Early biosynthetic pathway to abscisic acid in Cercospora cruenta

Unveiling Biosynthesis of the Phytohormone Abscisic Acid in Fungi: Unprecedented Mechanism of Core Scaffold Formation Catalyzed by an Unusual Sesquiterpene Synthase

Abscisic acid (ABA) is a well-known phytohormone that regulates abiotic stresses. ABA produced by fungi is also proposed to be a virulence factor of fungal pathogens. Although its biosynthetic pathway in fungi was proposed by a series of feeding experiments, the enzyme catalyzing the reaction from farnesyl diphosphate to α-ionylideneethane remains to be identified. In this work, we identified the novel type of sesquiterpene synthase BcABA3 and its unprecedented three-step reaction mechanism involving two neutral intermediates, β-farnesene and allofarnesene. Database searches showed that BcABA3 has no homology with typical sesquiterpene synthases and that the homologous enzyme genes are found in more than 100 bacteria, suggesting that these enzymes form a new family of sesquiterpene synthases.

Comparative transcriptome profiling of Pyropia yezoensis (Ueda) M.S. Hwang & H.G. Choi in response to temperature stresses

Background

Pyropia yezoensis is a model organism often used to investigate the mechanisms underlying stress tolerance in intertidal zones. The digital gene expression (DGE) approach was used to characterize a genome-wide comparative analysis of differentially expressed genes (DEGs) that influence the physiological, developmental or biochemical processes in samples subjected to 4 treatments: high-temperature stress (HT), chilling stress (CS), freezing stress (FS) and normal temperature (NT).

Results

Equal amounts of total RNAs collected from 8 samples (two biological replicates per treatment) were sequenced using the Illumina/Solexa platform. Compared with NT, a total of 2202, 1334 and 592 differentially expressed unigenes were detected in HT, CS and FS respectively. Clustering analysis suggested P. yezoensis acclimates to low and high-temperature stress condition using different mechanisms: In heat stress, the unigenes related to replication and repair of DNA and protein processing in endoplasmic reticulum were active; however at low temperature stresses, unigenes related to carbohydrate metabolism and energy metabolism were active. Analysis of gene differential expression showed that four categories of DEGs functioning as temperature sensors were found, including heat shock proteins, H2A, histone deacetylase complex and transcription factors. Heat stress caused chloroplast genes down-regulated and unigenes encoding metacaspases up-regulated, which is an important regulator of PCD. Cold stress caused an increase in the expression of FAD to improve the proportion of polyunsaturated fatty acids. An up-regulated unigene encoding farnesyl pyrophosphate synthase was found in cold stress, indicating that the plant hormone ABA also played an important role in responding to temperature stress in P. yezoensis.

Conclusion

The variation of amount of unigenes and different gene expression pattern under different temperature stresses indicated the complicated and diverse regulation mechanism in response to temperature stress in P. yezoensis. Several common metabolism pathways were found both in P. yezoensis and in higher plants, such as FAD in low-temperature stress and HSP in heat stress. Meanwhile, many chloroplast genes and unigene related to the synthesis of abscisic acid were detected, revealing its unique temperature-regulation mechanism in this intertidal species. This sequencing dataset and analysis may serve as a valuable resource to study the mechanisms involved in abiotic stress tolerance in intertidal seaweeds.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1586-1) contains supplementary material, which is available to authorized users.

Conservation between higher plants and the moss Physcomitrella patens in response to the phytohormone abscisic acid: a proteomics analysis

BACKGROUND

The plant hormone abscisic acid (ABA) is ubiquitous among land plants where it plays an important role in plant growth and development. In seeds, ABA induces embryogenesis and seed maturation as well as seed dormancy and germination. In vegetative tissues, ABA is a necessary mediator in the triggering of many of the physiological and molecular adaptive responses of the plant to adverse environmental conditions, such as desiccation, salt and cold.

RESULTS

In this study, we investigated the influence of abscisic acid (ABA) on Physcomitrella patens at the level of the proteome using two-dimensional gel electrophoresis (2-DE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Sixty-five protein spots showed changes in response to ABA treatment. Among them, thirteen protein spots were down-regulated; fifty-two protein spots were up-regulated including four protein spots which were newly induced. These proteins were involved in various functions, including material and energy metabolism, defense, protein destination and storage, transcription, signal transduction, cell growth/division, transport, and cytoskeleton. Specifically, most of the up-regulated proteins functioned as molecular chaperones, transcriptional regulators, and defense proteins. Detailed analysis of these up-regulated proteins showed that ABA could trigger stress and defense responses and protect plants from oxidative damage. Otherwise, three protein kinases involved in signal pathways were up-regulated suggesting that P. patens is sensitive to exogenous ABA. The down-regulated of the Rubisco small subunit, photosystem II oxygen-evolving complex proteins and photosystem assembly protein ycf3 indicated that photosynthesis of P. patens was inhibited by ABA treatment.

CONCLUSION

Proteome analysis techniques have been applied as a direct, effective, and reliable tool in differential protein expressions. Sixty-five protein spots showed differences in accumulation levels as a result of treatment with ABA. Detailed analysis these protein functions showed that physiological and molecular responses to the plant hormone ABA appear to be conserved among higher plant species and bryophytes.

Cloning and characterization of farnesyl pyphosphate synthase gene from the ABA-producing fungi Botrytis cinerea

Farnesyl pyphosphate synthase (FPPS) catalyzes the systhesis of farnesyl pyphosphate and appears to be a promising regulation site of Abscisic acid (ABA) biosynthesis pathway in fungi. Here we reported the isolation and characterization of Botrytis cinerea (FPPS) gene. The cloned FPPS gene carries an open reading frame of 1044-bp encoding a deduced protein of 347 amino acids with a molecular weight of 39.83 kDa, and the coding region is interrupted with a 63-bp intron. Comparison analysis showed that the deduced amino acids sequence share high similarity with other known FPPS gene. Southern blot revealed a single copy of FPPS gene in the genomic DNA. The result of transcription analysis indicated that the cloned FPPS gene expressed constitutively and was not induced in ABA accumulation phase.

