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PubMed2002

Dynamic and steady-state responses of inorganic nitrogen pools and NH(3) exchange in leaves of Lolium perenne and Bromus erectus to changes in root nitrogen supply.

Short- and long-term responses of inorganic N pools and plant-atmosphere NH(3) exchange to changes in external N supply were investigated in 11-week-old plants of two grass species, Lolium perenne and Bromus erectus, characteristic of N-rich and N-poor grassland ecosystems, respectively. A switch of root N source from NO(-)(3)to NH(4)(+) caused within 3 h a 3- to 6-fold increase in leaf apoplastic NH(4)(+) concentration and a simultaneous decrease in apoplastic pH of about 0.4 pH units in both species. The concentration of total extractable leaf tissue NH(4)(+) also increased two to three times within 3 h after the switch. Removal of exogenous NH(4)(+) caused the apoplastic NH(4)(+) concentration to decline back to the original level within 24 h, whereas the leaf tissue NH(4)(+)concentration decreased more slowly and did not reach the original level in 48 h. After growing for 5 weeks with a steady-state supply of NO(-)(3)or NH(4)(+), L. perenne were in all cases larger, contained more N, and utilized the absorbed N more efficiently for growth than B. erectus, whereas the two species behaved oppositely with respect to tissue concentrations of NO(-)(3), NH(4)(+), and total N. Ammonia compensation points were higher for B. erectus than for L. perenne and were in both species higher for NH(4)(+)- than for NO(-)(3)-grown plants. Steady-state levels of apoplastic NH(4)(+), tissue NH(4)(+), and NH(3) emission were significantly correlated. It is concluded that leaf apoplastic NH(4)(+) is a highly dynamic pool, closely reflecting changes in the external N supply. This rapid response may constitute a signaling system coordinating leaf N metabolism with the actual N uptake by the roots and the external N availability.

Ammonia↗

PubMed1992

Structure-Activity Relationships of Abscisic Acid Analogs Based on the Induction of Freezing Tolerance in Bromegrass (Bromus inermis Leyss) Cell Cultures.

The induction of freezing tolerance in bromegrass (Bromus inermis Leyss) cell culture was used to investigate the activity of absisic acid (ABA) analogs. Analogs were either part of an array of 32 derived from systematic alterations to four regions of the ABA molecule or related, pure optical isomers. Alterations were made to the functional group at C-1 (acid replaced with methyl ester, aldehyde, or alcohol), the configuration at C-2, C-3 (cis double bond replaced with trans double bond), the bond order at C-4, C-5 (trans double bond replaced with a triple bond), and ring saturation (C-2', C-3' double bond replaced with a single bond so that the C-2' methyl and side chain were cis). All deviations in structure from ABA reduced activity. A cis C-2, C-3 double bond was the only substituent absolutely required for activity. Overall, acids and esters were more active than aldehydes and alcohols, cyclohexenones were more active than cyclohexanones, and dienoic and acetylenic analogs were equally active. The activity associated with any one substituent was, however, markedly influenced by the presence of other substituents. cis, trans analogs were more active than their corresponding acetylenic analogs unless the C-1 was an ester. Cyclohexenones were more active than cyclohexanones regardless of oxidation level at C-1. An acetylenic side chain decreased the activity of cyclohexenones but increased the activity of cyclohexanones relative to their cis, trans counterparts. Trends suggested that for activity the configuration at C-1' has to be the same as in (S)-ABA, in dihydro analogs the C-2'-methyl and the side chain must be cis, small positional changes of the 7'-methyl are tolerable, and the C-1 has to be at the acid oxidation level.

Journal Article↗

PubMed1993

Molecular cloning of abscisic acid-responsive mRNAs expressed during the induction of freezing tolerance in bromegrass (Bromus inermis Leyss) suspension culture.

