Decrease In Abscisic Acid Levels Research Articles

The F-box protein ZmFBL41 negatively regulates disease resistance to Rhizoctonia solani by degrading the abscisic acid synthase ZmNCED6 in maize.

The maize F-box protein ZmFBL41 targets abscisic acid synthase 9-cis-epoxycarotenoid dioxygenase 6 for degradation, and this regulatory module is exploited by Rhizoctonia solani to promote infection. F-box proteins are crucial regulators of plant growth, development, and responses to abiotic and biotic stresses. Previous research identified the F-box gene ZmFBL41 as a negative regulator of maize (Zea mays) defenses against Rhizoctonia solani. However, the precise mechanisms by which F-box proteins mediate resistance to R. solani remain poorly understood. In this study, we show that ZmFBL41 interacts with an abscisic acid (ABA) synthase, 9-cis-epoxycarotenoid dioxygenase 6 (ZmNCED6), promoting its degradation via the ubiquitination pathway. We discovered that the ectopic overexpression of ZmNCED6 in rice (Oryza sativa) inhibited R. solani infection by activating stomatal closure, callose deposition, and jasmonic acid (JA) biosynthesis, indicating that ZmNCED6 enhances plant immunity against R. solani. Natural variation at ZmFBL41 across different maize haplotypes did not affect the ZmFBL41-ZmNCED6 interaction. These findings suggest that ZmFBL41 targets ZmNCED6 for degradation, leading to a decrease in ABA levels in maize, in turn, inhibiting ABA-mediated disease resistance pathways, such as stomatal closure, callose deposition, and JA biosynthesis, ultimately facilitating R. solani infection.

NO-mediated dormancy release of Avena fatua caryopses is associated with decrease in abscisic acid sensitivity, content and ABA/GAs ratios

Main conclusionNO releases caryopsis dormancy in Avena fatua, the effect being dependent on the level of dormancy. The NO effect involves also the reduction of caryopsis sensitivity to ABA and to a decrease in the ABA to GAs ratio due to a decrease in ABA levels and the lack of effect on GAs levels before germination is completed.Nitric oxide (NO) from various donors (i.e. SNP, GSNO and acidified KNO2), applied to dry caryopses or during initial germination, released primary dormancy in caryopses. Dormancy in caryopses was gradually lost during dry storage (after-ripening) at 25 °C, enabling germination at 20 °C in the dark. The after-ripening effect is associated with a decrease in NO required for germination. In addition, NO decreased the sensitivity of dormant caryopses to exogenous abscisic acid (ABA) and decreased the embryos’ ABA content before germination was completed. However, NO did not affect the content of bioactive gibberellins (GAs) from non-13-hydroxylation (GA4, GA7) and 13-hydroxylation (GA1, GA3, GA6.) pathways. Paclobutrazol (PAC), commonly regarded as a GAs biosynthesis inhibitor, counteracted the dormancy-releasing effect of NO and did not affect the GAs level; however, it increased the ABA content in embryos before germination was completed. Ascorbic acid, sodium benzoate and tiron, scavengers of reactive oxygen species (ROS), reduced the stimulatory effect of NO on caryopsis germination. This work provides new insight on the participation of NO in releasing A. fatua caryopses dormancy and on the relationship of NO with endogenous ABA and GAs.

The Marine Endophytic Polyamine-Producing Streptomyces mutabilis UAE1 Isolated From Extreme Niches in the Arabian Gulf Promotes the Performance of Mangrove (Avicennia marina) Seedlings Under Greenhouse Conditions

