Abscisic Acid Formation Research Articles

Quantitative Analysis of ABA and SA in Rice (Oryza sativa L.) Grown Under Drought Stress

Rice cultivation requires plenty of water for its proper growth, development, and productivity. The regular life cycle of rice plants is disrupted by moderate to severe drought. Both abscisic acid (ABA) and salicylic acid (SA) have involvement in rice physiology under drought. With the limited information, we aimed to study the relationship between ABA and SA concentration in leaves of rice plants under drought. The experiment was performed on a drought-sensitive variety of Swarna MTU 7029 rice. The HPLC method was used to analyse the endogenous ABA and SA content. This study provided data on ABA and SA content in 0.5 mM SA treated and untreated 56 days old rice plants at 7, 14 and 28 days of drought. The result showed that the concentration of ABA was enhanced by 74.6%, 82.8%, and 99.4% during 7, 14, and 28 days of drought, respectively, while it was increased by 64.6%, 74.3% and 78.5% in SA treated plants under 7, 14 and 28 days drought, respectively. The concentration of SA enhanced by 132.2%, 19.7%, and 3.0% during 7, 14, and 28 days of drought, while it was increased to 137.5%, 54.8%, and 23.2% in SA treated plants under 7, 14, and 28 days drought, respectively. This explains that on the 7th day of drought stress, SA may suppress the formation of ABA but not much on the 14th and 28th days of drought stress in treated rice plants. This outcome helps study the interaction of ABA and SA at the gene level under drought stress.

Strigolactone Levels in Dicot Roots Are Determined by an Ancestral Symbiosis-Regulated Clade of the PHYTOENE SYNTHASE Gene Family.

Strigolactones (SLs) are apocarotenoid phytohormones synthesized from carotenoid precursors. They are produced most abundantly in roots for exudation into the rhizosphere to cope with mineral nutrient starvation through support of root symbionts. Abscisic acid (ABA) is another apocarotenoid phytohormone synthesized in roots, which is involved in responses to abiotic stress. Typically low carotenoid levels in roots raise the issue of precursor supply for the biosynthesis of these two apocarotenoids in this organ. Increased ABA levels upon abiotic stress in Poaceae roots are known to be supported by a particular isoform of phytoene synthase (PSY), catalyzing the rate-limiting step in carotenogenesis. Here we report on novel PSY3 isogenes from Medicago truncatula (MtPSY3) and Solanum lycopersicum (SlPSY3) strongly expressed exclusively upon root interaction with symbiotic arbuscular mycorrhizal (AM) fungi and moderately in response to phosphate starvation. They belong to a widespread clade of conserved PSYs restricted to dicots (dPSY3) distinct from the Poaceae-PSY3s involved in ABA formation. An ancient origin of dPSY3s and a potential co-evolution with the AM symbiosis is discussed in the context of PSY evolution. Knockdown of MtPSY3 in hairy roots of M. truncatula strongly reduced SL and AM-induced C13 α-ionol/C14 mycorradicin apocarotenoids. Inhibition of the reaction subsequent to phytoene synthesis revealed strongly elevated levels of phytoene indicating induced flux through the carotenoid pathway in roots upon mycorrhization. dPSY3 isogenes are coregulated with upstream isogenes and downstream carotenoid cleavage steps toward SLs (D27, CCD7, CCD8) suggesting a combined carotenoid/apocarotenoid pathway, which provides “just in time”-delivery of precursors for apocarotenoid formation.

