Effects of postharvest storage and dormancy status on ABA content, metabolism, and expression of genes involved in ABA biosynthesis and metabolism in potato tuber tissues

At harvest, and for an indeterminate period thereafter, potato tubers will not sprout and are physiologically dormant. Abscisic acid (ABA) has been shown to play a critical role in tuber dormancy control but the mechanisms controlling ABA content during dormancy as well as the sites of ABA synthesis and catabolism are unknown. As a first step in defining the sites of synthesis and cognate processes regulating ABA turnover during storage and dormancy progression, gene sequences encoding the ABA biosynthetic enzymes zeaxanthin epoxidase (ZEP) and 9-cis-epoxycarotenoid dioxygenase (NCED) and three catabolism-related genes were used to quantify changes in their relative mRNA abundances in three specific tuber tissues (meristems, their surrounding periderm and underlying cortex) by qRT-PCR. During storage, StZEP expression was relatively constant in meristems, exhibited a biphasic pattern in periderm with transient increases during early and mid-to-late-storage, and peaked during mid-storage in cortex. Expression of two members of the potato NCED gene family was found to correlate with changes in ABA content in meristems (StNCED2) and cortex (StNCED1). Conversely, expression patterns of three putative ABA-8′-hydroxylase (CYP707A) genes during storage varied in a tissue-specific manner with expression of two of these genes rising in meristems and periderm and declining in cortex during storage. These results suggest that ABA synthesis and metabolism occur in all tuber tissues examined and that tuber ABA content during dormancy is the result of a balance of synthesis and metabolism that increasingly favors catabolism as dormancy ends and may be controlled at the level of StNCED and StCYP707A gene activities Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Chemically forced dormancy termination mimics natural dormancy progression in potato tuber meristems by reducing ABA content and modifying expression of genes involved in regulating ABA synthesis and metabolism

The length of potato tuber dormancy depends on both the genotype and the environmental conditions during growth and storage. Abscisic acid (ABA) has been shown to play a critical role in tuber dormancy control but the mechanisms regulating ABA content during dormancy, as well as the sites of ABA synthesis, and catabolism are unknown. Recently, a temporal correlation between changes in ABA content and certain ABA biosynthetic and catabolic genes has been reported in stored field tubers during physiological dormancy progression. However, the protracted length of natural dormancy progression complicated interpretation of these data. To address this issue, in this study the synthetic dormancy-terminating agent bromoethane (BE) was used to induce rapid and highly synchronous sprouting of dormant tubers. The endogenous ABA content of tuber meristems increased 2-fold 24 h after BE treatment and then declined dramatically. By 7 d post-treatment, meristem ABA content had declined by >80%. Exogenous [(3)H]ABA was readily metabolized by isolated meristems to phaseic and dihydrophaseic acids. BE treatment resulted in an almost 2-fold increase in the rate of ABA metabolism. A differential expression of both the StNCED and StCYP707A gene family members in meristems of BE-treated tubers is consistent with a regulatory role for StNCED2 and the StCYP707A1 and StCYP707A2 genes. The present results show that the changes in ABA content observed during tuber dormancy progression are the result of a dynamic equilibrium of ABA biosynthesis and degradation that increasingly favours catabolism as dormancy progresses.     

Regulation and Manipulation of the Biosynthesis of Abscisic Acid, Including the Supply of Xanthophyll Precursors

Mutant plants deficient in the phytohormone abscisic acid (ABA) are typically unable to control their stomatal behavior appropriately in response to water stress, leading to a “wilty” phenotype. In plant species showing strong seed dormancy, ABA deficiency of the seed results in a second clearly recognizable phenotype, that is, early germination. Mutants selected by means of this latter character are often collectively termed “viviparous.” These two broad classes include mutants that are defective in their ability to synthesize ABA. A number of these genetic lesions have been assigned to specific steps in ABA biosynthesis and have been invaluable in elucidating many important features of the pathway. Most of the genes encoding ABA biosynthetic enzymes have now been cloned and their expression has been studied and manipulated. Genetically modified plants constitutively overexpressing ABA biosynthesis genes have been produced and analyzed over the last 6 years. In some cases these plants have been found to have elevated ABA concentrations, leading to altered stomatal behavior and increased seed dormancy. Genetic manipulation of ABA synthesis in photosynthetic tissues has been most effectively achieved through overexpression of the key rate-limiting biosynthetic enzyme 9-cis-epoxycarotenoid dioxygenase, and downregulation of the major catabolic enzyme ABA 8′-hydroxylase. However in non-photosynthetic tissue manipulation of ABA synthesis is a more complex task because of the limiting supply of xanthophyll precursors. The recent cloning of genes encoding enzymes controlling important pathways of ABA catabolism has been reviewed elsewhere, and so only information relevant to the regulation and manipulation of ABA synthesis, including supply of xanthophyll precursors, is discussed in this review.     

