PHD finger proteins function in plant development and abiotic stress responses: an overview

The plant homeodomain (PHD) finger with a conserved Cys4-His-Cys3 motif is a common zinc-binding domain, which is widely present in all eukaryotic genomes. The PHD finger is the "reader" domain of methylation marks in histone H3 and plays a role in the regulation of gene expression patterns. Numerous proteins containing the PHD finger have been found in plants. In this review, we summarize the functional studies on PHD finger proteins in plant growth and development and responses to abiotic stresses in recent years. Some PHD finger proteins, such as VIN3, VILs, and Ehd3, are involved in the regulation of flowering time, while some PHD finger proteins participate in the pollen development, for example, MS, TIP3, and MMD1. Furthermore, other PHD finger proteins regulate the plant tolerance to abiotic stresses, including Alfin1, ALs, and AtSIZ1. Research suggests that PHD finger proteins, as an essential transcription regulator family, play critical roles in various plant biological processes, which is helpful in understanding the molecular mechanisms of novel PHD finger proteins to perform specific function.

PlantACT! - how to tackle the climate crisis

Greenhouse gas (GHG) emissions have created a global climate crisis which requires immediate interventions to mitigate the negative effects on all aspects of life on this planet. As current agriculture and land use contributes up to 25% of total GHG emissions, plant scientists take center stage in finding possible solutions for a transition to sustainable agriculture and land use. In this article, the PlantACT! (Plants for climate ACTion!) initiative of plant scientists lays out a road map of how and in which areas plant scientists can contribute to finding immediate, mid-term, and long-term solutions, and what changes are necessary to implement these solutions at the personal, institutional, and funding levels.

Core cellular and tissue-specific mechanisms enable desiccation tolerance in Craterostigma

Resurrection plants can survive prolonged life without water (anhydrobiosis) in regions with seasonal drying. This desiccation tolerance requires the coordination of numerous cellular processes across space and time, and individual plant tissues face unique constraints related to their function. Here, we analyzed the complex, octoploid genome of the model resurrection plant Craterostigma (C. plantagineum), and surveyed spatial and temporal expression dynamics to identify genetic elements underlying desiccation tolerance. Homeologous genes within the Craterostigma genome have divergent expression profiles, suggesting the subgenomes contribute differently to desiccation tolerance traits. The Craterostigma genome contains almost 200 tandemly duplicated early light-induced proteins, a hallmark trait of desiccation tolerance, with massive upregulation under water deficit. We identified a core network of desiccation-responsive genes across all tissues, but observed almost entirely unique expression dynamics in each tissue during recovery. Roots and leaves have differential responses related to light and photoprotection, autophagy and nutrient transport, reflecting their divergent functions. Our findings highlight a universal set of likely ancestral desiccation tolerance mechanisms to protect cellular macromolecules under anhydrobiosis, with secondary adaptations related to tissue function.

Superhydrophobic Terrestrial Cyanobacteria and Land Plant Transition

Plants and other organisms have evolved structures and mechanisms for colonizing land since the Early Ordovician. In this context, their surfaces, the crucial physical interface with the environment, are mainly considered barriers against water loss. It is suggested that extreme water repellency (superhydrophobicity) was an additional key innovation for the transition of algae from water to land some 400 mya. Superhydrophobicity enhances gas exchange on land and excludes aquatic competitors in water films. In a different context, in material science and surface technology, superhydrophobicity has also become one of the most important bioinspired innovations enabling the avoidance of water films and contamination. Here, we present data for an extremely water-repellent cyanobacterial biofilm of the desiccation tolerant Hassallia byssoidea providing evidence for a much earlier prokaryotic Precambrian (ca. 1-2 bya) origin of superhydrophobicity and chemical heterogeneities associated with land transition. The multicellular cyanobacterium is functionally differentiated in a submerged basal hydrophilic absorbing portion like a "rhizoid" and an upright emersed superhydrophobic "phyllocauloid" filament for assimilation, nitrogen fixation, and splash dispersed diaspores. Additional data are provided for superhydrophobic surfaces in terrestrial green algae and in virtually all ancestral land plants (Bryophytes, ferns and allies, Amborella, Nelumbo), slime molds, and fungi. Rethinking of superhydrophobicity as an essential first step for life in terrestrial environments is suggested.

