ABSCISIC ACID-DEFICIENT4 Has an Essential Function in Both cis-Violaxanthin and cis-Neoxanthin Synthesis

Abscisic acid activates transcription factor module MdABI5-MdMYBS1 during carotenoid-derived apple fruit coloration

Carotenoids are major pigments contributing to fruit coloration. We previously reported that the apple (Malus domestica Borkh.) mutant fruits of "Beni Shogun" and "Yanfu 3" show a marked difference in fruit coloration. However, the regulatory mechanism underlying this phenomenon remains unclear. In this study, we determined that carotenoid is the main factor influencing fruit flesh color. We identified an R1-type MYB transcription factor (TF), MdMYBS1, which was found to be highly associated with carotenoids and abscisic acid (ABA) contents of apple fruits. Overexpression of MdMYBS1 promoted, and silencing of MdMYBS1 repressed, β-branch carotenoids synthesis and ABA accumulation. MdMYBS1 regulates carotenoid biosynthesis by directly activating the major carotenoid biosynthetic genes encoding phytoene synthase (MdPSY2-1) and lycopene β-cyclase (MdLCYb). 9-cis-epoxycarotenoid dioxygenase 1 (MdNCED1) contributes to ABA biosynthesis, and MdMYBS1 enhances endogenous ABA accumulation by activating the MdNCED1 promoter. In addition, the basic leucine zipper domain TF ABSCISIC ACID-INSENSITIVE5 (MdABI5) was identified as an upstream activator of MdMYBS1, which promotes carotenoid and ABA accumulation. Furthermore, ABA promotes carotenoid biosynthesis and enhances MdMYBS1 and MdABI5 promoter activities. Our findings demonstrate that the MdABI5-MdMYBS1 cascade activated by ABA regulates carotenoid-derived fruit coloration and ABA accumulation in apple, providing avenues in breeding and planting for improvement of fruit coloration and quality.

DWARF27 and CAROTENOID CLEAVAGE DIOXYGENASE 7 genes regulate release, germination and growth of gemma in Marchantia polymorpha

Strigolactones (SLs), a class of carotenoid-derived hormones, play a crucial role in flowering plants by regulating underground communication with symbiotic arbuscular mycorrhizal fungi (AM) and controlling shoot and root architecture. While the functions of core SL genes have been characterized in many plants, their roles in non-tracheophyte plants like liverworts require further investigation. In this study, we employed the model liverwort species Marchantia polymorpha, which lacks detectable SL production and orthologs of key SL biosynthetic genes, including CAROTENOID CLEAVAGE DIOXYGENASE 8 (CCD8) and MORE AXILLARY GROWTH 1 (MAX1). However, it retains some SL pathway components, including DWARF27 (D27) and CCD7. To help elucidate the function of these remaining components in M. polymorpha, knockout mutants were generated for MpD27-1, MpD27-2 and MpCCD7. Phenotypic comparisons of these mutants with the wild-type control revealed a novel role for these genes in regulating the release of gemmae from the gemma cup and the germination and growth of gemmae in the dark. Mpd27-1, Mpd27-2, and Mpccd7 mutants showed lower transcript abundance of genes involved in photosynthesis, such as EARLY LIGHT INDUCED (ELI), and stress responses such as LATE EMBRYOGENESIS ABUNDANT (LEA) but exhibited higher transcript levels of ETHYLENE RESPONSE FACTORS (ERFs) and SL and carotenoid related genes, such as TERPENE SYNTHASE (TS), CCD7 and LECITHIN-RETINAL ACYL TRANSFERASE (LRAT). Furthermore, the mutants of M. polymorpha in the SL pathway exhibited increased contents of carotenoid. This unveils a previously unrecognized role for MpD27-1, MpD27-2 and MpCCD7 in controlling release, germination, and growth of gemmae in response to varying light conditions. These discoveries enhance our comprehension of the regulatory functions of SL biosynthesis genes in non-flowering plants.