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

Abscisic Acid, One of the Key Determinants of in Vitro Shoot Differentiation from Cotyledons of <i>Vigna radiata</i>

Differential regeneration potentiality of two cotyledons (Cot and Cot E) of Vigna radiata seed during in vitro shoot differentiation is now well established. In the present study, endogenous abscisic acid (ABA) level (both bound and free form) was estimated using high performance liquid chromatography technique from these two explant types prior to the induction of in vitro differentiation. Both free and conjugated forms of endogenous ABA were higher in Cot than Cot E. However, the bound form of ABA was higher than free or active form in both the explants. Effects of an ABA catabolic inhibitor, diniconazole on the endogenous ABA production potential were determined. Diniconazole inhibits ABA 8’-hydroxylase, the catabolizing enzyme, resulting in accumulation of free ABA in the cell. It was noted that diniconazole inhibited bound form of ABA formation in a concentration dependant manner with a concomitant increase in the free form and decrease in shoot differentiation from Cot E explants. Likewise, exogenously applied ABA in in vitro culture also resulted in decrease in shoot regeneration frequency from the cotyledonary explants ascertaining the differential level of endogenous ABA is one of the determinants of differential regeneration response of Cot and Cot E under in vitro cultural condition. Cytokinin antagonized inhibitory effect of ABA mediated by cytokinin responsive proteins, such proteins are up regulated differentially in Cot E has recently shown us through proteomic study confirming further the role of ABA.

Vacuolar Transport of Abscisic Acid Glucosyl Ester Is Mediated by ATP-Binding Cassette and Proton-Antiport Mechanisms in Arabidopsis

Abscisic acid (ABA) is a key plant hormone involved in diverse physiological and developmental processes, including abiotic stress responses and the regulation of stomatal aperture and seed germination. Abscisic acid glucosyl ester (ABA-GE) is a hydrolyzable ABA conjugate that accumulates in the vacuole and presumably also in the endoplasmic reticulum. Deconjugation of ABA-GE by the endoplasmic reticulum and vacuolar β-glucosidases allows the rapid formation of free ABA in response to abiotic stress conditions such as dehydration and salt stress. ABA-GE further contributes to the maintenance of ABA homeostasis, as it is the major ABA catabolite exported from the cytosol. In this work, we identified that the import of ABA-GE into vacuoles isolated from Arabidopsis (Arabidopsis thaliana) mesophyll cells is mediated by two distinct membrane transport mechanisms: proton gradient-driven and ATP-binding cassette (ABC) transporters. Both systems have similar Km values of approximately 1 mm. According to our estimations, this low affinity appears nevertheless to be sufficient for the continuous vacuolar sequestration of ABA-GE produced in the cytosol. We further demonstrate that two tested multispecific vacuolar ABCC-type ABC transporters from Arabidopsis exhibit ABA-GE transport activity when expressed in yeast (Saccharomyces cerevisiae), which also supports the involvement of ABC transporters in ABA-GE uptake. Our findings suggest that the vacuolar ABA-GE uptake is not mediated by specific, but rather by several, possibly multispecific, transporters that are involved in the general vacuolar sequestration of conjugated metabolites.

Identification of superoxide production by Arabidopsis thaliana aldehyde oxidases AAO1 and AAO3

Plant aldehyde oxidases (AOs) have gained great attention during the last years as they catalyze the last step in the biosynthesis of the phytohormone abscisic acid by oxidation of abscisic aldehyde. Furthermore, oxidation of indole-3-acetaldehyde by AOs is likely to represent one route to produce another phytohormone, indole-3-acetic acid, and thus, AOs play important roles in many aspects of plant growth and development. In the present work we demonstrate that heterologously expressed AAO1 and AAO3, two prominent members of the AO family from Arabidopsis thaliana, do not only generate hydrogen peroxide but also superoxide anions by transferring aldehyde-derived electrons to molecular oxygen. In support of this, superoxide production has also been found for native AO proteins in Arabidopsis leaf extracts. In addition to their aldehyde oxidation activity, AAO1 and AAO3 were found to exhibit NADH oxidase activity, which likewise is associated with the production of superoxide anions. According to these results and due to the fact that molecular oxygen is the only known physiological electron acceptor of AOs, the production of hydrogen peroxide and/or superoxide has to be considered in any physiological condition in which aldehydes or NADH serve as substrate for AOs. In this respect, conditions such as natural senescence and stress-induced stomatal movement, which both require simultaneously elevated levels of abscisic acid and hydrogen peroxide/superoxide, are likely to benefit from AOs in two ways, namely by formation of abscisic acid and by concomitant formation of reactive oxygen species.