Roles of Abscisic Acid and Gibberellins in Stem/Root Tuber Development

Root and tuber crops are of great importance. They not only contribute to feeding the population but also provide raw material for medicine and small-scale industries. The yield of the root and tuber crops is subject to the development of stem/root tubers, which involves the initiation, expansion, and maturation of storage organs. The formation of the storage organ is a highly intricate process, regulated by multiple phytohormones. Gibberellins (GAs) and abscisic acid (ABA), as antagonists, are essential regulators during stem/root tuber development. This review summarizes the current knowledge of the roles of GA and ABA during stem/root tuber development in various tuber crops.

Genetic background and cis-organization regulate ALDH7B4 gene expression in Eutrema salsugineum: a promoter analysis case study

ALDH7B4 promoter analysis in A. thaliana and E. salsugineum reveals that both genetic background and promoter architecture contribute to gene expression in response to stress in different species. Many genes are differentially regulated in a comparison of salinity-sensitive and salinity-tolerant plant species. The aldehyde dehydrogenase 7B4 (ALDH7B4) gene is turgor-responsive in A. thaliana and encodes a highly conserved detoxification enzyme in plants. This study compared the ALDH7B4 gene in A. thaliana (salinity-sensitive) and in the salinity-tolerant close relative Eutrema salsugineum. EsALDH7B4 in E. salsugineum is the ortholog of AtALDH7B4 and the expression is also salinity, drought, and wound responsive. However, E. salsugineum requires higher salinity stress to induce the EsALDH7B4 transcriptional response. The GUS expression driven either by the promoter AtALDH7B4 or EsALDH7B4 was induced under 300 mM NaCl treatment in A. thaliana while 600 mM NaCl treatment was required in E. salsugineum, suggesting that the genetic background plays a crucial role in regulation of gene expression. Promoter sequences of ALDH7B4 are less conserved than the protein coding region. A series of EsALDH7B4 promoter deletion fragments were fused to the GUS reporter gene and promoter activity was determined in A. thaliana. The promoter region that contains two conserved ACGT-containing motifs was identified to be essential for stress induction. Furthermore, a 38 bp "TC" rich motif in the EsALDH7B4 promoter, absent from the AtALDH7B4 promoter, negatively affects EsALDH7B4 expression. A MYB-like transcription factor was identified to bind the "TC" motif and to repress the EsALDH7B4 promoter activity. This study reveals that genetic background and cis-acting elements coordinately regulate gene expression.

Stress memory responses and seed priming correlate with drought tolerance in plants: an overview

Environmental-friendly techniques based on plant stress memory, cross-stress tolerance, and seed priming help sustainable agriculture by mitigating negative effects of dehydration stress. The frequently uneven rainfall distribution caused by global warming will lead to more irregular and multiple abiotic stresses, such as heat stress, dehydration stress, cold stress or the combination of these stresses. Dehydration stress is one of the major environmental factors affecting the survival rate and productivity of plants. Hence, there is an urgent need to develop improved resilient varieties. Presently, technologies based on plant stress memory, cross-stress tolerance and priming of seeds represent fruitful and promising areas of future research and applied agricultural science. In this review, we will provide an overview of plant drought stress memory from physiological, biochemical, molecular and epigenetic perspectives. Drought priming-induced cross-stress tolerance to cold and heat stress will be discussed and the application of seed priming will be illustrated for different species.

Systems biology of resurrection plants

Plant species that exhibit vegetative desiccation tolerance can survive extreme desiccation for months and resume normal physiological activities upon re-watering. Here we survey the recent knowledge gathered from the sequenced genomes of angiosperm and non-angiosperm desiccation-tolerant plants (resurrection plants) and highlight some distinct genes and gene families that are central to the desiccation response. Furthermore, we review the vast amount of data accumulated from analyses of transcriptomes and metabolomes of resurrection species exposed to desiccation and subsequent rehydration, which allows us to build a systems biology view on the molecular and genetic mechanisms of desiccation tolerance in plants.

