Turnover of 1-aminocyclopropane-1-carboxylic Acid synthase protein in wounded tomato fruit tissue

Recent advances in understanding the regulation of plant secondary metabolite biosynthesis by ethylene-mediated pathways

Plants produce a large repertoire of secondary metabolites. The pathways that lead to the biosynthesis of these metabolites are majorly conserved in the plant kingdom. However, a significant portion of these metabolites are specific to certain groups or species due to variations in the downstream pathways and evolution of the enzymes. These metabolites show spatiotemporal variation in their accumulation and are of great importance to plants due to their role in development, stress response and survival. A large number of these metabolites are in huge industrial demand due to their potential use as therapeutics, aromatics and more. Ethylene, as a plant hormone is long known, and its biosynthetic process, signaling mechanism and effects on development and response pathways have been characterized in many plants. Through exogenous treatments, ethylene and its inhibitors have been used to manipulate the production of various secondary metabolites. However, the research done on a limited number of plants in the last few years has only started to uncover the mechanisms through which ethylene regulates the accumulation of these metabolites. Often in association with other hormones, ethylene participates in fine-tuning the biosynthesis of the secondary metabolites, and brings specificity in the regulation depending on the plant, organ, tissue type and the prevailing conditions. This review summarizes the related studies, interprets the outcomes, and identifies the gaps that will help to breed better varieties of the related crops and produce high-value secondary metabolites for human benefits.

Targeted system approach to ethylene biosynthesis and signaling of a heat tolerant tomato cultivar; the impact of growing season on fruit ripening

Growing tomato in hot weather conditions is challenging for fruit production and yield. Tomato cv. Savior is a heat-tolerant cultivar which can be grown during both the Vietnamese winter (mild condition) and summer (hot condition) season. Understanding the mechanisms of ethylene biosynthesis and signaling are important for agriculture, as manipulation of these pathways can lead to improvements in crop yield, stress tolerance, and fruit ripening. The objective of this study was to investigate an overview of ethylene biosynthesis and signaling from target genes to proteins and metabolites and the impact of growing season on a heat tolerant tomato cultivar throughout fruit ripening and postharvest storage. This work also showed the feasibility of absolute protein quantification of ethylene biosynthesis enzymes. Summer fruit showed the delayed peak of ethylene production until the red ripe stage. The difference in postharvest ethylene production between winter and summer fruit appears to be regulated by the difference in accumulation of 1-aminocyclopropane-1-carboxylic acid (ACC) which depends on the putative up-regulation of SAM levels. The lack of differences in protein concentrations between winter and summer fruit indicate that heat stress did not alter the ethylene biosynthesis-related protein abundance in heat tolerant cultivar. The analysis results of enzymatic activity and proteomics showed that in both winter and summer fruit, the majority of ACO activity could be mainly contributed to the abundance of ACO5 and ACO6 isoforms, rather than ACO1. Likewise, ethylene signal transduction was largely controlled by the abundance of ethylene receptors ETR1, ETR3, ETR6, and ETR7 together with the constitute triple response regulator CTR1 for both winter and summer grown tomatoes. Altogether our results indicate that in the heat tolerant tomato cv. Savior, growing season mainly affects the ethylene biosynthesis pathway and leaves the signaling pathway relatively unaffected.

The Ubiquitin-26S Proteasome Pathway and Its Role in the Ripening of Fleshy Fruits

The 26S proteasome is an ATP-dependent proteolytic complex in eukaryotes, which is mainly responsible for the degradation of damaged and misfolded proteins and some regulatory proteins in cells, and it is essential to maintain the balance of protein levels in the cell. The ubiquitin-26S proteasome pathway, which targets a wide range of protein substrates in plants, is an important post-translational regulatory mechanism involved in various stages of plant growth and development and in the maturation process of fleshy fruits. Fleshy fruit ripening is a complex biological process, which is the sum of a series of physiological and biochemical reactions, including the biosynthesis and signal transduction of ripening related hormones, pigment metabolism, fruit texture changes and the formation of nutritional quality. This paper reviews the structure of the 26S proteasome and the mechanism of the ubiquitin-26S proteasome pathway, and it summarizes the function of this pathway in the ripening process of fleshy fruits.

