Ionic Regulation for Cytokinin-dependent Betacyanin Synthesis in Amaranthus Seedlings

Potassium ions at low concentrations stimulate cytokinin-dependent betacyanin synthesis in Amaranthus tricolor seedlings more than other alkali metal ions when tested as the chloride salts. The sequence of relative stimulation is K⁺ > Rb⁺ > (Na⁺ = Li⁺). Calcium and Mg²⁺ ions are inhibitory at concentrations > 1 millimolar when tested as chlorides. Anions also have an effect on the degree of alkali metal stimulation in the order PO₄³⁻ > NO₃⁻ > Cl⁻. The high activity of phosphate may be partly due to its chelating effect on inhibitory Ca²⁺ ions, or to effects on K⁺ uptake. A mixture of Na⁺ and K⁺ in the presence of phosphate is more effective than either cation alone. This result may be due either to effects on tyrosine transport or on the potassium uptake system. Phytochrome-dependent betacyanin synthesis shows the same stimulation by Na⁺ plus K⁺. The effect of a number of inhibitors of transport systems on betacyanin accumulation is reported. The possible role of the ionic environment of cells in their metabolic regulation is discussed, particularly in relation to cytokinin action.     

Effects of glycine hydroxamate, carbon dioxide, and oxygen on photorespiratory carbon and nitrogen metabolism in spinach mesophyll cells

The effects of added glycine hydroxamate on the photosynthetic incorporation of ¹⁴CO₂ into metabolites by isolated mesophyll cells of spinach (Spinacia oleracea L.) was investigated under conditions favorable to photorespiratory (PR) metabolism (0.04% CO₂ and 20% O₂) and under conditions leading to nonphotorespiratory (NPR) metabolism (0.2% CO₂ and 2.7% O₂). Glycine hydroxamate (GH) is a competitive inhibitor of the photorespiratory conversion of glycine to serine, CO₂ and NH₄⁺. During PR fixation, addition of the inhibitor increased glycine and decreased glutamine labeling. In contrast, labeling of glycine decreased under NPR conditions. This suggests that when the rate of glycolate synthesis is slow, the primary route of glycine synthesis is through serine rather than from glycolate. GH addition increased serine labeling under PR conditions but not under NPR conditions. This increase in serine labeling at a time when glycine to serine conversion is partially blocked by the inhibitor may be due to serine accumulation via the “reverse” flow of photorespiration from 3-P-glycerate to hydroxypyruvate when glycine levels are high. GH increased glyoxylate and decreased glycolate labeling. These observations are discussed with respect to possible glyoxylate feedback inhibition of photorespiration.     

Rapidly Induced Wound Ethylene from Excised Segments of Etiolated Pisum sativum L., cv. Alaska: II. Oxygen and Temperature Dependency

Wound-induced ethylene synthesis by subapical stem sections of etiolated Pisum sativum L., cv. Alaska seedlings, as described by Saltveit and Dilley (Plant Physiol 1978 61: 447-450), was half-saturated at 3.6% (v/v) O₂ and saturated at about 10% O₂. Corresponding values for CO₂ production during the same period were 1.1% and 10% O₂, respectively. Anaerobiosis stopped all ethylene evolution and delayed the characteristic pattern of wound ethylene synthesis. Exposing tissue to 3.5% CO₂ in air in a flow-through system reduced wound ethylene synthesis by 30%. Enhancing gas diffusivity by reducing the total pressure to 130 mm Hg almost doubled the rate of wound ethylene synthesis and this effect was negated by exposure to 250 μl liter⁻¹ propylene. Applied ethylene or propylene stopped wound ethylene synthesis during the period of application, but unlike N₂, no lag period was observed upon flushing with air. It is concluded that the characteristic pattern of wound-induced ethylene synthesis resulted from negative feedback control by endogenous ethylene.

No wound ethylene was produced for 2 hours after excision at 10 or 38 C. Low temperatures prolonged the lag period, but did not prevent induction of the wound response, since tissue held for 2 hours at 10 C produced wound ethylene immediately when warmed to 30 C. In contrast, temperatures above 36 C prevented induction of wound ethylene synthesis, since tissue cooled to 30 C after 1 hour at 40 C required 2 hours before ethylene production returned to normal levels. The activation energy between 15 and 36 C was 12.1 mole kilocalories degree⁻¹.