Molecular insights into plant desiccation tolerance: transcriptomics, proteomics and targeted metabolite profiling in Craterostigma plantagineum

The resurrection plant Craterostigma plantagineum possesses an extraordinary capacity to survive long-term desiccation. To enhance our understanding of this phenomenon, complementary transcriptome, soluble proteome and targeted metabolite profiling was carried out on leaves collected from different stages during a dehydration and rehydration cycle. A total of 7348 contigs, 611 proteins and 39 metabolites were differentially abundant across the different sampling points. Dynamic changes in transcript, protein and metabolite levels revealed a unique signature characterizing each stage. An overall low correlation between transcript and protein abundance suggests a prominent role for post-transcriptional modification in metabolic reprogramming to prepare plants for desiccation and recovery. The integrative analysis of all three data sets was performed with an emphasis on photosynthesis, photorespiration, energy metabolism and amino acid metabolism. The results revealed a set of precise changes that modulate primary metabolism to confer plasticity to metabolic pathways, thus optimizing plant performance under stress. The maintenance of cyclic electron flow and photorespiration, and the switch from C₃ to crassulacean acid metabolism photosynthesis, may contribute to partially sustain photosynthesis and minimize oxidative damage during dehydration. Transcripts with a delayed translation, ATP-independent bypasses, alternative respiratory pathway and 4-aminobutyric acid shunt may all play a role in energy management, together conferring bioenergetic advantages to meet energy demands upon rehydration. This study provides a high-resolution map of the changes occurring in primary metabolism during dehydration and rehydration and enriches our understanding of the molecular mechanisms underpinning plant desiccation tolerance. The data sets provided here will ultimately inspire biotechnological strategies for drought tolerance improvement in crops.

Profiling of phenolic compounds in desiccation-tolerant and non-desiccation-tolerant Linderniaceae

INTRODUCTION

Craterostigma plantagineum and Lindernia brevidens are resurrection plants, so these plants can tolerate desiccation of their vegetative tissues. Different components and mechanisms contribute to desiccation tolerance and secondary plant metabolites, like phenolic compounds, may play a role during these processes.

OBJECTIVES

Secondary plant metabolites of the two resurrection plants, C. plantagineum and L. brevidens as well as the closely related desiccation sensitive species, L. subracemosa, were investigated regarding the polyphenol profile.

MATERIAL AND METHODS

Secondary plant compounds were extracted with acidified methanol and analysed with ultra-high-performance liquid chromatography electrospray ionisation mass spectrometry (UHPLC-ESI-MS). Phenolic compounds were identified by comparing of ultraviolet (UV) and MSⁿ -spectra with published data. All compounds were quantified as verbascoside equivalents by external calibration at the compound specific wavelength.

RESULTS

In total, eight compounds that belong to the subclass of phenylethanoid glycosides and one flavone, luteolin hexoside pentoside, were identified. Two of these compounds exhibited a fragmentation pattern, which is closely related to phenylethanoid glycosides. The predominantly synthesised phenylethanoid in all of the three plant species and in every stage of hydration was verbascoside. The total content of phenolic compounds during the three stages of hydration, untreated, desiccated, and rehydrated revealed differences especially between C. plantagineum and L. brevidens as the latter one lost almost all phenolic compounds during rehydration.

CONCLUSION

The amount of verbascoside correlates with the degree of desiccation tolerance and verbascoside might play a role in the protective system in acting as an antioxidant.

