Yu et al. show that activation of the expression of cysteine protease genes by GmNAC039 and GmNAC018 is required for soybean nodule senescence The Plant Cell (2023).
By H Yu and Y Cao
Background: Soybean (Glycine max) roots have nodules in which symbiotic bacteria reduce nitrogen gas into ammonium, which nourishes the plants. The termination of symbiotic nitrogen fixation, termed nodule senescence, is an active process and soybean nodules are usually severely senescent when the plant begins flowering, which appears counterproductive, as seed maturation requires a large amount of nitrogen. While much has been discovered about symbiotic signaling and nodule organogenesis over the past two decades, knowledge about nodule senescence is largely limited to decreased nitrogenase activity and leghemoglobin content, physiological changes, and gene expression profiles of senescent nodules. One molecular marker of senescent nodules is an increase in the transcript levels of genes encoding cysteine proteases that break down proteins and degrade nodule cells. However, how nodule senescence is activated at the molecular level and what components activate the expression of senescence-related genes are largely unknown.
Question: What are the key components regulating nodule senescence in soybean?
Findings: We identified two NAC transcription factors from soybean, GmNAC039 and its paralog GmNAC018, as regulators of nodule senescence. GmNAC039 directly binds to the promoters of at least four cysteine protease genes and activates their expression. Knocking out these four cysteine protease genes in soybean delayed nodule senescence and increased nitrogenase activity. Thus, GmNAC039 and GmNAC018 activate the expression of cysteine protease genes to promote nodule senescence in soybean.
Next steps: Nodule senescence is associated with extensive cell degradation, which leads to programmed cell death in nodules. The expression of genes encoding proteins with proteolytic activities is required for nodule senescence. Given that cysteine proteases have key roles in soybean nodule senescence, identifying their target proteins is an important future research goal.
Haixiang Yu, Aifang Xiao, Jiashan Wu, Haoxing Li, Yan Duan, Qingshan Chen, Hui Zhu, and Yangrong Cao. (2023) GmNAC039 and GmNAC018 activate the expression of cysteine protease genes to promote soybean nodule senescence https://doi.org/10.1093/plcell/koad129