Pectinous cell wall thickenings formation-A response of moss protonemata cells to lead

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 4 mu M 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 [Krzeslowska, M., Wozny, 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 lead deposits. It indicated that JIM5 pectins which accumulated in CWTs were involved in immobilisation of high amounts of Pb2+. Because the region of lead accumulation occupied the largest volume of the CWTs, we concluded that CWTs appear to be a very important repository for Pb2+ in protonemata cells. Thus, we postulate that, CWTs localized at the tip of the apical cell-the main region of lead uptake [Krzeslowska, M., Wozny, A., 1996. Lead uptake localization and changes in cell ultrastructure of Funaria hygrometrica protonemata. Biol. Plant. 38, 253-259] rich in JIM5 pectins, callose and lipids function as the effective barrier against lead ions penetration into the protonema protoplast. The findings substantiate previous hypotheses that lead ions can be sequestered in cell walls and point to the possibility that capacity for lead binding might increase in cell response to lead. (C) 2008 Elsevier B.V. All rights reserved.