The Craterostigma plantagineum protein kinase CpWAK1 interacts with pectin and integrates different environmental signals in the cell wall

The cell wall protein CpWAK1 interacts with pectin, participates in decoding cell wall signals, and induces different downstream responses. Cell wall-associated protein kinases (WAKs) are transmembrane receptor kinases. In the desiccation-tolerant resurrection plant Craterostigma plantagineum, CpWAK1 has been shown to be involved in stress responses and cell expansion by forming a complex with the C. plantagineum glycine-rich protein1 (CpGRP1). This prompted us to extend the studies of WAK genes in C. plantagineum. The phylogenetic analyses of WAKs from C. plantagineum and from other species suggest that these genes have been duplicated after species divergence. Expression profiles indicate that CpWAKs are involved in various biological processes, including dehydration-induced responses and SA- and JA-related reactions to pathogens and wounding. CpWAK1 shows a high affinity for "egg-box" pectin structures. ELISA assays revealed that the binding of CpWAKs to pectins is modulated by CpGRP1 and it depends on the apoplastic pH. The formation of CpWAK multimers is the prerequisite for the CpWAK-pectin binding. Different pectin extracts lead to opposite trends of CpWAK-pectin binding in the presence of Ca²⁺ at pH 8. These observations demonstrate that CpWAKs can potentially discriminate and integrate cell wall signals generated by diverse stimuli, in concert with other elements, such as CpGRP1, pH_apo, Ca²⁺_[apo], and via the formation of CpWAK multimers.

Plant apocarotenoid metabolism utilizes defense mechanisms against reactive carbonyl species and xenobiotics

Carotenoid levels in plant tissues depend on the relative rates of synthesis and degradation of the molecules in the pathway. While plant carotenoid biosynthesis has been extensively characterized, research on carotenoid degradation and catabolism into apocarotenoids is a relatively novel field. To identify apocarotenoid metabolic processes, we characterized the transcriptome of transgenic Arabidopsis (Arabidopsis thaliana) roots accumulating high levels of β-carotene and, consequently, β-apocarotenoids. Transcriptome analysis revealed feedback regulation on carotenogenic gene transcripts suitable for reducing β-carotene levels, suggesting involvement of specific apocarotenoid signaling molecules originating directly from β-carotene degradation or after secondary enzymatic derivatizations. Enzymes implicated in apocarotenoid modification reactions overlapped with detoxification enzymes of xenobiotics and reactive carbonyl species (RCS), while metabolite analysis excluded lipid stress response, a potential secondary effect of carotenoid accumulation. In agreement with structural similarities between RCS and β-apocarotenoids, RCS detoxification enzymes also converted apocarotenoids derived from β-carotene and from xanthophylls into apocarotenols and apocarotenoic acids in vitro. Moreover, glycosylation and glutathionylation-related processes and translocators were induced. In view of similarities to mechanisms found in crocin biosynthesis and cellular deposition in saffron (Crocus sativus), our data suggest apocarotenoid metabolization, derivatization and compartmentalization as key processes in (apo)carotenoid metabolism in plants.

Identification and characterization of CTP:phosphocholine cytidylyltransferase CpCCT1 in the resurrection plant Craterostigma plantagineum

Phosphatidylcholine is a major phospholipid which is shown to be involved in stress adaptation. Phosphatidylcholine increased during dehydration in Craterostigma plantagineum, and therefore we characterized CTP:phosphocholine cytidylyltransferase (CpCCT1), a key regulatory enzyme for phosphatidylcholine synthesis in plants. The CpCCT1 gene from the resurrection plant C. plantagineum was cloned and the amino acid sequence was compared with homologs from other species including yeast and rat. CCT proteins have conserved catalytic and membrane-binding domains while the N-terminal and C-terminal domains have diverged. The tissue specific expression analysis indicated that CpCCT1 is expressed in all tested tissues and it is induced by dehydration and in response to 0.5 M NaCl solutions. In plants exposed to low temperature in the dark, the CpCCT1 transcript increased after 4 h at 4 °C. CpCCT1 expression also increased during mannitol and sorbitol treatments in a concentration dependent manner. Phytohormones such as abscisic acid and indole-3-acetic acid also trigged transcript accumulation. Comparisons of transcript and protein accumulations for different treatments (except for dehydration) suggest transcriptional and translational control mechanisms. Analysis of promoter activity and polysome occupancy suggest that CpCCT1 gene expression is mainly under translational regulation during dehydration.

