Fluridone affects quiescent centre division in the Arabidopsis thaliana root stem cell niche

Plants undergo cell division throughout their life in order to maintain their growth. It is well known that root and shoot tip of plants possess meristems, which contain quiescent cells. Fluridone (1-methyl-3-phenyl-5-(3-trifluoromethyl (phenyl))-4-(1H)-pyridinone) is an established inhibitor of both ABA and carotenoid biosynthesis. However, the other functions of fluridone remain undiscovered. In this report, we provide experimental evidence that fluridone plays a role in the division of the quiescent centre of the Arabidopsis root meristem. This study examined the effects of exogenous fluridone and ABA on the development of the stem cell niche in Arabidopsis root. We show that fluridone promoted the division of stem cells in the quiescent centre, whereas exogenous ABA suppressed quiescent centre division. Furthermore, we established a novel regulatory function for fluridone by demonstrating that it plays an important role in postembryonic development.     

The Arabidopsis Spontaneous Cell Death1 gene, encoding a ζ-carotene desaturase essential for carotenoid biosynthesis, is involved in chloroplast development,photoprotection and retrograde signalling

Carotenoids, a class of natural pigments found in all photosynthetic organisms, are involved in a variety of physiological processes, including coloration, photoprotection, biosynthesis of abscisic acid （ABA） and chloroplast biogenesis. Although carotenoid biosynthesis has been well studied biochemically, the genetic basis of the pathway is not well understood. Here, we report the characterization of two allelic Arabidopsis mutants, spontaneous cell death1-1 （spcl-1） and spc1-2. The weak allele spc1-1 mutant showed characteristics of bleached leaves, accumulation of superoxide and mosaic cell death. The strong mutant allele spc1-2 caused a complete arrest of plant growth and development shortly after germination, leading to a seedling-lethal phenotype. Genetic and molecular analyses indicated that SPC1 encodes a putative ζ-carotene desaturase （ZDS） in the carotenoid biosynthesis pathway. Analysis of carotenoids revealed that several major carotenoid compounds downstream of SPC 1/ZDS were substantially reduced in spc1-1, suggesting that SPC 1 is a functional ZDS. Consistent with the downregulated expression of CAO and PORB, the chlorophyll content was decreased in spc1-1 plants. In addition, expression of Lhcb1. 1, Lhcbl. 4 and RbcS was absent in spc1-2, suggesting the possible involvement of carotenoids in the plastid-to-nucleus retrograde signaling. The spc1-1 mutant also displays an ABA-deficient phenotype that can be partially rescued by the externally supplied phytohormone. These results suggest that SPC1/ZDS is essential for biosynthesis of carotenoids and plays a crucial role in plant growth and development.     

Chemical biology of abscisic acid

Chemical biology is a discipline that utilizes chemicals to elucidate biological mechanisms and physiological functions. Various abscisic acid (ABA) derivatives have revealed the structural requirement for the perception by ABA receptors while biotin or caged derivatives of ABA have disclosed the localization of several ABA-binding proteins. Recently, selective ABA agonist has been used to identify ABA receptors. Furthermore, ABA biosynthesis and catabolic inhibitors have contributed to the identification of new ABA functions in plant growth and development. The physiological function of ABA in non-plant organisms has gradually been revealed. In this review, we discuss the development of small bioactive chemicals and their significance in ABA research.     

Abscisic acid biosynthesis and catabolism and their regulation roles in fruit ripening

Abscisic acid (ABA) plays a series of significant physiology roles in higher plants including but not limited to promote bud and seed dormancy, accelerate foliage fall, induce stomatal closure, inhibit growth and enhance resistance. Recently, it has been revealed that ABA also has an important regulator role in the growth, development and ripening of fruit. In higher plants ABA is produced from an indirect pathway from the cleavage products of carotenoids. The accumulation of endogenous ABA levels in plants is a dynamic balance controlled by the processes of biosynthesis and catabolism, through the regulation of key ABA biosynthetic gene and enzyme activities. It has been hypothesized that ABA levels could be part of the signal that trigger fruit ripening, and that ABA may play an important role in the regulation of ripening and senescence of both non-climacteric and climacteric fruit. The expensive costs of natural ABA and labile active ABA for its chemical synthesis limit its application in scientific research and agricultural production. These findings that ABA has various of important roles in the regulation of growth and development, quality formation, coloring and softening, ripening and senescence of fruit, are providing opportunities and challenges for Horticultural Science. This is to elucidate the specific mechanism of response and biosynthesis, signal transduction, and receptor recognition of ABA in fruit, employing comprehensive research methods, such as molecular biology, plant physiology and molecular genetics. Further and more in-depth research about ABA has a great, realistic significance for knowing the mechanisms behind the process of fruit ripening.     