The regulation of ACC synthase protein turnover: a rapid route for modulating plant development and stress responses

The phytohormone ethylene regulates plant growth, development, and stress responses. The strict fine-tuning of the regulation of ethylene biosynthesis contributes to the diverse roles of ethylene in plants. Pyridoxal 5'-phosphate-dependent 1-aminocyclopropane-1-carboxylic acid synthase, a rate-limiting enzyme in ethylene biosynthesis, is central and often rate-limiting to regulate ethylene concentration in plants. The post-translational regulation of ACS is a major pathway controlling ethylene biosynthesis in response to various stimuli. We conclude that the regulation of ACS turnover may serve as a central hub for the rapid integration of developmental, environmental, and hormonal signals, all of which influence plant growth and stress responses.

Dynamic changes of the ethylene biosynthesis in 'Jonagold' apple

In this study, the short-term and dynamic changes of the ethylene biosynthesis of Jonagold apple during and after application of controlled atmosphere (CA) storage conditions were quantified using a systems biology approach. Rapid responses to imposed temperature and atmospheric conditions were captured by continuous online photoacoustic ethylene measurements. Discrete destructive sampling was done to understand observed changes of ethylene biosynthesis at the transcriptional, translational and metabolic level. Application of the ethylene inhibitor 1-methylcyclopropene (1-MCP) allowed for the discrimination between ethylene-mediated changes and ethylene-independent changes related to the imposed conditions. Online ethylene measurements showed fast and slower responses during and after application of CA conditions. The changes in 1-aminocyclopropane-1-carboxylate synthase (ACS) activity were most correlated with changes in ACS1 expression and regulated the cold-induced increase in ethylene production during the early chilling phase. Transcription of ACS3 was found ethylene independent and was triggered upon warming of CA-stored apples. Increased expression of ACO1 during shelf life led to a strong increase in 1-aminocyclopropane-1-carboxylate oxidase (ACO) activity, required for the exponential production of ethylene during system 2. Expression of ACO2 and ACO3 was upregulated in 1-MCP-treated fruit showing a negative correlation with ethylene production. ACO activity never became rate limiting.

Determining degradation and synthesis rates of arabidopsis proteins using the kinetics of progressive 15N labeling of two-dimensional gel-separated protein spots

The growth and development of plant tissues is associated with an ordered succession of cellular processes that are reflected in the appearance and disappearance of proteins. The control of the kinetics of protein turnover is central to how plants can rapidly and specifically alter protein abundance and thus molecular function in response to environmental or developmental cues. However, the processes of turnover are largely hidden during periods of apparent steady-state protein abundance, and even when proteins accumulate it is unclear whether enhanced synthesis or decreased degradation is responsible. We have used a (15)N labeling strategy with inorganic nitrogen sources coupled to a two-dimensional fluorescence difference gel electrophoresis and mass spectrometry analysis of two-dimensional IEF/SDS-PAGE gel spots to define the rate of protein synthesis (K(S)) and degradation (K(D)) of Arabidopsis cell culture proteins. Through analysis of MALDI-TOF/TOF mass spectra from 120 protein spots, we were able to quantify K(S) and K(D) for 84 proteins across six functional groups and observe over 65-fold variation in protein degradation rates. K(S) and K(D) correlate with functional roles of the proteins in the cell and the time in the cell culture cycle. This approach is based on progressive (15)N labeling that is innocuous for the plant cells and, because it can be used to target analysis of proteins through the use of specific gel spots, it has broad applicability.

Turnover of LeACS2, a wound-inducible 1-aminocyclopropane-1-carboxylic acid synthase in tomato, is regulated by phosphorylation/dephosphorylation

