NAC transcription factors transcriptionally fine-tune signal homeostasis in plant systemic acquired resistance.

Small signalling molecules, such as salicylic acid (SA) and N-hydroxypipecolic acid (NHP), act synergistically to activate systemic acquired resistance (SAR), a major form of plant inducible immunity. The transcriptional regulation of SA biosynthesis controlled by different transcription factors (TFs) has been well documented in SAR. Several TFs, e.g., SARD1, CBP60g and WRKY33, positively regulate NHP biosynthesis; however, direct negative modulators have remained elusive. Recently, Cai et al. (2024) identified a NAC triad composed of NAC-type TFs, NAC90, NAC36, and NAC61, that negatively regulates NHP and SA biosynthesis. NAC90 and NAC36 act as negative regulators of plant immunity by repressing transcription of ALD1, FMO1, and ICS1, the major NHP and SA biosynthetic genes, via direct binding to their promoters. These TFs, along with another NAC TF, NAC61, form heterodimers, further enhancing their repressive effects on NHP and SA biosynthesis. These findings establish the NAC90-NAC61-NAC36 triad as a negative regulator of NHP and SA levels. In this viewpoint article, we present our perspectives on further investigations to gain comprehensive insight into transcriptional regulation of SAR signal homeostasis.