The effects of oxidative stress on the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were studied in isolated chloroplasts from barley (Hordeum vulgare L. cv Angora). Active oxygen (AO) was generated by varying the light intensity, the oxygen concentration, or the addition of herbicides or ADP-FeCI,-ascorbate to the medium. Oxidative treatments stimulated association of Rubisco with the insoluble fraction of chloroplasts and partia1 proteolysis of the large subunit (LSU). The most prominent degradation product of the LSU of Rubisco showed an apparent molecular mass of 36 kD. The data suggest that an increase in the amount of AO photogenerated by O, reduction at photosystem I triggers Rubisco degradation. A possible relationship between AO-mediated denaturation of Rubisco and proteolysis of the LSU is discussed.

Previous investigations suggested that specific auxin spatial distribution due to auxin movements to particular embryonic regions was important for normal embryonic pattern formation. To gain information on the molecular mechanism(s) by which auxin acts to direct pattern formation in specific embryonic regions, the role of a plasma membrane (PM) ATPase was evaluated as downstream target of auxin in the present study. Western-blot analysis revealed that the PM H ϩ -ATPase expression level was significantly increased by auxin in wheat (Triticum aestivum) embryos (two-three times increase). In bilaterally symmetrical embryos, the spatial expression pattern of the PM H ϩ -ATPase correlates with the distribution pattern of the auxin analog, tritiated 5-azidoindole-3-acetic acid. A strong immunosignal was observed in the abaxial epidermis of the scutellum and in the epidermal cells at the distal tip of this organ. Pseudoratiometric analysis using a fluorescent pH indicator showed that the pH in the apoplast of the cells expressing the PM H ϩ -ATPase was in average more acidic than the apoplastic pH of nonexpressing cells. Cellulose staining of living embryos revealed that cells of the scutellum abaxial epidermis expressing the ATPase were longer than the scutellum adaxial epidermal cells, where the protein was not expressed. Our data indicate that auxin activates the proton pump resulting in apoplastic acidification, a process contributing to cell wall loosening and elongation of the scutellum. Therefore, we suggest that the PM H ϩ -ATPase is a component of the auxin-signaling cascade that may direct pattern formation in embryos.

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch‐activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m−3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.

Active oxygen (AO) species generated in plants under stress conditions trigger degradation of Rubisco (EC 4.1.1.39). To ®nd out whether AO species activate proteases or make the protein susceptible to proteolysis, puri®ed and 14 C-labelled Rubisco protein was incubated with stromal preparations obtained from barley (Hordeum vulgare L.) leaves. The protein was degraded into distinct fragments only after a treatment with AO. This result shows that AO-treated Rubisco has been modi®ed to become a substrate for stromal protease(s) and dismisses the possibility of protease activation. Upon degradation, distinct fragments accumulated with time. The fragmentation pattern was indistinguishable from that obtained with intact chloroplasts subjected to oxidative conditions (cf. M. Desimone et al., 1996, Plant Physiol 111: 789±796). Degradation required ATP-hydrolysis, since AMP, ADP or non-hydrolysable ATPanalogs did not support proteolysis. The ClpP-de®cient stromal preparations degraded AO-modi®ed Rubisco, making the involvement of the ClpC/P protease unlikely.

Abstract— Kinetic studies with the mustard seedling (Sinapis alba L.) support the hypothesis that the so‐called ‘high energy reaction’ of photomorphogenesis can be understood solely on the basis of phytochrome. Light‐induced anthocyanin synthesis (a typical ‘positive’ photoresponse(1) and light dependent inhibition of hypocotyl lengthening (a typical ‘negative’ photoresponse(1)) have been investigated. In order to explain the experimental data we have to assume that there are two different types of phytochrome 730 which differ greatly as far as their resistance to irreversible destruction is concerned. The existence of these two different types of phytochrome 730 has already been postulated on the basis of spectrophotometric measurements in vivo.(2)