1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) is the rate-limiting enzyme of the ethylene biosynthesis pathway. ACS is regulated both transcriptionally and post-translationally. We previously reported that LeACS2, a wound-inducible ACS in tomato (Solanum lycopersicum), is phosphorylated in vivo, and suggested that phosphorylation regulates protein stability rather than enzymatic activity. In this report, we demonstrate that phosphorylation/dephosphorylation of LeACS2 regulates its turnover upstream of the ubiquitin-26S-proteasome degradation pathway. Pulse-chase experiments coupled with treatment with protein kinase/phosphatase inhibitors demonstrated that LeACS2 is stabilized by phosphorylation and degraded after dephosphorylation. The amount of LeACS2 affected by the protein kinase/phosphatase inhibitors significantly influenced cellular ACS activity, ACC content, and ethylene production levels in tomato fruit tissue, suggesting that post-translational regulation by phosphorylation plays an important role in the control of ethylene production. A calcium-dependent protein kinase (CDPK), LeCDPK2, was isolated as one of the protein kinases that are able to phosphorylate LeACS2 at Ser-460. LeACS2 was immediately phosphorylated after translation by CDPK and mitogen-activated protein kinase at different sites in response to wound signaling and almost all functional LeACS2 molecules are phosphorylated in the cell. Phosphorylation at both sites was required for LeACS2 stability.

Characterization of transcriptional profiles of MA-ACS1 and MA-ACO1 genes in response to ethylene, auxin, wounding, cold and different photoperiods during ripening in banana fruit

The ripening-specific genes MA-ACS1 (Musa acuminata ACC synthase1) and MA-ACO1 (M. acuminata ACC oxidase 1) are regulated in response to a wide variety of factors. Here, we have studied the differential transcript accumulation pattern and protein levels of MA-ACS1 and MA-ACO1 genes in response to ethylene, auxin, wounding and low temperature in preclimacteric banana fruit. We have shown that exogenous application of ethylene and auxin induced the expression of MA-ACS1, while MA-ACO1 showed marginal expression following ethylene treatment in preclimacteric stage. Auxin did not induce MA-ACO1 expression. Thus, auxin-treated banana fruits showed lower ethylene production rate as compared to ethylene-treated fruits. Conversely, wounding and cold treatment down-regulated the expression of both the genes and thus inhibited ethylene production. Furthermore, we have detected a GCC-box putative ethylene-responsive element (ERE)- and an auxin-responsive element (ARE)-specific DNA-binding activity in the banana pulp and studied the ethylene and auxin responsive characteristics of the GCC-box and ARE (TGTCTC) containing synthetic promoter fragments. In addition, we have detected an enhanced ethylene production rate and expression level of MA-ACS1 and MA-ACO1 genes along with a strong GCC-box-specific DNA-binding activity following exposure to constant dark period for 8d at the preclimacteric stage. Together, our study provides interesting information about the regulation of expression of MA-ACS1 and MA-ACO1 genes in response to various factors during ripening in banana fruit, which may have physiological relevance concerning ethylene biosynthesis during post-harvest conditions.

MAPK phosphorylation-induced stabilization of ACS6 protein is mediated by the non-catalytic C-terminal domain, which also contains the cis-determinant for rapid degradation by the 26S proteasome pathway

Ethylene is an important hormone in plant growth, development and responses to environmental stimuli. The ethylene-signaling pathway is initiated by the induction of ethylene biosynthesis, which is under tight regulation at both transcriptional and post-transcriptional levels by exogenous and endogenous cues. 1-Aminocyclopropane-1-carboxylic acid synthase (ACS) is the rate-limiting enzyme that catalyzes the committing step of ethylene biosynthesis. Recently, we found that ACS2 and ACS6, two isoforms of the Arabidopsis ACS family, are substrates of a stress-responsive mitogen-activated protein kinase (MAPK) cascade. Phosphorylation of ACS2/ACS6 by MPK6 leads to the accumulation of ACS proteins and the induction of ethylene. In this report, we demonstrate that unphosphorylated ACS6 protein is rapidly degraded by the 26S proteasome pathway. The degradation machinery targets the C-terminal non-catalytic domain of ACS6, which is sufficient to confer instability to green fluorescent protein and luciferase reporters. Phosphorylation of ACS6 introduces negative charges to the C-terminus of ACS6, which reduces the turnover of ACS6 by the degradation machinery. Consistent with this, other nearby conserved negatively charged amino acid residues are essential for ACS6 stability regulation. Protein degradation and phosphorylation are two important post-translational modifications of proteins. This research reveals an intricate interplay between these two important processes in controlling the levels of cellular ACS activity, and thus ethylene biosynthesis. The post-translational nature of both processes ensures a rapid response of ethylene induction, which is detectable within minutes after plants are exposed to stress.

