Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Indole-3-acetaldehyde oxidase of pea seedlings

Indole-3-acetaldehyde oxidase was partially purified from the epicotyl of Pisum satiyum seedlings by column chromatography using CM-Sephadex and Sephadex G-150. The enzyme was only active in the presence of molecular oxygen. The activity was maximal at pH 8.0, and the Km value for indole-3-acetaldehyde was 1.4 × 10⁻³ M . The enzyme was inhibited strongly by p -hydroxymercuribenzoate, cyanide and hydroxylamine, suggesting that it contains sulfhydryl group(s) and a metal component such as iron.     

Growth and osmoregulation in Pisum sativum subhooks as affected by gibberellic acid and cotyledon excision

Growth stimulation by gibberellic acid (GA) of the Alaska pea ( Pisum sativum L.) subhook was observed within 6 h after its application; the stimulation being larger in cuttings with cotyledons than in decotylized ones. The osmotic potential in the subhook increased as it grew, the rate of its increase being faster in cuttings without than in cuttings with cotyledons. GA had no effect on the change in the osmotic potential until 8 h after GA application, but afterwards it suppressed the increase in cuttings with cotyledons. This GA effect was not observed in decotylized cuttings. Changes in the osmotic potential were well correlated with changes in the concentration of soluble sugars, but not with changes in amino acids and K⁺, Soluble sugars accumulated in the subhook of cuttings with and without cotyledons in proportion to growth, irrespective of the presence or absence of GA. Cotyledon excision suppressed sugar accumulation, and GA promoted it in cuttings with cotyledons but not in decotylized ones. These results suggest that GA stimulates the translocation of sugars from the cotyledons to the subhook and, thereby, maintains the osmotic potential low, resulting in enhanced growth.     

Citric acid production by the diploid strains of Aspergillus niger obtained by protoplast fusion

Summary Diploid strains were obtained following protoplast fusion between two citric acid producers of Aspergillus niger, one for the solid culture and the other for the shaking culture. In the shaking culture, all the diploid strains exhibited lower productivities than one parental strain. However, in the solid culture, some diploid strains exhibited higher productivities than either parental strain; the best diploid strain produced 1.2 times as much citric acid as the parental strain in solid culture.     

Furosemide: a Specific Inhibitor of Pi Transport across the Plasma Membrane of Plant Cells

A search was made for inhibitors of P_i uptake that act directlyon the P_i transporter in the plasma membranes of Catharanthusroseus cells to inhibit P_i uptake without inhibition of protonpumping. Using standard electrodes, we monitored changes inpH and in the concentration of K⁺ ions, as well as the rateof P_i uptake, when an inhibitor to be tested was applied tothe cells in unbuffered medium. A9C (28 µM), a blockerof anion channels, inhibited P_i uptake but it also inhibitedthe proton pump. However, a structurally similar inhibitor,furosemide, inhibited P_i uptake without inhibiting proton pumping. It is suggested that the carboxylic group of these inhibitorsinteracts with the P_i-binding site (probably an amino group)of the P_i transporter in the plasma membrane and that the hydrophobicstructure of these inhibitors facilitates their accumulationin the plasma membrane. ³Present address: Department of Biology, Hitotsubashi University,2-1 Naka, Kunitachi, Tokyo, 186 Japan     

Inhibition of the synthesis of cell wall polysaccharides in oat coleoptile segments by jasmonic acid: Relevance to its growth inhibition

The inhibitory mode of action of jasmonic acid (JA) on the growth of etiolated oat (Avena sativa L. cv. Victory) coleoptile segments was studied in relation to the synthesis of cell wall polysaccharides using [¹⁴C]glucose. Exogenously applied JA significantly inhibited indoleacetic acid (IAA)-induced elongation of oat coleoptile segments and prevented the increase of the total amounts of cell wall polysaccharides in both the noncellulosic and cellulosic fractions during coleoptile growth. JA had no effect on neutral sugar compositions of hemicellulosic polysaccharides but substantially inhibited the IAA-stimulated incorporation of [¹⁴C]glucose into noncellulosic and cellulosic polysaccharides. JA-induced inhibition of growth was completely prevented by pretreating segments with 30 mm sucrose for 4 h before the addition of IAA. The endogenous levels of UDP-sugars, which are key intermediates for the synthesis of cell wall polysaccharides, were not reduced significantly by JA. Although these observations suggest that the inhibitory mode of action of JA associated with the growth of oat coleoptile segments is relevant to sugar metabolism during cell wall polysaccharide synthesis, the precise site of inhibition remains to be investigated.Abbreviations JA jasmonic acid - ABA abscisic acid - IAA indoleacetic acid - T ₀ minimum stress relaxation time - TFA trifluoroacetic acid - TCA trichloroacetic acid - HPLC high-performance liquid chromatography - EtOAc ethyl acetate - TLC thin-layer chromatography - JA-Me methyl jasmonate - GLC-SIM gas-liquid chromatography-selected ion monitoring     

Acidification and Alkalinization of Culture Medium by Catharanthus roseus cellsIs Anoxic Production of Lactate a Cause of Cytoplasmic Acidification?

