Antagonisms between Kinetin and Amino Acids: Experiments on the Mode of Action of Cytokinins

The maintenance of chlorophyll in darkened first leaves of oats was used as a bioassay for cytokinins in pea (Pisum sativum) roots. No cytokinin was found (in contrast with earlier reports on sunflower roots); however, the extracts contained two or more substances antagonistic to cytokinin, i. e., promoting the yellowing in this test. Because the most active of these appeared to be an amino acid, individual amino acids were examined for their ability to modify the greening reaction. As a result, l-serine was found to have these properties. It promotes yellowing whether the greening agent is kinetin, indoleacetic acid, or adenine; it is, therefore, not functioning as a specific cytokinin antagonist. Its action is due to promoting proteolysis. Its d-isomer is inactive. l-Arginine, which alone does not cause chlorophyll retention and only weakly inhibits proteolysis, strongly antagonizes the action of l-serine, and thus prevents the yellowing; this effect is specific, and the only other effective serine antagonist found, although much weaker, is l-threonine. The action of arginine is not due to its preventing serine uptake, but rather the action parallels the serine-arginine antagonism previously described for nitrate reductase induction. A novel interpretation of the effect of amino acids on this process is therefore put forward. In studies of the RNase in darkened oat leaves, serine was found to have no effect; however, kinetin strongly inhibits the normal rise in the level of RNase which occurs in the isolated leaf. Kinetin also maintains the integrity of the cell membranes. A variety of evidence leads to the conclusion that the primary action of kinetin on the leaf is to inhibit proteolysis, rather than to promote protein synthesis.     

The role of protein synthesis in the senescence of leaves: I. The formation of protease

The senescence of oat leaves has been studied by following the loss of chlorophyll and protein and the increase of α-amino nitrogen, after detachment and darkening. Protein synthesis and the amounts of proteolytic enzymes in the leaves have been determined directly. The process of senescence is shown to be a sequential one in which protein synthesis,most probably the formation of a proteolytic enzyme with l-serine in its active center, is of prime importance. The evidence is as follows. Firstly, l-serine specifically enhances senescence, especially in presence of kinetin. Secondly, cycloheximide, which inhibits protein synthesis in other systems, delays senescence and prevents the serine enhancement. Although requiring higher concentrations, cycloheximide can be as effective as kinetin in inhibiting senescence. It is shown directly that cycloheximide prevents protein synthesis in oat leaves under the same conditions as when it prevents senescence. Thirdly, leaves have been shown to contain two proteinases, with pH optima at 3 and 7.5, whose activity increases during senescence, even though the total leaf protein is decreasing. The amounts of both these enzymes present after 3 days are clearly increased by serine, and are greatly decreased by cycloheximide or by kinetin. The role of kinetin in delaying senescence thus may rest on its ability to suppress protease formation.     

The Effect of Kinetin on Protein Level of Brassica Leaf Disks

The senescent leaf disks from Brassica rapa L. were floated on a medium containing ¹⁴C-L.-leucine in the presence or absence of kinetin. The increase in radioactivity in protein fraction of kinetin-treated disks was almost linear with time, while the increase in untreated disks incubated for 3 days started to slow down. The leaf disks were first incubated on the solution of ¹⁴C-L-leucine and then transferred to either kinetin solution or water in order to study whether the slower incorporation is due to the decreased rate of protein synthesis or that of protein retention. The decrease in radioactivity of disks floated on kinetin was slower than that of disks floated on water. The slower decrease in radioactivity caused by kinetin was not due to an increased turnover rate, since the same phenomena were observed in the presence of cold leucine or casein hydrolysate solution. These results suggest that kinetin retards the decomposition of protein in leaf disks and does not stimulate the synthesis of the bulk of protein.     