Ubiquitin, hormones and biotic stress in plants

BACKGROUND

The covalent attachment of ubiquitin to a substrate protein changes its fate. Notably, proteins typically tagged with a lysine48-linked polyubiquitin chain become substrates for degradation by the 26S proteasome. In recent years many experiments have been performed to characterize the proteins involved in the ubiquitylation process and to identify their substrates, in order to understand better the mechanisms that link specific protein degradation events to regulation of plant growth and development.

SCOPE

This review focuses on the role that ubiquitin plays in hormone synthesis, hormonal signalling cascades and plant defence mechanisms. Several examples are given of how targeted degradation of proteins affects downstream transcriptional regulation of hormone-responsive genes in the auxin, gibberellin, abscisic acid, ethylene and jasmonate signalling pathways. Additional experiments suggest that ubiquitin-mediated proteolysis may also act upstream of the hormonal signalling cascades by regulating hormone biosynthesis, transport and perception. Moreover, several experiments demonstrate that hormonal cross-talk can occur at the level of proteolysis. The more recently established role of the ubiquitin/proteasome system (UPS) in defence against biotic threats is also reviewed.

CONCLUSIONS

The UPS has been implicated in the regulation of almost every developmental process in plants, from embryogenesis to floral organ production probably through its central role in many hormone pathways. More recent evidence provides molecular mechanisms for hormonal cross-talk and links the UPS system to biotic defence responses.

Protein polyubiquitination plays a role in basal host resistance of barley

To study protein ubiquitination pathways in the interaction of barley (Hordeum vulgare) with the powdery mildew fungus (Blumeria graminis), we measured protein turnover and performed transient-induced gene silencing (TIGS) of ubiquitin and 26S proteasome subunit encoding genes in epidermal cells. Attack by B. graminis hyperdestabilized a novel unstable green fluorescent protein fusion that contains a destabilization domain of a putative barley 1-aminocyclopropane-1-carboxylate synthase, suggesting enhanced protein turnover. Partial depletion of cellular ubiquitin levels by TIGS induced extreme susceptibility of transformed cells toward the appropriate host pathogen B. graminis f. sp hordei, whereas papilla-based resistance to the nonhost pathogen B. graminis f. sp tritici and host resistance mediated by the mlo gene (for mildew resistance locus O) remained unaffected. Cells were rescued from TIGS-induced ubiquitin depletion by synthetic genes encoding wild-type or mutant barley monoubiquitin proteins. The strongest rescue was from a gene encoding a K63R mutant form of ubiquitin blocked in several ubiquitination pathways while still allowing Lys-48-dependent polyubiquitination required for proteasomal protein degradation. Systematic RNA interference of 40 genes encoding all 17 subunits of the proteasome 19S regulatory particle failed to induce hypersusceptibility against B. graminis f. sp hordei. This suggests a role for Lys-48-linked protein polyubiquitination, which is independent from the proteasome pathway, in basal host defense of barley.

Cloning and characterization of differentially expressed genes of internal breakdown in mango fruit (Mangifera indica)

Internal breakdown in mango fruits has become a major concern in recent years. This disorder renders the fruits unfit for human consumption. The overall loss due to this disorder is about 35-55%. Environmental and physiological factors like high temperature, humidity, respiration and low transpiration rates have been attributed to cause spongy tissue due to reduced loss of heat from fruits. Biochemical studies have shown that there is a reduction in pH, total soluble solids, ascorbic acid, total sugars and carotenoids, low reducing and non-reducing sugar contents, lower amylase and invertase activities and high acid and starch content in spongy tissue affected pulp. There are no reports on molecular studies to determine changes in gene expression in these tissues. The present study was conducted using PCR based subtractive hybridization and RNA gel blot analysis of a few selected genes. The latter showed a higher expression of catalase, ubiquitin, alcohol dehydrogenase, coproporphyrinogen oxidase and keratin associated protein. A lower expression of ribosomal gene, fructose bisphosphate aldolase and cysthathionine gamma synthase was also noticed in spongy tissue. Biochemical studies indicated a lower amylase activity and a lower content of the total and reducing sugars in spongy tissue as compared to healthy tissue. Analyses of results indicate that oxidative stress may be one of the causes for formation of spongy tissue, which affects the expression of many genes. The role of these genes in spongy tissue formation is discussed.

