Introduction:
Calamine flora, colonizing areas polluted with heavy metals, is an
excellent model for studying the microevolution of species/populations,
speciation and metallophyte origin, the genetic structure of populations
(Hildebrandt et al. 2006; Kuta et al. 2012; Słomka et al. 2011; Wąsowicz
et al. 2014; Wierzbicka and Rostański 2002), physiological adaptation to
harsh conditions (Słomka et al. 2008), colonization and seed banks
(Grodzińska et al. 2001), as well as the biology of reproduction (Bothe
& Słomka 2017; Izmaiłow et al. 2015).
Embryological characteristics are important traits to evaluate
metallophyte adaptation/tolerance. Impaired sexual reproduction of
plants colonizing polluted areas leading to reduced plant fertility and
seed set, could be considered as costs of adaptation. The more
disturbances in the reproductive processes and degenerations, the lower
the tolerance. This can be inferred from published studies on the
reproduction of angiosperm’s taxa representing different metallophyte
status from Polish post-industrial areas: e.g. Armeria maritima ,Capsella bursa-pastoris , Cardaminopsis arenosa ,Cirsium arvense , Chondrilla luncea , Echium vulgare,Vicia cracca or Viola tricolor and other violets (Czapik
2002; Izmaiłow 2000; Izmaiłow & Biskup 2003; Izmaiłow et al. 2015;
Kościńska-Pająk 2000; Kwiatkowska & Izmaiłow 2014; Siuta et al. 2005;
Słomka et al. 2012, 2017, 2018).
There is a lack of embryological data on B. laevigata , a
facultative metallophyte (plants can grow in metal-enriched
soil/substrate and also in unpolluted areas) which is a member of
European calamine flora. The species has been widely studied, including
its distribution, taxonomic and systematic position, karyotype analysis,
ploidy level (diploid vs tetraploid), genetic diversity and
genetic structure of European populations (e.g. Manton 1937; Olowokudejo
1992; Tremetsberger et al. 2002).
In Poland (Eastern Europe), B. laevigata (2n=18) (Babs-Kostecka
et al. 2014; Skalińska 1950), a glacial relict, occurs in the Tatra
Mountains and on over 100-year-old Zn-Pb waste-heap in Bolesław, an
isolated location in an anthropogenic, postindustrial site with elevated
concentrations of heavy metals. Recently, a new location on a lowland in
Central Poland has been discovered (Przemyski & Piwowarczyk 2012).
Until now, the studies on Polish populations of B. laevigata have
focused on: mycorrhizal status, adaptation and tolerance to heavy metal
stress, concentrations of heavy metals in plant organs and the genetic
structure of populations (e.g. Antosiewicz 1995; Babst-Kostecka et al.
2014; Godzik 1993; Gołda & Wojciechowska 1979; Orłowska et al. 2002;
Wąsowicz et al. 2014; Wierzbicka & Pielichowska 2004; Wierzbicka et al.
2017). B. laevigata was also used for restoriation in
post-industrial areas (Rostański 2014).
Plants that live in heavy metal-enriched habitats take metal ions from
the rhizosphere and transport them into plant cell/tissue/organs. In
cytosol, free metals are harmful as they cause oxidative stress or
damage to organelles. To deal with this negative impact, plants have to
bind metals and detoxify them. In the evolution of adaptation, plants
have developed two main strategies (cell level) that allow them to
survive in such extreme conditions: (1) metal accumulation, in some
species hyperaccumulation, and (2) metal exclusion (e.g. Baker 1981;
Hall 2002; Prasad 2004). The cell wall is the main structure that can be
actively modified under internal or external factors/stress and plays a
crucial role in defense strategies. In the metal exclusion strategy when
metals are taken from the environment, transported through the apoplast,
the cell wall is the barrier that stops metals entering the cell
symplast. In metal accumulation strategy, metals entering the cell are
sequestrated in the
cell wall. In particular, homogalacturonan, a major component of the
plant primary walls plays a crucial role in the binding and accumulation
of heavy metals in the cell wall. However, other compounds, such as
other polysaccharides, proteins, amino acids or phenolics, can also take
part in metal detoxification (le Gall et al. 2015; Krzesłowska 2011).
The aim of this study was to (1) estimate the frequency of developmental
disturbances and degenerations in male and female lineages ofBiscutella laevigata as the costs of adaptation/tolerance to
areas polluted with heavy metal; (2) how the un/low- and
high-methyl-esterified homogalacturonan (HG) recognized by LM19 and LM20
antibodies, respectively, are distributed in the cell wall of mature
embryos as a defense response to toxic metals.