Effect of hydroxylated carotenoid deficiency on ABA accumulation in Arabidopsis

Abscisic acid (ABA) is a sesquiterpene compound (C₁₅) derived from C₄₀ carotenoids. The immediate carotenoid precursors for ABA biosynthesis, 9- cis -violaxanthin and 9'- cis -neoxanthin, are produced from β -carotene by a series of hydroxylation, epoxidation, and isomerization reactions. Carotenoid hydroxylase deficient mutants contain severely reduced levels of violaxanthin and neoxanthin ( < 20% of wild type level) and provide a unique system to correlate carotenoid substrate availability and ABA production in photosynthetic tissues under non-stressed conditions. Quantitative measurements indicated that ABA levels in the carotenoid hydroxylase mutants are reduced nearly 50% compared to the wild type plants under non-stressed conditions. When drought-stressed, wild type plants showed up to a 17-fold increase in ABA levels, while ABA levels in the carotenoid hydroxylase mutants were only increased 6- to 7-fold (25% of wild type drought-stressed ABA levels). Expression of AtNCED3 ( Arabidopsis thaliana nine- cis -epoxycarotenoid dioxygenase 3, the rate-limiting activity for ABA biosynthesis) was induced in the carotenoid hydroxylase mutants, but to a lesser extent than the 40-fold increase in wild type plants. Therefore, the reduced ABA accumulation in response to drought-stress is at least partially due to the attenuated increase in AtNCED3 gene expression in the carotenoid hydroxylase mutants. The remaining violaxanthin and neoxanthin in the carotenoid hydroxylase mutants can not be converted into ABA, indicating that there is probably a separate pool of violaxanthin and neoxanthin that is not accessible to the cleavage enzymes, because it is sequestered in the light-harvesting complexes.     

The ABA1 gene and carotenoid biosynthesis are required for late skotomorphogenic growth in Arabidopsis thaliana

Several plant hormones, including auxin, brassinosteroids and gibberellins, are required for skotomorphogenesis, which is the etiolated growth that seedlings undergo in the absence of light. To examine the growth of abscisic acid (ABA)-deficient mutants in the dark, we analysed several aba1 loss-of-function alleles, which are deficient in zeaxanthin epoxidase. The aba1 mutants displayed a partially de-etiolated phenotype, including reduced hypocotyl growth, cotyledon expansion and the development of true leaves, during late skotomorphogenic growth. In contrast, only small differences in hypocotyl growth were found between wild-type seedlings and ABA-deficient mutants impaired in subsequent steps of the pathway, namely nced3, aba2, aba3 and aao3. Interestingly, phenocopies of the partially de-etiolated phenotype of the aba1 mutants were obtained when wild-type seedlings were dark-grown on medium supplemented with fluridone, an inhibitor of phytoene desaturase, and hence, of carotenoid biosynthesis. ABA supplementation did not restore the normal skotomorphogenic growth of aba1 mutants or fluridone-treated wild-type plants, suggesting a direct inhibitory effect of fluridone on carotenoid biosynthesis. In addition, aba1 mutants showed impaired production of the beta-carotene-derived xanthophylls, neoxanthin, violaxanthin and antheraxanthin. Because fluridone treatment of wild-type plants phenocopied the phenotype of dark-grown aba1 mutants, impaired carotenoid biosynthesis in aba1 mutants is probably responsible for the observed skotomorphogenic phenotype. Thus, ABA1 is required for skotomorphogenic growth, and beta-carotene-derived xanthophylls are putative regulators of skotomorphogenesis.     

Inhibition of Abscisic Acid Biosynthesis in Cercospora rosicola by Inhibitors of Gibberellin Biosynthesis and Plant Growth Retardants

The fungus Cercospora rosicola produces abscisic acid (ABA) as a secondary metabolite. We developed a convenient system using this fungus to determine the effects of compounds on the biosynthesis of ABA. Inasmuch as ABA and the gibberellins (GAs) both arise via the isoprenoid pathway, it was of interest to determine if inhibitors of GA biosynthesis affect ABA biosynthesis. All five putative inhibitors of GA biosynthesis tested inhibited ABA biosynthesis. Several plant growth retardants with poorly understood actions in plants were also tested; of these, six inhibited ABA biosynthesis to varying degrees and two had no effect. Effects of plant growth retardants on various branches of the isoprenoid biosynthetic pathway may help to explain some of the diverse and unexpected results reported for these compounds. Knowledge that certain inhibitors of GA biosynthesis also have the ability to inhibit ABA biosynthesis in C. rosicola indicates the need for further studies in plants on the mode of action of these compounds.     