Dalbergia odorifera undergoes massive molecular shifts in response to waterlogging combined with salinity

Field and greenhouse studies attempting to describe the molecular responses of plant species under waterlogging (WL) combined with salinity (ST) are almost nonexistent. We integrated transcriptional, metabolic, and physiological responses involving several crucial transcripts and common differentially expressed genes and metabolites in fragrant rosewood (Dalbergia odorifera) leaflets to dissect plant-specific molecular responses and patterns under WL combined with ST (SWL). We discovered that the synergistic pattern of the transcriptional response of fragrant rosewood under SWL was exclusively characterized by the number of regulated transcripts. The response patterns under SWL based on transcriptome and metabolome regulation statuses revealed different patterns (additive, dominant, neutral, minor, unilateral, and antagonistic) of transcripts or metabolites that were commonly regulated or expressed uniquely under SWL. Under SWL, the synergistic transcriptional response of several functional gene subsets was positively associated with several metabolomic and physiological responses related to the shutdown of the photosynthetic apparatus and the extensive degradation of starch into saccharides through α-amylase, β-amylase, and α-glucosidase or plastoglobuli accumulation. The dissimilarity between the regulation status and number of transcripts in plants under combined stresses led to nonsynergistic responses in several physiological and phytohormonal traits. As inferred from the impressive synergistic transcriptional response to morpho-physiological changes, combined stresses exhibited a gradually decreasing effect on the changes observed at the molecular level compared to those in the morphological one. Here, by characterizing the molecular responses and patterns of plant species under SWL, our study considerably improves our understanding of the molecular mechanisms underlying combined stress.

D27-like carotenoid isomerases: at the crossroads of strigolactone and abscisic acid biosynthesis

Strigolactones and abscisic acid (ABA) are apocarotenoid-derived plant hormones. Their biosynthesis starts with the conversion of trans-carotenes into cis forms, which serve as direct precursors. Iron-containing DWARF27 isomerases were shown to catalyse or contribute to the trans/cis conversions of these precursor molecules. D27 converts trans-β-carotene into 9-cis-β-carotene, which is the first committed step in strigolactone biosynthesis. Recent studies found that its paralogue, D27-LIKE1, also catalyses this conversion. A crucial step in ABA biosynthesis is the oxidative cleavage of 9-cis-violaxanthin and/or 9-cis-neoxanthin, which are formed from their trans isomers by unknown isomerases. Several lines of evidence point out that D27-like proteins directly or indirectly contribute to 9-cis-violaxanthin conversion, and eventually ABA biosynthesis. Apparently, the diversity of D27-like enzymatic activity is essential for the optimization of cis/trans ratios, and hence act to maintain apocarotenoid precursor pools. In this review, we discuss the functional divergence and redundancy of D27 paralogues and their potential direct contribution to ABA precursor biosynthesis. We provide updates on their gene expression regulation and alleged Fe-S cluster binding feature. Finally, we conclude that the functional divergence of these paralogues is not fully understood and we provide an outlook on potential directions in research.

Modulation of carotenoid biosynthesis in maize (Zea mays L.) seedlings by exogenous abscisic acid and salicylic acid under low temperature

KEY MESSAGE

Abscisic acid could regulate structural genes in the carotenoid biosynthesis pathway and alleviate the decrease of carotenoids in maize seedlings under low-temperature stress. Low temperature often hampers the development of maize seedlings and hinders the accumulation of carotenoids, which are functional against chilling stress for plants and providing health benefits for human. To explore effective approaches in reducing chilling stress and enhancing the potential nutritional values of maize seedlings, exogenous plant hormones abscisic acid (ABA) and salicylic acid (SA) that may affect carotenoid biosynthesis were applied on low-temperature-stressed maize seedlings. Results showed that low temperature significantly reduced the carotenoid levels in maize seedlings, only preserving 62.8% in comparison to the control. The applied ABA probably interacted with the ABA-responsive cis-acting elements (ABREs) in the promoter regions of PSY3, ZDS and CHYB and activated their expressions. Consequently, the total carotenoid concentration was apparently increased to 1121 ± 47 ng·g-1 fresh weight (FW), indicating the stress alleviation by ABA. The application of SA did not yield positive results in alleviating chilling stress in maize seedlings. However, neoxanthin content could be notably boosted to 52.12 ± 0.45 ng·g-1 FW by SA, offering a biofortification strategy for specific nutritional enhancement. Structural gene PSY1 demonstrated positive correlations with β-carotene and zeaxanthin (r = 0.93 and 0.89), while CRTISO was correlated with total carotenoids (r = 0.92), indicating their critical roles in carotenoid accumulation. The present study exhibited the effectiveness of ABA to mitigate chilling stress and improve the potential nutritional values in low-temperature-stressed maize seedlings, thereby promoting the production of plant-based food sources.