Ectopic expression of a fruit phytoene synthase from Citrus paradisi Macf. promotes abiotic stress tolerance in transgenic tobacco

Abscisic acid (ABA) is an important regulator of plant responses to environmental stresses and an absolute requirement for stress tolerance. Recently, a third phytoene synthase (PSY3) gene paralog was identified in monocots and demonstrated to play a specialized role in stress-induced ABA formation, thus suggesting that the first committed step in carotenogenesis is a key limiting step in ABA biosynthesis. To examine whether the ectopic expression of PSY, other than PSY3, would similarly affect ABA level and stress tolerance, we have produced transgenic tobacco containing a fruit-specific PSY (CpPSY) of grapefruit (Citrus paradisi Macf.). The transgenic plants contained a single- or double-locus insertion and expressed CpPSY at varying transcript levels. In comparison with the wild-type plants, the CpPSY expressing transgenic plants showed a significant increase on root length and shoot biomass under PEG-, NaCl- and mannitol-induced osmotic stress. The enhanced stress tolerance of transgenic plants was correlated with the increased endogenous ABA level and expression of stress-responsive genes, which in turn was correlated with the CpPSY copy number and expression level in different transgenic lines. Collectively, these results provide further evidence that PSY is a key enzyme regulating ABA biosynthesis and that the altered expression of other PSYs in transgenic plants may provide a similar function to that of the monocot's PSY3 in ABA biosynthesis and stress tolerance. The results also pave the way for further use of CpPSY, as well as other PSYs, as potential candidate genes for engineering tolerance to drought and salt stress in crop plants.

Characterization of phytoene synthases from cassava and their involvement in abiotic stress-mediated responses

Abiotic stress stimuli induce the increased synthesis of abscisic acid (ABA), which is generated through the cleavage of xanthophyll precursors. To cope with the increased xanthophyll demand, maize and rice contain a third stress-induced gene copy, coding for phytoene synthase (PSY), which catalyzes the first carotenoid-specific reaction in the pathway. To investigate whether this specific response extends beyond the Poaceae, cassava was analyzed, an important tropical crop known for its drought tolerance. We also found three PSY genes in cassava, one of which (MePSY3) forms a separate branch with the stress-specific Poaceae homologs. However, MePSY3 transcripts were virtually absent in all tissues investigated and did not change upon abiotic stress treatment. In contrast, the two remaining PSY genes contributed differentially to carotenoid biosynthesis in leaves, roots, and flower organs and responded towards drought and salt-stress conditions. Detailed analyses of PSY and 9-cis-epoxycarotenoid cleavage dioxygenase (MeNCED) expression and resulting ABA levels revealed MePSY1 as the main stress-responsive paralog. In the presence of high carotenoid levels in leaves, MePSY1 appeared to support, but not to be rate-limiting for ABA formation; MeNCED represented the main driver. The inverse situation was found in roots where carotenoid levels are low. Moreover, ABA formation and the relative induction kinetics showed discrimination between drought and salt stress. Compared to rice as a drought-intolerant species, the drought response in cassava followed a different kinetic regime. The difference is thought to represent a component contributing to the large differences in the adaptation towards water supply.

Mycorrhization of the notabilis and sitiens tomato mutants in relation to abscisic acid and ethylene contents

We examined whether the reduced mycorrhization of abscisic acid (ABA)-deficient tomato mutants correlates with their incapacity in ABA biosynthesis and whether this effect is dependent on ethylene production. The mycorrhization of notabilis and sitiens mutants, which have different ABA deficiencies and an excess of ethylene production, was analyzed. Comparative analysis of the ABA-deficient tomato mutants showed both quantitative and qualitative differences in the pattern of arbuscular mycorrhiza (AM) colonization between the two tomato mutant phenotypes. The sitiens mutant showed a great limitation in fungal colonization (mycorrhizal intensity and arbuscule formation) well correlated with their incapacity in ABA biosynthesis. The notabilis plants, which maintained normal ABA levels in roots under our experimental conditions, appeared to be less affected in their capacity for AM formation, and only a decrease in mycorrhizal intensity was noted at the end of the mycorrhization process. Blockage of ABA formation after tungstate application resulted in a reduction in mycorrhization of wild-type tomato plants. The transcript accumulation of the mycorrhiza-responsive LePT4 gene (tomato phosphate transporter) was clearly associated with the ABA content and mycorrhiza development in roots, as the tungstate treatment in wild-type plants and the inherent ABA deficiency in sitiens mutants led to a complete abolishment of their expression. Our results suggest that the decrease in arbuscular abundance in mycorrhizal sitiens roots is directly associated with their ABA biosynthesis deficiency, and the accumulation of ethylene, as a consequence of ABA deficiency in the mutants, primarily affects mycorrhizal intensity.

