Year: 2003
Type: review
Abstract: Plant growth and development are regulated by internal signals and by external environmental conditions. One important regulator that coordinates growth and development with responses to the environment is the sesquiterpenoid hormone abscisic acid (ABA). ABA plays important roles in many cellular processes including seed development, dormancy, germination, vegetative growth, and environmental stress responses. These diverse functions of ABA involve complex regulatory mechanisms that control its production, degradation, signal perception, and transduction. Because of the key role of ABA in plant stress responses, understanding these regulatory mechanisms will help devise rational strategies to breed or genetically engineer crop plants with increased tolerance to adverse environmental conditions. Since the discovery of ABA in the early 1960s, much effort has been devoted to understanding how ABA is synthesized. Through genetic and biochemical studies, the pathway for ABA biosynthesis in higher plants is now understood in great detail. Recently, all the major genes for the enzymes in the biosynthesis pathway have been identified (Schwartz et al., 2003). The new challenge is to understand how these biosynthesis genes, and the biosynthetic pathway as a whole, are regulated. Although much remains to be learned about the regulatory mechanism, evidence thus far indicates that ABA biosynthesis is subject to complex regulation during plant development and in response to environmental stresses. In this Update, we first present a brief overview of the functions of ABA and the biosynthesis pathway. We then focus on the regulation of ABA production and attempt to provide some future directions in ABA biosynthesis studies. Under non-stressful conditions, ABA in plant cells is maintained at low levels. Some low levels of ABA may be required for normal plant growth, as evidenced by reduced vigor observed in ABA-deficient mutant plants that can be restored to the wild-type level of growth by exogenous ABA (Finkelstein and Rock, 2002). Because all ABA-deficient mutants still have certain basal levels of ABA that are not dramatically lower than those in the wild type under normal growth conditions, it is difficult to uncover the cellular processes that require a very small amount of ABA. As a consequence, our knowledge of ABA functions has been gained mainly from observations with ABA at elevated levels, either from endogenous or exogenous sources. ABA levels can increase dramatically during seed maturation and in response to environmental stresses. Thus, ABA functions have been most extensively studied in these two processes. During seed development, ABA is known to initiate the following programs: embryo maturation, synthesis of storage reserves and late embryogenesis-abundant (LEA) proteins, and initiation of seed dormancy, although ABA is not the sole regulator of these processes. In particular, the induction of LEA protein synthesis to preserve the viability of embryos in the extremely dry condition of seeds is related to the role of ABA in promoting synthesis of LEA-like proteins in vegetative tissues to tolerate dehydration stress. Embryos from ABA antibody-expressing plants lose their viability as a result of desiccation intolerance (Phillips et al., 1997). In vegetative tissues, ABA levels increase when plants encounter adverse environmental conditions such as drought, salt, and to a lesser extent, low temperatures. Although a higher level of exogenous ABA inhibits plant growth under non-stressful conditions, an increased ABA content is beneficial for plants under environmental stress as a result of ABA-induced changes at the cellular and whole-plant levels. ABA promotes the closure of stomata to minimize transpirational water loss. It also mitigates stress damage through the activation of many stress-responsive genes that encode enzymes for the biosynthesis of compatible osmolytes and LEA-like proteins, which collectively increase plant stress tolerance (Hasegawa et al., 2000; Bray, 2002; Finkelstein et al., 2002). In addition, ABA has been shown to offset the inhibitory effect of stress-induced ethylene on plant growth (Sharp, 2002). Plant mutants defective in ABA biosynthesis are more susceptible to the environmental stresses and have been isolated in stress sensitivity screens (Xiong et al., 2002b). Importantly, manipulating ABA levels by changing the expression of key ABA biosynthetic genes provides an effective means to increase plant stress resistance. ABA-deficient mutants have been instrumental for revealing the pathway of ABA biosynthesis. By virtue of their precocious germination of seeds and the wilty appearance of the plants, mutants defective in ABA biosynthesis were isolated from a number of plant species including maize (Zea mays), tomato (Lycopersicon esculentum), tobacco (Nicotiana tabacum), potato (Solanum tuberosum), barley (Hordeum vulgare), and Arabidopsis. Before the molecular identities of the affected