Emission of Hydrogen Sulfide by Leaf Tissue in Response to l-Cysteine

Leaf discs and detached leaves exposed to l-cysteine emitted a volatile sulfur compound which was proven by gas chromatography to be H₂S. This phenomenon was demonstrated in all nine species tested (Cucumis sativus, Cucurbita pepo, Nicotiana tabacum, Coleus blumei, Beta vulgaris, Phaseolus vulgaris, Medicago sativa, Hordeum vulgare, and Gossypium hirsutum). The emission of volatile sulfur by cucumber leaves occurred in the dark at a similar rate to that in the light. The emission of leaf discs reached the maximal rate, more than 40 picomoles per minute per square centimeter, 2 to 4 hours after starting exposure to l-cysteine; then it decreased. In the case of detached leaves, the maximum occurred 5 to 10 h after starting exposure. The average emission rate of H₂S during the first 4 hours from leaf discs of cucurbits in response to 10 millimolar l-cysteine, was usually more than 40 picomoles per minute per square centimeter, i.e. 0.24 micromoles per hour per square decimeter. Leaf discs exposed to 1 millimolar l-cysteine emitted only 2% as much as did the discs exposed to 10 millimolar l-cysteine. The emission from leaf discs and from detached leaves lasted for at least 5 and 15 hours, respectively. However, several hours after the maximal emission, injury of the leaves, manifested as chlorosis, was evident. H₂S emission was a specific consequence of exposure to l-cysteine; neither d-cysteine nor l-cystine elicited H₂S emission. Aminooxyacetic acid, an inhibitor of pyridoxal phosphate dependent enzymes, inhibited the emission. In a cell free system from cucumber leaves, H₂S formation and its release occurred in response to l-cysteine. Feeding experiments with [³⁵S]l-cysteine showed that most of the sulfur in H₂S was derived from sulfur in the l-cysteine supplied and that the H₂S emitted for 9 hours accounted for 7 to 10% of l-cysteine taken up. ³⁵S-labeled SO₃²⁻ and SO₄²⁻ were found in the tissue extract in addition to internal soluble S²⁻. These findings suggest the existence of a sulfur cycle which converts l-cysteine to SO₄²⁻ through cysteine desulfhydration.     

The biosynthesis of polysaccharides. Incorporation of d-[1-14C]glucose and d-[6-14C]glucose into plum-leaf polysaccharides

1. The utilization of specifically labelled d-glucose in the biosynthesis of plum-leaf polysaccharides has been studied. After these precursors had been metabolized in plum leaves, the polysaccharides were isolated from the leaves, and their monosaccharide constituents isolated and purified. 2. Both the specific activities and the distribution of ¹⁴C along the carbon chains of the monosaccharides were determined. Significant ¹⁴C activity was found in units of d-galactose, d-glucose, d-xylose and l-arabinose, but their specific activities varied widely. The labelling patterns suggest that in the leaves the other monosaccharides all arise directly from d-glucose without any skeletal change in the carbon chain, other than the loss of a terminal carbon atom in the synthesis of pentoses. 3. The results indicated that within the leaf there are various precursor pools for polysaccharide synthesis and that these pools are not in equilibrium with one another.     

Incorporation of l-[C]leucine into egg proteins by liver slices from cod

1. Liver slices from cod (Gadus morhua L.) were incubated with l-[¹⁴C]leucine and the incorporation of label into total protein, precipitated with trichloroacetic acid, and into egg proteins, precipitated with an antibody after addition of carrier egg proteins, was measured. 2. Liver slices from immature male or female cod, and from male fish with developing testes, did not incorporate significant amounts of l-[¹⁴C]leucine into egg proteins, whereas with slices from female cod with developing ovaries the rate of incorporation into egg proteins was 8% of the rate of incorporation into total protein. 3. Liver slices from immature male or female fish that had received an intramuscular injection of oestradiol benzoate (1mg/kg) 5–8 days previously incorporated l-[¹⁴C]leucine into egg proteins at about 26% of the rate of incorporation into total protein. 4. Incorporation into total protein and into egg proteins was inhibited by puromycin, and 1.2 and 0.13μg of puromycin/mg of tissue protein, respectively, gave 50% inhibition.     

Biosynthesis of (+)-Tartaric Acid from l-[4-C]Ascorbic Acid in Grape and Geranium

The metabolic fate of l-[4-¹⁴C]ascorbic acid has been examined in the grape (Vitis labrusca L.) and lemon geranium (Pelargonium crispum L. L'Hér. cv. Prince Rupert) under conditions comparable to data from l-[1-¹⁴C]ascorbic acid and l-[6-¹⁴C]ascorbic acid experiments. In detached grape leaves and immature berries, l-[4-¹⁴C]ascorbic acid and l-[1-¹⁴C]ascorbic acid were equivalent precursors to carboxyl labeled (+)-tartaric acid. In geranium apices, l-[4-¹⁴C]ascorbic acid yielded internal labeled (+)-tartaric acid while l-[6-¹⁴C]ascorbic acid gave an equivalent conversion to carboxyl labeled (+)-tartaric acid. These findings clearly show that two distinct processes for the synthesis of (+)-tartaric acid from l-ascorbic acid exist in plants identified as (+)-tartaric acid accumulators. In grape leaves and immature berries, (+)-tartaric acid synthesis proceeds via preservation of a four-carbon fragment derived from carbons 1 through 4 of l-ascorbic acid while carbons 3 through 6 yield (+)-tartaric acid in geranium apices.     