Increased endogenous abscisic Acid maintains primary root growth and inhibits shoot growth of maize seedlings at low water potentials

Roots of maize (Zea mays L.) seedlings continue to grow at low water potentials that cause complete inhibition of shoot growth. In this study, we have investigated the role of abscisic acid (ABA) in this differential growth sensitivity by manipulating endogenous ABA levels as an alternative to external applications of the hormone. An inhibitor of carotenoid biosynthesis (fluridone) and a mutant deficient in carotenoid biosynthesis (vp 5) were used to reduce the endogenous ABA content in the growing zones of the primary root and shoot at low water potentials. Experiments were performed on 30 to 60 hour old seedlings that were transplanted into vermiculite which had been preadjusted to water potentials of approximately −1.6 megapascals (roots) or −0.3 megapascals (shoots). Growth occurred in the dark at near-saturation humidity. Results of experiments using the inhibitor and mutant approaches were very similar. Reduced ABA content by either method was associated with inhibition of root elongation and promotion of shoot elongation at low water potentials, compared to untreated and wild-type seedlings at the same water potential. Elongation rates and ABA contents at high water potential were little affected. The inhibition of shoot elongation at low water potential was completely prevented in fluridone-treated seedlings during the first five hours after transplanting. The results indicate that ABA accumulation plays direct roles in both the maintenance of primary root elongation and the inhibition of shoot elongation at low water potentials.     

Abscisic acid biosynthesis during corn embryo development

In this study we examined the biosynthesis of abscisic acid (ABA) by developing corn (Zea mays L.) embryos. Three comparisons were made: ABA biosynthesis in embryos isolated from kernels grown in vitro with those grown in the field; the developmental profile of ABA content with that of biosynthesis; and ABA biosynthesis in corn embryos lacking carotenoid precursors with ABA biosynthesis in normal embryos. Embryos were harvested at various times during seed development and divided into two groups. Endogenous levels of ABA were measured in one group of embryos and ABA biosynthetic capacity was measured in the other group. The ABA biosynthetic capacity was measured with and without tetcyclacis (an inhibitor of ABA degradation) in embryos from both field-grown and in-vitro-grown corn kernels. Reduced-carotenoid (either fluridone-treated or genetically viviparous) embryos were also included in the study. Corn kernels developing under field and in-vitro conditions differed from each other in their responses to tetcyclacis and in their profiles of ABA biosynthesis during development. Therefore, in-vitro kernel culture may not be an appropriate substitute for field conditions for studies of embryo development. The developmental profiles of endogenous ABA content differed from those of ABA biosynthesis in isolated embryos of both in-vitro-and field-grown kernels. This indicated that ABA levels in the developing embryos were determined by import from the maternal tissues available to the embryos rather than by in-situ biosynthesis. In embryos with reduced levels of carotenoids, either fluridone-treated or genetically viviparous embryos, ABA biosynthesis was low or nonexistent. This result is expected for the presence of an indirect pathway of ABA biosynthesis and in the absence of ABA precursors.Abbreviations ABA abscisic acid - DAP days after pollination     

异戊二烯类植物激素赤霉素和脱落酸的代谢调控

赤霉素和脱落酸在植物生理过程中具有重要的调控作用,其生物合成途径迄今已基本阐明。赤霉素与类胡萝卜素的生物合成途径具有共同前体牻牛儿基牻牛儿基二磷酸,而脱落酸则直接来自于类胡萝卜素。参与这两种植物激素和类胡萝卜素代谢过程的大多数酶基因已经从不同植物中获得克隆;各种调控方式也随着分子生物学的研究工作而得到鉴定。本文就近年来对赤霉素和脱落酸等代谢调控机制及其与植物类胡萝卜素代谢之间关系的研究工作做简要回顾。     