     

Enhanced inflorescence development in bougainvillea "san diego red" by removal of young leaves and cytokinin treatments

Removal of young leaves and application of the cytokinin, N-benzyla-α-(tetrahydro-2H-pyran-2yl)-adenine promote inflorescence development in Bougainvillea “San Diego Red”. Defoliation greatly increased the amount of assimilate accumulated at the shoot tip 1 to 2 days after treatment. Cytokinin applications further increased the amount accumulated and this increase was apparent 4 days before morphological changes could be detected at the inflorescence axes. Short days promoted inflorescence development and also increased assimilate accumulation at the reproductive axes; thus, it is suggested that the role of short day induction in bougainvillea may be that of redirecting the flow of assimilates, perhaps by its influence on cytokinin synthesis and distribution.     

Prion Switching in Response to Environmental Stress

Evolution depends on the manner in which genetic variation is translated into new phenotypes. There has been much debate about whether organisms might have specific mechanisms for “evolvability,” which would generate heritable phenotypic variation with adaptive value and could act to enhance the rate of evolution. Capacitor systems, which allow the accumulation of cryptic genetic variation and release it under stressful conditions, might provide such a mechanism. In yeast, the prion [PSI⁺] exposes a large array of previously hidden genetic variation, and the phenotypes it thereby produces are advantageous roughly 25% of the time. The notion that [PSI⁺] is a mechanism for evolvability would be strengthened if the frequency of its appearance increased with stress. That is, a system that mediates even the haphazard appearance of new phenotypes, which have a reasonable chance of adaptive value would be beneficial if it were deployed at times when the organism is not well adapted to its environment. In an unbiased, high-throughput, genome-wide screen for factors that modify the frequency of [PSI⁺] induction, signal transducers and stress response genes were particularly prominent. Furthermore, prion induction increased by as much as 60-fold when cells were exposed to various stressful conditions, such as oxidative stress (H₂O₂) or high salt concentrations. The severity of stress and the frequency of [PSI⁺] induction were highly correlated. These findings support the hypothesis that [PSI⁺] is a mechanism to increase survival in fluctuating environments and might function as a capacitor to promote evolvability.     

Replacement of light by dibutyryl-CAMP and CAMP in betacyanin synthesis

The effect of adenosine 3′,5′-cyclic monophosphate and its N⁶,O^2′- dibutyryl derivative (Bu₂-CAMP) on betacyanin formation in etiolated Amaranthus paniculatus seedlings was investigated. Both substances can replace the action of light in the synthesis of these pigments, the formation of which is controlled by phytochrome. The specificity of this mimicry is underlined by the observations that sodium butyrate does not promote any betacyanin formation and that theophylline enhances the effect of Bu₂-AMP. Puromycin inhibits the induction of betacyanin synthesis by Bu₂-CAMP just as it does the light-induced pigment formation. These findings suggest that phytochrome exerts its controlling role in the synthesis of betacyanins through the agency of CAMP.     

Dietary cardenolides enhance growth and change the direction of the fecundity‐longevity trade‐off in milkweed bugs (Heteroptera: Lygaeinae)

Sequestration, that is, the accumulation of plant toxins into body tissues for defense, was predicted to incur physiological costs and may require resistance traits different from those of non‐sequestering insects. Alternatively, sequestering species could experience a cost in the absence of toxins due to selection on physiological homeostasis under permanent exposure of sequestered toxins in body tissues. Milkweed bugs (Heteroptera: Lygaeinae) sequester high amounts of plant‐derived cardenolides. Although being potent inhibitors of the ubiquitous animal enzyme Na⁺/K⁺‐ATPase, milkweed bugs can tolerate cardenolides by means of resistant Na⁺/K⁺‐ATPases. Both adaptations, resistance and sequestration, are ancestral traits of the Lygaeinae. Using four milkweed bug species (Heteroptera: Lygaeidae: Lygaeinae) and the related European firebug (Heteroptera: Pyrrhocoridae: Pyrrhocoris apterus) showing different combinations of the traits “cardenolide resistance” and “cardenolide sequestration,” we tested how the two traits affect larval growth upon exposure to dietary cardenolides in an artificial diet system. While cardenolides impaired the growth of P. apterus nymphs neither possessing a resistant Na⁺/K⁺‐ATPase nor sequestering cardenolides, growth was not affected in the non‐sequestering milkweed bug Arocatus longiceps, which possesses a resistant Na⁺/K⁺‐ATPase. Remarkably, cardenolides increased growth in the sequestering dietary specialists Caenocoris nerii and Oncopeltus fasciatus but not in the sequestering dietary generalist Spilostethus pandurus, which all possess a resistant Na⁺/K⁺‐ATPase. We furthermore assessed the effect of dietary cardenolides on additional life history parameters, including developmental speed, longevity of adults, and reproductive success in O. fasciatus. Unexpectedly, nymphs under cardenolide exposure developed substantially faster and lived longer as adults. However, fecundity of adults was reduced when maintained on cardenolide‐containing diet for their entire lifetime but not when adults were transferred to non‐toxic sunflower seeds. We speculate that the resistant Na⁺/K⁺‐ATPase of milkweed bugs is selected for working optimally in a “toxic environment,” that is, when sequestered cardenolides are stored in the body.     

