Chloroplast Biogenesis 31: DETECTION OF AN INHIBITOR OF PROTOCHLOROPHYLL BIOSYNTHESIS IN CUCUMBER COTYLEDONS

An extract from cucumber cotyledons was shown to cause an inhibition of protochlorophyll biosynthesis and accumulation. The extract inhibited the net synthesis of protochlorophyll as well as the incorporation of δ-amino[¹⁴C]levulinic acid into protochlorophyllide and protochlorophyllide ester by excised cotyledons. The inhibition of δ-amino[¹⁴C]levulinic acid incorporation into the two protochlorophyll species was also observed in isolated etiochloroplasts before and after lysis of the plastids. The inhibition did not appear to involve the oxidation of the δ-aminolevulinic substrate or its translocation across the plastid membrane. Kinetic analysis of the rate of protochlorophyllide and protochlorophyllide ester biosynthesis in the presence and absence of the inhibitor suggested that the mode of inhibition of the two protochlorophyll species was different.     

THE PRODUCTION OF HORMONES IN HIGHER PLANTS

• 1 Although much is known about the effects of plant hormones and their role in the control of growth and differentiation, little is known about the way in which hormone production is itself controlled or about the cellular sites of hormone synthesis. The literature on hormone production is discussed in this review in an attempt to shed some light on these problems.
• 2 The natural auxin of plants, indol-3yl-acetic acid (IAA) is produced by a wide variety of living organisms. In animals, fungi and bacteria it is formed as a minor by-product of tryptophan degradation. The pathways of its production involve either the transamination or the decarboxylation of tryptophan. The transaminase route is the more important.
• 3 In higher plants auxin is also produced as a minor breakdown product of trypto phan, largely via transamination. In some species decarboxylation may occur but is of minor importance. Tryptophan can also be degraded by spontaneous reaction with oxidation products of certain phenols.
• 4 The unspecific nature of the enzymes involved in IAA production and the probable importance of spontaneous, nonenzymic reactions in the degradation of tryp to phan make it unlikely that auxin production from tryptophan can be regulated with any precision at the enzymic level. The limiting factor for auxin production is the availability of tryptophan, which in most cells is present in insufficient quantities for its degradation to occur to a significant extent. Tryptophan levels are, however, considerably elevated in cells in which net protein breakdown is taking place as a result of autolysis.
• 5 An indole compound, glucobrassicin, occurs in Brassica and a number of other genera. It breaks down readily to form a variety of products including indole aceto-nitrile, which can give rise to IAA. There is, however, no evidence to indicate that glucobrassicin is a precursor of auxin in vivo.
• 6 Conjugates of IAA, e.g. IAA-aspartic acid and IAA-glucose, are formed when IAA is supplied in unphysiologically high amounts to plant tissues. These and other IAA conjugates occur naturally in developing seeds and fruits. There is no persuasive evidence for the natural occurrence of IAA-protein complexes.
• 7 Tissues autolysing during prolonged extraction with ether produce IAA from tryptophan released by proteolysis. IAA is produced in considerable quantities by autolysing tissues in vitro.
• 8 During the senescence of leaves proteolysis results in elevated levels of trypto phan. Large amounts of auxin are produced by senescent leaves.
• 9 Coleoptile tips have a vicarious auxin economy which depends on a supply of IAA, IAA esters and other compounds closely related to IAA from the seed. These move acropetally in the xylem and accumulate at the coleoptile tip. The production of auxin in coleoptile tips involves the hydrolysis of IAA esters and the conversion of labile, as yet unidentified compounds, to IAA. There is no evidence for the de novo synthesis of IAA in coleoptiles.
• 10 Practically all the other sites of auxin production are sites of both meristematic activity and cell death. The production of auxin in developing anthers and fertilized ovaries takes place in the regressing nutritive tissues (tapetum, nucellus, endosperm) as the cells break down. In shoot tips, developing leaves, secondarily thickening stems, roots and developing fruits auxin is produced as a consequence of vascular differ entiation; the differentiation of xylem cells and most fibres involves a complete auto-lysis of the cell contents; the differentiation of sieve tubes involves a partial autolysis. There is no evidence that meristematic cells produce auxin.
• 11 The lysis and digestion of cells infected with fungi and bacteria results in elevated tryptophan levels and the production of auxin. Viral infections reduce the levels of tryptophan and are asSociated with reduced levels of auxin.
• 12 Crown-gall tissues produce auxin. It is suggested that the crown-gall disease may involve at any given time the death of a minority of the cells which produce auxin and other hormones as they autolyse; the other cells grow and divide in response to these hormones.
• 13 Auxin is produced in soils, particularly those rich in decaying organic matter, by micro-organisms. This environmental auxin may be important for the growth of roots.
• 14 There is no convincing evidence that auxin is a hormone in non-vascular plants. The induction of rhizoids in liverworts by low concentrations of auxin can be ex plained as a response to environmental auxin.
• 15 Abscisic acid is synthesized from mevalonic acid in living cells. It is possible that under certain circumstances, abscisic acid or closely related compounds are formed by the oxidation of carotenoids.
• 16 The sites of gibberellin production are sites of cell death. It is possible that precursors of gibberellins, such as kaurene, are oxidized to gibberellins when cells die.
• 17 Cytokinins are present in transfer-RNA (tRNA) of animals, fungi, bacteria and higher plants. They are probably formed in plants by the hydrolysis of tRNA in autolysing cells. There is evidence that they are also formed in living cells in root tips.
• 18 Ethylene is produced in senescent, dying or damaged cells by the breakdown of methionine.
• 19 It was shown many years ago that wounded and damaged cells produced sub stances which stimulate cell division. It now seems likely that the production of wound hormones and the normal production of hormones as a consequence of cell death are two aspects of the same phenomenon. Wounded cells can produce auxin, gibberellins, cytokinins and ethylene.
• 20 The control of hormone production in living cells is a biochemical problem which remains unsolved. The control of production of hormones formed as a con sequence of cell death depends on the control of cell death itself. Cell death is con trolled by hormones which are themselves produced as a consequence of cell death.
• 21 In spite of the fact that dying cells are present in all vascular plants, in all wounded and infected tissues, in certain differentiating tissues in animals, in cancerous tumours and in developing animal embryos, the biochemistry of cell death is a subject which has been almost completely ignored. Dying cells are an important source of hormones in plants; some of the many substances released by dying cells may also be of physiological significance in animals.