Multilayered regulation of developmentally programmed pre-anthesis tip degeneration of the barley inflorescence

Leaf and floral tissue degeneration is a common feature in plants. In cereal crops such as barley (Hordeum vulgare L.), pre-anthesis tip degeneration (PTD) starts with growth arrest of the inflorescence meristem dome, which is followed basipetally by the degeneration of floral primordia and the central axis. Due to its quantitative nature and environmental sensitivity, inflorescence PTD constitutes a complex, multilayered trait affecting final grain number. This trait appears to be highly predictable and heritable under standardized growth conditions, consistent with a developmentally programmed mechanism. To elucidate the molecular underpinnings of inflorescence PTD, we combined metabolomic, transcriptomic, and genetic approaches to show that barley inflorescence PTD is accompanied by sugar depletion, amino acid degradation, and abscisic acid responses involving transcriptional regulators of senescence, defense, and light signaling. Based on transcriptome analyses, we identified GRASSY TILLERS1 (HvGT1), encoding an HD-ZIP transcription factor, as an important modulator of inflorescence PTD. A gene-edited knockout mutant of HvGT1 delayed PTD and increased differentiated apical spikelets and final spikelet number, suggesting a possible strategy to increase grain number in cereals. We propose a molecular framework that leads to barley PTD, the manipulation of which may increase yield potential in barley and other related cereals.

The apocarotenoid production in microbial biofactories: An overview

Carotenoids are a vast group of natural pigments that come in a variety of colors ranging from red to orange. Apocarotenoids are derived from these carotenoids, which are hormones, pigments, retinoids, and volatiles employed in the textiles, cosmetics, pharmaceutical, and food industries. Due to the high commercial value and poor natural host abundance, they are significantly undersupplied. Microbes like Saccharomyces cerevisiae and Escherichia coli act as heterologous hosts for apocarotenoid production. This article briefly reviews categories of apocarotenoids, their biosynthetic pathway commencing from the MVA and MEP, its significance, the tool enzymes for apocarotenoid biosynthesis like CCDs, their biotechnological production in microbial factories, and future perspectives.

Role of secondary metabolites in distressed microalgae

Proficient photosynthetic microalgae/cyanobacteria produce a remarkable amount of various biomolecules. Secondary metabolites (SM) represent high value products for global biotrend application. Production improvement can be achieved by nutritional, environmental, and physiological stress as a first line tools for their stimulation. In recent decade, an increasing interest in algal stress biology and omics techniques have deepened knowledge in this area. However, deep understanding and connection of specific stress elucidator are missing. Hence, the present review summarizes recent evidence with an emphasis on the carotenoids, phenolic, and less-discussed compounds (glycerol, proline, mycosporins-like amino acids). Even when they are synthesized at very low concentrations, it highlights the need to expand knowledge in this area using genome-editing tools and omics approaches.

