Phylogeny and biogeography of Alyssum (Brassicaceae) based on nuclear ribosomal ITS DNA sequences

Root and Shoot Response to Nickel in Hyperaccumulator and Non-Hyperaccumulator Species

The soil-root interface is the micro-ecosystem where roots uptake metals. However, less than 10% of hyperaccumulators' rhizosphere has been examined. The present study evaluated the root and shoot response to nickel in hyperaccumulator and non-hyperaccumulator species, through the analysis of root surface and biomass and the ecophysiological response of the related aboveground biomass. Ni-hyperaccumulators Alyssoides utriculata (L.) Medik. and Noccaea caerulescens (J. Presl and C. Presl) F.K. Mey. and non-hyperaccumulators Alyssum montanum L. and Thlaspi arvense L. were grown in pot on Ni-spiked soil (0-1000 mg Ni kg^-1, total). Development of root surfaces was analysed with ImageJ; fresh and dry root biomass was determined. Photosynthetic efficiency was performed by analysing the fluorescence of chlorophyll a to estimate the plants' physiological conditions at the end of the treatment. Hyperaccumulators did not show a Ni-dependent decrease in root surfaces and biomass (except Ni 1000 mg kg^-1 for N. caerulescens). The non-hyperaccumulator A. montanum suffers metal stress which threatens plant development, while the excluder T. arvense exhibits a positive ecophysiological response to Ni. The analysis of the root system, as a component of the rhizosphere, help to clarify the response to soil nickel and plant development under metal stress for bioremediation purposes.

Heavy metal tolerance in contrasting ecotypes of Alyssum montanum

The response of metallicolous (M) and nonmetallicolous (NM) Alyssum montanum ecotypes to multi-metal stress was investigated under in vitro condition and compared in this study. Shoot cultures were simultaneously treated with 0.7 mM ZnSO₄, 3.0 μM Pb(NO₃)₂ and 16.4 μM CdCl₂ for 8 weeks and evaluated for their morphogenetic and ultrastructural reaction, growth tolerance as well as ability to Zn, Pb, and Cd uptake. Moreover, tissue localization and concentrations of antioxidant compounds were determined in order to elucidate the potential role of ROS-scavenging machinery in plant tolerance to metal toxicity. The results clearly demonstrated that M specimens treated with heavy metals showed less phytotoxic symptoms and low level of lipid peroxidation than reference NM one. The enhanced tolerance of M ecotype resulted from heavy metals detoxification in trichomes and intracellular leaf compartments as well as balanced ROS accumulation. The inactivation of ROS in M plants was based on peroxidase-flavonoid system, while in NM plants such relationship was not detected and amounts of antioxidant enzymes or phenolic compounds was comparable to untreated specimens or decreased significantly. Considering the procumbent growth of such hemicryptophyte which reproduce effectively in the presence of heavy metals but is characterized by low biomass production, it is proposed to exploit M ecotype of A. montanum in revegetation schemes of polluted calamine wastes to provide the prompt stabilization of areas prone to erosion.

Is hybridization driving the evolution of climatic niche in Alyssum montanum

PREMISE OF THE STUDY

After decades of interest, the contribution of hybridization to ecological diversification remains unclear. Hybridization is a potent source of novelty, but nascent hybrid lineages must overcome reproductive and ecological competition from their parental species. Here, we assess whether hybrid speciation is advantageous over alternative modes of speciation, by comparing the geographical and ecological ranges and climatic niche evolutionary rates of stabilized allopolyploid vs. autopolyploids in the Alyssum montanum species complex.

METHODS

We combined an extensive review of studies addressing the systematics and genetic diversity of A. montanum s.l., with flow cytometry and cloning of nuclear markers, to establish the ploidy level and putative hybrid nature of 205 populations. The respective geographic distribution and climatic niche evolution dynamics of the allo- and autopolyploids were investigated using multivariate analyses and comparative phylogenetic approaches.

KEY RESULTS

As expected by theory, allopolyploids occur mainly along contact zones and are generally spatially overlapping with their diploid counterparts. However, they demonstrate higher rates of niche evolution and expand into different climatic conditions than those of their diploid congeners. In contrast, autopolyploids show lower rates of niche evolution, occupy ecological niches similar to their ancestors and are restricted to less competitive and peripheral geographic areas.

CONCLUSIONS

Hybridization thus seems advantageous by promoting ecological niche evolution and more readily allowing escape from competitive exclusion.