Molecular identification of zeaxanthin epoxidase of Nicotiana plumbaginifolia, a gene involved in abscisic acid biosynthesis and corresponding to the ABA locus of Arabidopsis thaliana.

Abscisic acid (ABA) is a plant hormone which plays an important role in seed development and dormancy and in plant response to environmental stresses. An ABA-deficient mutant of Nicotiana plumbaginifolia, aba2, was isolated by transposon tagging using the maize Activator transposon. The aba2 mutant exhibits precocious seed germination and a severe wilty phenotype. The mutant is impaired in the first step of the ABA biosynthesis pathway, the zeaxanthin epoxidation reaction. ABA2 cDNA is able to complement N.plumbaginifolia aba2 and Arabidopsis thaliana aba mutations indicating that these mutants are homologous. ABA2 cDNA encodes a chloroplast-imported protein of 72.5 kDa, sharing similarities with different mono-oxigenases and oxidases of bacterial origin and having an ADP-binding fold and an FAD-binding domain. ABA2 protein, produced in Escherichia coli, exhibits in vitro zeaxanthin epoxidase activity. This is the first report of the isolation of a gene of the ABA biosynthetic pathway. The molecular identification of ABA2 opens the possibility to study the regulation of ABA biosynthesis and its cellular location.     

Auxotrophs produced by transposon mutagenesis in Streptomyces tendae ATCC 31160

Six auxotrophs were induced in Streptomyces tendae ATCC 31160 with transposon Tn 4560 . Insertion of the transposon into chromosomal DNA of these auxotrophs occurred at different sites suggesting that transposition may be random. Transposition frequency was 1 times 10^-3. Transposon-tagged antibiotic biosynthetic genes will provide a powerful tool for the isolation of genes involved in nikkomycin biosynthesis.     

Accumulation of an ABA analogue in the wilty tomato mutant,flacca

A new abscisic acid (ABA) analogue has been isolated from tomato plants. High levels of the compound are found in flacca mutants compared with normal isogenic controls. The analogue also accumulates in response to water stress. Three alternative structures, consistent with the mass spectrum, have been proposed. The possibility that the compound may be a biosynthetic precursor of ABA is considered.We are grateful for the financial support of the ARC.     

Analysis of the chromosomal distribution of transposon-carrying T-DNAs in tomato using the inverse polymerase chain reaction

We are developing a system for isolating tomato genes by transposon mutagenesis. In maize and tobacco, the transposon Activator (Ac) transposes preferentially to genetically linked sites. To identify transposons linked to various target genes, we have determined the RFLP map locations of Ac- and Dissociation (Ds)-carrying T-DNAs in a number of transformants. T-DNA flanking sequences were isolated using the inverse polymerase chain reaction (IPCR) and located on the RFLP map of tomato. The authenticity of IPCR reaction products was tested by several criteria including nested primer amplification, DNA sequence analysis and PCR amplification of the corresponding insertion target sequences. We report the RFLP map locations of 37 transposon-carrying T-DNAs. We also report the map locations of nine transposed Ds elements. T-DNAs were identified on all chromosomes except chromosome 6. Our data revealed no apparent chromosomal preference for T-DNA integration events. Lines carrying transposons at known map locations have been established which should prove a useful resource for isolating tomato genes by transposon mutagenesis.     

Control of abscisic acid synthesis

The abscisic acid (ABA) biosynthetic pathway involves the formation of a 9-cis-epoxycarotenoid precursor. Oxidative cleavage then results in the formation of xanthoxin, which is subsequently converted to ABA. A number of steps in the pathway may control ABA synthesis, but particular attention has been given to the enzyme involved in the oxidative cleavage reaction, i.e. 9-cis-epoxycarotenoid dioxygenase (NCED). Cloning of a gene encoding this enzyme in maize was first reported in 1997. Mapping and DNA sequencing studies indicated that a wilty tomato mutant was due to a deletion in the gene encoding an enzyme with a very similar amino acid sequence to this maize NCED. The potential use of this gene in altering ABA content will be discussed together with other genes encoding ABA biosynthetic enzymes.     

Complex regulation of ABA biosynthesis in plants.

