Page 53 - Plant-Based Remediation Processes

3 Metal/Metalloid Phytoremediation: Ideas and Future 41

one of the points below (Kotrba et al. 2009;Miransari 2011;Prasadetal.
2011). Selected bacterial strains, i.e. plant growth-promoting rhizobacteria
(PGPR) such as Azospirillum, Rhizobium, Enterobacter or Arthrobacter,may
be used to increase plant growth (Farina et al. 2012). These organisms are
capable of cooperating with plants by reducing the adverse effects of toxic
substances on their growth, stimulation of nutrient transport required for appro-
priate plant growth or formation of such compounds (Jha et al. 2011;Tang
et al. 2012). The presence of PGPR in the rhizosphere of plants used in
phytoremediation is particularly essential, as they have a positive effect on
the development of the root system (stimulation of growth and thus also uptake
of nutrients from soil) and limiting plant ageing processes by ethylene inhibi-
tion, i.e. blocking of its production by ACC-deaminase activity from bacteria
(ACC-1-aminocyclopropane-1-carboxylate) (Saleem et al. 2007). PGPR are
capable of producing many phytohormones, e.g. gibberellins, cytokinins or
indole-3-acetic acid (IAA) (Ma et al. 2011). For this reason, it may be assumed
that these organisms in the near future will be important subjects of studies on
enhancing resistance of plants growing in areas polluted with metals/metalloids,
as well as maintaining or increasing their growth (preventing a reduction of
biomass under conditions adverse for plant growth).
Another interesting group of growth promoting organisms, at the same time
enhancing efficiency of heavy metal uptake from polluted areas, comprises
endophytic bacteria (gram-positive and gram-negative) as well as siderophore-
producing bacteria (Pseudomonas putida, Bacillus megaterium or Ralstonia
metallidurans). The former are organisms colonising plant tissues having a
positive effect on plant growth and enhancing tolerance to the presence of toxic
trace elements. They exhibit several significant traits, e.g. they promote the
uptake of nutrients required for appropriate plant growth and they have a positive
effect on the capacity to limit the adverse effect of pathogens (Rajkumar et al.
2010). An even more interesting aspect of recently undertaken research is
connected with the applicability of low molecular chelators produced by fungi,
bacteria and plants, exhibiting high affinity to selected metal ions (Al, Cd, Cu, Fe
or Zn). Siderophore-producing bacteria (SPB) are also capable of stimulating
plant growth, yielding an increase of biomass and enhanced resistance to the
presence of heavy metals. They exhibit a capacity to increase the amount of
metals absorbed by plant tissues or enhance plant tolerance by stimulating growth
of individual plant organs (Rajkumar et al. 2010). In the near future, numerous
studies on phytoremediation are likely to focus on the application of new
specialised organisms which will promote plant growth and development and at
the same time will protect plants against the adverse effect of heavy metals
present in the soil. Moreover, such studies conducted in situ will make it possible
to develop optimal guidelines for the application of plants selected for growing in
polluted areas in order to achieve the highest possible efficiency of heavy metal
uptake from soil.