The effect of wilting on proline metabolism in excised bean leaves in the dark

The effects of wilting on the fate of proline and on the rates of nonprotein proline formation and utilization have been determined in excised bean leaves. Wilting did not alter the fate of exogenously added ¹⁴C-l-proline (2 mm) in either non-starved leaves (from plants previously in the light) or starved leaves (from plants previously in the dark). The fate of proline in nonstarved leaves was protein synthesis and in starved leaves was protein synthesis and oxidation to other compounds.     

Mechanism of plant growth stimulation by naphthenic Acid: effects on nitrogen metabolism of phaseolus vulgaris L

Fourteen-day-old Phaseolus vulgaris L. cv. Top Crop (bush bean) plants were sprayed with the plant growth stimulant, potassium naphthenate (20 mm). Seven days after treatment the contents of glutamic acid dehydrogenase, glutamic-oxaloacetic transaminase, nitrate reductase, glutamine synthetase, and cytochrome oxidase in the trifoliate leaf blades of treated plants were significantly larger, and the specific activity of the last four was significantly greater. Potassium nephthenate (1 μm) in the assay solutions did not significantly alter the activity of these enzymes in the cell-free extracts of untreated plants. Leaf discs from treated plants did not incorporate ¹⁴C-leucine into protein more actively. The protein content of leaves of treated plants was 15.3% greater, and the percentages of 16 individual amino acids in the hydrolysates of the proteins of control and treated plants showed numerous differences. The major changes were greater percentages of glutamic acid, glycine, and proline, and smaller values of arginine, lysine, tyrosine, and leucine in protein of treated plants. The content of ethanol-soluble (free) amino acids was greater by 7.5%. The principal changes in content of these acids were larger percentages of arginine and lysine, and smaller values for glutamic acid, serine, and proline in the leaves of potassium naphthenate-treated plants. The content of DNA, measured 1, 2, and 3 weeks after a foliar application of potassium naphthenate, was not significantly different from that of untreated plants, but the amount of RNA was significantly greater at all three times of measurement. The number and weight of green pods per plant 30 days after potassium naphthenate application were significantly larger, suggesting that the stimulative action of potassium naphthenate was in progress at the times of the assays. A mechanism, involving a genetic and a metabolic phase, is suggested for the stimulation of plant growth by naphthenate.     

Studies on Protein Synthesis by Senescing and Kinetin-treated Barley Leaves

Using sterile conditions, changes in total protein synthesis were followed. over an 8 day incubation period, in detached first seedling leaves of barley from 8 day old plants during senescence and after kinetin treatment. In senescing leaves, total ¹⁴C-alanine incorporation was enhanced by nearly 20% within 6 h of leaf detachment and by about 30 % after 24 h. Kinetin treatment stimulated protein synthesis even more, for total incorporation was promoted ca. 50 % after 6 h and by ca. 60 % after 24 h incubation. The leaf supernatant (30,000 ×g for 30 min) proteins were separated on DEAE-Sephadex (A-50) columns into approximately 14 fractions and changes in ¹⁴C labelling of these fractions were studied following leaf detachment and on incubation on water or kinetin for 6 days. In senescing leaves, ¹⁴C-incorporation into supernatant proteins was sustained, even as protein levels declined rapidly The varied stabilities of the different leaf proteins was suggested by the characteristically changing specific activities of the different protein fractions. Although kinetin greatly promoted incorporation into all protein fractions, no evidence was surmised of specific effects on individual leaf proteins. Studies of changes in total protein synthesis in attached senescing first seedling leaves taken from plants aged 7 to 27 days revealed a relatively small increase in ¹⁴C-incorporation. However, incorporation could be greatly increased in leaves up to 15 days old by detaching and preincubating such leaves for up to 2 days on water, prior to measurement. The promotion of ¹⁴C-incorporation into protcins follwing leaf excision could result from early changes in permeability and precursor pool size.     