Ethylene signal transduction

BACKGROUND

The phytohormone ethylene is a key regulator of plant growth and development. Components of the pathway for ethylene signal transduction were identified by genetic approaches in Arabidopsis and have now been shown to function in agronomically important plants as well.

SCOPE

This review focuses on recent advances in our knowledge on ethylene signal transduction, in particular on recently proposed components of the pathway, on the interaction between the pathway components and on the roles of transcriptional and post-transcriptional regulation in ethylene signalling.

CONCLUSIONS

Data indicate that the site of ethylene perception is at the endoplasmic reticulum and point to the importance of protein complexes in mediating the initial steps in ethylene signal transduction. The expression level of pathway components is regulated by both transcriptional and post-transcriptional mechanisms, degradation of the transcription factor EIN3 being a primary means by which the sensitivity of plants to ethylene is regulated. EIN3 also represents a control point for cross-talk with other signalling pathways, as exemplified by the effects of glucose upon its expression level. Amplification of the initial ethylene signal is likely to play a significant role in signal transduction and several mechanisms exist by which this may occur based on properties of known pathway components. Signal output from the pathway is mediated in part by carefully orchestrated changes in gene expression, the breadth of these changes now becoming clear through expression analysis using microarrays.

Related to ubiquitin 1 and 2 are redundant and essential and regulate vegetative growth, auxin signaling, and ethylene production in Arabidopsis

Related to Ubiquitin (RUB)/Nedd8 is a ubiquitin-like protein that covalently attaches to cullins, a subunit of the SCF (for Skp, Cdc53p/Cul1, and F-box protein) complex, an E3 ubiquitin ligase, and has been shown to be required for robust function of the complex. The effects of reducing protein levels for two Rub proteins, RUB1 and RUB2, were characterized in Arabidopsis thaliana. T-DNA insertional null lines homozygous at a single RUB-encoding locus were analyzed and found to have a wild-type phenotype. A double mutant was never recovered. More than one-quarter of the progeny from the self-fertilization of plants with a single functional RUB-encoding gene died as embryos at the two-cell stage. Outcrosses demonstrated reduced inheritance of the null allele from both the male and female parent. Hemigglutinin-tagged forms of RUB1 and RUB2 conjugate to the same cullin protein, CUL1, and produce the same conjugation pattern. To further understand the function of the RUB proteins, a construct designed to produce a double-stranded RUB1 mRNA was introduced into plants, and three lines with reduced levels of RUB1- and RUB2-encoding mRNA and RUB1/2 protein content were analyzed in detail. Mature plants were severely dwarfed, seedlings were insensitive to auxin in root assays, and dark-grown seedlings had a partial triple-response phenotype that was suppressed when seedlings were grown on ethylene perception or synthesis inhibitors. The dsrub lines produced threefold to fivefold more ethylene than the wild type. This study illustrates that RUB1 and RUB2 are genetically and biochemically redundant and demonstrates that RUB1/2 proteins are essential for early embryonic cell divisions and that they regulate diverse processes.

The eto1, eto2, and eto3 mutations and cytokinin treatment increase ethylene biosynthesis in Arabidopsis by increasing the stability of ACS protein

The Arabidopsis ethylene-overproducing mutants eto1, eto2, and eto3 have been suggested to affect the post-transcriptional regulation of 1-aminocyclopropane-1-carboxylic acid synthase (ACS). Here, we present the positional cloning of the gene corresponding to the dominant eto3 mutation and show that the eto3 phenotype is the result of a missense mutation within the C-terminal domain of ACS9, which encodes one isoform of the Arabidopsis ACS gene family. This mutation is analogous to the dominant eto2 mutation that affects the C-terminal domain of the highly similar ACS5. Analysis of purified recombinant ACS5 and epitope-tagged ACS5 in transgenic Arabidopsis revealed that eto2 does not increase the specific activity of the enzyme either in vitro or in vivo; rather, it increases the half-life of the protein. In a similar manner, cytokinin treatment increased the stability of ACS5 by a mechanism that is at least partially independent of the eto2 mutation. The eto1 mutation was found to act by increasing the function of ACS5 by stabilizing this protein. These results suggest that an important mechanism by which ethylene biosynthesis is controlled is the regulation of the stability of ACS, mediated at least in part through the C-terminal domain.