     

SILICON IN THE ROOTS OF HIGHER PLANTS

Silicon (Si) distribution in the roots of Sorghastrum nutans (L.) Nash and Sorghum bicolor (L.) Moench. was investigated by means of the electron-probe microanalyzer and scanning electron microscope. In both species, Si was confined to the inner tangential wall of the tertiary-phase endodermal cells in the form of nodular silica aggregates of similar morphology and X-ray intensity. The results are compared to those for six closely related genera, as well as to studies of Si in the roots of species of other tribes of the family Poaceae. The various types of root deposits occurring in the family are described, and their relationships discussed. It is concluded that the type of Si distribution exhibited is determined largely by the phylogenetic status of the genus, rather than by the basic pattern of root anatomy.     

NUCLEAR DNA AND CYTOPLASMIC DNA FROM TISSUES OF HIGHER PLANTS

Young wheat roots were labeled with ³²P-inorganic phosphate. Following the labeling period, roots were homogenized in a sucrose medium and fractionated into nuclei, cytoplasmic particles (including proplastids and mitochondria), and a soluble fraction containing most of the microsomes. DNA prepared from the particles had a higher buoyant density than that from the nuclei and showed a marked loss in total label if the roots were exposed to non-radioactive medium for 48 hours prior to fractionation of the cells.     