Biosynthetic considerations could assist the structure elucidation of host plant produced rhizosphere signalling compounds (strigolactones) for arbuscular mycorrhizal fungi and parasitic plants

Parasitic plants cause devastating losses to crop yields in several parts of the world. The root parasites, Striga and Orobanche species, use chemical signalling molecules that are exuded by the roots of plants in extremely low concentrations, and that can induce germination of the seeds of these parasites, to detect the vicinity of a suitable host. The majority of the so far identified germination stimulants belong to the strigolactones. It was recently discovered that this class of compounds can also induce hyphal branching in the symbiotic arbuscular mycorrhizal fungi, a process involved in root colonisation. The elucidation of the structure of new strigolactones is hindered by their low abundance and instability. In the present paper, we have used existing knowledge on the structure of strigolactones and combined it with recently obtained insight in the biosynthetic origin of these signalling compounds. This enabled us to postulate structures for strigolactones that have been isolated but for which so far the structure has not been elucidated, but also to propose structures of strigolactones that may be discovered in the future. Considering the strongly increased importance of the strigolactones, we expect that more groups will look for these compounds and also in systems so far not exploited. This could lead to the discovery of new strigolactones for which we expect the present biogenetic considerations will facilitate identification and structure elucidation.

Regulatory Networks of the Phytohormone Abscisic Acid

Structurally similar to retinoic acid (RA), the phytohormone abscisic acid (ABA) controls many developmental and physiological processes via complicated signaling networks that are composed of receptors, secondary messengers, protein kinase/phosphatase cascades, transcription factors, and chromatin-remodeling factors. In addition, ABA signaling is further modulated by mRNA maturation and stability, microRNA (miRNA) levels, nuclear speckling, and protein degradation. This chapter highlights the identified regulators of ABA signaling and reports their homologues in dicotyledonous and monocotyledonous plants.

Abscisic acid biosynthesis and catabolism

The level of abscisic acid (ABA) in any particular tissue in a plant is determined by the rate of biosynthesis and catabolism of the hormone. Therefore, identifying all the genes involved in the metabolism is essential for a complete understanding of how this hormone directs plant growth and development. To date, almost all the biosynthetic genes have been identified through the isolation of auxotrophic mutants. On the other hand, among several ABA catabolic pathways, current genomic approaches revealed that Arabidopsis CYP707A genes encode ABA 8'-hydroxylases, which catalyze the first committed step in the predominant ABA catabolic pathway. Identification of ABA metabolic genes has revealed that multiple metabolic steps are differentially regulated to fine-tune the ABA level at both transcriptional and post-transcriptional levels. Furthermore, recent ongoing studies have given new insights into the regulation and site of ABA metabolism in relation to its physiological roles.

The biosynthetic pathway to abscisic acid via ionylideneethane in the fungus Botrytis cinerea

The biosynthetic pathway to abscisic acid (ABA) from isopentenyl diphosphate in the fungus, Botrytis cinerea, was investigated. Labeling experiments with (18)O2 and H2(18)O indicated that all oxygen atoms at C-1, -1, -1' and -4' of ABA were derived from molecular oxygen, and not from water. This finding was inconsistent not only with the known carotenoid pathway via oxidative cleavage of carotenoids, but also with the classical direct pathway via cyclization of farnesyl diphosphate. The fungus produced new C15-compounds, 2E,4E-alpha-ionylideneethane and 2Z,4E-alpha-ionylideneethane, along with 2E,4E,6E-allofarnesene and 2Z,4E,6E-allofarnesene, but did not apparently produce carotenoids except for a trace of phytoene. The C15-compounds labeled with 13C were converted to ABA by the fungus, and the incorporation ratio of 2Z,4E-alpha-ionylideneethane was higher than that of 2E,4E-alpha-ionylideneethane. From these results, it was concluded that farnesyl diphosphate was reduced at C-1, desaturated at C-4, and isomerized at C-2 to form 2Z,4E,6E-allofarnesene before being cyclized to 2Z,4E-alpha-ionylideneethane; the ionylideneethane was then oxidized to ABA with molecular oxygen. This direct pathway via ionylideneethane means that the biosynthetic pathway to fungal ABA, not only before but also after isopentenyl diphosphate, differs from that to ABA in plants, since plant ABA is biosynthesized using the non-mevalonate and carotenoid pathways.

The P450 monooxygenase BcABA1 is essential for abscisic acid biosynthesis in Botrytis cinerea

The phytopathogenic ascomycete Botrytis cinerea is known to produce abscisic acid (ABA), which is thought to be involved in host-pathogen interaction. Biochemical analyses had previously shown that, in contrast to higher plants, the fungal ABA biosynthesis probably does not proceed via carotenoids but involves direct cyclization of farnesyl diphosphate and subsequent oxidation steps. We present here evidence that this "direct" pathway is indeed the only one used by an ABA-overproducing strain of B. cinerea. Targeted inactivation of the gene bccpr1 encoding a cytochrome P450 oxidoreductase reduced the ABA production significantly, proving the involvement of P450 monooxygenases in the pathway. Expression analysis of 28 different putative P450 monooxygenase genes revealed two that were induced under ABA biosynthesis conditions. Targeted inactivation showed that one of these, bcaba1, is essential for ABA biosynthesis: DeltaBcaba1 mutants contained no residual ABA. Thus, bcaba1 represents the first identified fungal ABA biosynthetic gene.