The Late Endosomal ClC-6 Mediates Proton/Chloride Countertransport in Heterologous Plasma Membrane Expression

Members of the CLC protein family of Cl⁻ channels and transporters display the remarkable ability to function as either chloride channels or Cl⁻/H⁺ antiporters. Due to the intracellular localization of ClC-6 and ClC-7, it has not yet been possible to study the biophysical properties of these members of the late endosomal/lysosomal CLC branch in heterologous expression. Whereas recent data suggest that ClC-7 functions as an antiporter, transport characteristics of ClC-6 have remained entirely unknown. Here, we report that fusing the green fluorescent protein (GFP) to the N terminus of ClC-6 increased its cell surface expression, allowing us to functionally characterize ClC-6. Compatible with ClC-6 mediating Cl⁻/H⁺ exchange, Xenopus oocytes expressing GFP-tagged ClC-6 alkalinized upon depolarization. This alkalinization was dependent on the presence of extracellular anions and could occur against an electrochemical proton gradient. As observed in other CLC exchangers, ClC-6-mediated H⁺ transport was abolished by mutations in either the “gating” or “proton” glutamate. Overexpression of GFP-tagged ClC-6 in CHO cells elicited small, outwardly rectifying currents with a Cl⁻ > I⁻ conductance sequence. Mutating the gating glutamate of ClC-6 yielded an ohmic anion conductance that was increased by additionally mutating the “anion-coordinating” tyrosine. Additionally changing the chloride-coordinating serine 157 to proline increased the NO₃⁻ conductance of this mutant. Taken together, these data demonstrate for the first time that ClC-6 is a Cl⁻/H⁺ antiporter.     

Growth regulator-induced betacyanin accumulation and dopa-4,5-dioxygenase (<Emphasis Type="Italic">DODA</Emphasis>) gene expression in euhalophyte <Emphasis Type="Italic">Suaeda salsa</Emphasis> calli

Suaeda salsa calluses cultured in darkness for 28 d were transferred to Murashige and Skoog (MS) media containing various growth regulators under white light conditions for 10 d to investigate cell growth, betacyanin accumulation, and expression of dopa-4,5-dioxygenase (DODA). Callus growth was markedly promoted when 0.2 mg·L⁻¹ 2,4-D and 0.5 mg·L⁻¹ 6-BA were added to the MS medium. Surprisingly, of the auxins tested, IAA had no effect on betacyanin content, but 2,4-D strongly decreased betacyanin content. Betacyanin content was positively correlated with 6-BA concentrations in the range of 0.1–2.0 mg·L⁻¹. DODA mRNA levels were consistent with the response of betacyanin content to exogenous growth regulators. These results suggest that betacyanin metabolism in S. salsa calluses is regulated under white light conditions by growth regulators through the regulation of genes such as DODA that are involved in betacyanin synthesis.     

Electrochemical Aging Responses in Pisum: Cellular Adaptations or Recovery from Injury?

Following excision, etiolated epicotyl segments of Pisum sativum L. cv. Alaska exhibit a marked hyperpolarization of membrane potential which is followed by a linear accumulation of K⁺ when segments are incubated in Higinbotham nutrient solution. Segments aged for several hours and then reexcised display only a slight depolarization of membrane potential and no delay in ion accumulation; thus, recovery from injury appears an unlikely explanation for these responses. Substances originating in either the plumule or the cotyledons do not seem to be directly involved in these “aging” responses. However, locally produced substances, such as ethylene, or substances originating in the roots have not been eliminated as causative factors. Cold temperatures and cycloheximide prolong the lag in K⁺ accumulation indicating a metabolic explanation for the induced K⁺ accumulation. However, similar specific activities of plasma membrane-bound ATPase were found in isolates from fresh and aged epicotyl segments. Reactivation of an ion transport mechanism, perhaps responsible for the osmotic control of growth in immature cells, is suggested as a possible explanation for the pattern of ion accumulation characteristic of excised pea epicotyl tissue.     