IAA and N(6)-benzyladenine inhibit ethylene-regulated expression of ACC oxidase and ACC synthase genes in mungbean hypocotyls

Regulation of expression of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (VR-ACO1) and ACC synthase (VR-ACS1) genes by ethylene, indole-3-acetic acid (IAA), and N(6)-benzyladenine (BA), was investigated in mungbean hypocotyl tissues. Exogenous ethylene markedly increased transcript level of VR-ACO1 and reduced that of VR-ACS1, whereas aminooxyacetic acid (AOA), an inhibitor of ethylene biosynthesis, decreased the level of VR-ACO1 mRNA and increased that of VR-ACS1, indicating that expression of VR-ACO1 and VR-ACS1 genes are under positive and negative feedback control by ethylene, respectively. However, IAA treatment reduced the level of VR-ACO1 transcripts and increased that of VR-ACS1, although the hormone greatly induced ethylene production. We have demonstrated that, in a system that separates the effect of IAA proper from the effect of IAA-induced ethylene, the amount of IAA-induced ethylene was enough to cause accumulation of VR-ACO1 mRNA and decrease of VR-ACS1 mRNA. We have also shown that the responsiveness of VR-ACO1 and VR-ACS1 to exogenous ethylene was greatly reduced in the presence of IAA. In addition, BA abolished ethylene responsiveness with respect to expression of VR-ACO1 and VR-ACS1. Based on these results, we suggest that IAA and BA inhibit ethylene action, resulting in suppression of VR-ACO1 expression and induction of VR-ACS1 expression.

Down-regulation of specific members of the glutamine synthetase gene family in alfalfa by antisense RNA technology

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH3 with glutamate to produce glutamine. In plants GS is an octameric enzyme and is located either in the cytoplasm (GS1) or in the chloroplast (GS2). Two distinct classes of GS1 genes with unique 3'-untranslated region (3'UTR) have been identified in alfalfa. We have demonstrated that the two classes exhibit differential expression pattern in the different plant organs suggesting different functional roles for the different isozymes. To determine the functional significance of the two classes of GS1 genes in alfalfa, we have utilized antisense gene constructs aimed specifically at the 3'UTR of the two GS1 genes and introduced them individually into alfalfa. Our data show that the gene constructs are effective in lowering the corresponding transcript level very effectively though there were organ-specific differences in the level of reduction. No transcript corresponding to the antisense gene construct was detected in any of the alfalfa transformants though they accumulated to significant levels in transgenic tobacco containing the same construct. This suggests that the antisense transcript was not stable in the presence of the homologous target sequence. Transgenic alfalfa with up to 80% reduction in the transcript level corresponding to each gene class, however, showed no reduction in GS activity or GS1 polypeptide level. The results suggest that GS1 mRNA levels are not rate-limiting for GS1 polypeptide synthesis and that GS levels are controlled both at the transcriptional and translational/post-translational level.

Differential induction of seven 1-aminocyclopropane-1-carboxylate synthase genes by elicitor in suspension cultures of tomato (Lycopersicon esculentum)

The key enzyme of ethylene biosynthesis, ACC synthase, is encoded by a multigene family. We describe three new DNA sequences encoding members of the ACC synthase family of the tomato. One of these sequences encodes a novel ACC synthase, LE-ACS6, which is phylogenetically related to the ACC synthases LE-ACS1A and LE-ACS1B. Gene-specific probes for seven tomato ACC synthase genes were prepared. They were used for RNase protection assays to study the accumulation of ACC synthase transcripts in suspension-cultured tomato cells after the addition of an elicitor. The ACC synthase genes LE-ACS2, LE-ACS5 and LE-ACS6 were strongly induced by the elicitor. In contrast, the genes LE-ACS1B, LE-ACS3 and LE-ACS4 were constitutively expressed and LE-ACS1B was present at all times at a particularly high level. Thus, there are two groups of ACC synthase transcripts expressed in these cells, either elicitor-induced or constitutive. A transcript of LE-ACS1A was not detected. Despite the presence of LE-ACS1B, LE-ACS2, LE-ACS3, LE-ACS4 and LE-ACS5, there was only little ethylene produced in the absence of the elicitor. Increased ethylene production is usually correlated with the accumulation of ACC synthase transcripts, indicating that ethylene production is controlled via the transcriptional activation of ACC synthase genes. However, the abundance of several ACC synthase mRNAs studied was not strictly correlated with the rate of elicitor-induced ethylene production. Our data provide evidence that the activity of these ACC synthases may not solely be controlled by the transcriptional activation of ACC synthase genes.