Mangrove forests in the Arabian Gulf are under continuous threat. To increase plantations of gray mangrove (Avicennia marina) in the United Arab Emirates (UAE), 27 endophytic actinobacterial isolates obtained from mangrove roots were in vitro pre-screened to detect the polyamines (PAs) [putrescine (Put), spermidine (Spd), and spermine (Spm)]. We also determined the abilities of the endophytic PA-producing actinobacterial isolates in enhancing the growth of A. marina under greenhouse conditions. Although three highly PA-producing isolates were recovered from inside mangrove root tissues, Streptomyces mutabilis UAE1 constantly colonized root and stem inner tissues for 12 weeks, suggesting an endophytic association between this actinobacterial isolate and mangrove seedlings. When roots were inoculated with S. mutabilis, mangrove growth was remarkably enhanced under gnotobiotic and greenhouse conditions. This was evident from the significant (P < 0.05) increases in dry weight and length of root (66.7 and 65.5%, respectively) and shoot (64.8 and 58.0%, respectively), number of branches (64.3%), total leaf area (40.2%), and photosynthetic pigments (54.5% chlorophyll a; 40.0% chlorophyll b; and 53.1% carotenoids) of mangrove compared to the PA-non-producing Streptomyces sp. or control treatment. Growth promotion in plants treated with S. mutabilis was also supported by significant (P < 0.05) elevations in the contents of mangrove in planta PAs, auxins, and cytokinins, accompanied by a decrease in abscisic acid levels. No difference was, however, detected in growth and amounts of PAs or any plant growth regulators (PGRs) in plants treated with or without the PA-non-producing isolate. Our findings indicate that plant growth parameters can be enhanced as a consequence of secretion of Put, Spd, and Spm by S. mutabilis resulting in regulation of endogenous PAs and other PGRs in mangrove tissues. This study is the first record that aims to improve sustainable agricultural management practices using plant growth promoting (PGP) actinobacteria, endophytic in mangrove tissues to promote mangrove growth under greenhouse conditions. Such research may allow this region to be a model to study the synergistic S. mutabilis–mangrove interactions and the future impacts on mangrove reforestation in the Arabian Gulf and elsewhere where mangrove forests are in threat.

Hydroelectrostatic hybrid priming stimulates germination performance via ABA and GA regulation: New promising evidence for tomato gene expression

Seed companies and farmers constantly demand seeds with proper agricultural performance. Seed priming technology is a feasible method to efficiently increase the production of agricultural crops by improving the morphophysiological pattern and metabolism of important enzymes, regulating phytohormones and reprogramming gene expression. The present study reports the ability of hydroelectrostatic hybrid priming, an ecofriendly combined method with multiple priming treatments, to enhance the germination indices and vigor of tomato seeds, as well as the effects of priming over a period of time. The dynamic levels of abscisic acid and gibberellins and the transcription levels of eleven genes associated with germination were tracked during the entire priming procedure and seed germination. Hydroelectrostatic hybrid priming enhances the germination index and vigor of tomato seeds, whereas unprimed seeds took longer to complete the germination process. An increase in gibberellin levels and a decrease in abscisic acid levels were detected during the priming procedures, allowing for the weakening of the endosperm cap quickly after sowing. In addition, significant changes in the transcription levels of SlNCED2 and SlDELLA were observed during the priming procedures; both genes play important roles during seed germination. The present investigation also demonstrated a potential synergism associated with the priming procedures, conferring fast germination of tomato seeds via hormone regulation and the reprogramming of gene expression. Regarding the effects of priming over a period of time, the results showed an enhancement of the germination indices and vigor responses of primed seeds compared to primed and unprimed seeds without a storage period. Hence, hydroelectrostatic hybrid priming is a promising and ecofriendly protocol for seed companies and farmers to meet the high demand for tomatoes.

Fig fruit ripening is regulated by the interaction between ethylene and abscisic acid.

Fleshy fruit ripening is typically regulated by ethylene in climacteric fruits and abscisic acid (ABA) in non-climacteric fruits. Common fig (Ficus carica) shows a dual-ripening mechanism, which is not fully understood. Here, we detected separate peaks of ethylene and ABA in fig fruits at the onset- and on-ripening stages, in conjunction with a sharp rise in glucose and fructose contents. In a newly-designed split-fruit system, exogenous ethylene failed to rescue fluridone-inhibited fruit ripening, whereas exogenous ABA rescued 2-amino-ethoxy-vinyl glycine (AVG)-inhibited fruit ripening. Transcriptome analysis revealed changes in the expression of genes key to both ABA and ethylene biosynthesis and perception during fig fruit ripening. At the de-greening stage, downregulation of FcACO2 or FcPYL8 retarded ripening, but downregulation of FcETR1/2 did not; unexpectedly, downregulation of FcAAO3 promoted ripening, but it inhibited ripening only before the de-greening stage. Furthermore, we detected an increase in ethylene emissions in the FcAAO3-RNAi ripening fruit and a decrease in ABA levels in the FcACO2-RNAi unripening fruit. Importantly, FcPYL8 can bind to ABA, suggesting that it functions as an ABA receptor. Our findings support the hypothesis that ethylene regulates the fig fruit ripening in an ABA-dependent manner. We propose a model for the role of the ABA-ethylene interaction in climacteric/non-climacteric processes.