Studying Lipid-Protein Interactions Using Protein-Lipid Overlay and Protein-Liposome Association Assays

The study of lipid-protein interactions is crucial for understanding reactions of proteins involved in lipid metabolism, lipid transport, and lipid signaling. Different detection methods can be employed for the identification of lipid-binding interactions. Isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) spectroscopy enable real-time monitoring of lipid protein interactions and provide thermodynamic parameters of the interacting partners. However, these technologies depend on the availability of the large equipment, limiting the practicability in many laboratories. Protein-lipid overlay assays are a simple first approach to screen for protein interactions with different lipids or lipid intermediates and are independent of large equipment. Subsequently, specific interactions can be analyzed in detail using protein-liposome association assays.

The dehydration- and ABA-inducible germin-like protein CpGLP1 from Craterostigma plantagineum has SOD activity and may contribute to cell wall integrity during desiccation

MAIN CONCLUSION

CpGLP1 belongs to the large group of germin-like proteins and comprises a cell wall-localized protein which has superoxide dismutase activity and may contribute towards ROS metabolism and cell wall folding during desiccation. The plant cell wall is a dynamic matrix and its plasticity is essential for cell growth and processing of environmental signals to cope with stresses. A few so-called resurrection plants like Craterostigma plantagineum survive desiccation by implementing protection mechanisms. In C. plantagineum, the cell wall shrinks and folds upon desiccation to avoid mechanical and oxidative damage which contributes to cell integrity. Despite the high toxic potential, ROS are important molecules for cell wall remodeling processes as they participate in enzymatic reactions and act as signaling molecules. Here we analyzed the C. plantagineum germin-like protein 1 (CpGLP1) to understand its contribution to cell wall folding and desiccation tolerance. The analysis of the CpGLP1 sequence showed that this protein does not fit into the current GLP classification and forms a new group within the Linderniaceae. CpGLP1 transcripts accumulate in leaves in response to dehydration and ABA, and mannitol treatments transiently induce CpGLP1 transcript accumulation supporting the participation of CpGLP1 in desiccation-related processes. CpGLP1 protein from cell wall protein extracts followed transcript accumulation and protein preparations from bacteria overexpressing CpGLP1 showed SOD activity. In agreement with cell wall localization, CpGLP1 interacts with pectins which have not been reported for GLP proteins. Our data support a role for CpGLP1 in the ROS metabolism related to the control of cell wall plasticity during desiccation in C. plantagineum.

S-Nitrosation impairs activity of stress-inducible aldehyde dehydrogenases from Arabidopsis thaliana

Nitric oxide (NO) is an intracellular messenger that mediates stress responses. Several plant aldehyde dehydrogenase (ALDH) genes are expressed during abiotic stress conditions to reduce the level of cytotoxic aldehydes. We investigated a possible interference between NO and ALDHs, using the isoform ALDH3H1 of Arabidopsis thaliana as model. The physiological NO donor; S-nitrosoglutathione (GSNO), inhibits ALDH3H1 in a time- and concentration-dependent manner. Mutagenesis and ESI-MS/MS analyses show that all Cys residues of ALDH3H1 are targets of GSNO-mediated S-nitrosation. Chemical labelling indicates that the deactivation is due to the conversion of the catalytic thiol into a catalytically non-active nitrosothiol. GSNO has the same effect on the chloroplastic ALDH3I1, suggesting that susceptibility of the catalytic Cys to NO is a common feature of ALDHs. S-Nitrosation and enzymatic inhibition of ALDH were reverted by reducing agents. Our study proves that the function of ALDHs does not exclusively depend on transcriptional regulation, with stress-induced expression, but may be also susceptible to posttranslational regulation through S-nitrosation. We discuss the potential involvement of S-nitrosoglutathione reductase (GSNOR), binding specific cofactors and reducing partners in a protective system of ALDHs in vivo, which will be experimentally corroborated in our forthcoming study.

