Abstract:
Arbuscules are the primary site of nutrient exchange in the arbuscular mycorrhizal symbiosis formed between land plants and Glomeromycotina fungi. Plants upregulate a suite of hydrolases and proteases to degenerate arbuscules at the end of their life cycles. MYB1, a transcription factor, was shown to be a regulator of this process in Medicago truncatula (Floss et al., 2017). It was hypothesised that MYB1 activates a variety of genes to direct the degeneration process. However, the exact genetic pathway leading to the degeneration has remained unknown. Here, we show that this process is mediated by NACa, a transcription factor belonging to the NAC family, which is also involved in nitrogen-induced nodule senescence in Lotus japonicus (Wang et al., 2023a, 2023b). We found that MYB1 activates the CP3 promoter via NACa when transiently expressed in Nicotiana benthamiana leaves. CP3 and NACa also show upregulation in constitutively overexpressing MYB1 roots further supporting our hypothesis. With YFP tagged proteins and promoters we found that NACa promoter showed activation across all arbuscule stages post-maturation however NACa protein had a bimodal expression in mature and collapsed arbuscules. Our results suggest that NACa plays an important role in arbuscule degeneration-related signaling; however, it might be involved in various other processes. We anticipate this study to open up new avenues for research in arbuscule degeneration to uncover the complete mechanism that drives this process. This information would then be useful to develop transgenic plants with more fungus retaining capabilities.