Cytochrome P450 mono-oxygenases in conifer genomes: discovery of members of the terpenoid oxygenase superfamily in spruce and pine

Diterpene resin acids, together with monoterpenes and sesquiterpenes, are the most prominent defence chemicals in conifers. These compounds belong to the large group of structurally diverse terpenoids formed by enzymes known as terpenoid synthases. CYPs (cytochrome P450-dependent mono-oxygenases) can further increase the structural diversity of these terpenoids. While most terpenoids are characterized as specialized or secondary metabolites, some terpenoids, such as the phytohormones GA (gibberellic acid), BRs (brassinosteroids) and ABA (abscisic acid), have essential functions in plant growth and development. To date, very few CYP genes involved in conifer terpenoid metabolism have been functionally characterized and were limited to two systems, yew (Taxus) and loblolly pine (Pinus taeda). The characterized yew CYP genes are involved in taxol diterpene biosynthesis, while the only characterized pine terpenoid CYP gene is part of DRA (diterpene resin acid) biosynthesis. These CYPs from yew and pine are members of two apparently conifer-specific CYP families within the larger CYP85 clan, one of four plant CYP multifamily clans. Other CYP families within the CYP85 clan were characterized from a variety of angiosperms with functions in terpenoid phytohormone metabolism of GA, BR, and ABA. The recent development of EST (expressed sequence tag) and FLcDNA (where FL is full-length) sequence databases and cDNA collections for species of two conifers, spruce (Picea) and pine, allows for the discovery of new terpenoid CYPs in gymnosperms by means of large-scale sequence mining, phylogenetic analysis and functional characterization. Here, we present a snapshot of conifer CYP data mining, discovery of new conifer CYPs in all but one family within the CYP85 clan, and suggestions for their functional characterization. This paper will focus on the discovery of conifer CYPs associated with diterpene metabolism and CYP with possible functions in the formation of GA, BR, and ABA in conifers.

Effects of sugars and abscisic acid on somatic embryogenesis from melon ( Cucumis melo L.) expanded cotyledon

Effects of sugars and abscisic acid (ABA) on somatic embryogenesis from melon ( Cucumis melo L.) expanded cotyledon and changes in ABA content on cultures on the various sucrose treatment media were investigated. Sucrose induced somatic embryogenesis, but mannitol did not. Two hundred millimolar sucrose treatments were optimal for somatic embryogenesis; the formation of somatic embryos was suppressed at lower or higher sucrose concentrations. The frequency of somatic embryogenesis, however, was increased by the addition of mannitol to the initial medium, even if the sucrose concentration was less than 200 mM. Somatic embryogenesis was enhanced by the addition of 0.5 μM ABA to 200 mM sucrose treatment. In the 200 mM sucrose treatment, in which somatic embryos were easily formed, endogenous levels of ABA in callus increased greatly after 2 weeks of culture. ABA content of embryogenic callus was markedly higher than that of non-embryonic callus at any given period. These results indicate that ABA formation, which was controlled by osmotic conditions induced by the sugars, could be involved in somatic embryogenesis.

GROWTH RETARDANTS: Effects on Gibberellin Biosynthesis and Other Metabolic Pathways.

