Natural Variation in Root System Development

Significant natural variation exists in root system size among Arabidopsis ecotypes (figure 1). This variation could be due to intrinsic developmental pathways (i.e. the root system of one ecotype is always larger than that of another ecotype when grown under identical conditions), or differences in the way individual ecotypes respond to environmental cues, or both. We observe that the root systems of 12-day-old seedlings of the Arabidopsis ecotype Landsberg erecta (Ler) are larger than those of ecotype Columbia (Col) when grown on nutrient media simulating no (0 mannitol) or mild (60 mM mannitol) osmotic stress. Furthermore, although the root system size of both ecotypes is reduced by osmotica, Ler appears to be less sensitive to osmotic stress than Col. Hence, Col and Ler differ in both intrinsic and environmental response pathways.

We have taken advantage of the natural variation in root system size between Col and Ler under mild osmotic stress to identify genetic loci responsible for at least part of this variation. Recombinant inbred lines (RILs) derived from Ler x Col were grown on 60 mM mannitol and the sum of all visible lateral roots per plant (TOT) was tallied for 6-10 plants per RIL (figure3). Mean TOT values and genotypic data for all RILs were subjected to QTL analysis and two robust loci were identified and named EDG1 and 2 (figure4; Elicitor of Drought Growth). Based on this analysis, the Ler allele of EDG1 and the Col allele of EDG2 are predicted to increase root system size. To test the biological significance of this prediction, two independent near-isogenic lines (NILs) were created for each of the EDG loci containing the relatively 'promotive' EDG allele in the opposite genetic background (figure 5A; Ler EDG1 in Col background and Col EDG2 in Ler backround). The NILs were then grown on 60mM mannitol and compared to appropriate, isogenic parental controls. As predicted, both NILs showed increased TOT values in three separate experimental replicates over their parental lines(figure 5B).

We were then interested in characterizing the growth and response of the NIL root systems in the absence and presence of osmotic stress, to enable us to assign roles for each in intrinsic and/or environmental response pathways. Our recent publication in Genetics (PDF) demonstrates details of how the EDG1 and EDG2 loci regulate both of these pathways.

At the cellular level, two parameters that affect root system size are the number of lateral root initiations and the percentage of those initiations that become lateral roots. Ler appears to initiate a greater number of lateral root primordia and also form a larger percentage of lateral roots from those initiations. While the number of initiations in both ecotypes seems to respond similarly to osmotic stress, the percentage of initiations that form lateral roots is significantly lower in Col compared to Ler. Further characterization of both parental ecotypes as well as the NILs is explained in detail in our recent publication (PDF). The stage is now set to identify genes at each of these loci whose allelic differences are responsible for the observed differences in root system size.