Explore the vast range of titles available.

An adequate carbohydrate supply contributes to the survival of seeds under conditions of limited oxygen availability. The amount of soluble, readily fermentable carbohydrates in dry cereal seeds is usually very limited, with starch representing the main storage compound. Starch breakdown during the germination of cereal seeds is the result of the action of hydrolytic enzymes and only through the concerted action of alpha-amylase (EC 3.2.1.1), beta-amylase (EC 3.2.1.2), debranching enzyme (EC 3.2.1.41), and alpha-glucosidase (EC 3.2.1.20) can starch be hydrolyzed completely. We present here data concerning the complete set of starch-degrading enzymes in three cereals, rice (Oryza sativa L.), which is tolerant to anaerobiosis, and wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), which are unable to germinate under anoxia. Among the cereal seeds tested under anoxia, only rice is able to degrade nonboiled, soluble starch, reflecting the ability to degrade the starch granules in vivo. This is explained by the presence of the complete set of enzymes needed to degrade starch completely either as the result of de novo synthesis (alpha-amylase, beta-amylase) or activation of preexisting, inactive forms of the enzyme (debranching enzyme, alpha-glucosidase). These enzymes are either absent or inactive in wheat and barley seeds kept under anaerobic conditions

A new starch-degrading enzyme activity is induced by storage of potato (Solanum tuberosum L.) tubers at low temperatures (L. Hill, R. Reimholz, R. Schroder, T.H. Nielsen, M. Stitt [1996] Plant Cell Environ 14: 1223-1237). The cold-induced activity was separated from other amylolytic activities in zymograms based on iodine staining of polyacrylamide gels containing amylopectin. A similar band of activity was detected at normal growth temperatures in leaves, stems, and growing tubers but was present only at low activity in warm-stored tubers. The cold-induced enzyme was separated by ion-exchange chromatography from other amylolytic activities. It has a broad neutral pH optimum. Characterization of its hydrolytic activity with different substrates showed that the cold-induced activity is a beta-amylase present at low activity in tubers stored at 20 degrees C and induced progressively when temperatures are decreased to 5 and 3 degrees C. The first clear induction of beta-amylase activity was observed within 3 d of storage at 3 degrees C, and the activity increased 4- to 5-fold within 10 d. The possible involvement of the cold-induced beta-amylase in sugar accumulation during cold storage is discussed

Metal ions play vital roles in enzymes. They may also show sensitivity to various sulfhydryl reagents and chelating reagents. Effect of some metal ions, EDTA and sulfhydryl reagents on the activity of partially purified β-amylase of radish root were studied. Amylolytic activity of purified enzyme was increased substantially in the presence of Ca2+, Mg2+, and Zn2+. Some other divalent cations Cu2+, Pb2+, Sn2+, and Hg2+ almost completely ceased the enzyme activity. Cobalt (II), Manganese (II), and Iron (III) exhibited moderate activating effects on the activity. Of the monovalent cations, Na+ and Ag+ reduced the β-amylase activity, while K+ increased. The chelating agent EDTA was found to be effective in the enzyme. Sulfhydryl reagents, Iodoacetic acid and N-Ethylmaleimide showed marginal inhibitory effect, but p-hydroxymercuribenzoic acid (PCMB) almost completely stopped the enzyme activity. The addition of thiol compounds such as cysteine could reverse the inhibitory effect of heavy metals and PCMB. The results indicate that sulfhydryl groups of radish root β-amylase were essential for the activity although it is not clear whether the sulfhydryl groups were directly involved in catalysis.