Abscisic acid (ABA) is a plant hormone that plays important roles during many phases of the plant life cycle, including seed development and dormancy, and in plant responses to various environmental stresses. Because many of these physiological processes are correlated with endogenous ABA levels, the regulation of ABA biosynthesis is a key element facilitating the elucidation of these physiological characteristics. Recent studies on the identification of genes encoding enzymes involved in ABA biosynthesis have revealed details of the main ABA biosynthetic pathway. At the same time, the presence of gene families and their respective organ-specific expression are indicative of the complex mechanisms governing the regulation of ABA biosynthesis in response to plant organ and/or environmental conditions. There have been recent advances in the study of ABA biosynthesis and new insights into the regulation of ABA biosynthesis in relation to physiological phenomena.     

Identification of StiR, the first regulator of secondary metabolite formation in the myxobacterium Cystobacter fuscus Cb f17.1

Myxobacteria are well established as proficient producers of natural products with numerous biological activities. Although some knowledge has been gained regarding the biosynthesis of secondary metabolites in this class of bacteria, almost nothing is known about the underlying regulatory mechanisms. In order to identify regulatory elements, we submitted the argyrin and stigmatellin producer Cystobacter fuscus to a random transposon mutagenesis strategy and screened 1,000 mutants for the occurrence of strains showing remarkably increased or decreased production of these compounds. In addition to the identification of the stigmatellin biosynthetic gene cluster, a novel positive regulator (stiR) of stigmatellin production was identified after transposon recovery. In order to exclude secondary mutagenesis effects, a double cross-over mutagenesis strategy was applied to the strain resulting in the repeated generation of the transposon genotype. This strain was shown to be equally effected in natural product biosynthesis.     

A collection of sequenced and mapped Ds transposon insertion sites in Arabidopsis thaliana

Insertional mutagenesis is a powerful tool for generating knockout mutations that facilitate associating biological functions with as yet uncharacterized open reading frames (ORFs) identified by genomic sequencing or represented in EST databases. We have generated a collection of Dissociation(Ds) transposon lines with insertions on all 5 Arabidopsischromosomes. Here we report the insertion sites in 260 independent single-transposon lines, derived from four different Ds donor sites. We amplified and determined the genomic sequence flanking each transposon, then mapped its insertion site by identity of the flanking sequences to the corresponding sequence in the Arabidopsisgenome database. This constitutes the largest collection of sequence-mapped Ds insertion sites unbiased by selection against the donor site. Insertion site clusters have been identified around three of the four donor sites on chromosomes 1 and 5, as well as near the nucleolus organizers on chromosomes 2 and 4. The distribution of insertions between ORFs and intergenic sequences is roughly proportional to the ratio of genic to intergenic sequence. Within ORFs, insertions cluster near the translational start codon, although we have not detected insertion site selectivity at the nucleotide sequence level. A searchable database of insertion site sequences for the 260 transposon insertion sites is available at http://sgio2.biotec.psu.edu/sr. This and other collections of Arabidopsislines with sequence-identified transposon insertion sites are a valuable genetic resource for functional genomics studies because the transposon location is precisely known, the transposon can be remobilized to generate revertants, and the Ds insertion can be used to initiate further local mutagenesis.     

Abscisic acid biosynthesis in tomato: regulation of zeaxanthin epoxidase and 9-cis-epoxycarotenoid dioxygenase mRNAs by light/dark cycles,water stress and abscisic acid

Two genes encoding enzymes in the abscisic acid (ABA) biosynthesis pathway, zeaxanthin epoxidase (ZEP) and 9-cis-epoxycarotenoid dioxygenase (NCED), have previously been cloned by transposon tagging in Nicotiana plumbaginifolia and maize respectively. We demonstrate that antisense down-regulation of the tomato gene LeZEP1 causes accumulation of zeaxanthin in leaves, suggesting that this gene also encodes ZEP. LeNCED1 is known to encode NCED from characterization of a null mutation (notabilis) in tomato. We have used LeZEP1 and LeNCED1 as probes to study gene expression in leaves and roots of whole plants given drought treatments, during light/dark cycles, and during dehydration of detached leaves. During drought stress, NCED mRNA increased in both leaves and roots, whereas ZEP mRNA increased in roots but not leaves. When detached leaves were dehydrated, NCED mRNA responded rapidly to small reductions in water content. Using a detached leaf system with ABA-deficient mutants and ABA feeding, we investigated the possibility that NCED mRNA is regulated by the end product of the pathway, ABA, but found no evidence that this is the case. We also describe strong diurnal expression patterns for both ZEP and NCED, with the two genes displaying distinctly different patterns. ZEP mRNA oscillated with a phase very similar to light-harvesting complex II (LHCII) mRNA, and oscillations continued in a 48 h dark period. NCED mRNA oscillated with a different phase and remained low during a 48 h dark period. Implications for regulation of water stress-induced ABA biosynthesis are discussed.     