Activation of Na+-permeant Cation Channel by Stretch and Cyclic AMP-dependent Phosphorylation in Renal Epithelial A6 Cells

It is currently believed that a nonselective cation (NSC) channel, which responds to arginine vasotocin (an antidiuretic hormone) and stretch, regulates Na⁺ absorption in the distal nephron. However, the mechanisms of regulation of this channel remain incompletely characterized. To study the mechanisms of regulation of this channel, we used renal epithelial cells (A6) cultured on permeable supports. The apical membrane of confluent monolayers of A6 cells expressed a 29-pS channel, which was activated by stretch or by 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of phosphodiesterase. This channel had an identical selectivity for Na⁺, K⁺, Li⁺, and Cs⁺, but little selectivity for Ca²⁺ (P_Ca/P_Na < 0.005) or Cl⁻ (P_Cl/P_Na < 0.01), identifying it as an NSC channel. Stretch had no additional effects on the open probability (P _o) of the IBMX-activated channel. This channel had one open (“O”) and two closed (short “C _S” and long “C _L”) states under basal, stretch-, or IBMX-stimulated conditions. Both stretch and IBMX increased the P _o of the channel without any detectable changes in the mean open or closed times. These observations led us to the conclusion that a kinetic model “C _L ↔ C _S ↔ O” was the most suitable among three possible linear models. According to this model, IBMX or stretch would decrease the leaving rate of the channel for C _L from C _S, resulting in an increase in P _o. Cytochalasin D pretreatment abolished the response to stretch or IBMX without altering the basal activity. H89 (an inhibitor of cAMP-dependent protein kinase) completely abolished the response to both stretch and IBMX, but, unlike cytochalasin D, also diminished the basal activity. We conclude that: (a) the functional properties of the cAMP-activated NSC channel are similar to those of the stretch-activated one, (b) the actin cytoskeleton plays a crucial role in the activation of the NSC channel induced by stretch and cAMP, and (c) the basal activity of the NSC channel is maintained by PKA-dependent phosphorylation but is not dependent on actin microfilaments.     

Nucleic Acid Metabolism in Germinating Onion: I. Changes in Root Tip Nucleic Acid during Germination

Nucleic acid synthesis in the G₁ cell population of the 1-millimeter apex of the Allium cepa embryo was studied during the initial 73 hours of germination. Quantitative data indicate that the total amount of RNA per cell began to increase after 18 hours of germination while the initial DNA per cell increase did not occur until some 20 hours later. Polyacrylamide gel electrophoresis patterns of ³H-uridine-labeled total nucleic acid samples indicated that synthesis of all detectable RNA fractions present in the pre-emergent 1-millimeter apex (i.e., cytoplasmic and “chloroplast-like” RNA) began at approximately the same time (18 hours). Synthesis of the various cytoplasmic RNA fractions continued throughout the germination period. Data indicating synthesis of the “chloroplast-like” RNA were obtained only for the initial 36 hours of germination. Specific radioactivity of ³H-uridine-labeled total nucleic acid increased during the first 41.5 hours of germination but then decreased while the accumulation of RNA per cell continued to increase throughout the 73-hour period. In addition, a method is described which reduced the bacterial contamination of Allium seed to a level not detectable by incorporation of radioactive precursors into bacterial ribosomal RNA.     

Chaperone Proteins Select and Maintain [PIN+] Prion Conformations in Saccharomyces cerevisiae

Prions are proteins that can adopt different infectious conformations known as “strains” or “variants,” each with a distinct, epigenetically inheritable phenotype. Mechanisms by which prion variants are determined remain unclear. Here we use the Saccharomyces cerevisiae prion Rnq1p/[PIN⁺] as a model to investigate the effects of chaperone proteins upon prion variant determination. We show that deletion of specific chaperone genes alters [PIN⁺] variant phenotypes, including [PSI⁺] induction efficiency, Rnq1p aggregate morphology/size and variant dominance. Mating assays demonstrate that gene deletion-induced phenotypic changes are stably inherited in a non-Mendelian manner even after restoration of the deleted gene, confirming that they are due to a bona fide change in the [PIN⁺] variant. Together, our results demonstrate a role for chaperones in regulating the prion variant complement of a cell.     