Physiological and biochemical characterization of bud dormancy: Evolution of carbohydrate and antioxidant metabolisms and hormonal profile in a low chill peach variety

In this work, we studied the interplay among the levels of starch, sugars, abscisic acid (ABA) and bioactive gibberellins (GAs), and the antioxidant metabolism during bud dormancy release in a low chill peach variety. The experiments were performed in two consecutive years and two geographical areas with different annual media temperature.In the cold area, peach plants accumulated more starch and soluble sugars than in the temperate area, sorbitol and sucrose being the most abundant sugars.The enzymatic activities of the ascorbate-glutathione (ASC-GSH) cycle and the ROS-scavengers enzymes superoxide dismutase (SOD), catalase and peroxidase (POX) were measured. The increased activity of the ASC-GSH cycle enzymes and POX suggested a mild oxidative stress at endodormancy and at dormancy release periods. In this sense, the presence of H2O2-sensitive antioxidant enzymes, including Fe-SOD, Cu,Zn-SOD, POX and APX in the floral buds could trigger the oxidative signaling leading to dormancy release. Interestingly, the induction of a new POX isoenzyme occurred at dormancy release, suggesting its use as a marker for this event in low chill peach varieties.GAs levels decreased at the end of the dormancy period, whereas ABA levels showed a strong decline throughout the dormancy period, especially in buds from the cold area. As a consequence, a sharp decrease in the ABA/GAs ratio was observed, a phenomenon closely related to dormancy release. This response was more evident in the cold area. Results suggested that a decrease in ABA levels, as well as in the ABA/GAs ratio, may be necessary for the bud dormancy release in peach, at least in low chill varieties. Overall, we observed an interaction among sugars, antioxidant enzymes and plant hormones in the dormancy period. Glucose levels correlated with chill accumulation, peaking at ecodormancy and dormancy release. This response was parallel with increases in some antioxidant enzymes, the induction of a new POX isoenzyme and the decline in ABA levels and the ratio ABA/GAs.

Ethylene, not ABA, is closely linked to the recovery of gas exchange after drought in four Caragana species.

Drought is a cyclical phenomenon in natural environments. During dehydration, stomatal closure is mainly regulated by abscisic acid (ABA) dynamics that limit transpiration in seed plants, but following rehydration, the mechanism of gas exchange recovery is still not clear. In this study, leaf water potential (ψleaf ), stomatal conductance (gs ), leaf hydraulic conductance (Kleaf ), foliar ABA level, ethylene emission rate in response to dehydration and rehydration were investigated in four Caragana species with isohydric (Caragana spinosa and C. pruinosa) and anisohydric (C. intermedia and C. microphylla) traits. Two isohydric species with ABA-induced stomatal closure exhibited more sensitive gs and Kleaf to decreasing ψleaf than two anisohydric species which exhibited a switch from ABA to water potential-driven stomatal closure during dehydration. Following rehydration, the recovery of gas exchange was not associated with a decrease in ABA level but was strongly limited by the degradation of the ethylene emission rate in all species. Furthermore, two anisohydric species with low drought-induced ethylene production exhibited more rapid recovery in gas exchange upon rehydration. Our results indicated that ethylene is a key factor regulating the drought-recovery ability in terms of gas exchange, which may shape species adaptation to drought and potential species distribution.