The Tomato E8 Gene Influences Ethylene Biosynthesis in Fruit but Not in Flowers

We investigated the function of the tomato (Lycopersicon esculentum) E8 gene. Previous experiments in which antisense suppression of E8 was used suggested that the E8 protein has a negative effect on ethylene evolution in fruit. E8 is expressed in flowers as well as in fruit, and its expression is high in anthers. We introduced a cauliflower mosaic virus 35S-E8 gene into tomato plants and obtained plants with overexpression of E8 and plants in which E8 expression was suppressed due to co-suppression. Overexpression of E8 in unripe fruit did not affect the level of ethylene evolution during fruit ripening; however, reduction of E8 protein by cosuppression did lead to elevated levels during ripening. Levels for ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), and ACC oxidase mRNA were increased approximately 7-fold in fruit of plants with reduced E8 protein. Levels of ACC synthase 2 mRNA were increased 2.5-fold, and ACC synthase 4 mRNA was not affected. Reduction of E8 protein in anthers did not affect the accumulation of ACC or of mRNAs encoding enzymes involved in ethylene biosynthesis. Our results suggest that the product of the E8 reaction participates in feedback regulation of ethylene biosynthesis during fruit ripening.

Proteolysis in plants: mechanisms and functions

Proteolysis is essential for many aspects of plant physiology and development. It is responsible for cellular housekeeping and the stress response by removing abnormal/misfolded proteins, for supplying amino acids needed to make new proteins, for assisting in the maturation of zymogens and peptide hormones by limited cleavages, for controlling metabolism, homeosis, and development by reducing the abundance of key enzymes and regulatory proteins, and for the programmed cell death of specific plant organs or cells. It also has potential biotechnological ramifications in attempts to improve crop plants by modifying protein levels. Accumulating evidence indicates that protein degradation in plants is a complex process involving a multitude of proteolytic pathways with each cellular compartment likely to have one or more. Many of these have homologous pathways in bacteria and animals. Examples include the chloroplast ClpAP protease, vacuolar cathepsins, the KEX2-like proteases of the secretory system, and the ubiquitin/26S proteasome system in the nucleus and cytoplasm. The ubiquitin-dependent pathway requires that proteins targeted for degradation become conjugated with chains of multiple ubiquitins; these chains then serve as recognition signals for selective degradation by the 26S proteasome, a 1.5 MDa multisubunit protease complex. The ubiquitin pathway is particularly important for developmental regulation by selectively removing various cell-cycle effectors, transcription factors, and cell receptors such as phytochrome A. From insights into this and other proteolytic pathways, the use of phosphorylation/dephosphorylation and/or the addition of amino acid tags to selectively mark proteins for degradation have become recurring themes.

ACS4, a primary indoleacetic acid-responsive gene encoding 1-aminocyclopropane-1-carboxylate synthase in Arabidopsis thaliana. Structural characterization, expression in Escherichia coli, and expression characteristics in response to auxin [corrected]