Ethylene Production and Respiratory Behavior of the rin Tomato Mutant

Little or no change in ethylene or CO₂ production occurred in rin tomato mutant fruits monitored for up to 120 days after harvest. Of the abnormally ripening tomatoes investigated, including “Never ripe” (Nr Y a h, Nr c l₂ r), “Evergreen” (gf r) and “Green Flesh” (gf), only rin did not show a typical climacteric and ethylene rise.     

Temperature-dependent water and ion transport properties of barley and sorghum roots : I. Relationship to leaf growth

Root temperature strongly affects shoot growth, possibly via “nonhydraulic messengers” from root to shoot. In short-term studies with barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) seedlings, the optimum root temperatures for leaf expansion were 25° and 35°C, respectively. Hydraulic conductance (L_p) of both intact plants and detached exuding roots of barley increased with increasing root temperature to a high value at 25°C, remaining high with further warming. In sorghum, the L_p of intact plants and of detached roots peaked at 35°C. In both species, root temperature did not affect water potentials of the expanded leaf blade or the growing region despite marked changes in L_p. Extreme temperatures greatly decreased ion flux, particularly K⁺ and NO₃⁻, to the xylem of detached roots of both species. Removing external K⁺ did not alter short-term K⁺ flux to the xylem in sorghum but strongly inhibited flux at high temperature in barley, indicating differences in the sites of temperature effects. Leaf growth responses to root temperature, although apparently “uncoupled” from water transport properties, were correlated with ion fluxes. Studies of putative root messengers must take into account the possible role of ions.     

Nitrate-Dependent Ferrous Iron Oxidation by Anaerobic Ammonium Oxidation (Anammox) Bacteria

We examined nitrate-dependent Fe²⁺ oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “Candidatus Brocadia sinica” anaerobically oxidized Fe²⁺ and reduced NO₃⁻ to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein⁻¹ min⁻¹, respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of “Ca. Brocadia sinica” (10 to 75 nmol NH₄⁺ mg protein⁻¹ min⁻¹). A ¹⁵N tracer experiment demonstrated that coupling of nitrate-dependent Fe²⁺ oxidation and the anammox reaction was responsible for producing nitrogen gas from NO₃⁻ by “Ca. Brocadia sinica.” The activities of nitrate-dependent Fe²⁺ oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO₃⁻ ± SD of “Ca. Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe²⁺ oxidation was further demonstrated by another anammox bacterium, “Candidatus Scalindua sp.,” whose rates of Fe²⁺ oxidation and NO₃⁻ reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein⁻¹ min⁻¹, respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe²⁺ oxidation and the anammox reaction decreased the molar ratios of consumed NO₂⁻ to consumed NH₄⁺ (ΔNO₂⁻/ΔNH₄⁺) and produced NO₃⁻ to consumed NH₄⁺ (ΔNO₃⁻/ΔNH₄⁺). These reactions are preferable to the application of anammox processes for wastewater treatment.     

Relationship between Intracellular Na+ Concentration and Reduced Na+ Affinity in Na+,K+-ATPase Mutants Causing Neurological Disease

The neurological disorders familial hemiplegic migraine type 2 (FHM2), alternating hemiplegia of childhood (AHC), and rapid-onset dystonia parkinsonism (RDP) are caused by mutations of Na⁺,K⁺-ATPase α₂ and α₃ isoforms, expressed in glial and neuronal cells, respectively. Although these disorders are distinct, they overlap in phenotypical presentation. Two Na⁺,K⁺-ATPase mutations, extending the C terminus by either 28 residues (“+28” mutation) or an extra tyrosine (“+Y”), are associated with FHM2 and RDP, respectively. We describe here functional consequences of these and other neurological disease mutations as well as an extension of the C terminus only by a single alanine. The dependence of the mutational effects on the specific α isoform in which the mutation is introduced was furthermore studied. At the cellular level we have characterized the C-terminal extension mutants and other mutants, addressing the question to what extent they cause a change of the intracellular Na⁺ and K⁺ concentrations ([Na⁺]_i and [K⁺]_i) in COS cells. C-terminal extension mutants generally showed dramatically reduced Na⁺ affinity without disturbance of K⁺ binding, as did other RDP mutants. No phosphorylation from ATP was observed for the +28 mutation of α₂ despite a high expression level. A significant rise of [Na⁺]_i and reduction of [K⁺]_i was detected in cells expressing mutants with reduced Na⁺ affinity and did not require a concomitant reduction of the maximal catalytic turnover rate or expression level. Moreover, two mutations that increase Na⁺ affinity were found to reduce [Na⁺]_i. It is concluded that the Na⁺ affinity of the Na⁺,K⁺-ATPase is an important determinant of [Na⁺]_i.     