Elucidation of the mechanism of reflowering in tree peony (Paeonia suffruticosa) ‘Zi Luo Lan’ by defoliation and gibberellic acid application

In this study, the reflowering mechanism of tree peony (Paeonia suffruticosa ‘Zi Luo Lan’) after defoliation and gibberellic acid (GA) application (autumn-flowering treatment) was investigated by monitoring the morphological changes, measuring the endogenous GA3 and abscisic acid (ABA) contents, and determining the expression patterns of six GA- and two ABA-related genes. The results show that autumn-flowering treatment induced tree peony reflowering in autumn, which was accompanied by nutrient absorption in buds. The application of exogenous GA3 induced a simultaneous increase in GA3 and decrease in ABA levels, suggesting that the high ratios of GA3/ABA may play a key role in inducing tree peony reflowering. RT-qPCR analysis shows that PsCPS and PsGA2ox were significantly induced and inhibited by GA3 application, respectively, which supports the hypothesis that GA3 treatment induces endogenous GA3 production. In addition, GA3 treatment inhibited the expression of the PsGID1c, but its effect on PsGAI1 was limited, whereas the expression of PsGAMYB could be GA- or ABA-related. Furthermore, autumn-flowering treatment significantly inhibited the expression of PsNCED and PsbZIP, which coincides with the observed changes in ABA levels. Therefore, we postulate that autumn-flowering treatment induces tree peony reflowering by inhibiting the function of ABA accumulation and signaling.

Vernalization of Oriental hybrid lily 'Sorbonne': changes in physiology metabolic activity and molecular mechanism.

Oriental hybrid lily 'Sorbonne' was used to investigate molecular changes during the storage at 4 °C for dormancy-release besides physiology metabolic activity observations. In physiological mechanism, endogenous abscisic acid (ABA) concentration level of lily bulbs decreased as the cold preservation time increased, and it kept at a stable level after being preserved for 35 days. The level of soluble sugars concentrations also changed during the cold preservation time, and it increased as the cold preservation time raised to 49 days then decreased afterward. On molecule level, a new transcriptome providing comprehensive sequence profiling data of variation during dormancy-release in lily was constructed. 34,367 unigenes expressed differentially between the control and the treatment was analyzed. 14 genes including 8 MADS-box family genes, 4 genes related to plant hormone, and 2 DNA methylation genes were selected to identify the levels of their expression by qRT-PCR. Our results show that the decrease of ABA level during cold storage, as well as changes in plant hormone genes was correlated with dormancy-release; MADS-box family genes VRN2, FLC, FT, SOC1 a, as well as LFY, MIKC and ARF, MYB transcription factor were included in lily floral induction and DNA methylation was correlated to lily vernalization under low temperature. According to the results of the present studies, we predicted that plant hormone pathway, energy metabolic pathway, vernalization pathway, and DNA methylation played important roles during vernalization; these data provided the foundation for future studies of vernalization to induce flowering of lily.

Abscisic Acid Plays an Important Role in the Regulation of Strawberry Fruit Ripening

The plant hormone abscisic acid (ABA) has been suggested to play a role in fruit development, but supporting genetic evidence has been lacking. Here, we report that ABA promotes strawberry (Fragaria ananassa) fruit ripening. Using a newly established Tobacco rattle virus-induced gene silencing technique in strawberry fruit, the expression of a 9-cis-epoxycarotenoid dioxygenase gene (FaNCED1), which is key to ABA biosynthesis, was down-regulated, resulting in a significant decrease in ABA levels and uncolored fruits. Interestingly, a similar uncolored phenotype was observed in the transgenic RNA interference (RNAi) fruits, in which the expression of a putative ABA receptor gene encoding the magnesium chelatase H subunit (FaCHLH/ABAR) was down-regulated by virus-induced gene silencing. More importantly, the uncolored phenotype of the FaNCED1-down-regulated RNAi fruits could be rescued by exogenous ABA, but the ABA treatment could not reverse the uncolored phenotype of the FaCHLH/ABAR-down-regulated RNAi fruits. We observed that down-regulation of the FaCHLH/ABAR gene in the RNAi fruit altered both ABA levels and sugar content as well as a set of ABA- and/or sugar-responsive genes. Additionally, we showed that exogenous sugars, particularly sucrose, can significantly promote ripening while stimulating ABA accumulation. These data provide evidence that ABA is a signal molecule that promotes strawberry ripening and that the putative ABA receptor, FaCHLH/ABAR, is a positive regulator of ripening in response to ABA.

Phytochrome- and gibberellin-mediated regulation of abscisic acid metabolism during germination of photoblastic lettuce seeds.