Genome-Wide Characterization and Expression Profiling of ABA Biosynthesis Genes in a Desert Moss Syntrichia caninervis

Syntrichia caninervis can survive under 80-90% protoplasmic water losses, and it is a model plant in desiccation tolerance research. A previous study has revealed that S. caninervis would accumulate ABA under dehydration stress, while the ABA biosynthesis genes in S. caninervis are still unknown. This study identified one ScABA1, two ScABA4s, five ScNCEDs, twenty-nine ScABA2s, one ScABA3, and four ScAAOs genes, indicating that the ABA biosynthesis genes were complete in S. caninervis. Gene location analysis showed that the ABA biosynthesis genes were evenly distributed in chromosomes but were not allocated to sex chromosomes. Collinear analysis revealed that ScABA1, ScNCED, and ScABA2 had homologous genes in Physcomitrella patens. RT-qPCR detection found that all of the ABA biosynthesis genes responded to abiotic stress; it further indicated that ABA plays an important role in S. caninervis. Moreover, the ABA biosynthesis genes in 19 representative plants were compared to study their phylogenetic and conserved motifs; the results suggested that the ABA biosynthesis genes were closely associated with plant taxa, but these genes had the same conserved domain in each plant. In contrast, there is a huge variation in the exon number between different plant taxa; it revealed that ABA biosynthesis gene structures are closely related to plant taxa. Above all, this study provides strong evidence demonstrating that ABA biosynthesis genes were conserved in the plant kingdom and deepens our understanding of the evolution of the phytohormone ABA.

The Arabidopsis D27-LIKE1 is a cis/cis/trans-β-carotene isomerase that contributes to Strigolactone biosynthesis and negatively impacts ABA level

The enzyme DWARF27 (D27) catalyzes the reversible isomerization of all-trans- into 9-cis-β-carotene, initiating strigolactone (SL) biosynthesis. Genomes of higher plants encode two D27-homologs, D27-like1 and -like2, with unknown functions. Here, we investigated the enzymatic activity and biological function of the Arabidopsis D27-like1. In vitro enzymatic assays and expression in Synechocystis sp. PCC6803 revealed an unreported 13-cis/15-cis/9-cis- and a 9-cis/all-trans-β-carotene isomerization. Although disruption of AtD27-like1 did not cause SL deficiency phenotypes, overexpression of AtD27-like1 in the d27 mutant restored the more-branching phenotype, indicating a contribution of AtD27-like1 to SL biosynthesis. Accordingly, generated d27 d27like1 double mutants showed a more pronounced branching phenotype compared to d27. The contribution of AtD27-like1 to SL biosynthesis is likely a result of its formation of 9-cis-β-carotene that was present at higher levels in AtD27-like1 overexpressing lines. By contrast, AtD27-like1 expression correlated negatively with the content of 9-cis-violaxanthin, a precursor of ABA, in shoots. Consistently, ABA levels were higher in shoots and also in dry seeds of the d27like1 and d27 d27like1 mutants. Transgenic lines expressing GUS driven by the AtD27LIKE1 promoter and transcript analysis of hormone-treated Arabidopsis seedlings revealed that AtD27LIKE1 is expressed in different tissues and affects ABA and auxin. Taken together, our work reports a cis/cis-β-carotene isomerase that affects the content of both cis-carotenoid-derived plant hormones, ABA and SLs.

Do Opposites Attract? Auxin-Abscisic Acid Crosstalk: New Perspectives

Plants are constantly exposed to a variety of different environmental stresses, including drought, salinity, and elevated temperatures. These stress cues are assumed to intensify in the future driven by the global climate change scenario which we are currently experiencing. These stressors have largely detrimental effects on plant growth and development and, therefore, put global food security in jeopardy. For this reason, it is necessary to expand our understanding of the underlying mechanisms by which plants respond to abiotic stresses. Especially boosting our insight into the ways by which plants balance their growth and their defense programs appear to be of paramount importance, as this may lead to novel perspectives that can pave the way to increase agricultural productivity in a sustainable manner. In this review, our aim was to present a detailed overview of different facets of the crosstalk between the antagonistic plant hormones abscisic acid (ABA) and auxin, two phytohormones that are the main drivers of plant stress responses, on the one hand, and plant growth, on the other.