Involvement of a lipoxygenase-like enzyme in abscisic Acid biosynthesis

Several lines of evidence indicate that abscisic acid (ABA) is derived from 9′-cis-neoxanthin or 9′-cis-violaxanthin with xanthoxin as an intermediate. ¹⁸O-labeling experiments show incorporation primarily into the side chain carboxyl group of ABA, suggesting that oxidative cleavage occurs at the 11, 12 (11′, 12′) double bond of xanthophylls. Carbon monoxide, a strong inhibitor of heme-containing P-450 monooxygenases, did not inhibit ABA accumulation, suggesting that the oxygenase catalyzing the carotenoid cleavage step did not contain heme. This observation, plus the ability of lipoxygenase to make xanthoxin from violaxanthin, suggested that a lipoxygenase-like enzyme is involved in ABA biosynthesis. To test this idea, the ability of several soybean (Glycine max L.) lipoxygenase inhibitors (5,8,11-eicosatriynoic acid, 5,8,11,14-eicosatetraynoic acid, nordihydroguaiaretic acid, and naproxen) to inhibit stress-induced ABA accumulation in soybean cell culture and soybean seedlings was determined. All lipoxygenase inhibitors significantly inhibited ABA accumulation in response to stress. These results suggest that the in vivo oxidative cleavage reaction involved in ABA biosynthesis requires activity of a nonheme oxygenase having lipoxygenase-like properties.     

The Chloroplast and Leaf Developmental Mutant, pale cress, Exhibits Light-conditional Severity and Symptoms Characteristic of its ABA Deficiency

The PALE CRESS gene (PAC) is essential for proper chloroplastand leaf development in Arabidopsis thaliana. The ability ofpac mutants to accumulate significantly more chlorophyll whengrown in low light conditions than in high light conditionssuggests that carotenoid deficiency is at least partly responsiblefor premature cessation of chloroplast development. In additionto accumulation of low levels of chlorophyll and carotenoidpigments,pac mutants are abscisic acid (ABA) deficient and havecharacteristics which may be explained by this deficiency. Theseinclude reduced seed viability and, in enclosed growth conditions,increased leaf growth. Plants transformed with an antisensePAC construct often bear viviparous embryos which may be symptomaticof a deficiency in ABA. Since carotenoids are precursors ofABA, a role for PAC in carotenoid biosynthesis is further supported.The nuclear-encoded, chloroplast-localized PAC protein has beenimplicated in the maturation of plastid-encoded mRNAs. Thus,PAC may affect the abundance of one or more chloroplast proteinswhich function in the synthesis or stability of carotenoids.Using thePROLIFERA gene as a marker for cell division, it isshown that cell division profiles in the pac shoot apex aredisrupted. pac leaves are relatively normal in size and shapedespite the light intensity-induced variability of leaf celldefects. Copyright 2000 Annals of Botany Company Abscisic acid, carotenoid, chloroplast development, leaf development, organismal theory, PALE CRESS,PROLIFERA , vivipary     

Ectopic expression of a tomato 9-cis-epoxycarotenoid dioxygenase gene causes over-production of abscisic acid

The tomato mutant notabilis has a wilty phenotype as a result of abscisic acid (ABA) deficiency. The wild-type allele of notabilis, LeNCED1, encodes a putative 9-cis-epoxycarotenoid dioxygenase (NCED) with a potential regulatory role in ABA biosynthesis. We have created transgenic tobacco plants in which expression of the LeNCED1 coding region is under tetracycline-inducible control. When leaf explants from these plants were treated with tetracycline, NCED mRNA was induced and bulk leaf ABA content increased by up to 10-fold. Transgenic tomato plants were also produced containing the LeNCED1 coding region under the control of one of two strong constitutive promoters, either the doubly enhanced CaMV 35S promoter or the chimaeric 'Super-Promoter'. Many of these plants were wilty, suggesting co-suppression of endogenous gene activity; however three transformants displayed a common, heritable phenotype that could be due to enhanced ABA biosynthesis, showing increased guttation and seed dormancy. Progeny from two of these transformants were further characterized, and it was shown that they also exhibited reduced stomatal conductance, increased NCED mRNA and elevated seed ABA content. Progeny of one transformant had significantly higher bulk leaf ABA content compared to the wild type. The increased seed dormancy was reversed by addition of the carotenoid biosynthesis inhibitor norflurazon. These data provide strong evidence that NCED is indeed a key regulatory enzyme in ABA biosynthesis in leaves, and demonstrate for the first time that plant ABA content can be increased through manipulating NCED.