Germination of lettuce (Lactuca sativa) 'Grand Rapids' seeds is regulated by phytochrome. The action of phytochrome includes alterations in the levels of gibberellin (GA) and abscisic acid (ABA). To determine the molecular mechanism of phytochrome regulation of ABA metabolism, we isolated four lettuce cDNAs encoding 9-cis-epoxycarotenoid dioxygenase (biosynthesis; LsNCED1-LsNCED4) and four cDNAs for ABA 8'-hydroxylase (catabolism; LsABA8ox1-LsABA8ox4). Measurements of ABA and its catabolites showed that a decrease in ABA level coincided with a slight increase in the level of the ABA catabolite phaseic acid after red light treatment. Quantitative reverse transcription-polymerase chain reaction analysis indicated that ABA levels are controlled by phytochrome through down-regulation of LsNCED2 and LsNCED4 expression and up-regulation of LsABA8ox4 expression in lettuce seeds. Furthermore, the expression levels of LsNCED4 decreased after GA(1) treatment, whereas the levels of expression of the other two genes were unaffected. The LsNCED4 expression was also down-regulated by red light in lettuce seeds in which GA biosynthesis was suppressed by AMO-1618, a specific GA biosynthesis inhibitor. These results indicate that phytochrome regulation of ABA metabolism is mediated by both GA-dependent and -independent mechanisms. Spatial analysis showed that after red light treatment, the ABA decrease on the hypocotyl side was greater than that on the cotyledon side of lettuce seeds. Moreover, phytochrome-regulated expression of ABA and GA biosynthesis genes was observed on the hypocotyl side, rather than the cotyledon side, suggesting that this regulation occurs near the photoperceptive site.

Field studies on the regulation of abscisic acid content and germinability during grain development of barley: molecular and chemical analysis of pre-harvest sprouting

To investigate whether the regulation of abscisic acid (ABA) content was related to germinability during grain development, two cDNAs for 9-cis-epoxycarotenoid dioxygenase (HvNCED1 and HvNCED2) and one cDNA for ABA 8'-hydroxylase (HvCYP707A1), which are enzymes thought to catalyse key regulatory steps in ABA biosynthesis and catabolism, respectively, were cloned from barley (Hordeum vulgare L.). Expression and ABA-quantification analysis in embryo revealed that HvNCED2 is responsible for a significant increase in ABA levels during the early to middle stages of grain development, and HvCYP707A1 is responsible for a rapid decrease in ABA level thereafter. The change in the embryonic ABA content of imbibing grains following dormancy release is likely to reflect changes in the expression patterns of HvNCEDs and HvCYP707A1. A major change between dormant and after-ripened grains occurred in HvCYP707A1; the increased expression of HvCYP707A1 in response to imbibition, followed by a rapid ABA decrease and a high germination percentage, was observed in the after-ripened grains, but not in the dormant grains. Under field conditions, HvNCED2 showed the same expression level and pattern during grain development in 2002, 2003, and 2004, indicating that HvNCED2 expression is regulated in a growth-dependent manner in the grains. By contrast, HvNCED1 and HvCYP707A1 showed a different expression pattern in each year, indicating that the expression of these genes is affected by environmental conditions during grain development. The varied expression levels of these genes during grain development and imbibition, which would have effects on the activity of ABA biosynthesis and catabolism, might be reflected, in part, in the germinability in field-grown barley.

Non-lethal freezing effects on seed degreening in Brassica napus

The effects of a non-lethal freezing stress on chlorophyll content, moisture level and distribution, and abscisic acid (ABA) levels were examined in siliques and seeds of Brassica napus (canola). A non-lethal freezing stress resulted in the retention of chlorophyll in seed at harvest that was most pronounced for seeds 28, 32 and 36 days after flowering (DAF). This increase was primarily due to an increased retention of chlorophyll a relative to chlorophyll b. Chlorophyll retention in seeds exposed to a non-lethal freezing stress correlated with an increased ABA catabolism, as measured 1, 3 or 7 days after the stress treatment. Although the non-lethal freezing stress had no significant effect on moisture content in seeds of siliques stressed at 28-44 DAF, moisture distribution, as viewed by magnetic resonance imaging, showed an uneven drying of 32 and 40 DAF siliques after exposure to the non-lethal freezing stress. Moisture was initially lost more rapidly from the silique wall between seeds, than in control non-stressed siliques. Increased moisture loss was not due to structural changes in the vasculature of the silique/seed of stressed tissues. These results are consistent with the hypothesis that a non-lethal freezing stress-induced decrease in ABA level, during seed maturation, effects an inhibition of normal chlorophyll a catabolism resulting in mature but green B. napus seed.