genes were known, a major route for ABA biosynthesis was revealed by profiling ABA biosynthetic intermediates in combination with feeding assays using these mutants. These studies suggested that ABA in higher plants is synthesized from an “indirect” pathway through the cleavage of a C40 carotenoid precursor, followed by a two-step conversion of the intermediate xanthoxin to ABA via ABA-aldehyde (Fig. 1; Taylor et al., 2000; Finkelstein and Rock, 2002; Seo and Koshiba, 2002; Schwartz et al., 2003). By now, major ABA-deficient mutants, genes, and enzymes have been characterized in Arabidopsis (Schwartz et al., 2003). The information from Arabidopsis is applicable to other plant species because the pathway and the respective genes are highly conserved in angiosperms. To avoid confusion, in this Update, genes are named after their products instead of the respective genetic loci. Regulation of ABA biosynthesis. ABA is derived from C40 epoxycarotenoid precursors through an oxidative cleavage reaction in plastids. The C15 intermediate xanthoxin is converted to ABA by a two-step reaction via ABA-aldehyde in cytosol. Abiotic stresses such as drought and salt activate the biosynthetic genes (italicized), probably through a Ca2+-dependent phosphorelay cascade as shown on the left. ABA feedback stimulates the expression of the biosynthetic genes, which is also likely through a Ca2+-dependent phosphoprotein cascade. Among the biosynthetic genes, NCED is strongly upregulated by stress (indicated with a thick arrow), whereas SDR is regulated by sugar. ABA biosynthetic enzymes are shown in small ovals. The NCED step probably limits ABA biosynthesis in leaves (indicated with a dashed arrow). ZEP, zeaxanthin epoxidase; NCED, 9-cis-epoxycarotenoid dioxygenase; AAO, ABA-aldehyde oxidase; MCSU, MoCo sulfurase. The first step that is more specific to the ABA biosynthesis pathway is the epoxidation of zeaxanthin and antheraxanthin to violaxanthin, which occurs in plastids. This step is catalyzed by a zeaxanthin epoxidase (ZEP), whose molecular identity was first revealed in tobacco (Marin et al., 1996). After a series of structural modifications, violaxanthin is converted to 9-cis-epoxycarotenoid. Oxidative cleavage of the major epoxycarotenoid 9-cis-neoxanthin by the 9-cis-epoxycarotenoid dioxygenase (NCED) yields a C15 intermediate, xanthoxin (Schwartz et al., 1997). This step was considered the first committed step in the ABA biosynthesis pathway. The ZmNCED gene was isolated using the maize vp14 mutant (Tan et al., 1997). The product xanthoxin is then exported to the cytosol, where it is converted to ABA through a two-step reaction via ABA-aldehyde. A short-chain alcohol dehydrogenase/reductase (SDR), encoded by the AtABA2 gene (Rook et al., 2001; Cheng et al., 2002; Gonzalez-Guzman et al., 2002), catalyzes the first step of this reaction and generates ABA aldehyde. ABA aldehyde oxidase (AAO) then catalyzes the last step in the biosynthesis pathway. Mutations in either the aldehyde oxidase apoprotein (e.g. Seo et al., 2000b) or molybdenum cofactor (MoCo) synthase (e.g. Ataba3 mutant) would impair ABA biosynthesis. The AtABA3 gene encodes a MoCo sulfurase that catalyzes the sulfurylation of a dioxo form of MoCo to a sulfurylated mono-oxo form (Bittner et al., 2001; Xiong et al., 2001b). This form of MoCo is required by aldehyde oxidase and xanthine dehydrogenase for their activities. Seed maturation and germination expose the young embryo to dramatic osmotic stresses. ABA is the major factor that is required to escort the embryo upon entering and exiting its quiescent state. ABA in developing seeds can either be derived from maternal tissues or be synthesized de novo in the embryo. Studies in Arabidopsis suggest that during seed development, there appear to be two peaks of ABA accumulation (for review, see Bentsink and Koornneef, 2002; Finkelstein et al., 2002). The first one occurs about halfway during seed development (approximately 10 d after pollination). This ABA is likely to be derived from maternal tissues because in reciprocal crosses, the peak only occurred when the wild-type but not ABA-deficient mutants were used as the female. ABA at this stage promotes the synthesis of storage proteins. Embryos from ABA antibody-expressing plants either did not accumulate or accumulated a much lower level of storage proteins relative to the wild-type embryos (Phillips et al., 1997). The second peak with less significant ABA accumulation (about one-third of the first peak) is from biosynthesis in the embryo and may activate the synthesis of LEA proteins that prepare the embryo for desiccation. This peak of ABA also initiates seed dormancy. ABA levels fall rapidly in the later stage of seed maturation and are very low in dry seeds. During seed imbibition, de novo ABA biosynthesis in the embryo is a determinant of seed dormancy (Bentsink and Koornneef, 2002; Finkelstein et al., 2002). ABA at this stage also maintains, within a narrow time window, the imbibed embryo in a reversible