FLAVOPROTEINS IN THE LEAVES OF HIGHER PLANTS CATALYZING THE OXIDATION OF L-GLUTAMATE

1. Two forms of enzyme capable of catalyzing the oxidation of L-glutamate(and L-aspartate) were isolated from the leaves of spinach andseparated from each other by column-chromatographic purificationon calcium phosphate and anion exchangers. They were distinguishedas GD₁ (L-glutamate dehydrogenase 1) and GD₂ (L-glutamate dehydrogenase2). The purification procedures and some fundamental propertiesof the partially purified enzymes were investigated.
2. It wasdiscovered that the enzymes did not require any cofactor,ie., neither dialysis nor precipitation with ammonium sulfatecaused a fall in enzyme activities and the addition of DPN andTPN to the reaction mixture did not accelerate the reactionrate
3. From the results of spectroscopic investigation GD₁ andGD₂were shown to be flavoproteins, although their prostheticgrouphas not yet been identified The activity of GD₁ was enhancedby the addition of FAD or FMN, while GD₂ was not acceleratedby these factors.
4. The characteristics of the two enzymes includingsubstrate specificity,MICHAELIS constant, optimum pH of thereaction and specificityfor electron acceptors were compared.
5. From the stoichiometric study of the oxidation of L-glutamatewith these enzymes, it was confirmed that the reaction is representedby the following equation: L-glutamate+oxidized dye+h₂o
6. Among various inhibitors tested,molecular oxygen which couldfunction as electron acceptor ofL-glutamate oxidation in thepresence of GD₁ was found to causea strong inhibition uponthe same reaction with TTC as el acceptor.The inhibition wasconfirmed to be due to hydrogen peroxideproduced as a resultof the aerobic oxidation of L-glutamate.

(Received July 25, 1962; )     

METABOLISM OF MANNITOL IN HIGHER PLANTS

To determine to what extent higher plants can metabolize mannitol-C¹⁴, it was introduced into 26 species belonging to 17 families. Fifteen species respired mannitol as measured by the evolution of C¹⁴O₂. In several species, including Fraxinus americana and Syringa vulgaris, mannitol was respired at rates comparable to those of fructose and glucose. In others, including Avena sativa, mannitol was respired only slightly. A lag period in the production of C¹⁴O₂ from mannitol-C¹⁴, which does not occur after offering glucose-C¹⁴ or fructose-C¹⁴, was considered to be due to a slow penetration of mannitol to the site of its metabolism. The first step in the dissimilation of mannitol was shown to be its oxidation to fructose, possibly via phosphorylated intermediates. Mannitol was not found to be a constituent of polysaccharides of Syringa vulgaris.     

高等植物性别分化研究的某些进展

高等植物性别分化研究的某些进展邵宏波（四平师范学院生物工程研究室吉林四平１３６０００）关键词高等植物,性别分化,基因表达ＳＯＭＥＡＤＶＡＮＣＥＳＩＮＴＨＥＳＥＸＵＡＬＤＩＦＦＥＲＥＮＴＩＡＴＩＯＮＲＥＳＥＡＲＣＨＯＦＨＩＧＨＥＲＰＬＡＮＴＳ￥Ｓｈａｏ...     

植物小孢子母细胞胞间细胞融合通道的超显微结构研究

采用常规的透射电子显微镜术研究了百合和黑麦花药小孢子母细胞减数第一次分裂前期孢间细胞融合道（ＩＣＣ）的精细结构。首次发现ＩＣＣｓ内部除包含早期研究所看到的染色质,或核糖体、质体、线粒体等细胞器外,其中还存在有一些形态学特征与微丝和微管类似的细胞骨架系统。该系统穿过ＩＣＣｓ,从一个细胞延伸到相邻细胞的细胞质内,表面与周围的染色质颗粒、核糖体、质体等细胞器相连,方向与细胞质或核物质细胞间运动的方向一致