Molecular biology and regulation of abscisic acid biosynthesis in plants

The phytohormone abscisic acid (ABA) is involved in seed dormancy and the response to various environmental stresses. Our understanding of the ABA biosynthetic pathway has been increased recently through the use of plant mutants and the cloning of many of the genes encoding for the enzymes involved. C₄₀ Xanthophylls are precursors of ABA and are now known to be derived from isopentenyl phosphate (IPP) synthesized in plastids via a mevalonate-independent pathway. Enzyme reactions downstream of zeaxanthin have recently been reported to be important for the precise regulation of ABA levels. Zeaxanthin epoxidase (ZEP) catalyses the conversion of zeaxanthin to violaxanthin. Changes in ZEP gene expression appear to regulate ABA biosynthesis in seeds and roots, but not in leaves which might be expected considering the important role of epoxy-carotenoids in photosynthesis and photoprotection. The isomerization of the resulting all-trans-violaxanthin to 9-cis-epoxy-carotenoids awaits elucidation. Although 9-cis-epoxy-carotenoid dioxygenase (NCED), which subsequently cleaves the resulting carotenoids could use the 9-cis isomers of both violaxanthin and neoxanthin as substrates in vitro, the in vivo substrates remain to be determined. NCEDs are apparently encoded by multigene families and identification of the various members is required to determine their relative contribution to the regulation of ABA levels. Studies on those already available indicate that their up-regulation upon water stress is compatible with a key role in the modulation of ABA levels. The genes encoding for the enzymes that convert the cleavage product xanthoxin to ABA are not yet known, although recently cloned aldehyde oxidases may act on ABA-aldehyde.     

Identification of an abscisic acid gene cluster in the grey mold Botrytis cinerea

Like several other phytopathogenic fungi, the ascomycete Botrytis cinerea is known to produce the plant hormone abscisic acid (ABA) in axenic culture. Recently, bcaba1, the first fungal gene involved in ABA biosynthesis, was identified. Neighborhood analysis of bcaba1 revealed three further candidate genes of this pathway: a putative P450 monooxygenase-encoding gene (bcaba2), an open reading frame without significant similarities (bcaba3), and a gene probably coding for a short-chain dehydrogenase/reductase (bcaba4). Targeted inactivation of the genes proved the involvement of BcABA2 and BcABA3 in ABA biosynthesis and suggested a contribution of BcABA4. The close linkage of at least three ABA biosynthetic genes is strong evidence for the presence of an abscisic acid gene cluster in B. cinerea.     

Assignment of 44 Ds Insertions to the Linkage Map of Arabidopsis

Transposon tagging is a useful tool for biological studies. Transposon insertions have been used to obtain new mutants which are extremely helpful in understanding gene function. These insertions immediately provide a means to isolate the corresponding genes. Transposon tagging has also been used to clone genes previously defined by point mutations. In addition, transposon insertions into cloned genes that lack mutations can be generated to facilitate functional analysis. The maize Ac/Ds transposon elements are known to transpose to local sites with high frequencies and have been shown to function in several dicots. To generate a collection of Ds elements for the purpose of targeted insertional mutagenesis of mapped genes in Arabidopsis, we have mapped 44 Ds insertions by simple sequence length polymorphism (SSLP). Because the Arabidopsis genome project is advancing rapidly, many genes will be discovered whose functions are unknown. The mapped 44 Ds insertions will be a useful resource for post-genome analysis of gene functions in Arabidopsis.     

MAX2 affects multiple hormones to promote photomorphogenesis

Ubiquitin-26S proteasome system (UPS) has been shown to play central roles in light and hormone-regulated plant growth and development. Previously, we have shown that MAX2, an F-box protein, positively regulates facets of photomorphogenic development in response to light. However, how MAX2 controls these responses is still unknown. Here, we show that MAX2 oppositely regulates GA and ABA biosynthesis to optimize seed germination in response to light. Dose-response curves showed that max2 seeds are hyposensitive to GA and hypersensitive to ABA in seed germination responses. RT-PCR assays demonstrated that the expression of GA biosynthetic genes is down-regulated, while the expression of GA catabolic genes is up-regulated in the max2 seeds compared to wild-type. Interestingly, expression of both ABA biosynthetic and catabolic genes is up-regulated in the max2 seeds compared to wild-type. Treatment with an auxin transport inhibitor, NPA, showed that increased auxin transport in max2 seedlings contributes to the long hypocotyl phenotype under light. Moreover, light-signaling phenotypes are restricted to max2, as the biosynthetic mutants in the strigolactone pathway, max1, max3, and max4, did not display any defects in seed germination and seedling de-etiolation compared to wild-type. Taken together, these data suggest that MAX2 modulates multiple hormone pathways to affect photomorphogenesis.