state between dormancy and germination by regulating the basic Leu Zip transcription factor ABI5 (Lopez-Molina et al., 2001). These important roles of ABA and its dynamics of accumulation in the embryo suggest that ABA biosynthesis is under tight developmental regulation in the embryo. Transcripts for all the ABA biosynthetic genes are detected in embryos/developing seeds, although more detailed analysis of the expression of individual genes during seed development has not been reported except for AtZEP1. In situ hybridization detected AtZEP expression in the embryo from globular to desiccation stages (Audran et al., 2001), which indicates that the ZEP gene was during the first peak of ABA accumulation in developing seeds. In to a role of ABA in it also an which ZEP other ABA biosynthetic in developing Although the of a developmental signal that one or more of the ABA biosynthetic genes be evidence that osmotic and ABA are likely to be the signals that activate ABA biosynthesis in developing seeds. ABA has been shown to be required for the first peak accumulation of ABA in developing seeds, it is not the ABA was derived from maternal tissues or that maternal ABA only as a signal for de novo synthesis of ABA in developing This is because ABA can its biosynthesis by ABA biosynthetic genes Because ZEP, and genes are all by to et al., 2002), levels may ABA biosynthesis in the embryo. This is more important for because is not by either osmotic stress or ABA. the of seed maturation, osmotic stress may more important in de novo ABA which is for embryo desiccation tolerance and dormancy. The activation of individual genes in seeds may be for ABA biosynthesis and accumulation in seeds. the expression of its between one-third and of seed development, which with increased ABA accumulation during this (Audran et al., seeds from plants a higher ABA level and dormancy et al., that may ABA biosynthesis in seeds and during seed in NCED may also ABA levels in the seeds. of the gene increased ABA levels in imbibed seeds and seed dormancy et al., These suggest that ABA biosynthesis in developing and seeds may be regulated at environmental signals such as have been suggested to ABA biosynthesis or The environmental conditions that most dramatically activate ABA are drought and salt stress. ABA levels under these stresses result mainly from increased de novo biosynthesis. The of ABA to be by stress and by ABA and stress and salt stresses ABA biosynthesis through regulation of ABA biosynthetic genes because transcription by using transcription stress-induced ABA biosynthesis. regulation of ABA biosynthetic genes the key to understanding how ABA biosynthesis is although regulation of the specific of ABA biosynthesis enzymes also ZEP was the first gene in the ABA biosynthesis pathway to be and its expression and regulation have been in a number of plant ZEP genes were in plant with a higher basal expression in leaves (Audran et al., Xiong et al., It was that ZEP not ABA biosynthesis in tissues because on a the amount of 9-cis-epoxycarotenoid of the in tissues such as leaves is higher than the amount of ABA during stress. In tobacco and tomato plants, the levels of ZEP genes in leaves were also not regulated by drought stress but were to be regulated with levels in the which may regulation by the (Audran et al., et al., than the of water changes in the in levels, in the level of ZEP protein was The regulation be related to the of ZEP products in the complex but not to the role of ZEP proteins in ABA biosynthesis. In to the amount of epoxycarotenoid precursors is lower in where ZEP, may ABA biosynthesis. with this ZEP genes in were regulated by drought stress. levels increased after drought stress in tobacco and tomato plants (Audran et al., et al., The regulation of ABA biosynthetic genes may not only between plant and developmental stages but also between plant the ZEP genes in tobacco and the Arabidopsis ZEP gene also a basal level under non-stressful conditions. drought, salt, and increased its expression level in the and in the (Xiong et al., that the AtZEP gene is under stress in the regulation of the ZEP genes observed in may be related to the relative basal levels. A basal level may stress of the studies with other ABA biosynthetic genes MCSU, and are less the that the cleavage step is in ABA the expression of NCED has stress were shown to NCED expression in maize (Tan et al., tomato et al., and Arabidopsis et al., 2001), et al., and and in NCED levels can be detected within to after or dehydration and et al., that the activation of NCED genes can be In with the of whose expression not to be regulated by stress et al., 2002; Gonzalez-Guzman et al., 2002), all the other ABA biosynthetic genes are by drought and salt stress et al., et al., 2001; Xiong et al., Xiong et al., although their protein levels have not been in most Because ABA biosynthesis increase dramatically upon stress it is that the protein levels of these genes increase after the levels, as was with the