Einfluss möglicher phosphodiesterase-inhibitoren und cAMP auf die betacyan-akkumulation

The influence of cyclic AMP, theophylline, papaverine and NH₄NO₃ on the accumulation of betacyanin in callus of Portulaca grandiflora, var. JR, were studied in relation to amounts of dihydroxyphenylalanine (DOPA), dopamine, phenylalanine, tyrosine, protein, ‘lipid’ and the nucleotides CMP, AMP, GMP and UMP present. Inhibition of betacyanin formation is characterized by reduced amounts of DOPA and dopamine and a constant rise of GMP and CMP (GMP/CMP = 8). Protein accumulation is also inhibited. The increase in pigment accumulation due to theophylline and NH₄NO₃ is characterized by a raised DOPA and protein concentration and a lower GMP/CMP ratio (=3). The increase in betacyanin accumulation is due to de novo synthesis of enzymes. Inhibition is probably due to the regulation of the callus phosphodiesterase by papaverine and theophylline (≦ 10^?5 M/1) which triggers a change in concentration of nucleotides, which eventually regulates tyrosinase biosynthesis.     

Analysis of Minimal Functions of Simian Virus 40 II. Enhancement of Oncogenic Transformation in Vitro by UV Irradiation

After light UV irradiation (5,000 to 10,000 ergs/mm²) “complete” and “defective” simian virus 40 (SV40) showed an enhancement of oncogenic transformation capacity in Syrian hamster kidney cells in vitro up to 180 and 270% of the controls, respectively. Simultaneously with the enhancement of transformation, an increase in T-antigen induction was observed in CV-1 cells infected with light UV-irradiated SV40; infectivity, however, was correspondingly reduced by 1 log₁₀. After strong UV irradiation (10,000 to 80,000 ergs/mm²) of “complete” and “defective” SV40, transformation capacity in vitro proved to be the most resistant viral function. It was only slightly reduced in comparison with a 4 to 5 log₁₀ reduction of infectivity. T-antigen induction of SV40 was also equally resistant to strong UV irradiation. We found no evidence of “multiplicity reactivation” involved in the high resistance of transformation capacity of SV40 after UV irradiation. Syrian hamster kidney cells transformed in vitro by UV-irradiated SV40 contained the SV40-specific T-antigen and showed the same morphology and growth characteristics as cells transformed by non-irradiated “complete” or “defective” SV40. They induced malignant tumors after subcutaneous inoculation into Syrian hamsters.     

NAD+ accumulation as a metabolic off switch for orthodox pollen

Terrestrial plant pollen is classified into two categories based on its metabolic status: pollen with low-metabolism are termed “orthodox” and pollen with high-metabolism are termed “recalcitrant.” Nicotinamide adenine dinucleotide (NAD) is crucial for a number of metabolisms in all extant organisms. It has recently been shown that NAD homeostasis plays an important role in a broad range of developmental processes and responses to environment. Recently, a reverse genetic approach shed light on the significance of NAD biosynthesis on pollen fate. In orthodox Arabidopsis pollen, NAD⁺ that was accumulated in excess at dispersal dramatically decreased on rehydration. The lack of a key gene that is involved in NAD biosynthesis compromised the excess accumulation. Moreover, absence of the excess accumulation phenocopied the so-called recalcitrant pollen, as demonstrated by the germination inside anthers and the loss of desiccation tolerance. Upon rehydration, NAD⁺-consuming inhibitors impaired tube germination. Taken together, our results suggest that accumulation of NAD⁺ functions as a physiochemical molecular switch for suspended metabolism and that the decrease of NAD⁺ plays a very important role during transitions in metabolic states. Shifting of the redox state to an oxidizing environment may efficiently control the comprehensive metabolic network underlying the onset of pollen germination.