Plant growth retardants are applied in agronomic and horticultural crops to reduce unwanted longitudinal shoot growth without lowering plant productivity. Most growth retardants act by inhibiting gibberellin (GA) biosynthesis. To date, four different types of such inhibitors are known: (a) Onium compounds, such as chlormequat chloride, mepiquat chloride, chlorphonium, and AMO-1618, which block the cyclases copalyl-diphosphate synthase and ent-kaurene synthase involved in the early steps of GA metabolism. (b) Compounds with an N-containing heterocycle, e.g. ancymidol, flurprimidol, tetcyclacis, paclobutrazol, uniconazole-P, and inabenfide. These retardants block cytochrome P450-dependent monooxygenases, thereby inhibiting oxidation of ent-kaurene into ent-kaurenoic acid. (c) Structural mimics of 2-oxoglutaric acid, which is the co-substrate of dioxygenases that catalyze late steps of GA formation. Acylcyclohexanediones, e.g. prohexadione-Ca and trinexapac-ethyl and daminozide, block particularly 3ss-hydroxylation, thereby inhibiting the formation of highly active GAs from inactive precursors, and (d) 16,17-Dihydro-GA5 and related structures act most likely by mimicking the GA precursor substrate of the same dioxygenases. Enzymes, similar to the ones involved in GA biosynthesis, are also of importance in the formation of abscisic acid, ethylene, sterols, flavonoids, and other plant constituents. Changes in the levels of these compounds found after treatment with growth retardants can mostly be explained by side activities on such enzymes.

Regulation of abscisic acid metabolism: towards a metabolic basis for abscisic acid-cytokinin antagonism

xanthine dehydrogenase (XDH) activity and a model illustrating the proposed metabolic interrelationship is The penultimate step in abscisic acid (ABA) biosyn- presented. thesis involves oxidation of xanthoxal (XAN) catalysed by a molybdenum-cofactor (MoCo)-containing alde- Key words: Abscisic acid, aldehyde oxidase, allopurinol, hyde oxidase (AO) and represents one potential site avocado, cytokinin, molybdenum cofactor, tungstate, of regulation of ABA in plant tissues. In an attempt xanthoxal. to understand the biochemical basis for cytokinin‐ abscisic acid (CK-ABA) antagonism the effect of sev- Introduction eral CKs, molybdate, tungstate and allopurinol (an inhibitor of xanthine oxidase activity and purine meta- Cytokinins appear to antagonize many physiological probolism) on the formation of XAN, ABA and related cesses thought to be mediated, all or in part, by ABA. catabolites in mesocarp of ripening avocado (Persea For example, ABA-induced stomatal closure, leaf senesamericana Mill. cv. Hass) was investigated. Treatment cence, and leaf and fruit abscission are reversed by with either adenine (Ade), isopentenyladenine (2iP) or exogenous application of CK while CK-mediated release zeatin (Z) enhanced conversion of ABA to phaseic acid of seed dormancy contrasts with ABA inhibition of (PA) and caused a reduction in the amount of radio- germination (Salisbury, 1994). This implied antagonism activity incorporated from 3R-[2-14C] mevalonolactone may be the result of metabolic interaction particularly as (MVL) into ABA by stimulating overall ABA metabolism. CKs share, at least in part, a common biosynthetic origin Ancymidol and N-(2-chloro-4-pyridyl)-N-phenylurea with ABA. CK biosynthesis appears to involve the addi(CPPU), while not affecting formation of PA and DPA, tion of dimethylallyl pyrophosphate to the N6 position appeared to retard ABA biosynthesis which resulted of AMP and subsequent conversion of N6D2-isopentenyl in the accumulation of XAN. Tungstate caused accu- AMP to N6D2-isopentenyl adenosine and 2iP, followed mulation of XAN at the expense of ABA and related by hydroxylation of 2iP to yield ‘zeatin-like’ CKs (Binns, acidic metabolites while molybdate and allopurinol 1994). By comparison, ABA is regarded as an apocarotaccelerated ABA metabolism, i.e. formation of XAN, enoid derived from the metabolism of isopentenyl pyroABA, PA, and DPA. These findings are discussed in phosphate via zeaxanthin, 9ae-cis-neoxanthin and XAN, terms of the regulation of the ABA biosynthetic path- which is then oxidized to ABA (Cowan and Richardson, 1997; Duckham et al., 1991; Parry, 1993; Rock and way in avocado fruit by CK-induced suppression of