Inhibition of protein synthesis in Krebs 2 ascites cells and cell-free systems by phenylalanine and its effect on leucine and lysine in the amino acid pool

1. The inhibition of incorporation of ¹⁴C-labelled amino acids into protein of whole cells by phenylalanine has been reproduced in a cell-free system. In both cases only the l-isomer was inhibitory. 2. The effect of phenylalanine on incorporation of [¹⁴C]leucine and [¹⁴C]lysine into protein was different in both whole cells and cell-free systems. 3. In whole cells inhibition of incorporation of leucine at 2·5μg./ml. was very rapid, but when the concentration was increased to 100μg./ml. the inhibition was not apparent for about 1hr. The kinetics of inhibition of lysine was the same at both these concentrations and was similar to that found with leucine at 100μg./ml. 4. Neither a lower specific radioactivity of the two amino acids in the pool nor a decrease in their pool size could be consistently related with inhibition of protein synthesis. 5. In the cell-free system l-phenylalanine inhibited the incorporation of leucine but not of lysine. 6. Charging of transfer RNA by leucine was markedly decreased in the presence of phenylalanine, whereas charging of transfer RNA by lysine was not.     

Engineering of Corynebacterium glutamicum for High-Yield l-Valine Production under Oxygen Deprivation Conditions

We previously demonstrated efficient l-valine production by metabolically engineered Corynebacterium glutamicum under oxygen deprivation. To achieve the high productivity, a NADH/NADPH cofactor imbalance during the synthesis of l-valine was overcome by engineering NAD-preferring mutant acetohydroxy acid isomeroreductase (AHAIR) and using NAD-specific leucine dehydrogenase from Lysinibacillus sphaericus. Lactate as a by-product was largely eliminated by disrupting the lactate dehydrogenase gene ldhA. Nonetheless, a few other by-products, particularly succinate, were still produced and acted to suppress the l-valine yield. Eliminating these by-products therefore was deemed key to improving the l-valine yield. By additionally disrupting the phosphoenolpyruvate carboxylase gene ppc, succinate production was effectively suppressed, but both glucose consumption and l-valine production dropped considerably due to the severely elevated intracellular NADH/NAD⁺ ratio. In contrast, this perturbed intracellular redox state was more than compensated for by deletion of three genes associated with NADH-producing acetate synthesis and overexpression of five glycolytic genes, including gapA, encoding NADH-inhibited glyceraldehyde-3-phosphate dehydrogenase. Inserting feedback-resistant mutant acetohydroxy acid synthase and NAD-preferring mutant AHAIR in the chromosome resulted in higher l-valine yield and productivity. Deleting the alanine transaminase gene avtA suppressed alanine production. The resultant strain produced 1,280 mM l-valine at a yield of 88% mol mol of glucose⁻¹ after 24 h under oxygen deprivation, a vastly improved yield over our previous best.     

Developmental changes in the potential for h(2)s emission in cucurbit plants

Based on results obtained with leaf discs exposed to sulfate, leaves on cucurbit plants (Cucurbita pepo L. cv Small Sugar Pumpkin and Cucumis sativus cv Chipper) 1 to 2.5 weeks old have a low potential for H₂S emission (less than 10 picomoles per min per cm² leaf area) in response to sulfate, whereas discs from most of the leaves on plants 3 to 4 weeks old emit H₂S at a higher rate (50 to 150 picomoles per min per cm² leaf area). This difference is determined by the age of the plant, and is independent of the leaves' age or developmental stage. In response to l-cysteine, however, discs from leaves on cucurbit plants 1 to 2.5 weeks old emit H₂S at higher rates (15 to 50 picomoles per min per cm² leaf area) than in response to sulfate. Furthermore, the potential for H₂S emission in response to l-cysteine decreases with increasing age of the individual leaf. Thus, most of the potential for H₂S emission in response to l-cysteine is developed during germination and the early growth of cucurbit plants, but most of the potential for H₂S emission in response to sulfate arises later in the development of the plants.     