Abscisic acid (ABA) increases the freezing tolerance of bromegrass (Bromus inermis Leyss) cell-suspension cultures at 23 degrees C and elicits many metabolic changes similar to those observed during cold acclimation. Induction and maintenance of freezing tolerance by ABA is accompanied by the expression of novel polypeptides and translatable RNAs. The objective of this study was to isolate and characterize ABA-responsive cDNAs associated with ABA-induced freezing tolerance in bromegrass cell cultures. Among the 16 ABA-responsive cDNA clones isolated, 9 were expressed only with ABA treatment, 7 showed increased transcript level, and 1 was transiently expressed. Cold responsiveness was determined in three clones with increased transcript levels and in the transiently expressed clone. Deacclimation of ABA-hardened cells was a relatively slow process, because all of the novel transcripts persisted for at least 7 d after cells were cultured in ABA-free medium. Preliminary sequencing of cDNAs has identified several clones that share high sequence homology with genes associated with sugar metabolism, osmotic stress, and protease activity. Clone pBGA61 was fully sequenced and tentatively identified as an NADPH-dependent aldose reductase. The predicted amino acid sequence of the coding region shared 92% similarity with that predicted for barley aldose reductase cDNA. It is proposed that expression of genes related to sugar metabolism and osmotic stress may be required for ABA-induced hardening.

Abscisic Acid↗

PubMed1994

Abscisic acid-induced heat tolerance in Bromus inermis Leyss cell-suspension cultures. Heat-stable, abscisic acid-responsive polypeptides in combination with sucrose confer enhanced thermostability.

Increased heat tolerance is most often associated with the synthesis of heat-shock proteins following pre-exposure to a nonlethal heat treatment. In this study, a bromegrass (Bromus inermis Leyss cv Manchar) cell suspension cultured in a medium containing 75 microM abscisic acid (ABA) without prior heat treatment had a 87% survival rate, as determined by regrowth analysis, following exposure to 42.5 degrees C for 120 min. In contrast, less than 1% of the control cells survived this heat treatment. The heat tolerance provided by treatment with 75 microM ABA was first evidenced after 4 d of culture and reached a maximum tolerance after 11 d of culture. Preincubation with sucrose partially increased the heat tolerance of control cells and rendered ABA-treated cells tolerant to 45 degrees C for 120 min (a completely lethal heat treatment for control cells). Comparative two-dimensional polyacrylamide gel electrophoresis of cellular protein isolated from heat-tolerant cells identified 43 ABA-responsive proteins of which 26 were heat stable (did not coagulate and remained soluble after 30 min at 90 degrees C). Eight heat-stable, ABA-responsive proteins ranging from 23 to 45 kD had similar N-terminal sequences. The ABA-responsive (43-20 kD), but none of the control heat-stable, proteins cross-reacted to varying degrees with a polyclonal antibody directed against a conserved, lysine-rich dehydrin sequence. A group of 20- to 30-kD heat-stable, ABA-responsive proteins cross-reacted with both the anti-dehydrin antibody and an antibody directed against a cold-responsive winter wheat protein (Wcs 120). In ABA-treated cells, there was a positive correlation between heat- and pH-induced coagulation of a cell-free homogenate and the heat tolerance of these cells. At 50 degrees C, control homogenates coagulated after 8 min, whereas cellular fractions from ABA-treated cells showed only marginal coagulation after 15 min. In protection assays, addition of heat-stable, ABA-responsive polypeptides to control fractions reduced the heat-induced coagulation of cell-free homogenates. Sucrose (8%) alone and control, heat-stable fractions enhanced the thermostability of control fractions, but the most protection was conferred by ABA-responsive, heat-stable proteins in combination with sucrose. These data suggest that stress-tolerance mechanisms may develop as a result of cooperative interactions between stress proteins and cell osmolytes, e.g. sucrose. Hypotheses are discussed implicating the role of these proteins and osmolytes in preventing coagulation and denaturation of cellular proteins and membranes.

Abscisic Acid↗

PubMed1994

Effects of Abscisic Acid Metabolites and Analogs on Freezing Tolerance and Gene Expression in Bromegrass (Bromus inermis Leyss) Cell Cultures.