Carotenogenic and abscisic acid biosynthesizing activity in a cell‐free system

Abscisic acid is considered an apocarotenoid formed by cleavage of a C‐40 precursor and subsequent oxidation of xanthoxin and abscisic aldehyde. Confirmation of this reaction sequence is still awaited, and might best be achieved using a cell‐free system capable of both carotenoid and abscisic acid biosynthesis. An abscisic acid biosynthesizing cell‐free system, prepared from flavedo of mature orange fruits, was used to demonstrate conversion of farnesyl pyrophosphate, geranylgeranyl pyrophosphate and all‐trans‐β‐carotene into a range of β,β‐xanthophylls, xanthoxin, xanthoxin acid, 1′,4′‐trans‐abscisic acid diol and abscisic acid. Identification of product carotenoids was achieved by high‐performance liquid chromatography and on‐line spectral analysis of individual components together with co‐chromatography. Putative C‐15 intermediates and product abscisic acid were identified by combined capillary gas chroma‐tography‐mass spectrometry. Kinetic studies revealed that β‐carotene, formed from either famesyl pyrophosphate or geranylgeranyl pyrophosphate, reached a maximum within 30 min of initiation of the reaction. Thereafter, β‐carotene levels declined exponentially. Catabolism of substrate β‐carotene into xanthophylls, putative abscisic acid precursors and product abscisic acid was restricted to the all‐trans‐isomer. However, when a combination of all‐trans‐ and 9‐cis‐β‐carotene in the ratio 1:1 was used as substrate, formation of abscisic acid and related metabolites was enhanced. Biosyn‐thetically prepared [14C]‐all‐trans‐violaxanthin, [14C]‐all‐trans‐neoxanthin and [14C]‐9′‐cis‐neoxanthin were used as substrates to confirm the metabolic interrelationship between carotenoids and abscisic acid. The results are consistent with 9′‐cis‐neoxan‐thin being the immediate carotenoid precursor to ABA, which is oxidatively cleaved to produce xanthoxin. Formation of abscisic aldehyde was not observed. Rather, xanthoxin appeared to be converted to abscisic acid via xanthoxin acid and 1′,4′‐trans‐abscisic acid diol. An alternative pathway for abscisic acid biosynthesis is therefore proposed.

Genotype-specific differences in chilling tolerance of maize in relation to chilling-induced changes in water status and abscisic acid accumulation.

Four inbred maize lines differing in chilling tolerance were used to study changes in water status and abscisic acid (ABA) levels before, during and after a chilling period. Seedlings were raised in fertilized soil at 24/22°C (day/night), 70% relative humidity. and a 12-h photoperiod with 200 μmol m-2 s-1 from fluorescent tubes. At an age of 2 weeks the plants were conditioned at 14/12°C for 4 days and then chilled for 5 days at 5/3°C. The other conditions (relative humidity, quantum flux, photoperiod) were unchanged. After the chilling period the plants were transferred to the original conditions for recovery. The third leaves were used to study changes in leaf necrosis, ion efflux, transpiration, water status and ABA accumulation. Pronounced differences in chilling tolerance between the 4 lines as estimated by necrotic leaf areas, ion efflux and whole plant survival were observed. Conditioning significantly increased tolerance against chilling at 5/3°C in all genotypes. The genotypes with low chilling tolerance had lower water and osmotic potentials than the more tolerant genotypes during a chilling period at 5/3°C. These differences were related to higher transpiration rates and lower diffusive resistance values of the more susceptible lines. During chilling stress at 5/3°C ABA levels were quadrupled. Only a small rise was measurable during conditioning at 14/12°C. However, conditioning enhanced the rise of ABA during subsequent chilling. ABA accumulation in the two lines with a higher chilling tolerance was triggered at a higher leaf water potential and reached higher levels than in the less tolerant lines. We conclude that chilling tolerance in maize is related to the ability for fast and pronounced formation of ABA as a protective agent against chilling injury.