Transcriptome and Gene Regulatory Network Analyses Reveal New Transcription Factors in Mature Fruit Associated with Harvest Date in Prunus persica

Harvest date is a critical parameter for producers and consumers regarding agro-industrial performance. It involves a pleiotropic effect controlling the development of other fruit quality traits through finely controlling regulatory mechanisms. Fruit ripening is a process in which various signals and biological events co-occur and are regulated by hormone signaling that produces the accumulation/degradation of multiple compounds. However, the regulatory mechanisms that control the hormone signaling involved in fruit development and ripening are still unclear. To investigate the issue, we used individuals with early, middle and late harvest dates from a peach segregating population to identify regulatory candidate genes controlling fruit quality traits at the harvest stage and validate them in contrasting peach varieties for this trait. We identified 467 and 654 differentially expressed genes for early and late harvest through a transcriptomic approach. In addition, using the Arabidopsis DAP-seq database and network analysis, six transcription factors were selected. Our results suggest significant hormonal balance and cell wall composition/structure differences between early and late harvest samples. Thus, we propose that higher expression levels of the transcription factors HB7, ERF017 and WRKY70 in early harvest individuals would induce the expression of genes associated with the jasmonic acid pathway, photosynthesis and gibberellins inhibition. While on the other hand, the high expression levels of LHY, CDF3 and NAC083 in late harvest individuals would promote the induction of genes associated with abscisic acid biosynthesis, auxins and cell wall remodeling.

Abscisic acid and its role in the modulation of plant growth, development, and yield stability

Abscisic acid (ABA) is known to confer stress tolerance; however, at elevated levels it impairs plant growth under prolonged stress. Paradoxically, at its basal level, ABA plays many vital roles in promoting plant growth and development, including modulation of tillering, flowering, and seed development, as well as seed maturation. In this review, we provide insight into novel discoveries of ABA fluxes, ABA signaling responses, and their impact on yield stability. We discuss ABA homeostasis implicated under pre- and postanthesis drought and its impact on productive tillers, grain number determination, and seed development to address yield stability in cereal crops while considering the new knowledge that emerged from the model plant systems.

D27-LIKE1 isomerase has a preference towards trans/cis and cis/cis conversions of carotenoids in Arabidopsis

Carotenoids contribute to a variety of physiological processes in plants, functioning also as biosynthesis precursors of ABA and strigolactones (SLs). SL biosynthesis starts with the enzymatic conversion of all-trans-β-carotene to 9-cis-β-carotene by the DWARF27 (D27) isomerase. In Arabidopsis, D27 has two closely related paralogs, D27-LIKE1 and D27-LIKE2, which were predicted to be β-carotene-isomerases. In the present study, we characterised D27-LIKE1 and identified some key aspects of its physiological and enzymatic functions in Arabidopsis. d27-like1-1 mutant does not display any strigolactone-deficient traits and exhibits a substantially higher 9-cis-violaxanthin content, which is accompanied by a slightly higher ABA level. In vitro feeding assays with recombinant D27-LIKE1 revealed that the protein exhibits affinity to all β-carotene isoforms but with an exclusive preference towards trans/cis conversions and the interconversion between 9-cis, 13-cis and 15-cis-β-carotene forms, and accepts zeaxanthin and violaxanthin as substrates. Finally, we present evidence showing that D27-LIKE1 mRNA is phloem mobile and D27-LIKE1 is an ancient isomerase with a long evolutionary history. In summary, we demonstrate that D27-LIKE1 is a carotenoid isomerase with multi-substrate specificity and has a characteristic preference towards the catalysation of cis/cis interconversion of carotenoids. Therefore, D27-LIKE1 is a potential regulator of carotenoid cis pools and, eventually, SL and ABA biosynthesis pathways.

