BR signaling and modulate nitrogen uptake. Moreover, 2,4-epibrassinolide (eBL) treatment enhanced plant tolerance to LN stress, corresponding to the reduction in anthocyanin and the increase in chlorophyll content. Meanwhile, eBL-treated plants also enhanced the net nitrate (NO 3 - ) influx rate, nitrogen content, and nitrogen metabolism by enhancing the enzyme activities of nitrate reductase (NR), glutamine synthetase (GS), nitrite reductase (NiR), and glutamate synthase (GOGAT) under both LN and NN (normal nitrate) conditions. The transcript levels of NRT genes were increased in eBL-treated plants. Furthermore, RNA-seq analysis revealed that BR-mediated pathways, such as nitrogen metabolism, starch and sucrose metabolism, tryptophan metabolism, plant hormone signal transduction pathways, were activated in response to nitrogen availability. These results indicated that BR promoted nitrogen uptake and physiological responses in rapeseed responding to nitrogen availability, offering a strategy for improvement of nitrogen use efficiency.