Genes coding for two major proteins of the tuberous root of sweet potato (Ipomoea batatas), namely, sporamin and beta-amylase, are inducible in leaves and petioles when they are supplied with high concentrations of sucrose or other metabolizable sugars, such as glucose and fructose, and the accumulation of a large amount of starch accompanies this induction. Three inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A), namely, okadaic acid, microcystin-LR, and calyculin A, strongly inhibited the sucrose-inducible accumulation of mRNAs for sporamin, beta-amylase, and the small subunit of ADP-glucose pyrophosphorylase in petioles. However, these inhibitors did not have any major effect on the steady-state levels of mRNAs for catalase and glyceraldehyde-3-phosphate dehydrogenase, and the sucrose-inducible increase in the level of sucrose synthase mRNA was enhanced by okadaic acid. Inhibitors of PP1 and PP2A also inhibited sucrose-inducible expression of a fusion gene, consisting of the promoter of the sweet potato gene for beta-amylase and the coding sequence for beta-glucuronidase (GUS), in leaves of transgenic tobacco (Nicotiana tabacum). The inhibition was not due to inhibition of uptake and cleavage of sucrose, since okadaic acid also inhibited induction of the fusion gene by glucose or fructose. Addition of okadaic acid to leaves that had been treated with sucrose for 6 h inhibited further increases in GUS activity. These results suggest that the continuous dephosphorylation of proteins is required in the transduction of carbohydrate metabolic signals to the transcriptional activation of at least some sugar-inducible genes in plant

The sugar-inducible expression of genes for sporamin and beta-amylase in leaf explants of sweet potato (Ipomoea batatas) and that of a beta-glucuronidase-fusion gene, with the promoter of the gene for beta-amylase in leaves of tobacco (Nicotiana tabacum), requires Ca2+ signaling (M. Ohto, K. Hayashi, M. Isobe, K. Nakamura [1995] Plant J 7: 297-307), and it was inhibited by staurosporin and K252a, inhibitors of protein kinases. Autophosphorylation activities of several potential protein kinases in leaves of tobacco were significantly higher in younger leaves than in mature leaves. However, the autophosphorylation activities of these proteins in mature leaves, especially those of the major autophosphorylatable proteins with apparent molecular masses of 56 and 54 kD, increased upon treatment of leaf discs with a 0.3 M solution of sucrose, glucose, or fructose, did not increase with sorbitol or mannitol treatments, and the increase by sucrose was inhibited by cycloheximide. Autophosphorylation of the 56- and 54-kD protein in vitro was dependent on Ca2+ and inhibited by staurosporine, K-252a, and by W-7. These results suggest that they belong to the family of calcium-dependent protein kinases. They were concentrated in the plasma membrane fraction and were released from membrane vesicles by high salt or with sodium carbonate. The possible functions of these sugar-inducible calcium-dependent protein kinases associated with the plasma membrane are discussed

Expression of a beta-amylase gene of Arabidopsis thaliana (AT beta-Amy) is regulated by sugars. We identified a mutant, hba1, in which the level of expression of AT beta-Amy in leaves of plants that had been grown in a medium with 2% sucrose was significantly higher than that in wild-type plants. Higher than wild-type levels of beta-amylase in hba1 plants depended on the presence of 1 to 2% sucrose or 1% glucose in the medium, whereas leaves of mutant plants grown with higher levels of sugars had beta-amylase activities similar to those in leaves of wild-type plants. The hba1 phenotype was recessive and did not affect levels of sugars and starch in leaves. It is proposed that expression of AT beta-Amy is regulated by a combination of both positive and negative factors, dependent on the level of sugars, and that HBA1 might function to maintain low-level expression of AT beta-Amy until the level of sugars reaches some high level. Results of crosses of hba1 plants with transgenic plants that harbored an AT beta-Amy:GUS transgene with 1587 bp of the 5'-upstream region suggested that HBA1 affects expression of AT beta-Amy in trans. The hba1 plants also had growth defects and elevated levels of anthocyanin in their petioles. However, sugar-regulated changes in levels of several mRNAs other than beta-amylase mRNA were unaffected in hba1 plants, suggesting that only a subset of sugar-regulated genes is under the control of HBA1

