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Trees are considered good candidates for phytoremediation of soils contaminated with trace elements (TE), e.g. mine tailings. Using two year-old Tilia cordata plants, we demonstrated the nature and the scale of root architecture, especially root apices, as an indicator of mining sludge toxicity and plant capability to cope with these stress conditions. The novelty of our research is the analysis of the root response to substrate with extremely high concentrations of numerous toxic TE, and the 3D illustration of the disorders in root apex architecture using a clarity technique for confocal microscopy. The analysis demonstrates (1) a marked reduction in the size of the root apex zones (2) the occurrence of vascular tissues abnormally close to the root apex (3) collapse of the internal tissues in many root apices. Simultaneously, at the cellular level we observed some signs of a defensive response - such as a common increase of cell wall (CW) thickness and the formation of local CW thickenings - that enlarge the CW capacity for TE sequestration. However, we also detected harmful effects. Among others, a massive deposition of TE in the middle lamella which caused major damage - probably one of the reasons why the inner tissues of the root apex often collapsed – and the formation of incomplete CWs resulting in the occurrence of extremely large cells. Moreover, many cells of the root apex exhibited degenerated protoplasts. All these alterations indicate the harsh conditions for lime growth and survival and simultaneously, the manifestation of a defensive response. The obtained results allowed us to conclude that analysis of the nature and scale of structural alterations in roots can be useful indicators of plant ability to cope with stress conditions, e.g. in prospect of using the examined plants for reclamation of soils contaminated with TE. Image 1 • Clearing technique excellently documented the nature of root architecture alterations. • Root architecture analysis clearly shows the extremely high toxicity of the substrate. • Mining sludge exposure caused an increase of CW capacity for TE sequestration. • CW thickening occurred not only in the laboratory but also in similar to field conditions. • Presence of incomplete CW indicates a strong cell dysfunction. The nature and scale of alterations in root apex architecture can be considered as an indicator of mining sludge toxicity and the degree of plant resistance to contaminated substrate. [ABSTRACT FROM AUTHOR]

Lead poisoning constitutes one of most detrimental environmental hazards to all living organisms. Plants developed a variety of avoidance and tolerance mechanisms that are activated in response to lead exposure. Plant cell walls were suggested to play important role in these reactions by creating an efficient barrier to lead entry to the protoplasts, but the molecular mechanisms involved in such shielding reaction have not been elucidated. Tip growing protomemata of Funaria hygrometrica (Hedw.) were used as model for studying effects of lead exposure on plant cell walls (CWs). Forty-eight hour-treatment 4μM PbCl2 resulted in the appearance of cell wall thickenings (CWTs) at the tip of the apical cell, which is the lead entry site to the cell protoplast [Krzesłowska, M., Woźny, A., 1996. Lead uptake localization and changes in cell ultrastructure of Funaria hygrometrica protonemata. Biol. Plant. 38, 253–259]. The nature of these thickenings differed from the one of cell wall in unexposed plants as revealed by immunolabelling with monoclonal antibodies and histochemical analyses. The most striking difference was the appearance high amount of low-esterified (JIM5 epitope) and unesterified (PAM1 epitope) homogalacturonan, which were absent from the tip cell wall of control protonemata and are known as the compounds able to bind and immobilise Pb2+. Furthermore, the cell wall thickenings commonly contained callose and at least two kinds of lipid compounds known as the substances preventing metal ions entry to the protoplast. Observations in transmission electron microscope (TEM) showed that CWTs contained a few distinct, varied structurally regions. The dominant one was the region of a granular structure—never found in the control CW. This region contained both the highest amount of JIM5 pectins—and the most numerous lead deposits. In many cases gold particles, identifying JIM5 pectins, appeared to be bound to... [Copyright &y& Elsevier]

The article discusses about the accumulation of Pb on root tip tissues of plants and also the effect of the cell wall compound which is able to bind metal ions (Low methylesterified pectins level) for Pb accumulation level and its distribution on root tips.

The article discusses about a study in which the effects of lead exposure on plant cell walls were observed by using tip growing protonemata of Funaria hygrometrica.