Properties and Partial Purification of 1-Aminocyclopropane-1-carboxylate Synthase

We studied the regulation of 1-aminocyclopropane-1-carboxylate (ACC) synthase activity in tomato (Lycopersicon esculentum Mill.) fruit tissue and attempted the purification of this enzyme. The increase of ACC synthase activity in wounded tomato pericarp was inhibited by cordycepin and cycloheximide. Density labeling studies showed a 0.75% increase in the buoyant density of ACC synthase isolated from tomato pericarp tissue that had been incubated on ²H₂O as compared to ACC synthase from H₂O-treated tissue. These data are consistent with the hypothesis that ACC synthase is synthesized de novo following wounding of tomato pericarp tissue. SDS-gel electrophoresis and fluorography showed that the pattern of incorporation of l-[³⁵S]methionine into protein changed with time after wounding of the tissue. Radioactive protein bands that were not detected 1 hour after wounding, became apparent 2 to 3 hours after wounding.     

Stress-Induced Translational Control in Potato Tubers May Be Mediated by Polysome-Associated Proteins

Potato tubers exhibit distinct responses to wounding and hypoxia that include selective translation of stress-induced mRNAs. Newly synthesized wound-response mRNAs are bound to polysomes, whereas preexisting mRNAs are displaced and degraded. mRNAs that are induced and translated during hypoxic conditions are bound to ribosomes as expected. However, preexisting wound-response mRNAs whose translation is inhibited during hypoxia remain bound to polysomes, indicating that there are at least two distinct mechanisms by which translation is regulated in response to stress conditions. A 32-kD phosphoprotein is associated with polyribosomes from wounded tubers. This protein remains polysome bound as long as wound-response mRNAs are present, even during hypoxia when these mRNAs are no longer translated. However, association of the 32-kD protein with polysomes is not elicited by hypoxic stress alone. The kinase that phosphorylates this protein is active only for the first 24 hr after wounding and is not active during periods of hypoxia. This protein may mediate recognition of the wound-response mRNAs by ribosomes.     

Methods of measuring the prevalence of Phoma exigua on potatoes and in soil

Methods are described for assessing inoculum of Phoma exigua by directly wounding potato test tubers and by inoculating soil into Arran Banner test tubers or slices. Large samples of tubers were wounded by dropping onto perforated metal sheet or grading on farm riddles, but standardization was achieved by inflicting four uniform wounds with shaped brass teeth. Tubers were then incubated for 12 wk at 5 ^oC. Soil samples to be tested were inoculated into crush wounds on surface-sterilized test tubers or tuber slices which were incubated for 8 wk at 5 ^oC. Assessments were speeded by immersing wounded slices in a mixture of Agrimycin and isopropylphenyl carbamate (IPC) solutions before inoculating, and then incubating for 2–3 wk at 10 ^oC. Although the sensitivity of test tubers is limited by their latent infection, comparisons of the tests in 3–5 years showed that the test tuber methods were frequently more sensitive than direct wounding, especially when amounts of inoculum were small.     

Extensin and Phenylalanine Ammonia-Lyase Gene Expression Altered in Potato Tubers in Response to Wounding, Hypoxia, and Erwinia carotovora Infection

Potato (Solanum tuberosum L.) tubers are susceptible to infection by Erwinia carotovora, causal agent of bacterial soft rot, when wounded and subjected to wet, hypoxic environments. The expression of two putative plant defense genes, extensin and phenylalanine ammonia-lyase (PAL), was examined by monitoring their respective mRNA levels and cell wall hydroxyproline levels in tuber tissues under various conditions leading to susceptibility or resistance and after inoculation with E. carotovora in order to assess the possible roles of these genes and their products in this plant-pathogen interaction. Extensin and PAL mRNA levels as well as cell wall hydroxyproline levels accumulated markedly in response to wounding and subsequent aerobic incubation. Extensin and PAL mRNA levels as well as cell wall hydroxyproline levels decreased in response to wounding and subsequent anaerobic incubation; these changes were correlated with high susceptibility of tuber tissue to E. carotovora infection. Inoculation of wound sites with E. carotovora caused some additional accumulation of the wound-regulated extensin and PAL mRNAs under certain aerobic conditions, but never under anaerobic conditions.     

