Pollution of soil and water with met-

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					                                                Metal tolerance in Ljubljana


                                                Plants Against
                                                Pollution
                                                Accumulation of metals in soil poses a threat to plant growth and survival.
                                                Marjana Regvar from the University of Ljubljana investigates how some tough
  Zinc and cadmium hyperaccumulator:            species deal with the stress. A question which is becoming increasingly impor-
  Thlaspi praecox                               tant for our own food chain, too.


P
       ollution of soil and water with met-     branches which are called arbuscules and           is really toxic,” Regvar says. “Some people
       als and metalloids increases as hu-      contact the membranes of the cells, mak-           even suggested that we should wear protec-
       man civilisations grow. Mining and       ing the exchange of molecules possible.            tive clothes while working, and in the same
smelting, the burning of fossil fuels, use of   But they do not just help their hosts to gain      time the inhabitants of the valley grow their
pesticides and the disposal of batteries and    minerals and water from the soil. They also        food on this soil. A third of the children suf-
other municipal wastes are typical sources.     bind toxic metals and avoid their uptake           fers from heavy diseases and has more lead
The effects are long lasting because metals     into the plant’s roots.                            in their blood than allowed.”
cannot be chemically degraded. They ac-                                                                 Since 1998, the Regvar group and their
cumulate and are taken up into plant tis-       Time for applied ecology                           partners in Slovenia, Germany and Spain
sues where they pose toxic stress, leading to       Regvar studied Mycorrhiza systems              have made good progress. This year, for ex-
growth disturbances and even to death. As       for her masters thesis and her PhD, be-            ample, they found evidence for Arbuscu-
a consequence, postindustrial landslides of-    tween 1992 and 1998, but was searching             lar Mycorrhiza colonisation in common and
ten develop into wastelands. But even only      for the signals that help plants to distin-        tartary buckwheat, which had been doubt-
slightly polluted soils may pose a danger, as   guish between parasitic fungi and profit-          ed before (Likar et al., Mycorrhiza (2008),
                        Annie von Heijne




contaminated agricultural crops increasing-     able Ecto-Mycorrhiza symbionts (Ecto-My-           18: 309-315). This finding gives rise to the
ly affect human food safety.                    corrhiza fungi do not form arbuscules al-          hope that plants could be inoculated with
    Besides costly and technically complex      though they colonise cortical root tissues).       a mixture of the symbiotic fungi, thus ben-
physical procedures to remove metals from       In 1998 she changed direction. Her profes-         efiting agricultural outcome in the future.
soil, scientists are search-                                       sor, Nada Gogala, retired       A second research focus led to the discov-
ing for cheap and sustaina-                                        and Regvar took over the        ery of cadmium and zinc hyperaccumula-
ble solutions. One of these                                        chair for Plant Physiology.     tion in the Thlaspi praecox. This plant be-
scientists is Marjana Reg-                                         At this point she gained a      longs to the Brassicaceae family, a group
var from the University of                                         project in the highly pollut-   with numerous hyperaccumulating plants,
Ljubljana in Slovenia. “In                                         ed Mežica Valley in North-      but it shows some special features. “This
the last decades it has been                                       ern Slovenia, previously an     discovery was very interesting” says Reg-
discovered that some plants                                        area crammed with heavy         var. “That’s why we are still studying this
themselves might do the                                            industry.                       plant today.”
job, using mechanisms de-                                              “It was the time when
veloped for their own pro-                                         applied ecology started         Rare expertise
tection,” she says. In her                                         to be more important for            In order to investigate the mechanisms
plant physiology group at                                          governments and indus-          by which T. praecox stores toxic metals in
the Department of Biology                                          try than basic research,”       its organs Regvar’s team had to develop a
she investigates so-called                                         Regvar remembers. She           technique that would make it possible to
hyperaccumulating plants                                           and her team of two as-         measure metal concentrations in plant tis-
that assimilate high con-                                          sistants and one techni-        sues. The solution was delivered by Bogdan
centrations of metals and                                          cian switched to Arbuscu-       Povh of the Max-Planck-Institute for Nucle-
store them in sites where                                          lar Mycorrhiza and metal        ar Physics in Heidelberg, who introduced
they cannot do any physi-                                          hyperaccumulating plants.       them to a technology based on so-called mi-
ological harm. Such plants Marjana Regvar                          They began to ask how the       cro-proton induced x-ray emission (micro-
may be used for phytoreme-                                         highly degradated postin-       PIXE). The basis of this technique is a small
diation in the future because they extract      dustrial soils in Northern Slovenia could be       beam of protons which makes it possible to
metals from the soil.                           restored. The situation in the Mežica Val-         measure the typical spectral energies of el-
    Another strategy is some plants’ abili-     ley is dramatic, especially for Slovenians,        ements against the cellulosis background
ty to form arbuscular mycorrhiza symbios-       because most of them grow vegetables in            of plant tissue. Additionally, the research-
es. Some fungi enter plant root tissue, form    their own gardens. “The soil in this valley        ers gain information about the thickness of
 Journal Club                                                                                         6-2008         Lab Times           page 35