The Dynamic Responses of Cell Walls in Resurrection Plants During Dehydration and Rehydration

Plant cell walls define the shape of the cells and provide mechanical support. They function as osmoregulators by controlling the transport of molecules between cells and provide transport pathways within the plant. These diverse functions require a well-defined and flexible organization of cell wall components, i.e., water, polysaccharides, proteins, and other diverse substances. Cell walls of desiccation tolerant resurrection plants withstand extreme mechanical stress during complete dehydration and rehydration. Adaptation to the changing water status of the plant plays a crucial role during this process. This review summarizes the compositional and structural variations, signal transduction and changes of gene expression which occur in cell walls of resurrection plants during dehydration and rehydration.

Craterostigma plantagineum cell wall composition is remodelled during desiccation and the glycine-rich protein CpGRP1 interacts with pectins through clustered arginines

Craterostigma plantagineum belongs to the desiccation-tolerant angiosperm plants. Upon dehydration, leaves fold and the cells shrink which is reversed during rehydration. To understand this process changes in cell wall pectin composition, and the role of the apoplastic glycine-rich protein 1 (CpGRP1) were analysed. Cellular microstructural changes in hydrated, desiccated and rehydrated leaf sections were analysed using scanning electron microscopy. Pectin composition in different cell wall fractions was analysed with monoclonal antibodies against homogalacturonan, rhamnogalacturonan I, rhamnogalacturonan II and hemicellulose epitopes. Our data demonstrate changes in pectin composition during dehydration/rehydration which is suggested to affect cell wall properties. Homogalacturonan was less methylesterified upon desiccation and changes were also demonstrated in the detection of rhamnogalacturonan I, rhamnogalacturonan II and hemicelluloses. CpGRP1 seems to have a central role in cell adaptations to water deficit, as it interacts with pectin through a cluster of arginine residues and de-methylesterified pectin presents more binding sites for the protein-pectin interaction than to pectin from hydrated leaves. CpGRP1 can also bind phosphatidic acid (PA) and cardiolipin. The binding of CpGRP1 to pectin appears to be dependent on the pectin methylesterification status and it has a higher affinity to pectin than its binding partner CpWAK1. It is hypothesised that changes in pectin composition are sensed by the CpGRP1-CpWAK1 complex therefore leading to the activation of dehydration-related responses and leaf folding. PA might participate in the modulation of CpGRP1 activity.

Transcriptional and metabolic changes in the desiccation tolerant plant Craterostigma plantagineum during recurrent exposures to dehydration

Multiple dehydration/rehydration treatments improve the adaptation of Craterostigma plantagineum to desiccation by accumulating stress-inducible transcripts, proteins and metabolites. These molecules serve as stress imprints or memory and can lead to increased stress tolerance. It has been reported that repeated exposure to dehydration may generate stronger reactions during a subsequent dehydration treatment in plants. This stimulated us to address the question whether the desiccation tolerant resurrection plant Craterostigma plantagineum has a stress memory. The expression of four representative stress-related genes gradually increased during four repeated dehydration/rehydration treatments in C. plantagineum. These genes reflect a transcriptional memory and are trainable genes. In contrast, abundance of chlorophyll synthesis/degradation-related transcripts did not change during dehydration and remained at a similar level as in the untreated tissues during the recovery phase. During the four dehydration/rehydration treatments the level of ROS pathway-related transcripts, superoxide dismutase (SOD) activity, proline, and sucrose increased, whereas H₂O₂ content and electrolyte leakage decreased. Malondialdehyde (MDA) content did not change during the dehydration, which indicates a gain of stress tolerance. At the protein level, increased expression of four representative stress-related proteins showed that the activated stress memory can persist over several days. The phenomenon described here could be a general feature of dehydration stress memory responses in resurrection plants.