No abstract

Stem extension in light‐grown plants of Chenopodium rubrum L. ecotype selection 184 (50°10′N; 150°35′W) was recorded continuously for periods up to one week at constant temperature. Stem extension rate measurements were made with linear voltage‐displacement transducer devices. At the beginning of experiments, the 3rd intenode above the cotyledons was about 5 mm long. Stem extension rate exhibited a rhythmic behaviour in continuous white light (20 W m−2), and in continuous darkness with a period of approximately 23 h. In continuous darkness, the amplitude of the rhythm damped out very quickly after 24 h and a second peak was just measurable. The mean value of the stem extension rate was dependent on the light fluence before the experiments. This overt rhythm, which could be observed at the individual plant or even internode level, exhibited the characteristics of an endogenous circadian rhythm. There was no correlation of the peak time to local time. The peak time was determined by the time of transfer from dark to light for dark periods equal to or longer than 8 h, and the phase was shifted by the time of transfer from light to dark at the proper phase of a pre‐existing rhythm.

Clutathione S-transferases (CSTs) with additional activities as fatty acid hydroperoxidases were investigated in soybean (Glycine max 1.) hypocotyls. Aside from the GSTs present in total soluble tissue extracts, enzyme activities and distinct immunoreactive GST polypeptides were also detected in the intercellular washing fluid. Whereas the intracellular isoenzymes were both constitutive and inducible, apoplastic CST and glutathione peroxidase was detectable only in tissues treated with the known GST inducer 2,3,5-triiodobenzoic acid. Monensin inhibited the induced accumulation of apoplastic CST but did not affect the intracellular isoforms. The discovery of apoplastic inducible CST will be discussed in light of the putative function of these enzymes in plants.The GSTs (EC 2.5.1.18) are a family of proteins with severa1 activities (Wilce and Parker, 1994;Marrs, 1996). GSTs catalyze the nucleophilic attack of the thiol of GSH to electrophilic substrates, typically resulting in the formation of GSH conjugates. GSTs are usually dimeric proteins with subunit molecular masses of 24 to 30 kD; they are organized in gene families that produce multiple isoenzymes in eukaryotic organisms. GSTs are mostly soluble cytoplasmic enzymes, but microsomal isoforms are also known in both plants and animals. GSTs conjugate GSH to various xenobiotics, e.g. drugs and pesticides, which is often a key step in their metabolic detoxification and elimination from the cytoplasm (Lamoureux and Rusness, 1989;Kreuz et al., 1996;Reinemer et al., 1996).The multitude of GST isoenzymes in plants and their inducibility by diverse biotic and abiotic factors has been the primary focus of research in recent years. Yet, knowledge about the physiological substrate(s) and function(s) of plant GSTs is only slowly beginning to emerge. Thus, GSTs have been implicated in the conjugation of cinnamic acid and of anthocyanins and in the binding of IAA (Edwards and Dixon, 1991;Marrs, 1996). Some GST isoenzymes display additional activities; for example, the selenium-independent GSH peroxidases catalyze the reduction of fatty acid hydroperoxides with concomitant formation of GSSG (Bartling et al., 1993). Such hydroperoxides are formed by the action of active oxygen species that are generated both as normal by-products of aerobic metabolism and as the result of pathogen infection or exposure to certain abiotic * Corresponding author; e-mail klaus-eugen.kreuz@chbs.mhs. ciba.com; fax 41-61-697-8455.agents. Organic hydroperoxides are potentially cytotoxic, and their removal by GSH peroxidase activity has thus been implicated in the protection of tissues against oxidative stress (Ketterer et al., 1990).In this report we describe the isolation and characterization of GSTs from soybean (Glycine max L.) that display high GSH peroxidase activity toward hydroperoxides of linolenic acid and arachidonic acid. In addition to the soluble intracellular isoenzymes, GST and GSH peroxidase were also found in the intercellular washing fluid of hypocotyls. These apoplastic enzyme ac...