BnaNAC19 promotes low-nitrogen-induced leaf senescence and nitrogen remobilization in Brassica napus L.

Nitrogen (N) remobilization from senescing leaves is essential for sustaining crop growth and yield under N-limited conditions, yet the regulatory mechanisms underlying this process remain poorly understood in rapeseed (Brassica napus L.). Here, we investigated the role of the NAC transcription factor BnaNAC19 in leaf senescence and N remobilization. BnaNAC19 was preferentially expressed in older leaves and siliques, and its transcript level was markedly induced by low-N stress. Under low-N conditions, BnaNAC19-overexpressing lines displayed accelerated senescence of older leaves, as indicated by enhanced leaf yellowing and reduced SPAD values and chlorophyll contents. This premature senescence was accompanied by improved N utilization efficiency and enhanced N remobilization, with higher 15N accumulation in young leaves and greater 15N depletion from older leaves. Transcriptome analysis of older leaves revealed that overexpression of BnaNAC19 was associated with the up-regulation of senescence-associated genes, including BnaA7.ORE1 and BnaC2.PAO4, and N transport-related genes, including BnaA7.NRT1.7, BnaC5.NRT1.5, BnaA9.AAP1, and BnaC2.DUR3. At the maturity stage, BnaNAC19-overexpressing plants produced higher seed biomass, seed N accumulation, and nitrogen harvest index under reduced N supply, without a concomitant increase in shoot N accumulation. Field evaluation further showed that BnaNAC19 overexpression increased seed yield per plant under both reduced and sufficient N inputs. These results suggest that BnaNAC19 promotes a low-N-responsive senescence-remobilization program that facilitates N redistribution from older leaves to developing sinks, providing a promising genetic target for improving N utilization efficiency in rapeseed.