Massive Tandem Proliferation of ELIPs Supports Convergent Evolution of Desiccation Tolerance across Land Plants

Desiccation tolerance was a critical adaptation for the colonization of land by early nonvascular plants. Resurrection plants have maintained or rewired these ancestral protective mechanisms, and desiccation-tolerant species are dispersed across the land plant phylogeny. Although common physiological, biochemical, and molecular signatures are observed across resurrection plant lineages, features underlying the recurrent evolution of desiccation tolerance are unknown. Here we used a comparative approach to identify patterns of genome evolution and gene duplication associated with desiccation tolerance. We identified a single gene family with dramatic expansion in all sequenced resurrection plant genomes and no expansion in desiccation-sensitive species. This gene family of early light-induced proteins (ELIPs) expanded in resurrection plants convergent through repeated tandem gene duplication. ELIPs are universally highly expressed during desiccation in all surveyed resurrection plants and may play a role in protecting against photooxidative damage of the photosynthetic apparatus during prolonged dehydration. Photosynthesis is particularly sensitive to dehydration, and the increased abundance of ELIPs may help facilitate the rapid recovery observed for most resurrection plants. Together, these observations support convergent evolution of desiccation tolerance in land plants through tandem gene duplication.

Desiccation Tolerance Evolved through Gene Duplication and Network Rewiring in Lindernia

Although several resurrection plant genomes have been sequenced, the lack of suitable dehydration-sensitive outgroups has limited genomic insights into the origin of desiccation tolerance. Here, we utilized a comparative system of closely related desiccation-tolerant (Lindernia brevidens) and -sensitive (Lindernia subracemosa) species to identify gene- and pathway-level changes associated with the evolution of desiccation tolerance. The two high-quality Lindernia genomes we assembled are largely collinear, and over 90% of genes are conserved. L. brevidens and L. subracemosa have evidence of an ancient, shared whole-genome duplication event, and retained genes have neofunctionalized, with desiccation-specific expression in L. brevidens Tandem gene duplicates also are enriched in desiccation-associated functions, including a dramatic expansion of early light-induced proteins from 4 to 26 copies in L. brevidens A comparative differential gene coexpression analysis between L. brevidens and L. subracemosa supports extensive network rewiring across early dehydration, desiccation, and rehydration time courses. Many LATE EMBRYOGENESIS ABUNDANT genes show significantly higher expression in L. brevidens compared with their orthologs in L. subracemosa Coexpression modules uniquely upregulated during desiccation in L. brevidens are enriched with seed-specific and abscisic acid-associated cis-regulatory elements. These modules contain a wide array of seed-associated genes that have no expression in the desiccation-sensitive L. subracemosa Together, these findings suggest that desiccation tolerance evolved through a combination of gene duplications and network-level rewiring of existing seed desiccation pathways.

The ATAF1 transcription factor is a key regulator of aldehyde dehydrogenase 7B4 (ALDH7B4) gene expression in Arabidopsis thaliana

ALDH7B4 expression contributes to abiotic stress tolerance. The NAC transcription factor ATAF1 is a main regulator of expression of the ALDH7B4 gene in Arabidopsis thaliana as shown by ATAF1 mutants. The aldehyde dehydrogenase 7B4 (ALDH7B4) protein has important roles in detoxification of excessive aldehydes, elimination of reactive oxygen species (ROS) and inhibition of lipid peroxidation when plants are exposed to abiotic stress. However, the regulation of the expression of the ALDH7B4 gene under stress is largely unknown. Promoter studies revealed crucial cis-elements in the ALDH7B4 promoter in response to heat and stress combinations. Using a yeast one-hybrid assay, several NAC transcription factors, including ATAF1 were isolated. These transcription factors play an important role in plant adaptation to abiotic stress. ATAF1 activates the expression of the ALDH7B4 gene by directly binding to the promoter. Overexpression of ATAF1 in Arabidopsis plants results in elevated expression of ALDH7B4 in seeds, seedlings, and mature plants, whereas ATAF1 knock-out mutant plants abolished the expression of ALDH7B4. This study implies that ATAF1 may confer stress tolerance by up-regulating the target gene ALDH7B4.