<Emphasis Type="Italic">Strboh A</Emphasis> homologue of NADPH oxidase regulates wound-induced oxidative burst and facilitates wound-healing in potato tubers

During 30-months of storage at 4°C, potato (Solanum tuberosum L.) tubers progressively lose the ability to produce superoxide in response to wounding, resist microbial infection, and develop a suberized wound periderm. Using differentially aged tubers, we demonstrate that Strboh A is responsible for the wound-induced oxidative burst in potato and aging attenuates its expression. In vivo superoxide production and NADPH oxidase (NOX) activity from 1-month-old tubers increased to a maximum 18–24 h after wounding and then decreased to barely detectable levels by 72 h. Wounding also induced a 68% increase in microsomal protein within 18 h. These wound-induced responses were lost over a 25- to 30-month storage period. Superoxide production and NOX activity were inhibited by diphenylene iodonium chloride, a specific inhibitor of NOX, which in turn effectively inhibited wound-healing and increased susceptibility to microbial infection and decay in 1-month-old tubers. Wound-induced superoxide production was also inhibited by EGTA-mediated destabilization of membranes. The ability to restore superoxide production to EGTA-treated tissue with Ca⁺² declined with advancing tuber age, likely a consequence of age-related changes in membrane architecture. Of the five homologues of NOX (Strboh A-D and F), wounding induced the expression of Strboh A in 6-month-old tubers but this response was absent in tubers stored for 25–30 months. Strboh A thus mediates the initial burst of superoxide in response to wounding of potato tubers; loss of its expression increases the susceptibility to microbial infection and contributes to the age-induced loss of wound-healing ability.     

Plant Desiccation and Protein Synthesis : VI. Changes in Protein Synthesis Elicited by Desiccation of the Moss Tortula ruralis are Effected at the Translational Level

Upon rehydration of the moss Tortula ruralis following desiccation at a rapid or slow rate, there is increasing utilization of newly synthesized-poly(A)⁺ RNA for protein synthesis. Initially, poly(A)⁺ RNA conserved in the dry moss is associated with polysomes, but by 2 hours of rehydration there is an overwhelming recruitment of newly synthesized poly(A)⁺ RNA, at the expense of conserved messages. In rehydrated moss, there is a marked synthesis in vivo of new proteins, which are separable by two-dimensional electrophoresis, and identifiable by fluorography. These new proteins, termed rehydration proteins, are synthesized after both rapid and slow desiccation, but their synthesis persists longer after rapid desiccation. The protein patterns obtained following in vitro translation of bulk RNA from hydrated, desiccated, and rehydrated moss were qualitatively identical. Thus the differences in protein patterns observed in vivo must result from preferential selection of specific mRNAs from the same pool, which is indicative of control of protein synthesis at the translational level. The implications of these observations in relation to the response of the moss to drying in its natural environment are discussed.     

Coordinate expression of AOS genes and JA accumulation: JA is not required for initiation of closing layer in wound healing tubers

