Tomato (Lycopersicon esculentum) fruit ripening is a devel

Leaves of tomato (Lycopersicon esculentum) plants grown in soil in which moisture was lowered from field capacity to levels approaching permanent wilting point show a 10-fold increase in abscisic acid (ABA) and a 60 to 70 percent decrease in rbcS and cab steady-state mRNA levels. As indicated by transcription runon Phytohormones regulate plant growth, development, and response to stress. ABA, one of the five classes of plant hormones identified, has been shown to induce changes in gene expression in several angiosperms (2,8,19,20). Both developmental and stress processes lead to changes in ABA levels that correlate with changes in gene expression. During embryo development, ABA levels and the synthesis of specific mRNAs and proteins increase during the period before desiccation (19,20). In leaves, water deficit results in higher ABA levels as well as changes in gene expression (2,8,15,19).Although most studies on the effects of ABA on gene expression have focused on positive regulation, other genes are down-regulated by this phytohormone (2, 4). With the exception of rbcS, which is down-regulated by ABA in immature wheat embryos (20), genes whose expression is negatively regulated by this phytohormone have not been identified. The rbcS genes are nuclear and code for the small subunit of Rubisco (RbuP2Case; 3-phospho-i-glycerate carboxylase [dimerizing]; EC 4.1.1.39) (12). The large subunit of this enzyme is coded for by the chloroplast gene rbcL (5).Regulation by light of cab genes, which code for Chl a/b binding proteins, and rbcS genes has been extensively studied (6,7,22,23,25,26). However, published results indicate that light is not the only factor controlling the expression of these 291 genes. In addition to the aforementioned results with wheat embryos, it has been reported that ABA depresses the accumulation of Rubisco protein in pea embryos (14), although the level of this regulation is not known. Another plant hormone, cytokinin, has been shown to decrease the levels of translatable rbcS and cab mRNA in Lemna gibba (26). In addition to these hormonal effects, other reports indicate that developmental processes regulate rbcS and cab expression (4,13,18,22,23). Thus, factors other than light also regulate the expression of photosynthesis-related nuclear genes. The biochemical nature of the signal(s) controlling these events is not currently known.Treatment of wheat (20) and pea (14) embryos with ABA results in the decline in rbcS mRNA and Rubisco levels, respectively, suggesting that this phytohormone negatively controls the expression of rbcS genes. However, since these observations were made by applying ABA, it was not clear whether an increase in endogenous ABA would elicit a similar response.We wished to examine the effects of increased levels of endogenous leaf ABA on the expression of rbcS and cab, two photosynthesis-related nuclear gene families. Here we describe the effect of water deficit on rbcS and cab expression in leaves ofwild-type and sit' tomato plants. ABA-mediated effects can ...

SummaryThe halophyte Mesembryanthemum crystallinum is an inducible crassulacean acid metabolism (CAM) plant native to seasonally arid coastal environments that has been widely used to study plant responses to environmental stress. On exposure of plants to salt, the activities of both the tonoplast (vacuolar) H+-ATPase (V-ATPase) and Na+/ H + antiporter increase in leaf cells, thereby energizing vacuolar salt accumulation. To investigate the molecular basis of this response, a cDNA (Vmacl) encoding the H +-conducting c subunit (16.6 kDa) of an M. crystallinum V-ATPase has been cloned. Northern analysis of RNA from leaves of plants treated with NaCl or with isoosmotic mannitoL solutions demonstrated (i) that NaCI increased steady-state transcript levels for the V-ATPase c subunit, and (ii) that this effect was caused by the ionic rather than the osmotic component of salt stress. Southern analysis of genomic DNA suggested the probable existence of more than one gene for this subunit of the V-ATPasa in M.crystallinum. Expression studies using the 3"-untranslated region of the Vmacl cDNA as a probe showed that the corresponding salt-inducible transcript was preferentially expressed in leaves. Induction by salt was also observed in juvenile plants in addition to adult ones. These findings, as well as the inability of mannitol to upregulate mRNA levels for this gene, clearly differentiate between the induction of transcript for the V-ATPase c subunit and for genes involved in the CAM pathway in M. crystallinum.Further, the plant growth regulator abscisic acid (ABA) was able to mimic the effect of salt on transcript levels for the V-ATPasa c subunit, suggesting the possible involvement of ABA in a distinct signal-transduction pathway linked to vacuolar salt accumulation in this highly salt-tolerant species.

Through the development and application of a liquid chromatography-mass spectrometry-based procedure for measuring the transport of complex organic molecules by vacuolar membrane vesicles in vitro, it is shown that the mechanism of uptake of sulfonylurea herbicides is determined by the ligand, glucose, or glutathione, to which the herbicide is conjugated. ATP-dependent accumulation of glucosylated chlorsulfuron by vacuolar membrane vesicles purified from red beet (Beta vulgaris) storage root approximates Michaelis-Menten kinetics and is strongly inhibited by agents that collapse or prevent the formation of a transmembrane H+gradient, but is completely insensitive to the phosphoryl transition state analog, vanadate. In contrast, ATP-dependent accumulation of the glutathione conjugate of a chlorsulfuron analog, chlorimuron-ethyl, is incompletely inhibited by agents that dissipate the transmembrane H+ gradient but completely abolished by vanadate. In both cases, however, conjugation is essential for net uptake because neither of the unconjugated parent compounds are accumulated under energized or nonenergized conditions. That the attachment of glucose to two naturally occurring phenylpropanoids, p-hydroxycinnamic acid and p-hydroxybenzoic acid via aromatic hydroxyl groups, targets these compounds to the functional equivalent of the transporter responsible for chlorsulfuron-glucoside transport, confirms the general applicability of the H+ gradient dependence of glucoside uptake. It is concluded that H+gradient-dependent, vanadate-insensitive glucoside uptake is mediated by an H+ antiporter, whereas vanadate-sensitive glutathione conjugate uptake is mediated by an ATP-binding cassette transporter. In so doing, it is established that liquid chromatography-mass spectrometry affords a versatile high-sensitivity, high-fidelity technique for studies of the transport of complex organic molecules whose synthesis as radiolabeled derivatives is laborious and/or prohibitively expensive.