beta-Amylase of sweet potato (Ipomoea batatas L.), which constitutes about 5% of the total soluble protein of the tuberous root, is absent or is present in only small amounts in organs other than the tuberous roots of the normal, field-grown plants. However, when leaf-petiole cuttings from such plants were supplied with a solution that contained sucrose, the accumulation of 6-amylase was induced in both leaf and petiole portions of the explants. The sucrose-induced accumulation of 6-amylase in leaf-petiole cuttings occurred concomitant with the accumulation of starch and of sporamin, the most abundant storage protein of the tuberous root The accumulation of beta-amylase, of sporamin and of starch in the petioles showed similar dependence on the concentration of sucrose, and a 6% solution of sucrose gave the highest levels of induction when assayed after 7 days of treatment. The induction of mRNAs for beta-amylase and sporamin in the petiole could be detected after 6 hours of treatment with sucrose, and the accumulation of beta-amylase and sporamin polypeptides, as well as that of starch, continued for a further 3 weeks. In addition to sucrose, glucose or fructose, but not mannitol or sorbitol, also induced the accumulation of beta-amylase and sporamin, suggesting that metabolic effects of sucrose are important in the mechanism of this induction. Treatment of leaf-petiole cuttings with water under continuous light, but not in darkness, also caused the accumulation of small amounts of these components in the petioles, probably as a result of the endogenous supply of sucrose by photosynthesis. These results suggest that the expression of the gene for beta-amylase is under metabolic control which is coupled with the expression of sink function of cells in the sweet potato

Sporamin and beta-amylase are two major proteins of tuberous storage root of sweet potato (Ipomoea batatas) and their accumulation can be induced concomitantly with the accumulation of starch in leaves and petioles by sucrose (K. Nakamura, M. Ohto, N. Yoshida, K. Nakamura [1991] Plant Physiol 96: 902-909). Although mechanical wounding of leaves of sweet potato only occasionally induced the expression of sporamin and beta-amylase genes, their expression could be reproducibly induced in leaf-petiole cuttings when these explants were dipped in a solution of polygalacturonic acid or chitosan at their cut edges. Polygalacturonic acid seemed to induce expression of the same genes coding for sporamin and beta-amylase that are induced by sucrose. Because polygalacturonic acid and chitosan are known to mediate the induction of wound-inducible defense reactions, these results raise an interesting possibility that beta-amylase, in addition to sporamin, may have some role in the defense reaction. Expression of sporamin and beta-amylase genes could also be induced by abscisic acid, and this induction by abscisic acid, as well as induction by polygalacturonic acid or sucrose, was repressed by gibberellic acid. By contrast, methyl jasmonate did not cause the significant induction of either sporamin or beta-amylase mRNAs. Induction of expression of sporamin and beta-amylase genes by polygalacturonic acid or sucrose was inhibited by cycloheximide, suggesting that de novo synthesis of proteins is required for both of the induction processes

A maize (Zea mays L.) cDNA clone (pZMB2) encoding beta-amylase was isolated from a cDNA library prepared from the aleurone RNA of germinating kernels. The cDNA encodes a predicted product of 488 amino acids with significant similarity to known beta-amylases from barley (Hordeum vulgare), rye (Secale cereale), and rice (Oryza sativa). Glycine-rich repeats found in the carboxyl terminus the endosperm-specific beta-amylase of barley and rye are absent from the maize gene product. The N-terminal sequence of the first 20 amino acids of a beta-amylase peptide derived from purified protein is identical to the 5th through 24th amino acids of the predicted cDNA product, indicating the absence of a conventional signal peptide in the maize protein. Recombinant inbred mapping data indicate that the cDNA clone is single-copy gene that maps to chromosome 7L at position 83 centimorgans. Northern blot analysis and in vitro translation-immunoprecipitation data indicate that the maize beta-amylase is synthesized de novo in the aleurone cells but not in the scutellum during seed germination

To investigate how beta -amylase expression is regulated in A. thaliana , we isolated five cDNA clones from a lambda -Zap expression library using immunoaffinity-purified anti- beta -amylase IgG. Restriction mapping and partial sequence analysis indicated that the coding regions of all five cDNAs were identical. Here, we present the sequence of the longest complete cDNA which contains 2469 bases and a single open reading frame encoding a protein of 498 amino acids. The mol wt of the protein from the deduced amino acid sequence is 56,069.