Wounding induces a series of coordinated physiological responses essential for protection and healing of the damaged tissue. Wound-induced formation of jasmonic acid (JA) is important in defense responses in leaves, but comparatively little is known about the induction of JA biosynthesis and its role(s) in tuber wound-healing. In this study, the effects of wounding on JA content, expression of JA biosynthetic genes, and the involvement of JA in the initiation of closing layer formation in potato tubers were determined. In addition, the role of abscisic acid (ABA) in wound-induced JA accumulation was examined. The basal JA content in non-wounded tuber tissues was low (<3 ng g⁻¹ FW). Two hours after wounding, the JA content increased by >5-fold, reached a maximum between 4 and 6 h after wounding, and declined to near-basal levels thereafter. Tuber age (storage duration) had little effect on the pattern of JA accumulation. The expressions of the JA biosynthetic genes (StAOS2, StAOC, and StOPR3) were greatly increased by wounding reaching a maximum 2-4 h after wounding and declining thereafter. A 1-h aqueous wash of tuber discs immediately after wounding resulted in a 94% inhibition of wound-induced JA accumulation. Neither JA treatment nor inhibition of JA accumulation affected suberin polyphenolic accumulation during closing layer development indicating that JA was not essential for the initiation of primary suberization. ABA treatment did not restore JA accumulation in washed tuber tissues suggesting that leaching of endogenous ABA was either not involved or not solely involved in this loss of JA accumulation by washing. Collectively, these results indicate that JA is not required for the induction of processes essential to the initiation of suberization during closing layer development, but do not exclude the possibility that JA may be involved in other wound related responses.     

Wound-induced superoxide production and PAL activity decline with potato tuber age and wound healing ability

Wound healing of potato tubers involves the concerted action of several enzymes that facilitate polymerization of phenolics into suberin at the wound site. A decline in the efficiency of healing and resistance to pathogens with advancing tuber age was associated with reduced ability of older tubers to produce superoxide radicals (FRs) in response to wounding. Autophotographs of luminol‐treated longitudinal sections of tissue from 6‐, 18‐ and 30‐month‐old tubers revealed a substantial decline in superoxide production at the wound surface with advancing age. Older tubers were less able to respond to wounding by increasing phenylalanine ammonia lyase (PAL) activity. This enzyme produces t‐cinnamic acid, which constitutes a component of the phenolic domain of suberin, and is normally induced by wounding and/or ethylene. Interestingly, the ability of wounded tissue to oxidize exogenous 1‐aminocyclopropane‐1‐carboxylic acid (ACC) to C₂H₄ also decreased with advancing tuber age. The oxidation of ACC was inhibited by the FR scavenger, n‐propyl gallate (PG), and inhibition was greatest in tissue from younger tubers, reflecting their greater ability to produce superoxide radicals upon wounding. Regardless of tuber age, 1‐aminocyclobutane‐1‐carboxylic acid, an ACC oxidase inhibitor, did not inhibit C₂H₄ generation from exogenous ACC. Hence, C₂H₄ production from ACC by wounded tuber tissue is largely non‐enzymatic and FR‐driven, and thus serves as an indicator of the ability of wounded tissue to produce superoxide. Age‐induced reduction in PAL activity and FR production at the wound surface probably limited the oxidative polymerization of phenolics into suberin during wound periderm formation. The age‐induced loss in ability of wounded tissue to heal and resist pathogens is thus consistent with reduced synthesis and polymerization of phenolic adducts into suberin, a consequence of reduced FR and PAL activity at the wound surface.     

Regulation by Cytokinins of Endogenous Levels of Jasmonic and Salicylic Acids in Mechanically Wounded Tobacco Plants

Plants respond differentially to wounding and pathogens usingdistinct signaling pathways, so that wound signals are transmittedto jasmonic acid (JA) which induces basic pathogenesis-related(PR) proteins, whereas pathogenic signals cause, in additionto JA, accumulation of salicylic acid (SA) which stimulatesproduction of acidic PR proteins. Transgenic tobacco plantsexpressing a gene for a small GTP binding protein respond abnormallyto mechanical wounding to produce SA and consequently acidicPR proteins, suggesting that wound signals cross with pathogensignaling pathways [Sano et al. (1994) Proc. Natl. Acad. Sci.USA 91: 10556]. This unusual signal crossing is associated witha highly sensitive wound-response of transgenic plants which,upon wounding, produce JA at least eighteen hours earlier thanwild-type plants. When wildtype plants are wounded in the presenceof the synthetic cytokinin, benzylaminopurine, production ofJA begins six hours earlier than in untreated samples, and alsoSA begins to accumulate. The cytokinin antagonist, 2-chloro-4-cyclohexylamino-6-ethylamino-s-triazine,erases these effects. Because transgenic plants constitutivelyproduce four-to six-fold higher amounts of endogenous cytokininsthan wild-type plants, it is concluded that cytokinins are indispensablefor control of endogenous levels of SA and JA. (Received April 23, 1996; Accepted June 10, 1996)     

