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GENETICS, GENOMICS, AND MOLECULAR EVOLUTION

Characterization of the Plant-Specific BREVIS RADIX Gene Family Reveals Limited Genetic Redundancy Despite High Sequence Conservation¹

Department of Plant Molecular Biology, University of Lausanne, CH–1015 Lausanne, Switzerland; and Biology Department, McGill University, Montreal, Quebec, Canada H3A 1B1

To date, the function of most genes in the Arabidopsis (Arabidopsis thaliana) genome is unknown. Here we present the first analysis of the novel, plant-specific BRX (BREVIS RADIX) gene family. BRX has been identified as a modulator of root growth through a naturally occurring loss-of-function allele. The biochemical function of BRX is enigmatic, however several domains in BRX are conserved in the proteins encoded by the related BRX-like (BRXL) genes. The similarity between Arabidopsis BRXL proteins within these domains ranges from 84% to 93%. Nevertheless, analysis of brx brx-like multiple mutants indicates that functional redundancy of BRXLs is limited. This results mainly from differences in protein activity, as demonstrated by assaying the propensity of constitutively expressed BRXL cDNAs to rescue the brx phenotype. Among the genes tested, only BRXL1 can replace BRX in this assay. Nevertheless, BRXL1 does not act redundantly with BRX in vivo, presumably because it is expressed at a much lower level than BRX. BRX and BRXL1 similarity is most pronounced in a characteristic tandem repeat domain, which we named BRX domain. One copy of this domain is also present in the PRAF (PH, RCC1, and FYVE)-like family proteins. The BRX domain mediates homotypic and heterotypic interactions within and between the BRX and PRAF protein families in yeast (Saccharomyces cerevisiae), and therefore likely represents a novel protein-protein interaction domain. The importance of this domain for BRX activity in planta is underscored by our finding that expression of the C-terminal fragment of BRX, comprising the two BRX domains, is largely sufficient to rescue the brx phenotype.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Christian Hardtke (christian.hardtke{at}unil.ch).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.075382.

^* Corresponding author; e-mail christian.hardtke{at}unil.ch; fax 41–21–692–4195.

Received December 14, 2005; returned for revision February 3, 2006; accepted February 14, 2006.

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