Time-series transcriptomics reveals a drought-responsive temporal network and crosstalk between drought stress and the circadian clock in foxtail millet

Drought stress is a serious factor affecting crop growth and production worldwide. The circadian clock has been identified as key to improving regional adaptability of plants. However, our understanding of the contribution of the circadian clock to drought response and the impacts of drought stress on the circadian clock in plants is still limited. To explore the interactions between the circadian clock and drought stress, foxtail millet seedlings were treated with simulated drought (20% polyethylene glycol-6000) treatment starting at the day (DD) onset zeitgeber time 0 (ZT0, lights on) and at the night (DN) onset zeitgeber time 16 (ZT16, lights off). A high temporal-resolution transcriptomic investigation was performed using DD and DN samples collected at intervals of 2 or 4 h within a 24-h drought-treatment period. Overall, we identified 13 294 drought-responsive genes (DRGs). Among these DRGs, 7931 were common between DD and DN samples, 2638 were specific to DD, and 2725 were specific to DN. Additionally, we identified 1257 circadian genes, of which 67% were DRGs. Interestingly, with drought treatment starting at the day for 8, 12 or 16 h, the circadian phase shifted to 12 h. We also found that the circadian clock led to different day and night drought-responsive pathways. The identification of DRG_Clock (DRG and circadian clock) and DRG_NonClock (DRG and not circadian clock) genes provides a reference for selecting candidate drought resistance genes. Our work reveals the temporal drought-response process and crosstalk between drought stress and the circadian clock in foxtail millet.

Plant carotenoids: recent advances and future perspectives

Carotenoids are isoprenoid metabolites synthesized de novo in all photosynthetic organisms. Carotenoids are essential for plants with diverse functions in photosynthesis, photoprotection, pigmentation, phytohormone synthesis, and signaling. They are also critically important for humans as precursors of vitamin A synthesis and as dietary antioxidants. The vital roles of carotenoids to plants and humans have prompted significant progress toward our understanding of carotenoid metabolism and regulation. New regulators and novel roles of carotenoid metabolites are continuously revealed. This review focuses on current status of carotenoid metabolism and highlights recent advances in comprehension of the intrinsic and multi-dimensional regulation of carotenoid accumulation. We also discuss the functional evolution of carotenoids, the agricultural and horticultural application, and some key areas for future research.

The Genetic Components of a Natural Color Palette: A Comprehensive List of Carotenoid Pathway Mutations in Plants

Carotenoids comprise the most widely distributed natural pigments. In plants, they play indispensable roles in photosynthesis, furnish colors to flowers and fruit and serve as precursor molecules for the synthesis of apocarotenoids, including aroma and scent, phytohormones and other signaling molecules. Dietary carotenoids are vital to human health as a source of provitamin A and antioxidants. Hence, the enormous interest in carotenoids of crop plants. Over the past three decades, the carotenoid biosynthesis pathway has been mainly deciphered due to the characterization of natural and induced mutations that impair this process. Over the year, numerous mutations have been studied in dozens of plant species. Their phenotypes have significantly expanded our understanding of the biochemical and molecular processes underlying carotenoid accumulation in crops. Several of them were employed in the breeding of crops with higher nutritional value. This compendium of all known random and targeted mutants available in the carotenoid metabolic pathway in plants provides a valuable resource for future research on carotenoid biosynthesis in plant species.

Seed Dormancy and Pre-Harvest Sprouting in Rice-An Updated Overview

Pre-harvest sprouting is a critical phenomenon involving the germination of seeds in the mother plant before harvest under relative humid conditions and reduced dormancy. As it results in reduced grain yield and quality, it is a common problem for the farmers who have cultivated the rice and wheat across the globe. Crop yields need to be steadily increased to improve the people’s ability to adapt to risks as the world’s population grows and natural disasters become more frequent. To improve the quality of grain and to avoid pre-harvest sprouting, a clear understanding of the crops should be known with the use of molecular omics approaches. Meanwhile, pre-harvest sprouting is a complicated phenomenon, especially in rice, and physiological, hormonal, and genetic changes should be monitored, which can be modified by high-throughput metabolic engineering techniques. The integration of these data allows the creation of tailored breeding lines suitable for various demands and regions, and it is crucial for increasing the crop yields and economic benefits. In this review, we have provided an overview of seed dormancy and its regulation, the major causes of pre-harvest sprouting, and also unraveled the novel avenues to battle pre-harvest sprouting in cereals with special reference to rice using genomics and transcriptomic approaches.