Water relationships and incorporation of C assimilates in tubers of potato plants differing in potassium nutrition

Potato (Solanum tuberosum L. cv Saturna) plants were grown in pots with varying supplies of K fertilizer (1.25, 5, and 10 grams K₂O per 12 kilograms soil). Four weeks after midflower, plants were supplied with ¹⁴CO₂ for 12 hours and osmotic and water potential in tubers were determined. Assimilation of ¹⁴CO₂ increased from 504 (K₁) to 1860 (K₅) and 1922 kilobecquerels per plant per 12 hours (K₁₀). In all treatments, about half of the ¹⁴C was translocated to the tubers within 12 hours, although calculated turgor pressure in tubers which could be considered as a potential counter-pressure to phloem unloading increased from +5.3 (K₁) to +5.9 (K₅) and +6.0 bars (K₁₀), respectively. Incorporation of ¹⁴C per gram tuber dry weight as well as per gram tuber starch was significantly higher in K₅ than in K₁ and slightly increased even further in the K₁₀ treatment, where tuber pressure sap contained the highest K concentration (179 millimolar K).     

Identification of a Thiol/Disulfide Redox Switch in the Human BK Channel That Controls Its Affinity for Heme and CO

Heme is a required prosthetic group in many electron transfer proteins and redox enzymes. The human BK channel, which is a large-conductance Ca²⁺ and voltage-activated K⁺ channel, is involved in the hypoxic response in the carotid body. The BK channel has been shown to bind and undergo inhibition by heme and activation by CO. Furthermore, evidence suggests that human heme oxygenase-2 (HO2) acts as an oxygen sensor and CO donor that can form a protein complex with the BK channel. Here we describe a thiol/disulfide redox switch in the human BK channel and biochemical experiments of heme, CO, and HO2 binding to a 134-residue region within the cytoplasmic domain of the channel. This region, called the heme binding domain (HBD) forms a linker segment between two Ca²⁺-sensing domains (called RCK1 and RCK2) of the BK channel. The HBD includes a CXXCH motif in which histidine serves as the axial heme ligand and the two cysteine residues can form a reversible thiol/disulfide redox switch that regulates affinity of the HBD for heme. The reduced dithiol state binds heme (K_d = 210 nm) 14-fold more tightly than the oxidized disulfide state. Furthermore, the HBD is shown to tightly bind CO (K_d = 50 nm) with the Cys residues in the CXXCH motif regulating affinity of the HBD for CO. This HBD is also shown to interact with heme oxygenase-2. We propose that the thiol/disulfide switch in the HBD is a mechanism by which activity of the BK channel can respond quickly and reversibly to changes in the redox state of the cell, especially as it switches between hypoxic and normoxic conditions.     

Acetazolamide-based [18F]-PET tracer: In vivo validation of carbonic anhydrase IX as a sole target for imaging of CA-IX expressing hypoxic solid tumors

Carbonic anhydrase IX is overexpressed in many solid tumors including hypoxic tumors and is a potential target for cancer therapy and diagnosis. Reported imaging agents targeting CA-IX are successful mostly in clear cell renal carcinoma as SKRC-52 and no candidate was approved yet in clinical trials for imaging of CA-IX. To validate CA-IX as a valid target for imaging of hypoxic tumor, we designed and synthesized novel [¹⁸F]-PET tracer (1) based on acetazolamide which is one of the well-known CA-IX inhibitors and performed imaging study in CA-IX expressing hypoxic tumor model as 4T1 and HT-29 in vivo models other than SKRC-52. [¹⁸F]-acetazolamide (1) was found to be insufficient for the specific accumulation in CA-IX expressing tumor. This study might be useful to understand in vivo behavior of acetazolamide PET tracer and can contribute to the development of successful PET imaging agents targeting CA-IX in future. Additional study is needed to understand the mechanism of poor targeting of CA-IX, as if CA-IX is not reliable as a sole target for imaging of CA-IX expressing hypoxic solid tumors.     