Catharanthus roseus(L.) G. Don cells acidified Mura-shige-Skoogmedium rapidly. Upon transfer to fresh medium, the medium pH(initially5.3) dropped below 4 within 2 d. This acidificationwas reversed under hypoxic conditions. The cells induced a similaracidification in a simple medium consisting of CaCl₂, KCl, andglucose: medium pH dropped below 4 within 6 h. The acidificationwas accompanied by an influx of K⁺ at a H⁺(efflux)/K⁺ ratioof ca 0.6 as well as by an expansion of endogenous organic acidpool, in which malic and citric acids were the major components.Anoxia reversed all these processes: the direction of both K⁺and H⁺ fluxes reversed with a H⁺/K⁺ ratio of 1.70. Anoxia induceda cytoplasmic acidification from pH 7.6 (aerobic) to 7.4 asmeasured by 31P-NMR, accompanied by a rapid, long-lasting lactateaccumulation at expense of malic and citric acids. Evidencesuggested that accumulation of lactic acid was not a cause ofcytoplasmic acidification under anoxia, but a result of pH regulationby the biochemical pH-stat [Davies (1973) Symp. Soc. Exp. Biol.27: 513]. The anoxic acidification of the cytoplasm was ascribedto the influx of H⁺ from the medium. (Received April 18, 1997; Accepted July 8, 1997)     

Purification of Xyloglucan Hydrolase/Endotransferase from Cell Walls of Azuki Bean Epicotyls

An enzyme involved in the breakdown of xyloglucans was purifiedfrom an extract of cell walls of azuki bean epicotyls obtainedwith 1 M NaCl and purified by column chromatography on severaldifferent resins. The purified enzyme gave a single band ofa protein with a molecular mass of about 32 kDa after SDS-PAGE.The enzyme hydrolyzed the xyloglucans of high molecular massfrom azuki cell walls to yield fragments of about 50 kDa withoutproduction of any oligo- or monosaccharides. Moreover, the enzymehad hardly any effect on xyloglucans of less than 60 kDa. Theenzyme also hydrolyzed xyloglucans from tamarind, but it didnot react with cellulose derivatives. In the presence of pyridylamino-labeledxyloglucan oligosac-charides as acceptor substrates, the enzymecatalyzed the transfer of 50-kDa products to the oligosaccharides.The K_m value of the enzyme for xyloglucans of 540 kDa was similarin the presence and in the absence of xyloglucan oligosaccharidesas acceptors: 1.0 mg ml^–1. These results suggest thatthe enzyme was an endotransferase but had unusual acceptor specificity,preferring smaller acceptors such as water. (Received September 9, 1996; Accepted March 16, 1997)     

Osmotic Stress-Induced Growth Suppression of Dark-Grown Wheat (Triticum aestivum L.) Coleoptiles

Application of 60 mM polyethylene glycol (PEG) to dark-grownwheat (Triticum aestivum L.) roots substantially reduced growthof coleoptiles. However, when PEG was removed, the growth rateof these coleoptiles greatly increased. Cell walls of stressedcoleoptiles remained loosened as compared with those of unstressedones. The osmotic potential of the stressed coleoptiles decreasedto that of the 60 mM PEG solution. On the other hand, the extentof decrease in the osmotic potential of stressed roots was smallerthan that of stressed coleoptiles. The osmotic potential difference() between the cell sap and the incubation medium of stressedroots was substantially higher than that of unstressed ones.The amount of ink moved from roots, where it was applied, tothe apical region of coleoptiles was significantly reduced underosmotic stress conditions. When water was exogenously appliedto abraded coleoptiles, the growth of these stressed coleoptileswas greatly promoted. These results suggest that inhibitionof coleop-tile growth under osmotic stress conditions is notdirectly related to a decrease in cell wall extensibility orto loss of the capacity to maintain osmotic potential gradients,but is caused by the reduction of the water supply from theroots to the coleoptiles. (Received August 26, 1996; Accepted December 18, 1996)