Iron Deficiency Reprograms Lateral Root Growth via TAR2-Dependent Auxin Biosynthesis in Arabidopsis

Auxin plays a central role in shaping root system architecture (RSA) by regulating cell division, differentiation, primary root elongation, and lateral root (LR) initiation and emergence. Iron (Fe) is an essential micronutrient required for photosynthesis, chlorophyll biosynthesis, and redox metabolism. Fe availability has a significant impact on plant health, development, and yield. Here, we investigate the role of the auxin biosynthetic gene TRYPTOPHAN AMINOTRANSFERASE RELATED2 (TAR2) in coordinating LR development and Fe homeostasis in Arabidopsis thaliana. Fe deficiency increased auxin accumulation in roots, as observed through DR5rev:GFP reporter activity, and this response required TAR2 function to drive Fe deficiency-induced modifications to RSA. The tar2-1 mutant displayed significantly reduced visible LR numbers, total LR length and LR density. Yeast one-hybrid assays identified several Fe deficiency-responsive bHLH transcription factors, including bHLH34, bHLH38, bHLH39, and PYE, that directly bind the TAR2 promoter, indicating a regulatory link between Fe signaling and auxin biosynthesis. Together, our findings support that TAR2-dependent local auxin biosynthesis is a major contributor to LR development and the adaptive reprogramming of RSA in response to Fe deficiency.