Regulatory Peptides That Control Root Architecture

In higher plants, root system architecture (RSA) emanates from an interaction between intrinsic reiterative developmental programs and environmental influences. How these factors interact is not understood but traits that influence RSA are of keen interest and the optimization of RSA is an important agronomic goal. Recently, endogenous peptides have been shown to regulate root development and here we describe a family of regulatory peptides (RAR; Root Architecture Regulator) that exert a strong influence on several aspects of legume root development. RAR loci are encoded by a multigene family in Medicago truncatula. The archetypical member MtRAR1 is regulated by environmental cues such as nitrogen starvation or limitation and elevated CO₂. Adding the predicted MtRAR1 peptide to roots or over-expressing MtRAR1 in transgenic roots leads to phenotypic changes in RSA: sustained and reduced lateral root formation, increased nodulation and the formation of unique periodic root swellings typified by circumferential cell proliferation (CCP) and elevated root hair numbers.  The phenotypes induced by RAR peptides follow strict structure function relationships. Inoculation of the symbiotic bacterium Sinohizobium onto Medicago roots over-expressing MtRAR1 gene or exposed to RAR1 peptide leads to enhanced nodulation. Nodulation is partially tolerant to a range of nitrate levels. Serial sectioning of roots indicate that MtRAR1 over-expression or peptide addition inhibits lateral root initiation at an early developmental point. The concomitant increase in nodulation suggests a developmental link between lateral root and nodule development. RAR genes may find utility in breeding programs aimed at optimising root architecture.