NCED gene and a with suggested that its protein levels did not as the levels et al., it is not this is only an the conditions In to the regulation of these genes by drought and salt the expression of AtZEP (Xiong et al., NCED and and (Xiong et al., was not by This is with the that the of increase in ABA in plants to was much less than that in plants The regulation of the SDR gene is was detected mainly in Thus, its expression tissues appear to be from those for other ABA biosynthetic genes that are more et al., 2002). The gene is at a low level and is not by drought stress. its expression is by sugar. During seed development, changes in levels in the seeds may have an on ABA as levels also and are by stress. stress under conditions effect on et al., 2002). biosynthetic are regulated by their ABA has been to ABA accumulation by its enzymes and The of a ABA which catalyzes the first step of ABA degradation, was by exogenous ABA (e.g. and Studies with tobacco NCED gene under an that ABA with the of the acid and 2002). These studies the that ABA its accumulation by its degradation, at under non-stressful conditions. the other ABA can or its biosynthesis was Because the NCED gene product has been suggested to the step in the ABA biosynthesis or not this gene is regulated by ABA is very to the of ABA can its biosynthesis. In tomato plants, it was that the NCED gene was not by exogenous ABA et al., in ABA was to activate NCED genes et al., These observations would the that ABA may its but not its when the expression of ABA biosynthetic genes was genetic a To our we have that ZEP, and in Arabidopsis are all by in to regulated by stress. ABA the expression of these genes (Xiong et al., these genes appear also to be regulated by endogenous ABA. It was observed that in of the ABA-deficient mutants or the levels for all the ABA biosynthetic genes under stress conditions were lower than those in the wild-type plants (Xiong et al., although their basal levels were in these mutants under non-stressful conditions (Audran et al., 2001; Xiong et al., Thus, ABA in the Arabidopsis mutants may not be a of the in the biosynthetic enzymes but also may be because of the reduced expression of other ABA biosynthetic genes as a result of the it was that the NCED gene was by ABA in certain genetic (e.g. ABA-deficient mutants and certain such as Xiong et al., Recently, Cheng et also reported that the gene AtZEP and be by ABA in the In addition, levels under drought and salt stress were reduced in the ABA-deficient mutants and as with those in wild-type that ABA is required for activation of by osmotic stress (Xiong et al., these observations strongly suggest a feedback regulation of ABA biosynthesis by ABA. This may a stress where an induction of ABA biosynthesis rapidly stimulates biosynthesis of ABA through this feedback (Fig. To understand this feedback regulatory it is important to which may this of ABA biosynthetic One such is to ABA and drought A in to exogenous ABA during seed germination, vegetative growth, and in gene expression (Xiong et al., mutant plants are also in ABA biosynthesis. of that the mutant was defective in the because the ABA regulation of the and genes (Xiong et al., gene products are required for the last step of ABA the conversion of ABA aldehyde to ABA (Fig. a feeding that the conversion of ABA-aldehyde to ABA was in the mutant (Xiong et al., This may be for the ABA biosynthesis in the It is likely that an small that is to be in and degradation, may the for the of an early ABA The may in the feedback One such is likely to be an protein because stress induction of this gene was in (Xiong et al., Although a in the protein also reduced the expression of this was not ABA et al., 2001). Thus, at some of the including may be in this as Arabidopsis and are protein that may ABA The and ABA in seed germination, vegetative growth, and the expression of certain genes (Finkelstein et al., 2002). In the of ABA biosynthesis is because in this ABA to activate the expression of the NCED gene and this reduced the levels of ZEP and under ABA all these genes were in (Xiong et al., their and and are to have roles in ABA it was shown that ABA-induced species production was in but not in In and closure in but not in et al., 2001). Because is in ABA and probably also in regulating ABA biosynthesis et al., 2001), it is that this of ABA biosynthetic genes may be by through a protein cascade (Fig. In the of in the feedback that for ABA biosynthesis is ABA and that there is between the pathway for ABA biosynthesis and the pathway for ABA studies are to the between these two and the of of ABA biosynthesis in ABA accumulation under stress conditions. in ABA biosynthesis either as a or a gene and the may be to regulation as whose has been the of ABA biosynthetic genes are known It that ZEP, MCSU, and SDR are genes, whereas NCED and to gene suggest that for those to gene is regulated by stresses. In addition, may be in a and developmental was in leaves but not in the and was mainly in