Citric acid alters Arabidopsis root morphology and development through ROS-dependent and ROS-independent mechanisms

Citric acid is an integral component of primary metabolism and cellular energetics and plays extensive roles in other cellular processes, such as signaling, chelating, and exudation. Here, we characterized the unique effects that citric acid has on Arabidopsis (Arabidopsis thaliana) root structure and development. In particular, we investigated how citric acid modifies 2 root types in opposing ways: by inhibiting primary root growth while promoting anchor root growth. To understand the mechanisms driving these different growth patterns within the same organism, we analyzed nutrient and transcriptomic responses to citric acid treatment in anchor roots and primary roots. High-spatial resolution elemental analysis revealed that root meristems and the root-hypocotyl junction are regions of strong nutrient enrichment, but that citric acid treatment has little effect on nutrient levels in these regions. Transcriptional analysis revealed major differences between primary roots and anchor roots in response to citric acid. In particular, citric acid acted as a reactive oxygen species (ROS) scavenger through increased Class III peroxidase transcription, effectively reducing H2O2 levels both in vitro and in vivo. Altering the ROS balance at the root-hypocotyl junction was sufficient to induce anchor root formation. Citric acid treatment also differentially upregulated lignin biosynthesis, lignin assembly, and ETHYLENE RESPONSE FACTOR 115 expression in primary roots and anchor roots. ETHYLENE RESPONSE FACTOR 115 regulates the quiescent center and root columella, and we found that citric acid treatment induces developmental defects in this tissue. Overall, this study reveals that a vital organic acid produced and secreted at relatively high concentrations has both widespread and specific effects on plant development and root architecture.