Effects of Phenylmercuric Acetate on Stomatal Movement and Transpiration of Excised Retula papyrifera Marsh. Leaves

Effects of 10⁻³m, 10⁻⁴m, and 10⁻⁵m phenylmercuric acetate (PMA) on stomatal movement and transpiration of excised Betula papyrifera leaves were investigated. Duco cement leaf prints and transpiration decline curves were used for the analysis of stomatal condition. PMA induced stomatal closure and decreased transpiration. Stomata of leaves treated with any of the 3 PMA concentrations closed earlier and at a higher relative water content than did stomata of untreated leaves. As determined from transpiration decline curves, PMA at 10⁻³m caused an increase in apparent “cuticular” transpiration. However, the increase appeared to result largely from some PMA-poisoned stomata which remained open for prolonged periods. Considerable PMA toxicity was observed, with 10⁻³m and 10⁻⁴m concentrations causing browning of leaves. PMA treatment caused a decrease in chlorophyll content, even at a low PMA concentration (10⁻⁵m) which influenced stomatal response only slightly and did not cause evident browning of leaves. The time and degree of stomatal opening varied with stomatal size. Large stomata tended to open earlier and close later than small stomata. Hence, in Betula papyrifera stomata of various size classes were considered as physiologically different populations.     

Incorporation of methanol into pectic substance

Others have shown that l-methionine is utilized in the biosynthesis of methyl ester groups in pectic substance. Methanol, like l-methionine, is used for methyl ester biosynthesis by detached parsley leaves (Petroselinum crispum). When a combination of methanol-³H and methanol-¹⁴C is given to parsley leaves, methanol recovered from pectic substance by alkaline hydrolysis has a ³H/¹⁴C ratio about one-fourth that of the mixture administered. Unlike l-methionine, methanol is oxidized prior to its utilization as a carbon source for methyl ester biosynthesis.     

alpha-l-Arabinofuranosidase from Radish (Raphanus sativus L.) Seeds

An α-l-arabinofuranosidase has been purified 1043-fold from radish (Raphanus sativus L.) seeds. The purified enzyme was a homogeneous glycoprotein consisting of a single polypeptide with an apparent molecular weight of 64,000 and an isoelectric point value of 4.7, as evidenced by denaturing gel electrophoresis and reversed-phase or size-exclusion high-performance liquid chromatography and isoelectric focusing. The enzyme characteristically catalyzes the hydrolysis of p-nitrophenyl α-l-arabinofuranoside and p-nitrophenyl β-d-xylopyranoside in a constant ratio (3:1) of the initial velocities at pH 4.5, whereas the corresponding α-l-arabinopyranoside and β-d-xylofuranoside are unsusceptible. The following evidence was provided to support that a single enzyme with one catalytic site was responsible for the specificity: (a) high purity of the enzyme preparation, (b) an invariable ratio of the activities toward the two substrates throughout the purification steps, (c) a parallelism of the activities in activation with bovine serum albumin and in heat inactivation of the enzyme as well as in the inhibition with heavy metal ions and sugars such as Hg²⁺, Ag⁺, l-arabino-(1→4)-lactone, and d-xylose, and (d) results of the mixed substrate kinetic analysis using the two substrates. The enzyme was shown to split off α-l-arabinofuranosyl residues in sugar beet arabinan, soybean arabinan-4-galactan, and radish seed and leaf arabinogalactan proteins. Arabinose and xylose were released by the action of the enzyme on oat-spelt xylan. Synergistic action of α-l-arabinofuranosidase and β-d-galactosidase on radish seed arabinogalactan protein resulted in the extensive degradation of the carbohydrate moiety.     

Carbohydrates stimulate ethylene production in tobacco leaf discs : I. Interaction with auxin and the relation to auxin metabolism

Various naturally occurring carbohydrates, applied at a concentration range of 1 to 100 mm, stimulated ethylene production for several days in indoleacetic acid (IAA)-treated or untreated tobacco (Nicotiana tabacum L. cv `Xanthi') leaf discs. The lag period for this sugar-stimulated ethylene production was 8 to 12 hours after excision in the untreated leaf discs, but less than 2 hours in the IAA-treated ones. Among the tested carbohydrates, 12 were found to increase synergistically ethylene production, with d-galactose, sucrose, and lactose being the most active; mannitol and l-glucose had no effect. The extent and duration of the increased ethylene production was dependent upon the type of sugar applied, the tissue's age, and the existence of both exogenous IAA and sugar in the medium. Sucrose appeared to elicit a continuous IAA effect for 48 hours, as expressed by increased ethylene production, even when IAA was removed from the medium after a 4-hour pulse. Sucrose stimulated both the uptake and decarboxylation of [1-¹⁴C]IAA, as well as the hydrolysis of the esteric and amide IAA conjugates formed in the tissue after application of free IAA. This gradual hydrolysis was accompanied by a further accumulation of a third IAA metabolite. Moreover, synthetic indole-3-acetyl-l-alanine increased ethylene production mainly with sucrose, and this effect was accompanied by its increased decarboxylation and turnover pattern suggesting that release of free IAA was involved. An esteric IAA conjugate, tentatively identified by GC retention time was found to be the major component (84%) of the naturally occurring IAA conjugates in tobacco leaves. Accordingly the sucrose-stimulated ethylene production in tobacco leaves can be ascribed mainly to the sucrose-stimulated hydrolysis of the esteric IAA conjugate.     

