A bHLH121/URI-ANAC017-MAT1/SAM1 regulatory cascade controls iron homeostasis in Arabidopsis.

Abstract

Iron (Fe) deficiency is a critical environmental constraint limiting plant growth, yield, and quality. Although plants have evolved sophisticated mechanisms to enhance Fe uptake, transport, and utilization, key components of the underlying regulatory network remain unclear. Here, we identify the NAM, ATAF1/2, and CUP-SHAPED COTYLEDON 2 (NAC) transcription factor ANAC017 as an essential regulator of Fe homeostasis in Arabidopsis. ANAC017 expression is strongly induced under Fe deficiency, and loss of ANAC017 function leads to reduced Fe accumulation in both roots and shoots, impaired root elongation, and pronounced leaf chlorosis. Notably, anac017 mutants show excessive Fe retention in the cell wall-particularly within the hemicellulose fraction-under Fe-deficient conditions, suggesting that ANAC017 contributes to the mobilization and reutilization of apoplastic Fe. Mechanistically, ANAC017 functions within the canonical Fe signaling cascade. We show that the core Fe regulator, the BASIC HELIX-LOOP-HELIX (bHLH) transcription factor bHLH121/URI, directly activates ANAC017 transcription, and overexpression of ANAC017 partially rescues the Fe-sensitive phenotype of the bhlh121/uri mutant. Further experiments revealed that the transcription factor ANAC017 binds to the promoter of MAT1/SAM1 (encoding S-adenosylmethionine synthetase), potentially modulating its expression. In conclusion, this study reveals that ANAC017 functions downstream of the bHLH121/URI pathway to regulate Fe homeostasis via two separate mechanisms: facilitating cell wall Fe reutilization and mediating S-adenosylmethionine synthetase metabolism by regulating MAT1/SAM1.