Seasonal variations in the pattern of RNA metabolism of tuber tissue in response to excision and culture

Summary Between December 1975 and June 1976 expiants excised from Jerusalem artichoke tubers were cultured both in the presence and in the absence of 2,4-D, the cells in the tissue dividing only in the presence of 2,4-D, in which the length of the first cell cycle increased nonlinearly from 18 hours to 40 hours as the tubers aged in storage at 4 °C. Simultaneously the amount of RNA in the tissue declined linearly from 8 to 5 g RNA per explant. Detailed examination of the RNA metabolism in dividing and in non-dividing cells during February and June 1976 revealed superimposed but independent responses to wounding during excision and to stimulation into growth by 2,4-D. The responses to wounding involved only a very low level of metabolic activity, were complete within a few hours of excision and changed very little with the storage of the tubers. Tissue treated with 2,4-D showed a much higher level of metabolic activity including the periodic accumulation of RNA coupled to its discontinuous synthesis. The features of these growth-related responses changed considerably during the investigation.     

Role of Protease Inhibitors in Potato Protection

Mechanical wounding or infection of potatoes with Phytophthora infestans caused an accumulation of only serine protease inhibitors in exudates of potato tubers. Among them, proteins prevailed that are structurally similar to those present in healthy tubers: a 22-kDa trypsin inhibitor, a 21-kDa serine protease inhibitor consisting of two polypeptide chains, and a 8-kDa potato chymotrypsin I inhibitor produced de novo. The accumulated proteins inhibited the growth of hyphae and germination of zoospores of P. infestans. Treatment with elicitors, jasmonic and arachidonic acids, intensified the accumulation of these inhibitors in tubers in response to the wound stress, whereas salicylic acid blocked this process. These results suggest that lipoxygenase metabolism plays a substantial role in signal transduction of the protective system of resting potato tubers.     

Severe ethanol stress induces the preferential synthesis of mitochondrial disaggregase Hsp78 and formation of DUMPs in Saccharomyces cerevisiae

Severe ethanol stress (>9% v/v) induces pronounced translation repression in yeast cells. However, some proteins, which are exceptionally synthesized even under translation repression, play important roles in ethanol tolerance. These proteins are expected to provide important clues for elucidating the survival strategies of yeast cells under severe ethanol stress. In this study, we identified Hsp78 as a protein effectively synthesized under severe ethanol stress. As Hsp78 is involved in mitochondrial protein quality control, we investigated the effect of severe ethanol stress on mitochondrial proteins and found that Ilv2, Kgd1, and Aco1 aggregated with Hsp78 under severe ethanol stress, forming mitochondrial deposition sites for denatured proteins, called DUMPs (Deposits of Unfolded Mitochondrial Proteins). Aggregation of mitochondrial proteins and formation of DUMPs were accelerated in hsp78? cells compared with those in wild-type cells. During the recovery process after ethanol removal, aggregated Ilv2 and DUMP levels rapidly decreased in wild-type cells but were maintained for a long time (>180 min) in hsp78Δ cells. Furthermore, the frequency of respiration-deficient mutants caused by severe ethanol stress was higher in hsp78? cells than in wild-type cells. These results indicate that severe ethanol stress damaged mitochondrial proteins and that Hsp78 was preferentially synthesized to cope with the damage, thereby suppressing the rapid increase in aggregated protein levels under stress and achieving proper clearance of aggregated proteins during the recovery process. This study provides novel insights into the adverse effects of ethanol on mitochondria and yeast response to severe ethanol stress.