Optical isomers and racemic mixtures of abscisic acid (ABA) and the ABA metabolites abscisyl alcohol (ABA alc), abscisyl aldehyde (ABA ald), phaseic acid (PA), and 7[prime]hydroxyABA (7[prime]OHABA) were studied to determine their effects on freezing tolerance and gene expression in bromegrass (Bromus inermis Leyss) cell-suspension cultures. A dihydroABA analog (DHABA) series that cannot be converted to PA was also investigated. Racemic ABA, (+)-ABA, ([plus or minus])-DHABA, and (+)-DHABA were the most active in inducing freezing tolerance, (-)-ABA, ([plus or minus])-7[prime]OHBA, (-)-DHABA, ([plus or minus])-ABA ald, and ([plus or minus])-ABA alc had a moderate effect, and PA was inactive. If the relative cellular water content decreased below 82%, dehydrin gene expression increased. Except for (-)-ABA, increased expression of dehydrin genes and increased accumulation of responsive to ABA (RAB) proteins were linked to increased levels of frost tolerance. PA had no effect on the induction of RAB proteins; however, ([plus or minus])- and (+)-DHABA were both active, which suggests that PA is not involved in freezing tolerance. Both (+)-ABA and (-)-ABA induced dehydrin genes and the accumulation of RAB proteins to similar levels, but (-)-ABA was less effective than (+)-ABA at increasing freezing tolerance. The (-)-DHABA analog was inactive, implying that the ring double bond is necessary in the (-) isomers for activating an ABA response.

Journal Article↗

PubMed1994

Comparison of Dehydrin Gene Expression and Freezing Tolerance in Bromus inermis and Secale cereale Grown in Controlled Environments, Hydroponics, and the Field.

There have been very few reports on the expression of stress-responsive genes in field-grown material. A barley dehydrin cDNA was used to investigate the expression of dehydrin-like transcripts after low-temperature and abscisic acid-induced acclimation of bromegrass (Bromus inermis Leyss) suspension cells and of bromegrass and rye (Secale cereale) plants grown in the field and under controlled environmental conditions. Field-acclimated plants accumulated high levels of dehydrin transcripts and were very freezing tolerant. Plants grown in pots and hydroponics under controlled environments also accumulated dehydrin transcripts and showed increased freezing tolerance. Simulation of a combined drought and freezing stress in pots resulted in expression of dehydrin-like transcripts comparable to those observed in field-acclimated material.

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PubMed1983

Nitrogen Turnover and Assimilation during Regrowth in Trifolium subterraneum L. and Bromus mollis L.

Subterranean clover (Trifolium subterraneum L. cv Woogenellup) and soft chess grass (Bromus mollis L. cv Blando) were grown in monocultures with (15)NH(4)Cl added to the soil to study nitrogen movement during regrowth following shoot removal. Four clipping treatments were imposed. Essentially all available (15)N was assimilated from the soil prior to the first shoot harvest. Measurements of total reduced nitrogen and (15)N contained within that nitrogen fraction in roots, crowns, and shoots at each harvest showed large, significant (P </= 0.001) declines in excess (15)N of crowns and roots in both species between the first and fourth harvests. There was no significant decline in total reduced nitrogen in the same organs over that period. Similar responses were evident in plants defoliated three times. The simplest interpretation of these data is that reduced nitrogen compounds turn over in plant roots and crowns during shoot regrowth. Calculations for grass and clover plants clipped four times during the growing season indicated that 100 to 143% of the nitrogen present in crowns and roots turned over between the first and fourth shoot harvest in both species, assuming nitrogen in those organs was replaced with nitrogen containing the lowest available concentration of (15)N. If other potential sources of nitrogen were used for the calculations, it was necessary to postulate that larger amounts of total nitrogen flowed through the crown and root to produce the measured dilution of (15)N compounds. These data provide the first quantitative estimates of the amount of internal nitrogen used by plants, in addition to soil nitrogen or N(2), to regenerate shoots after defoliation.

Journal Article↗

PubMed1987

Factors Influencing the Induction of Freezing Tolerance by Abscisic Acid in Cell Suspension Cultures of Bromus inermis Leyss and Medicago sativa L.

