Plant Regulatory Peptides Underpin Root Architecture in Response to Environmental Cues

In plants, cellular proliferation and differentiation are coordinated by cell-to-cell communication. Secreted peptides are signal molecules that have recently emerged as key mediators of plant growth and development including root system architecture (RSA). RSA underpins plant fitness and productivity and is particularly influenced by nitrogen limitation. Here, we report identification and characterisation of novel plant peptide coding gene families using bioinformatic predictions and biochemical validations. In particular, we present RARs (Root Architecture Regulators) that are intercellular signals that are unique to higher plants and which coordinate root development with nutritional cues. In the model legume Medicago truncatula RARs are induced by limited nitrogen and elevated CO₂. MtRAR1 over-expression elicits root developmental phenotypes that include the formation of periodic “bumps”, inhibition of root branching, increased numbers of nitrogen fixing nodules and altered root hair development. RAR over-expression also alters the transcript levels of M. truncatula homologs of the Arabidopsis transcription factors ANR1 (ARABIDOPSIS NITRATE REGULATED1) and LBD38 (LATERAL ORGAN BOUNDARY DOMAIN38) and the high affinity nitrate transporter NRT2.5. Synthetic M. truncatula RAR peptides are bioactive and phenocopy MtRAR1 over-expression. MtRAR1 over-expression in an auxin reporter line implicates altered auxin distribution at bump sites. We propose that RARs important positional signals that that coordinate root lateral organ development in response to nutritional cues.