Characterization of the Role of the Neoxanthin Synthase Gene BoaNXS in Carotenoid Biosynthesis in Chinese Kale

Chinese kale (Brassica oleracea var. alboglabra) is rich in carotenoids, and neoxanthin is one of the most important carotenoids in Chinese kale. In this study, the function of the neoxanthin synthase gene (BoaNXS) in Chinese kale was investigated. BoaNXS, which had a 699-bp coding sequence, was cloned from the white flower cultivar of Chinese kale and was expressed in all developmental stages and organs of Chinese kale; its expression was highest in young seeds. The subcellular localization indicated that BoaNXS was localized in the chloroplast. BoaNXS-overexpressed plants were obtained via Agrobacterium-mediated transient overexpression methodology, and the gene overexpression efficiencies ranged from 2.10- to 4.24-fold. The color in the leaves of BoaNXS-overexpressed plants changed from green to yellow-green; the content of total and individual carotenoids, such as neoxanthin, violaxanthin, and lutein, was significantly increased, and the expression levels of most carotenoid biosynthetic genes were notably increased. These findings indicated that BoaNXS is of vital importance in carotenoid biosynthesis in Chinese kale and could be used as a candidate gene for enriching the carotenoid accumulation and color of Chinese kale and other Brassica vegetables.

ABA Metabolism and Homeostasis in Seed Dormancy and Germination

Abscisic acid (ABA) is a key hormone that promotes dormancy during seed development on the mother plant and after seed dispersal participates in the control of dormancy release and germination in response to environmental signals. The modulation of ABA endogenous levels is largely achieved by fine-tuning, in the different seed tissues, hormone synthesis by cleavage of carotenoid precursors and inactivation by 8'-hydroxylation. In this review, we provide an overview of the current knowledge on ABA metabolism in developing and germinating seeds; notably, how environmental signals such as light, temperature and nitrate control seed dormancy through the adjustment of hormone levels. A number of regulatory factors have been recently identified which functional relationships with major transcription factors, such as ABA INSENSITIVE3 (ABI3), ABI4 and ABI5, have an essential role in the control of seed ABA levels. The increasing importance of epigenetic mechanisms in the regulation of ABA metabolism gene expression is also described. In the last section, we give an overview of natural variations of ABA metabolism genes and their effects on seed germination, which could be useful both in future studies to better understand the regulation of ABA metabolism and to identify candidates as breeding materials for improving germination properties.

Plant apocarotenoids: from retrograde signaling to interspecific communication

Carotenoids are isoprenoid compounds synthesized by all photosynthetic and some non-photosynthetic organisms. They are essential for photosynthesis and contribute to many other aspects of a plant's life. The oxidative breakdown of carotenoids gives rise to the formation of a diverse family of essential metabolites called apocarotenoids. This metabolic process either takes place spontaneously through reactive oxygen species or is catalyzed by enzymes generally belonging to the CAROTENOID CLEAVAGE DIOXYGENASE family. Apocarotenoids include the phytohormones abscisic acid and strigolactones (SLs), signaling molecules and growth regulators. Abscisic acid and SLs are vital in regulating plant growth, development and stress response. SLs are also an essential component in plants' rhizospheric communication with symbionts and parasites. Other apocarotenoid small molecules, such as blumenols, mycorradicins, zaxinone, anchorene, β-cyclocitral, β-cyclogeranic acid, β-ionone and loliolide, are involved in plant growth and development, and/or contribute to different processes, including arbuscular mycorrhiza symbiosis, abiotic stress response, plant-plant and plant-herbivore interactions and plastid retrograde signaling. There are also indications for the presence of structurally unidentified linear cis-carotene-derived apocarotenoids, which are presumed to modulate plastid biogenesis and leaf morphology, among other developmental processes. Here, we provide an overview on the biology of old, recently discovered and supposed plant apocarotenoid signaling molecules, describing their biosynthesis, developmental and physiological functions, and role as a messenger in plant communication.