Mechanisms of habitat adaptation
in Arabidopsis arenosa

Flowering time differences associated with habitat:

Arabidopsis arenosa is unusual compared to its relatives in its ability to survive in a range of distinct habitats. One of the most striking contrasts is between populations found on railway versus shaded outcrop sites. Plants growing in railway sites are exposed to greater drought risk, elevated temperature, higher UV exposure, poor soil quality, and annual herbicide applications. How did these plants manage this habitat shift? Are they just very plastic, or did they adapt genetically? Our initial common garden experiments suggest that there was at least some genetic adaptation; the plants have heritable differences in life history and developmental traits. We are using genomic analyses and genetic mapping to ask which genes might have been involved in adaptation to these distinct habitats, with an initial focus on differences in reproductive timing.

 

Examples of two typical A. arenosa habitats. Left, a shaded limestone rock outcrop, and right, a railway bed (Photos: left, K Bomblies; right, Sang-Tae Kim).

 

It was previously described that A. arenosa from railways are rapid-cycling annuals and populations from outcrops are late-flowering perennials. We have confirmed in the lab that this difference is at least in part genetic: in our controlled conditions, progeny of outcrop plants flower much later than those of railway plants. When plants are treated with extended cold (vernalization), the flowering of outcrop plants, but not railway plants, is greatly accelerated. Thus railway plants are early flowering in the lab because they have the lost the vernalization response. This is very similar to what has been observed in latitude-associated variation in A. thaliana, and raises the transcription factor FLC as a likely candidate. FLC is epigenetically regulated and suppressed by cold. It has also been implicated in perennialism in A. alpina. Thus we are exploring this gene, as well as two other related genes that were recovered from our genome scanning experiments, as candidates controlling differences in the flowering behavior of plants from these two habitats. Additional candidates may be recovered in our mapping experiments.

 

Flowering time of non-vernalized (left) and vernalized(right) plants. Populations are connected by lines. Outcrop populations (red) are late flowering, but have a much stronger response to vernalization than railway (yellow) populations. A river cobble population shows an intermediate response.

 

 

Substrate:

We have been working together with David Salt (University of Aberdeen) to study substrate adaptation in A. arenosa. The Salt lab analyzed over 100 A. arenosa isolates from our collection using their Ionomics platform to measure nutrient uptake. Several exciting trends have emerged, for example one population is extremely efficient at phosphorous uptake and accumulation. Another, originating from serpentine, excludes magnesium. There is extensive variation for sodium uptake. We will test candidate genes for these differences.

 

Our genomic analyses have also identified a number of candidate genes involved in substrate adaptation to limestone (calcareous) versus silicaceous soils. These include numerous ion channels and heavy metal scavenging proteins, as well as numerous genes implicated in redox regulation. Over time we hope to understand better the molecular mechanisms these plants have used for substrate adaptation, how easily substrate shifts are achieved, whether these edaphic adaptations restrict gene flow among populations, and whether populations found on similar substrate, but in geographically discontiguous regions, migrated or “re-adapted” from neighboring populations. In cases where independent adaptations to similar substrates occurred, we can ask whether the same genes are involved.

 

Limestone outcrop in the Košice region in Slovakia (photo B Arnold).

 

Non-calcareous rock outcrop site in a forest in the Steiermark region of central Austria (photo B Arnold and J Hollister).

 

Brian Arnold and Jesse Hollister also collected A. arenosa plants from a serpentine site in Austria, where beautiful green rocks abound. Serpentine is a geochemically challenging substrate for plants, with a high level of nickel and low Calcium to magnesium ratios (photo B Arnold and J Hollister).

 

Mechanisms of morphological evolution
in Arabidopsis arenosa

 

Arabidopsis arenosa populations vary extensively in morphology. We see particularly striking morphological variation in root system architecture and gravitropism, inflorescence branch outgrowth, and leaf shape.

 

• Root architecture: Subspecies arenosa grows in flat ruderal sits, especially railways, while subspecies borbasii, grows on steeply sloped rock outcrops and scree slopes. Interestingly, subspecies arenosa has gravitropic roots (like A. thaliana and A. lyrata) but subspecies borbasii has "wandering" roots (see picture below). Root architecture differences of this sort have been implicated in other species in adaptation to growing on slopes, nutrient and moisture acquisition.
• Inflorescence architecture: Subspecies arenosa also has more extensive branch outgrowththan subspecies borbasii. Rosette branches are greater in number and growth rate. Rate of growth and height at flowering of the main inflorescence is also higher in subspecies arenosa. There is, however, no difference in the lateral branches that grow out from the main inflorescence.
• Leaf shape: There is extensive variation among A. arenosa populations in leaf shape. Though not a hard-and-fast rule, subspecies arenosa tends to have simpler leaves (see bottom image below for leaf scans from wild plants).

 

We are using genomic analyses to generate lists of candidate genes for these morphological differences. We will narrow down the lists of candidates by performing wider molecular evolution and phenotype/genotype association studies. Once we have narrowed the list to a manageable number of candidates, we will use transgenic approaches to test gene functions in the two morphologically distinct A. arenosa subspecies.We will also do Bulk-segregant mapping experiments to test gene effects and ask how well genome-sequence data can predict candidates for morphological change.

 

Roots of A. arenosa, A. lyrata and A. thaliana plants grown on vertical plates at pH5 or pH8. Notice that A. arenosa roots grow better at pH 8 than at pH 5, which is not true of A. thaliana. The strains used here originate from limestone outcrops that naturally have high pH (~7.5-8.5). A. lyrata also shows a preference for elevated pH, but the difference is less dramatic than for A. arenosa. The roots of A. arenosa also show a tendency to “wander” from vertical and show horizontally growing rather than downward curving lateral roots. This feature is unique to our outcrop samples. Plants from railway populations have roots that grow vertically.

 

Arabidopsis arenosa plants grown in laboratory conditions showing genetically controlled branching differences among strains.

 

Leaves from wild A. arenosa plants showing extensive variation. Left of the line are leaves from plants from limestone outcrops in the Swabian Alb region of southwestern Germany, while right of the line are leaves from sandy forest sites in the Rhine valley area.