One variation, two regulatory modes? Novel insights into the transcriptional regulation of 1-FEH isoforms in chicory under abiotic stress

Background Chicory taproots are a major source of industrially extracted inulin. However, low temperatures at the end of the growing season cause significant inulin degradation. Inulin loss has been attributed to fructan 1-exohydrolase (1-FEH) activity, which is mediated by three isoforms, 1-FEH1, 1-FEH2a and 1-FEH2b, respectively. Previous breeding studies have indicated a prominent role for 1-FEH2b, while the relative contributions of different 1-FEH isoforms have not yet been clearly defined. Results We demonstrate that promoters of 1-FEH1 and 1-FEH2b ( p1-FEH1 , p1-FEH2b ) are differentially activated by cold-inducible transcription factors CiNAC5 and CiDREB2, respectively. Furthermore, differential responses were observed for promoters of the closely related genes 1-FEH2a and 1-FEH2b , caused by a single-base variation in their DRE element core motif: While in p1-FEH2b the core motif GTCG mediates effective binding to and activation by CiDREB2, the respective sequence in p1-FEH2a ( TTCG ) renders this promoter insensitive to CiDREB2. Furthermore, we observed a strong co-activation of p1-FEH2b by CiDREB2B and CiMYB5, corroborated by direct protein-protein interaction of these transcription factors. Remarkably, all three 1-FEH genes are also activated by high temperatures and drought, i.e., climate change-related abiotic stress factors. Conclusions The distinct expression patterns of 1-FEH1 , 1-FEH2a , and 1-FEH2b under cold treatment suggest that these isoforms have specific roles in responding to cold stress. However, these genes are also likely to be involved in chicory’s response to climate change-related abiotic stress factors such as high temperatures and drought. The differential regulation of their corresponding promoters reveals distinct signaling pathways. In summary, our observations shed light on the role of inulin metabolism in chicory’s response to abiotic stress and identify potential targets for improving chicory inulin quality via targeted promoter editing.