element clusters and can compute an ele-         says Regvar. How the metal gets there is un-         “The presence and absence of individ-
ment’s concentrations.                           certain, the plant somehow fails to avoid        ual glucosinolates in the roots is likely to
    “Using micro-PIXE we were able to scan       its transport. Nevertheless, it succeeds in      affect formation of Arbuscular Mycorrhiza
plant tissues and determine the store sites      keeping the cadmium from embryonic cells         symbioses,” assumes Regvar. “Some glu-
of essential and non essential minerals and      in the deeper layers of the embryo, storing      cosinolates may inhibit this formation and
metals,” Regvar says. Only a few labs in the     them at the surface. In any case, Thlaspi        some may contribute to it.” This finding
world have the expertise assembled in Reg-       praecox is worth further investigation. And      might be important when it comes to the
var’s group and its collaborating lab at the     who knows, in future, farmers might use it       inoculation of agricultural soils with fungi
Josef-Stefan-Institute in Ljubljana. Their ex-   on their fields as a metal extractor.            to strengthen the plants’ metal resistance
periments showed that T. praecox accumu-                                                          and nutrition. And even if it’s far too early
lates zinc in the large vacuolated cells of a    With a little help from some fungi               for manipulating GS concentrations in ag-
leaf’s epidermis, indicating that it might be        The special nature of Thlaspi praecox is     ricultural crops, it might be at least helpful
stored and detoxified in the vacuoles (Vo-       highlighted by another fact. Unlike other        to understand why some plants fail to form
gel-Mikus K. et al., Plant, Cell and Environ.    Brassicaceae the plant is able to form Ar-       Arbuscular Mycorrhiza systems.
(2008), 31: 1484-96). Cadmium turned             buscular Mycorrhiza symbioses. In anoth-
out to be stored in the vacuoles of leaf mes-    er project, Regvar’s group investigated the      Phytoremediation in sight?
ophyll cells and in this way was kept away       role that fungi play in the plant life cycle          “Today the phytoremediation efforts are
from the mechanisms of photosynthesis.           and how plants might control them (Pon-          still limited; we actually know too little,”
Lead seems to accumulate in the vacuoles         grac et al., J. Chem. Ecol. (2008), 34: 1038-    says Regvar. The knowledge that she and
of mesophyll cells as well as in cell walls,     1044). To do this, they determined the de-       the ecologist community have accumulated
where it might be bound chemically as a          gree of colonisation by symbionts in differ-     is still largely theoretical. It is almost certain
second detoxification mechanism.                 ent plant organs and during different phas-      that in heavily polluted areas plants might
                                                 es of the year. At the same time, they meas-     survive better with the help of Mycorrhiza.
Close to the embryo                              ured the concentration of glucosinolates         But inoculation with profitable microbes is
    These experiments affirmed potential         (GS) which are sulphur-containing sec-           far from being easily applicable since not
metal accumulation strategies that had al-       ondary compounds known to deter path-            every plant forms symbiosis and not eve-
ready been shown for other plants. But they      ogens. It turned out that the total GS con-      ry species of symbiotic Mycorrhiza fungus
revealed something unexpected, too. “The         centrations and profiles of nine individual      protects against toxic metals. Much more
scholar opinion tells us that metal hyper-       GS differed during life cycle stages, with the   work needs to be done to understand the
accumulating plants keep the toxic met-          highest peak of general GS concentration         molecular and ecological relationships be-
als away from their seeds,” explains Reg-        in the rosette leaves and during the vegeta-     tween fungi, the complex microsystems of
var. Normally, embryonic cells do not share      tive period. That is, when leaves are young      the soil and plants. The same holds true for
plasmodesmata with mother cells; they are        and worth protecting. The interesting thing      the use of hyperaccumulating plants as ex-
disconnected from the mother plant and           was that some of the GS peaks in the roots       tractors of metal, but research funding is
thus saved from toxic materials which could      coincided with the highest peak of symbi-        not easy to come by in a country of just 2
cross borders. “But in Thlaspi praecox seeds     ont colonisation: glucotropaeolin reached        million people. Nethertheless, Regvar and
we found considerable amounts of cadmi-          its highest concentration and glucobrassi-       her team persevere to stick at it.
um in the epidermal cells of the cotyledons,”    canapin its lowest.                                                              MATTHIAS NAWRAT

				
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