Protein metabolism in senescing wheat leaves : determination of synthesis and degradation rates and their effects on protein loss

Wheat leaves (Triticum aestivum L.) at the moment of their maximum expansion were detached and put in darkness. Their protein, RNA and DNA contents, as well as their rates of protein synthesis and degradation, were measured at different times from 0 to 5 days after detachment. Rates of protein synthesis were measured by incorporation into proteins of large amounts of [³H]leucine. Fractional rates of protein degradation were estimated either from the difference between the rates of synthesis and the net protein change or by the disappearance of radioactivity from proteins previously labeled with [³H]leucine or [¹⁴C]proline.

Protein loss reached a value of 20% during the first 48 hours of the process. RNA loss paralleled that of protein, whereas DNA content proved to be almost constant during the first 3 days and decreased dramatically thereafter.

Measurements of protein synthesis and degradation indicate that, in spite of a slowdown in rate of protein synthesis, an increased rate of protein breakdown is mainly responsible for the observed rapid protein loss.

     

Arabinogalactan-Proteins from Primary and Mature Roots of Radish (Raphanus sativus L.)

Organ-specific variations in blood group H-like activity were observed in developing radish plants. A temporary increase in serological activity was found to occur in the roots at the earlier stages of development. Arabinogalactan-proteins (AGPs) were isolated from primary and mature roots, and investigated for changes in their physicochemical properties, structure, and serological activities. These root AGPs were composed mainly of l-arabinose and d-galactose but were distinguishable from each other in their contents of l-fucose as well as of protein and hydroxyproline. The structures of the carbohydrate moieties of the root AGPs were essentially similar to those of AGPs isolated from seeds and mature leaves in that they consisted of consecutive (1→3)-linked β-d-galactosyl backbone chains having side chains of (1→6)-linked β-d-galactosyl residues, to which α-l-arabinofuranosyl residues were attached in the outer regions. One prominent feature of the primary root AGPs was that they contained appreciable amounts of l-fucose, which was presumably responsible for expression of the serological activity. In their immunological reactions with rabbit anti-radish leaf AGP antibody, the root AGPs were shown to share common antigenic determinant(s) with those of seed and leaf AGPs.     

Light dependence of catalase synthesis and degradation in leaves and the influence of interfering stress conditions

The enzyme catalase (EC 1.11.1.6) is light sensitive and subject to a rapid turnover in light, similar to the D1 reaction center protein of photosystem II. After 3 h of preadaptation to darkness or to different light intensities (90 and 520 μmol m⁻² s⁻¹ photosynthetic photon flux density), sections of rye leaves (Secale cereale L.) were labeled for 4 h with l-[³⁵S]methionine. From leaf extracts, catalase was immunoprecipitated with an antiserum prepared against the purified enzyme from rye leaves. Both incorporation into catalase and degradation of the enzyme polypeptide during a subsequent 16-h chase period increased with light intensity. At a photon flux density of 520 μmol m⁻² s⁻¹, the apparent half-time of catalase in rye leaves was 3 to 4 h, whereas that of the D1 protein was much shorter, about 1.5 h. Exposure to stress conditions, such as 0.6 m NaCl or a heat-shock temperature of 40°C, greatly suppressed both total protein synthesis and incorporation of the label into catalase and into the D1 protein. Immunoblotting assays indicated that in light, but not in darkness, steady-state levels of catalase and of the D1 protein strongly declined during treatments with salt, heat shock, or translation inhibitors that block repair synthesis. Because of the common property of rapid photodegradation and the resulting dependence on continuous repair, declines in catalase as well as of the D1 protein represent specific and sensitive indicators for stress conditions that suppress the translational activities of leaves.