Natural Genetic Variation

Researchers: Dr. Laura Ragni, Dr. Eavan Dorcey, Stéphanie Plantegenet

Our main interest is to isolate genes that are responsible for the intra- and interspecific morphological variation in plants. Therefore, the analysis of natural genetic variation is the starting point for the majority of individual projects in our lab.

For the isolation of genes involved in morphological variation, exploiting natural genetic variation offers some distinct advantages as compared to classical mutagenesis experiments. Most importantly, it allows the identification of naturally occurring, differentially active gene alleles that are likely targets for the evolution of morphological variation. Because naturally occurring lines do not carry major deleterious mutations that impair their survival in the wild, the approach also counter-selects against the isolation of mutant alleles that result in strong phenotypes as commonly detected in mutagenesis experiments. This applies in particular to wild populations with a high level of inbreeding, such as in our favorite model organism, Arabidopsis thaliana.

So why has this approach not been followed earlier? Mainly because the isolation of genes that modify quantitative traits has technically not been feasible. However, the rich biological and genomic resources, such as the fully sequenced genome, have turned Arabidopsis into an ideal model system to investigate the molecular basis of natural genetic variation at the gene level.

Natural morphological variation of root system architecture between wild isolates of Arabidopsis. Most of this variation results from the influence of genetic factors.

Plant organs are formed in a continuous, post-embryonic manner by ordered cell divisions and expansions. The extent of growth ultimately determines organ shape. Thus, we are primarily interested in genes that modulate growth rate. To isolate such genes, we focus on the root system. This is because of our interest in root biology per se, but also because roots grow from a so-called meristematic region at their tip, which produces concentric tissue layers through controlled cell proliferation, elongation and differentiation. This feature enables us to reduce the problem of accurately measuring growth from three dimensions to a single one. Nevertheless, analysis of root system growth requires care and highly reproducible growth conditions, because root system development is very plastic and responds to many macro- and micro-environmental cues. The figure above shows the natural variation in root system morphology between Arabidopsis accessions collected from different locations and grown in identical growth chamber conditions on tissue culture media.

So far, we have isolated an important regulator of root system architecture from the accession Uk-1. This gene is a major so-called quantitative trait locus (QTL), which we gave the name BREVIS RADIX (BRX), latin for "short root". Other efforts in this area include attempts to isolate two other QTLs for root growth located in other accessions.

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