title: Heuristic aspect of the lateral root initiation index: A case study of the role of nitric oxide in root branching.
abstract: OBJECTIVE
  Lateral root (LR) initiation (LRI) is a central process in root branching. Based on LR and/or LR primordium densities, it has been shown that nitric oxide (NO) promotes LRI. However, because NO inhibits primary root growth, we hypothesized that NO may have an opposite effect if the analysis is performed on a cellular basis. Using a previously proposed parameter, the LRI index (which measures how many LRI events take place along a root portion equivalent to the length of a single file of 100 cortical cells of average length), we addressed this hypothesis and illustrate here that the LRI index provides a researcher with a tool to uncover hidden but important information about root initiation. •
  
  
  RESULTS
  Arabidopsis thaliana roots were treated with an NO donor (sodium nitroprusside [SNP]) and/or an NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide [cPTIO]). LRI was analyzed separately in the root portions formed before and during the treatment. In the latter, SNP caused root growth inhibition and an increase in the LR density accompanied by a decrease in LRI index, indicating overall inhibitory outcome of the NO donor on branching. The inhibitory effect of SNP was reversed by cPTIO, showing the NO-specific action of SNP on LRI. •
  
  
  CONCLUSIONS
  Analysis of the LRI index permits the discovery of otherwise unknown modes of action of a substance on the root system formation. NO has a dual action on root branching, slightly promoting it in the root portion formed before the treatment and strongly inhibiting it in the root portion formed during the treatment.
authors.affiliation: Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Apdo. Postal 510-3, 62250 Cuernavaca, Morelos, México ; Laboratorio de Fisiología y Desarrollo Vegetal, Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209 Cuernavaca, Morelos, México.
authors.forename: Verónica
authors.initials: V
authors.lastname: Lira-Ruan
authors.affiliation: Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Apdo. Postal 510-3, 62250 Cuernavaca, Morelos, México.
authors.forename: Selene Napsucialy
authors.initials: SN
authors.lastname: Mendivil
authors.affiliation: Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Apdo. Postal 510-3, 62250 Cuernavaca, Morelos, México.
authors.forename: Joseph G
authors.initials: JG
authors.lastname: Dubrovsky
keywords: lateral root initiation; nitric oxide; root branching; root system architecture