Seminars

Plants anchor and shape the root system in the ground where minerals are distributed in patches and gradients. The local availability of macronutrient nitrogen (N) strongly impacts the root growth behavior, such as root elongation, emergence of lateral roots, and angles of root growth, thus altering the root system architecture. The plant hormone auxin plays a central role in these developmental response mechanisms. However, the concerted actions of auxin and N signaling pathways remain largely uncharacterized. Root transcriptome profiling and gene expression analysis indicate the N-responsive CLAVATA3/EMBRYO SURROUNDING REGION-RELATED3 (CLE3) peptide and the cognate receptor kinase CLAVATA1 (CLV1) control expression of a phospholipid-binding mobile flowering repressor protein Arabidopsis thaliana CENTRORADIALIS homologue (ATC) at the juncture of N signaling and auxin-mediated pathways controlling root gravitropism. The CLE3-CLV1-ATC pathway integrates to promote expression of the PIN3 auxin exporter in the root-tip columella cells establishing asymmetric auxin distribution and directing root gravitropism when N supply is limited. Split-root experiments with the quadruple mutant line for the ammonium transporters indicate both the local and systemic N signals are involved in modulating ATC expression. Transcriptome profiles of the split-root segments further implicate potential impact of local ammonium supply and subsequent long-distance N signaling on phytoalexin biosynthesis and metabolite secretion to the rhizosphere. The CLE3-CLV1-ATC pathway integration and the transcriptome network regulation by systemic N signals represent previously uncharacterized N-controlled mechanisms modulating root development and metabolism at the root-soil interface.