Programmed cell death (PCD) is a key element in normal plant growth and development which may also be induced by various abiotic and biotic stress factors including salt stress. In the present study, morphological, biochemical, and physiological responses of the theoretically immortal unicellular freshwater green alga Micrasterias denticulata were examined after salt (200 mM NaCl or 200 mM KCl) and osmotic stress induced by iso-osmotic sorbitol. KCl caused morphological changes such as cytoplasmic vacuolization, extreme deformation of mitochondria, and ultrastructural changes of Golgi and ER. However, prolonged salt stress (24 h) led to the degradation of organelles by autophagy, a special form of PCD, both in NaCl- and KCl-treated cells. This was indicated by the enclosure of organelles by ER-derived double membranes. DNA of NaCl- and KCl-stressed cells but not of sorbitol-treated cells showed a ladder-like pattern on agarose gel, which means that the ionic rather than the osmotic component of salt stress leads to the activation of the responsible endonuclease. DNA laddering during salt stress could be abrogated by addition of Zn2+. Neither cytochrome c release from mitochondria nor increase in caspase-3-like activity occurred after salt stress. Reactive oxygen species could be detected within 5 min after the onset of salt and osmotic stress. Respiration, photosynthetic activity, and pigment composition indicated an active metabolism which supports programmed rather than necrotic cell death in Micrasterias after salt stress.

Plant secondary metabolism significantly contributes to defensive measures against adverse abiotic and biotic cues. To investigate stress-induced, transcriptional alterations of underlying effector gene families, which encode enzymes acting consecutively in secondary metabolism and defense reactions, a DNA array (MetArray) harboring gene-specific probes was established. It comprised complete sets of genes encoding 109 secondary product glycosyltransferases and 63 glutathione-utilizing enzymes along with 62 cytochrome P450 monooxygenases and 26 ABC transporters. Their transcriptome was monitored in different organs of unstressed plants and in shoots in response to herbicides, UV-B radiation, endogenous stress hormones, and pathogen infection. A principal component analysis based on the transcription of these effector gene families defined distinct responses and crosstalk. Methyl jasmonate and ethylene treatments were separated from a group combining reactions towards two sulfonylurea herbicides, salicylate and an avirulent strain of Pseudomonas syringae pv. tomato. The responses to the herbicide bromoxynil and UV-B radiation were distinct from both groups. In addition, these analyses pinpointed individual effector genes indicating their role in these stress responses. A small group of genes was diagnostic in differentiating the response to two herbicide classes used. Interestingly, a subset of genes induced by P. syringae was not responsive to the applied stress hormones. Small groups of comprehensively induced effector genes indicate common defense strategies. Furthermore, homologous members within branches of these effector gene families displayed differential expression patterns either in both organs or during stress responses arguing for their non-redundant functions.

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Aquaporins are ubiquitous membrane channel proteins that facilitate and regulate the permeation of water across biological membranes. Aquaporins are members of the MIP family and some of them seem to be also able to transport other molecules such as urea or glycerol. In the plant kingdom, a single plant expresses a considerably large number of MIP homologues. These homologues can be subdivided into four groups (PIP, TIP, NIP, SIP) with highly conserved amino acid sequences and intron positions in each group. Since their discovery, advancing knowledge of their structure led to an understanding of the basic features of the water transport mechanism. An optimal water balance is essential to the homeostasis of most organisms, and aquaporins may be one of the mechanisms involved under changing environmental and developmental conditions. In fact, this may be one reason for the abundance and diversity of aquaporins, in particular in plants. In addition, exposure to different types of stress alters water relations and thus, aquaporins may be involved in stress responses as well. The transcriptional and/or post-translational regulation of aquaporins would determine changes in membrane water permeability. Both phosphorylation and translocation to/from vesicles have been reported as post-translational mechanisms. However, translocation in plants has not yet been shown. Although significant advances have been achieved, complete understanding of aquaporin function and regulation remains elusive.

In the present study we investigate whether the unicellular green alga Micrasterias denticulata is capable of executing programmed cell death (PCD) upon experimental induction and by which morphological, molecular and physiological hallmarks it is characterized. This is particularly interesting as unicellular fresh water green algae growing in shallow bog ponds are exposed to extreme environmental conditions and the capability to perform PCD may provide an important strategy to guarantee survival of the population. The theoretically "immortal" alga Micrasterias is an ideal object for such investigations as it has served as a cell biological model system since many years and details on its growth properties, physiology and ultrastructure throughout the cell cycle are well known. Treatment with low concentrations of H 2 O 2 known to induce PCD in other organisms resulted in severe ultrastructural changes of organelles as observed in TEM. These include deformation and partly disintegration of mitochondria, abnormal dilatation of cisternal rims of dictyosomes, the occurrence of multivesicular bodies, an increase in the number of ER compartments and slight condensation of chromatin. Additionally, a statistically significant increase in caspase-3-like activity could be detected which was abrogated by a caspase-3 inhibitor. Photosynthetic activity measured by fast chlorophyll fluorescence decreased as a consequence of H 2 O 2 exposure whereas pigment composition, except of a reduction in carotenoids, was the same as in untreated controls. TUNEL positive staining and ladder-like degradation of DNA, both frequently regarded as PCD hallmark in higher plants could only be detected in dead Micrasterias cells.