1-Aminocyclopropane-1-carboxylic acid (ACC) synthase is the key regulatory enzyme in the biosynthetic pathway of the plant hormone ethylene. The enzyme is encoded by a divergent multigene family in Arabidopsis thaliana, comprising at least five genes, ACS1-5 (Liang, X., Abel, S., Keller, J.A., Shen,N. N.F., and Theologis, A. (1992) Poc. Natl. Acad. Sci. U.S.A. 89, 11046-11050). In etiolated seedlings, ACS4 is specifically induced by indoleacetic acid (IAA). The response to IAA is rapid (within 25 min) and insensitive to protein synthesis inhibition, suggesting that the ACS4 gene expression is a primary response to IAA. The ACS4 mRNA accumulation displays a biphasic dose-response curve which is optimal at 10 microM of IAA. However, IAA concentrations as low as 100 microM are sufficient to enhance the basal level of ACS4 mRNA. The expression of ACS4 is defective in the Arabidopsis auxin-resistant mutant lines axr1-12, axr2-1, and aux1-7. ACS4 mRNA levels are severely reduced in axr1-12 and axr2-1 but are only 1.5-fold lower in aux1-7. IAA inducibility is abolished in axr2-1. The ACS4 gene was isolated and structurally characterized. The promoter contains four sequence motifs reminiscent of functionally defined auxin-responsive cis-elements in the early auxin-inducible genes PS-IAA4/5 from pea and GH3 from soybean. Conceptual translation of the coding region predicts a protein with a molecular mass of 53,795 Da and a theoretical isoelectric point of 8.2. The ACS4 polypeptide contains the 11 invariant amino acid residues conserved between aminotransferases and ACC synthases from various plant species. An ACS4 cDNA was generated by reverse transcriptase-polymerase chain reaction, and the authenticity was confirmed by expression of ACC synthase activity in Escherichia coli.

The Apparent Turnover of 1-Aminocyclopropane-1-Carboxylate Synthase in Tomato Cells Is Regulated by Protein Phosphorylation and Dephosphorylation

In suspension-cultured cells of tomato (Lycopersicon esculentum Mill.), the activity of 1-aminocyclopropane-1-carboxylate synthase (ACC-S) rapidly increases in response to fungal elicitors. The effect of inhibitors of protein kinases and protein phosphatases on the regulation of ACC-S was studied. K-252a, an inhibitor of protein kinases, prevented induction of the enzyme by elicitors and promoted its apparent turnover in elicitor-stimulated cells, causing a 50% loss of activity within 4 to 8 min in both the presence and absence of cycloheximide. Calyculin A, an inhibitor of protein phosphatases, caused a rapid increase of ACC-S in the absence of elicitors and an immediate acceleration of the rate of ACC-S increase in elicitor-stimulated cells. In the presence of cycloheximide there was no such increase, indicating that the effect depended on protein synthesis. Cordycepin, an inhibitor of mRNA synthesis, did not prevent the elicitor-induced increase in ACC-S activity but strongly reduced the K-252a-induced decay and the calyculin A-induced increase of its activity. In vitro, ACC-S activity was not affected by K-252a and calyculin A or by treatments with protein phosphatases. These results suggest that protein phosphorylation/dephosphorylation is involved in the regulation of ACC-S, not by regulating the catalytic activity itself but by controlling the rate of turnover of the enzyme.

Use of a tomato mutant constructed with reverse genetics to study fruit ripening, a complex developmental process

Fruit ripening is one of the most dramatic developmental transitions associated with extensive alteration in gene expression. The plant hormone ethylene is considered to be the causative ripening agent. Transgenic tomato plants were constructed expressing antisense or sense RNA to the key enzyme in the ethylene (C2H4) biosynthetic pathway, 1-aminocyclopropane-1-carboxylate (ACC) synthase using the constitutive CaMV 35S and fruit specific E8 promoters. Fruits expressing antisense LE-ACS2 RNA produce less ethylene and fail to ripen only when ethylene production is suppressed by more than 99% (> 0.1 nl/g fresh weight). Ethylene production is considerably inhibited (50%) in fruits expressing sense LE-ACS2 RNA. Antisense fruits accumulate normal levels of polygalacturonase (PG), ACC oxidase (pTOM13), E8, E17, J49, and phytoene desaturase (D2) mRNAs which were previously thought to be ethylene-inducible. E4 gene expression is inhibited in antisense fruits and its expression is not restored by treatment with exogenous propylene (C3H6). Antisense fruits accumulate PG mRNA, but it is not translated. Immunoblotting experiments indicate that the PG protein is not expressed in antisense fruits but its accumulation is restored by propylene (C3H6) treatment. The results suggest that at least two signal-transduction pathways are operating during tomato fruit ripening. The independent (developmental) pathway is responsible for the transcriptional activation of genes such as PG, ACC oxidase, E8, E17, D2, and J49. The ethylene-dependent pathway is responsible for the transcriptional and posttranscriptional regulation of genes involved in lycopene, aroma biosynthesis, and the translatability of developmentally regulated genes such as PG.