The Arabidopsis cytochrome P450 CYP707A encodes ABA 8'-hydroxylases: key enzymes in ABA catabolism.

The hormonal action of abscisic acid (ABA) in plants is controlled by the precise balance between its biosynthesis and catabolism. In plants, ABA 8'-hydroxylation is thought to play a predominant role in ABA catabolism. ABA 8'-hydroxylase was shown to be a cytochrome P450 (P450); however, its corresponding gene had not been identified. Through phylogenetic and DNA microarray analyses during seed imbibition, the candidate genes for this enzyme were narrowed down from 272 Arabidopsis P450 genes. These candidate genes were functionally expressed in yeast to reveal that members of the CYP707A family, CYP707A1-CYP707A4, encode ABA 8'-hydroxylases. Expression analyses revealed that CYP707A2 is responsible for the rapid decrease in ABA level during seed imbibition. During drought stress conditions, all CYP707A genes were upregulated, and upon rehydration a significant increase in mRNA level was observed. Consistent with the expression analyses, cyp707a2 mutants exhibited hyperdormancy in seeds and accumulated six-fold greater ABA content than wild type. These results demonstrate that CYP707A family genes play a major regulatory role in controlling the level of ABA in plants.

Changes in abscisic acid levels in embryo axes of Norway maple and sycamore seeds during maturation and dehydration

Changes in the abscisic acid (ABA) levels in embryo axes of seeds, belonging to the orthodox (Norway maple — Acer platanoides L.) and recalcitrant (sycamore — Acer pseudoplatanus L.) categories, were investigated throughout maturation using an ELISA (enzyme-linked immunosorbent assay) test. Concentration of ABA in embryo axes substantially differed depending on species and sampling date. ABA was always higher in Norway maple except at the end of seed maturation when ABA content was similar in both species. During maturation ABA decreased in both species but the decline was more marked in Norway maple than in sycamore (11 vs. 3 fold). These species also differed in the pattern of ABA changes, which in sycamore embryo axes was very regular, while in Norway maple a sharp decrease was recorded after acquisition by the seeds of tolerance to desiccation. Dehydration of embryo axes of Norway maple caused a further significant decrease of ABA level. In contrast, in dehydrated sycamore embryo axes ABA content did not decrease, but slightly increased. The role of ABA in desiccation tolerance and dormancy of Norway maple and sycamore seeds is discussed.

Fluctuation in levels of endogenous hormones after decapitation and 6-benzyl amino purine treatment in azalea, and their relationship to apical dominance

Endogenous levels of indole-3-acetic acid (IAA), cytokinins, gibberellins (GAs) and abscisic acid (ABA) in ‘Siji’ azalea ( Rhododendron obtusum), which had been sprayed once with 1000 mg/1 6-benzyl amino purine (6-BA) or decapitated, were determined by enzyme-linked immunosorbent assays. It was found that decapitation could significantly decrease the level of IAA in the shoot segment (0.5 cm in length just below the terminal bud, where axillary buds will emerge). A single application of 6-BA elicited a decrease of IAA level in the shoot segment, whereas an increase of IAA level in the terminal bud, and elicited an increase of isopentenyladenine and isopentenyladenosine, zeatin and zeatin riboside, in both the shoot segment and the terminal bud. It was also found that both the 6-BA treatment and decapitation could elicit an increase of GA 1+3 level and a decrease of ABA level in the shoot segment (and the terminal bud).

Confirmation that abscisic acid accumulation is required for maize primary root elongation at low water potentials