et al., whereas may be less in seeds. Thus, mutant seeds were not in dormancy et al., which is from other known ABA-deficient mutants whose seeds are much less This may mutants were not isolated in germination It is also likely that may be regulated by stresses or may have of stress ABA biosynthetic genes and their regulation in Arabidopsis all the genes are in ABA those known to be are in are in ABA biosynthetic genes and their regulation in Arabidopsis all the genes are in ABA those known to be are in are in Because the ABA biosynthesis pathway gene there be a step in the pathway. this step is important for genetic of the pathway. the complex regulation mechanisms for individual ABA biosynthetic genes that may plant with the of ABA or its it would be difficult to a the regulatory of these genes or the step for ABA biosynthesis in the it was that the step catalyzed by NCED, the oxidative cleavage of is (Tan et al., and Taylor et al., 2000; et al., This may be in where most studies on ABA biosynthesis are with this or of the NCED genes in increased ABA biosynthesis and reduced water et al., 2001; and 2002; et al., the step is at the NCED, then one would that of other ABA biosynthetic genes would not increase ABA at not in the evidence that the AtZEP whose product catalyzes the step considered in the result in an increased stress gene induction in Arabidopsis (Xiong et al., of to increased seed dormancy and seed germination in tobacco et al., as a result of increased ABA biosynthesis in imbibed These observations would suggest that ZEP ABA biosynthesis in tissues such as seeds and It would be to see the expression level of other ABA biosynthetic genes can ABA biosynthesis. The that of the ABA biosynthetic genes probably have an on ABA biosynthesis may in have to with the in the This is because a increase in ABA biosynthesis from a ABA biosynthetic gene may result in a increased induction of other ABA biosynthetic genes through the (Fig. This can be by the expression of other ABA biosynthetic genes in the In addition, the that are either not upregulated or by ABA may the NCED step to late in ABA biosynthesis in As a consequence, regulating NCED genes may have a more significant on ABA biosynthesis. the regulation of ABA biosynthesis is of great in ABA level and plant stress responses and developmental Because ABA biosynthetic genes the and in their (Xiong et al., Bray, 2002), it is that these genes may be regulated as the of stress-responsive genes (Xiong et al., 2002b). evidence has suggested that the signal for stress-induced ABA biosynthesis may involve and protein and biochemical or genetic studies of of these processes in to ABA biosynthesis have been In the molecular studies the in ABA biosynthetic genes and the respective transcription that are for the activation of these genetic analysis will also be to such a complex signal screens of seed germination, either in the of synthesis or of salt, have identified in ABA biosynthesis in ABA These mutants also were in screens for sensitivity to (Finkelstein et al., 2002; et al., 2002). In some were identified in more than that ABA biosynthesis has in these Because and ABA biosynthesis have a in seed germination and in some other processes et al., 2000; and 2000; Gonzalez-Guzman et al., 2002), there is a that some of the in other hormone response may be in ABA biosynthesis regulation but have our One such may be response to ABA plants have a basal ABA level of that in the wild which is probably to increased biosynthesis because the ZEP level was also higher in et al., the effect of the on ABA biosynthesis be a of the complex between than a specific regulation of ABA biosynthesis. from genetic of the regulatory the in regulatory in screens may also have to with the that these screens are not for the of the To uncover the regulatory for ABA one may the roles of ABA in stress responses at the vegetative screens on stress responses in vegetative tissues would likely new that are important in regulating ABA biosynthesis or to the in manipulating drought stress in a and screens for drought tolerance have not been screens such as those for regulation (e.g. et al., and (e.g. sensitivity to may that ABA accumulation or Because gene expression is more to stress regulation than are some of the molecular genetic such as the one used in the for stress signal mutants et al., may be more in signal In this the of ABA biosynthetic genes particular, and can be to a mutants with gene in response to stress can be These screens may uncover regulatory mechanisms in ABA biosynthesis. the of the information of Arabidopsis and of expression also the of new regulatory in the pathway to ABA biosynthesis. A understanding of the regulation of ABA biosynthesis will require a combination of molecular and biochemical
Source: PLANT PHYSIOLOGY
Authors: Liming Xiong, Jian-Kang Zhu
Institutions: University of Arizona, Donald Danforth Plant Science Center
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: bronze
OpenAlex ID: W2130686286