Wood cells, unlike most other cells in plants, grow by a unique combination of intrusive and symplastic growth. Fibers grow in diameter by diffuse symplastic growth, but they elongate solely by intrusive apical growth penetrating the pectin-rich middle lamella that cements neighboring cells together. In contrast, vessel elements grow in diameter by a combination of intrusive and symplastic growth. We demonstrate that an abundant pectin methyl esterase (PME; EC 3.1.1.11) from wood-forming tissues of hybrid aspen (Populus tremula 3 tremuloides) acts as a negative regulator of both symplastic and intrusive growth of developing wood cells. When PttPME1 expression was up-and down-regulated in transgenic aspen trees, the PME activity in wood-forming tissues was correspondingly altered. PME removes methyl ester groups from homogalacturonan (HG) and transgenic trees had modified HG methylesterification patterns, as demonstrated by two-dimensional nuclear magnetic resonance and immunostaining using PAM1 and LM7 antibodies. In situ distributions of PAM1 and LM7 epitopes revealed changes in pectin methylesterification in transgenic trees that were specifically localized in expanding wood cells. The results show that en block deesterification of HG by PttPME1 inhibits both symplastic growth and intrusive growth. PttPME1 is therefore involved in mechanisms determining fiber width and length in the wood of aspen trees.

Heavy metals are aggresive environmental pollutants. They are easily taken up by plants and they are strong stess factors for plant metabolism. Heavy metals influence includes also disturbances in plant mineral nutrition by competition with other nutrients. Typical symptoms of heavy metals toxicity are often similar or even the same like symptoms of some essential nutrients deficiency. The aim of this paper is to discuss mechanisms of some heavy metals uptake and possible ways of their influence on some basic and trace nutrients availability for plants. Attention is also paid to regulatory relationships among groups of heavy metals themselves. Moreover, possibilities and limitations in revitalization of heavy metal-treated plants by micro-, and macronutrient supply are discussed

T. 1993. Inhibition of electron flow around photosystem I in chloroplasts of Cd-treated maize plants is due to Cd-indueed iron deficiency. Photosystem I activity of ehloroplasts isolated from 21 days old maize seedlings {Zeu mays L. ev. Hidosil) cultivated in a nutrient solution containing different concentrations of Cd (10, 20, 30 \i.M) was investigated. Cd markedly decreased ferredoxin(Fd)dependent NADP* photoreduetion, while it had no effect on electron transport from 2,6-diehlorophenolindophenol to methyl viologen, indicating that the metal interferred with electron transport on the reducing side of photosystem I. The decrease in electron transport correlated with a low Fd content, which in turn was correlated with a low Fe concentration, suggesting Cd-indueed Fe deficiency. In in vitro experiments direet Cd inhibition of Fd-dependent NADP* photoreduetion required much higher Cd coneentrations than those observed in Cd-treated plants.