Abstract Previous work showed that primary root elongation in maize (Zeamays L.) seedlings at low water potentials was severely inhibited when accumulation of abscisic acid (ABA) was decreased using either fluridone or the vp5 mutant to inhibit carotenoid (and ABA) biosynthesis. The objective of this study was to confirm that the inhibition of root elongation resulting from these treatments was indeed attributable to the decrease in ABA levels. Seedlings were transplanted after germination to vermiculite at water potentials of −1.6 or − 0.3 MPa. ABA was mixed at various concentrations with the vermiculite to test whether the effects of fluridone and the vp5 mutation on root elongation could be overcome. In both treatments, restoration of ABA levels in the root apical 10 mm, which encompassed the elongation zone, resulted in recovery of root elongation rate at both −1.6 MPa and −0.3 MPa. Analysis of the spatial distribution of elongation rate showed that the recovery of overall root elongation resulted from restoration of the profile of elongation, and did not involve over-promotion of local elongation rate at any position. The recovery of root elongation was shown to be independent of effects of ABA status on shoot growth. When ABA was applied at high water potential, such that levels of ABA in the root tip reached those associated with maintenance of elongation at low water potentials, root elongation was inhibited. Thus, the response of root elongation to bulk tissue ABA content varied with the tissue water status. The results confirm that accumulation of ABA is required for the maintenance of maize primary root elongation at low water potentials.

Changes in abscisic acid levels in bean leaves during the initial stages of host and nonhost reactions to rust fungi

Changes in abscisic acid (ABA) levels were determined in leaf tissues of French bean ( Phaseolus vulgaris) inoculated with Uromyces appendiculatus var. appendiculatus (pathogenic) and U. vignae (nonpathogenic) using an indirect enzyme-linked immunosorbent assay (ELISA) and HPLC. There was a significant decrease in abscisic acid levels both in leaf tissues and intercellular washing fluids of rust-infected bean leaves at 6 h post inoculation. Similar decreases were induced by both fungi. ABA levels remained low until at least 48 h after inoculation. No evidence was found for production of ABA by differentiated urediospore germlings of either U. appendiculatus or U. vignae. The results suggest that the decrease in ABA levels is a nonspecific result of changes brought about by infection, prior to cell penetration, and that this growth regulator is not a determinant of successful fungal invasion.

Maternal Abscisic Acid Transport in Developing Embryos of Sunflower

Summary When [2- 14 C] abscisic acid (ABA) was applied to «blocks» excised from sunflower heads, containing achenes and leaving a layer of head tissue below the base of the achenes, its uptake by the embryo was about 2 times higher than when achenes were placed on the surface of the culture medium; this suggested that in the mother plant, preferential anatomical connections allowed maternal (exogenous) ABA to be transported to the embryo. This transport of ABA through the head tissue was possible even during late embryogenesis. Changes, with embryo age, in [2- 14 C] ABA uptake by isolated embryos proved that the regulation of external ABA accumulation took place at the level of the embryo itself. During the first part of embryo development, an increasing intracellular pH of cotyledonary cells contributed to the increasing accumulation of unmetabolized ABA. Thereafter, an increasing catabolism played a major role in the decrease of ABA level. The importance of this catabolism and its particular orientation towards the formation of alkaline non-hydrolysable compounds presented the problem of the physiological significance of these conjugates.

ABA Levels and Effects in Chilled and HardenedPhaseolus vulgaris

Leaf abscisic acid (ABA) levels of chilled P. vulgaris were measured after 18 h chilling at 5 °C, at a saturation deficit of 1-24 g m-3 (SD), and after chilling in a water-saturated atmosphere. Changes were also followed during a chill hardening period of 4 d at 12 °C, 2-1 g m~3 SD. It was found that hardening resulted in an almost 5-fold increase in ABA levels after 3 d at 12 °C, and this decreased to approximately control levels on the fourth day. Subsequent chilling of hardened plants produced no change in ABA levels from that of control plants (22 °C). In contrast, non-hardened plants chilled at 1-24 g m-3 SD had ABA levels almost 3 times the level of control plants. However, chilling in a water-saturated atmosphere resulted in a decrease in ABA levels. In addition, the response of leaf diffusion resistance (LDR) to exogenous ABA fed via the transpiration stream was measured at 5 °C and 22 °C in hardened and non-hardened plants. Use of tritium-labelled ABA was made to calculate the stomatal sensitivity to ABA. It was found that exogenous ABA caused an increased in LDR at 22 °C in both hardened and non-hardened plants. However, the sensitivity of the hardened plants to ABA was greater in terms of rate of closure and amount of ABA required to close the stomata. At 5 °C, however, ABA caused stomatal opening and the maintainance of open stomata in non-hardened plants. In hardened plants, ABA caused stomatal closure at 5 °C. These results are discussed in relation to the locking-open response of chilled P. vulgaris stomata.