A 2-gram fresh weight inoculum of bromegrass (Bromus inermis Leyss. culture BG970) cell suspension culture treated with 7.5 x 10(-5) molar abscisic acid (ABA) for 7 days at 25 degrees C survived slow cooling to -60 degrees C. Over 80% of the cells in ABA treated cultures survived immersion in liquid N(2) after slow cooling to -40 or -60 degrees C. In contrast, a 6-gram fresh weight inoculum only attained a hardiness level of -28 degrees C after 5 days of ABA treatment. Ethanol (2 x 10(-2) molar) added to the culture medium at the time of ABA addition, inhibited the freezing tolerance of bromegrass cells by 25 degrees C. A 6-gram inoculum of both control and ABA treated bromegrass cells altered the pH of the medium more than a 2-gram inoculum. ABA inhibited the increase in fresh weight of bromegrass by 20% after 4 days. Both control and ABA (10(-4) molar) treated alfalfa cells (Medicago sativa L.) grown at 25 degrees C hardened from an initial LT(50) of -5 degrees C to an LT(50) of -23 degrees C by the third to fifth day after subculture. Thereafter, the cells dehardened but the ABA treated cells did not deharden to the same level as the control cells. ABA inhibited the increase in fresh weight of alfalfa by 50% after 5 days.

Journal Article↗

PubMed1987

Protein Synthesis in Bromegrass (Bromus inermis Leyss) Cultured Cells during the Induction of Frost Tolerance by Abscisic Acid or Low Temperature.

Bromus inermis Leyss cell cultures treated with 75 micromolar abscisic acid (ABA) at both 23 and 3 degrees C developed more freezing resistance than cells cultured at 3 degrees C. Protein synthesis in cells induced to become freezing tolerant by ABA and low temperature was monitored by [(14)C]leucine incorporation. Protein synthesis continued at 3 degrees C, but net cell growth was stopped. Most of the major proteins detected at 23 degrees C were synthesized at 3 degrees C. However, some proteins were synthesized only at low temperatures, whereas others were inhibited. ABA showed similar effects on protein synthesis at both 23 and 3 degrees C. Comparative electrophoretic analysis of [(14)C]leucine labeled protein detected the synthesis of 19, 21 and 47 kilodalton proteins in less than 8 hours after exposure to exogenous ABA. Proteins in the 20 kilodalton range were also synthesized at 3 degrees C. In addition, a 31 kilodalton protein band showed increased expression in freezing resistant ABA treated cultures after 36 hours growth at both 3 and 23 degrees C. Quantitative analysis of [(14)C]leucine labeled polypeptides in two-dimensional gels confirmed the increased expression of the 31 kilodalton protein. Two-dimensional analysis also resolved a 72 kilodalton protein enriched in ABA treated cultures and identified three proteins (24.5, 47, and 48 kilodaltons) induced by low temperature growth.

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PubMed1988

Identification of Proteins Correlated with Increased Freezing Tolerance in Bromegrass (Bromus inermis Leyss. cv Manchar) Cell Cultures.

Cellular and extracellular protein profiles from Bromus inermis Leyss. cv Manchar cell suspension cultures cold hardened by low temperature and abscisic acid (ABA) treatment were analyzed by one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis. Cellular proteins (25, 165, 190, and 200 kilodaltons) increased by low temperature growth and cellular proteins (20, 25, 28, 30, 32, 37, 40, 45, 200 kilodaltons) increased by exogenous ABA treatment were identified. Low temperature treatment inhibited the synthesis of a 22 kilodalton protein and ABA treatment resulted in the synthesis of two extracellular proteins (17 and 21 kilodaltons). Low temperature and ABA-induced hardening conditions increased or induced a 25 and a 200 kilodalton protein. The 25 and a 30 kilodalton protein previously shown to be enriched by ABA-induced hardening conditions at both 3 and 23 degrees C temperatures co-fractionated with the crude membrane fraction (30,000g sediment). The 200 kilodalton protein was detected in the 30,000g supernatant. Two-dimensional analysis of the crude membrane fraction resolved the 30 kilodalton protein band into a major polypeptide with an apparent isoelectric point of 6.85.

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PubMed1960