Abscisic Acid Biosynthesis in Isolated Embryos of Zea mays L.

Previous labeling experiments with (18)O(2) have supported the hypothesis that stress-induced abscisic acid (ABA) is synthesized through an indirect pathway involving an oxygenated carotenoid (xanthophyll) as a precursor. To investigate ABA formation under nonstress conditions, an (18)O(2) labeling experiment was conducted with isolated embryos from in vitro grown maize (Zea mays L.) kernels. Of the ABA produced during the incubation in (18)O(2), three-fourths contained a single (18)O atom located in the carboxyl group. Approximately one-fourth of the ABA synthesized during the experiment contained two (18)O atoms. These results suggest that ABA synthesized in maize embryos under nonstress conditions also proceeds via the indirect pathway, requiring a xanthophyll precursor. It was also found that the newly synthesized ABA was preferentially released into the surrounding medium.

Regulation of Abscisic Acid Formation inMycosphaerella(Cercospora)Rosicolaby Phosphate

The regulation by phosphate of abscisic acid (ABA) production by Mycosphaerella (Cercospora) rosicola was examined in defined media, submerged shake cultures. Phosphate concentrations below 0.5 mM were suboptimal for growth and caused ABA synthesis to begin earlier than at 0.5 mM, the optimal concentration for maximal yield of ABA. Higher concentrations of phosphate gave progressively greater suppression of ABA formation while dry weight yield was essentially constant from 0.5 mM to 10 mM. Addition of 5 mM phosphate to phosphate-limited cultures at various times showed that there was a critical period during the first 3-5 da of incubation during logarithmic growth and before ABA synthesis had begun when the fungus was sensitive to phosphate suppression. Mycosphaerella rosicola B. H. Davis, a member of the Loculoascomycetidae (Cercospora rosicola Pass., imperfect state), was shown to produce the plant growth regulator abscisic acid (ABA) by Assante et al. (1977). Norman et al. (1981a, b) have designed a defined medium for growth and ABA formation by this fungus. We examined the role of phosphate limitation in regulating ABA because this nutrient is an important regulator of secondary metabolism in other fungi (Weinberg, 1974), and because in a preliminary survey of growth limitation by various nutrients, phosphate limitation was the only one that caused earlier synthesis of ABA (Griffin and Walton, unpublished).

Regulation of Abscisic Acid Formation in Mycosphaerella (Cercospora) rosicola by Phosphate

The regulation by phosphate of abscisic acid (ABA) production by Mycosphaerella (Cercospora) rosicola was examined in defined media, submerged shake cultures. Phosphate concentrations below 0.5 mM were suboptimal for growth and caused ABA synthesis to begin earlier than at 0.5 mM, the optimal concentration for maximal yield of ABA. Higher concentrations of phosphate gave progressively greater suppression of ABA formation while dry weight yield was essentially constant from 0.5 mM to 10 mM. Addition of 5 mM phosphate to phosphate-limited cultures at various times showed that there was a critical period during the first 3–5 da of incubation during logarithmic growth and before ABA synthesis had begun when the fungus was sensitive to phosphate suppression.

Apical Dominance in the ‘Rogue’ Tomato

The rogue tomato exhibits less apical dominance than the normal plant though the degree of correlative inhibition varies considerably between winter- and summer-sown plants. An examination of the level of endogenous hormones in both rogue and normal plants at both times of year indicates that the degree of branching is strongly associated with the levels of auxin in the tissue. It is suggested that this hormone has an effect on apical dominance by virtue of its role in hormone-directed transport and by its effect on the formation of abscisic acid in the region of the lateral buds. The results are discussed in relation to current hypotheses of the mechanism of apical dominance.

Abscisic Acid Translocation and Influence of Water Stress on Grain Abscisic Acid Content

Abscisic Acid Translocation and Influence of Water Stress on Grain Abscisic Acid Content