AOBPreview published online on January 16, 2009
Annals of Botany, doi:10.1093/aob/mcn267
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The lateral root initiation index: an integrative measure of primordium formation
2
1 Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), A. P. 510-3, 62250 Cuernavaca, Morelos, Mexico
2 Department of Horticulture, ALS 4017, Oregon State University, Corvallis, OR 97331, USA
3 Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis, OR 97331, USA
* For correspondence. E-mail jdubrov{at}ibt.unam.mx
Received: 15 September 2008 Returned for revision: 20 October 2008 Accepted: 26 November 2008
Background and Aims: Lateral root initiation is an essential and continuous process in the formation of root systems; therefore, its quantitative analysis is indispensable. In this study a new measure of lateral root initiation is proposed and analysed, namely the lateral root initiation index (ILRI), which defines how many lateral roots and/or primordia are formed along a parent-root portion corresponding to 100 cortical cells in a file.
Methods: For data collection, a commonly used root clearing procedure was employed, and a new simple root clearing procedure is also proposed. The ILRI was determined as 100dl, where d is the density of lateral root initiation events (number mm–1) and l is the average fully elongated cortical cell length (mm).
Key Results: Analyses of different Arabidopsis thaliana genotypes and of a crop plant, tomato (Solanum lycopersicum), showed that ILRI is a more precise parameter than others commonly used as it normalizes root growth for variations in cell length. Lateral root primordium density varied in the A. thaliana accessions Col, Ler, Ws, and C24; however, in all accessions except Ws, ILRI was similar under the same growth conditions. The nitrogen/carbon ratio in the growth medium did not change the lateral root primordium density but did affect ILRI. The ILRI was also modified in a number of auxin-related mutants, revealing new root branching phenotypes in some of these mutants. The rate of lateral root initiation increased with Arabidopsis seedling age; however, ILRI was not changed in plants between 8 and 14 d post-germination.
Conclusions: The ILRI allows for a more precise comparison of lateral root initiation under different growth conditions, treatments, genotypes and plant species than other comparable methods.
Key words: Arabidopsis thaliana, auxin, lateral root density, lateral root initiation index, mutant phenotype, pericycle, root architecture, root branching, root primordium, Solanum lycopersicum
Present address: Department of Plant Physiology, Faculty of Science, Charles University in Prague, Vinicna 5, 128 44 Prague 2, Czech Republic.