Protection of photosynthesis in desiccation-tolerant resurrection plants

Inhibition of photosynthesis is a central, primary response that is observed in both desiccation-tolerant and desiccation-sensitive plants affected by drought stress. Decreased photosynthesis during drought stress can either be due to the limitation of carbon dioxide entry through the stomata and the mesophyll cells, due to increased oxidative stress or due to decreased activity of photosynthetic enzymes. Although the photosynthetic rates decrease in both desiccation-tolerant and sensitive plants during drought, the remarkable difference lies in the complete recovery of photosynthesis after rehydration in desiccation-tolerant plants. Desiccation of sensitive plants leads to irreparable damages of the photosynthetic membranes, in contrast the photosynthetic apparatus is deactivated during desiccation in desiccation-tolerant plants. Desiccation-tolerant plants employ different strategies to protect and/or maintain the structural integrity of the photosynthetic apparatus to reactivate photosynthesis upon water availability. Two major mechanisms are distinguished. Homoiochlorophyllous desiccation-tolerant plants preserve chlorophyll and thylakoid membranes and require active protection mechanisms, while poikilochlorophyllous plants degrade chlorophyll in a regulated manner but then require de novo synthesis during rehydration. Desiccation-tolerant plants, particularly homoiochlorophyllous plants, employ conserved and novel antioxidant enzymes/metabolites to minimize the oxidative damage and to protect the photosynthetic machinery. De novo synthesized, stress-induced proteins in combination with antioxidants are localized in chloroplasts and are important components of the protective network. Genome sequence informations provide some clues on selection of genes involved in protecting photosynthetic structures; e.g. ELIP genes (early light inducible proteins) are enriched in the genomes and more abundantly expressed in homoiochlorophyllous desiccation-tolerant plants. This review focuses on the mechanisms that operate in the desiccation-tolerant plants to protect the photosynthetic apparatus during desiccation.

Relation between water status and desiccation-affected genes in the lichen photobiont Trebouxia gelatinosa

The relation between water status and expression profiles of desiccation -related genes has been studied in the desiccation tolerant (DT) aeroterrestrial green microalga Trebouxia gelatinosa, a common lichen photobiont. Algal colonies were desiccated in controlled conditions and during desiccation water content (WC) and water potential (Ψ) were measured to find the turgor loss point (Ψ_tlp). Quantitative real-time PCR was performed to measure the expression of ten genes related to photosynthesis, antioxidant defense, expansins, heat shock proteins (HSPs), and desiccation related proteins in algal colonies collected during desiccation when still at full turgor (WC > 6 g H₂O g^-1 dry weight), immediately before and after Ψ_tlp (-4 MPa; WC ∼ 1 g H₂O g^-1 dry weight) and before and after complete desiccation (WC < 0.01 g H₂O g^-1 dry weight), quantifying the HSP70 protein levels by immunodetection. Our analysis showed that the expression of eight out of ten genes changed immediately before and after Ψ_tlp. Interestingly, the expression of five out of ten genes changed also before complete desiccation, i.e. between 0.2 and 0.01 g H₂O g^-1 dry weight. However, the HSP70 protein levels were not affected by changes in water status. The study provides new evidences of the link between the loss of turgor and the expression of genes related to the desiccation tolerance of T. gelatinosa, suggesting the former as a signal triggering inducible mechanisms.

Genome Analysis of the Ancient Tracheophyte Selaginella tamariscina Reveals Evolutionary Features Relevant to the Acquisition of Desiccation Tolerance

Resurrection plants, which are the "gifts" of natural evolution, are ideal models for studying the genetic basis of plant desiccation tolerance. Here, we report a high-quality genome assembly of 301 Mb for the diploid spike moss Selaginella tamariscina, a primitive vascular resurrection plant. We predicated 27 761 protein-coding genes from the assembled S. tamariscina genome, 11.38% (2363) of which showed significant expression changes in response to desiccation. Approximately 60.58% of the S. tamariscina genome was annotated as repetitive DNA, which is an almost 2-fold increase of that in the genome of desiccation-sensitive Selaginella moellendorffii. Genomic and transcriptomic analyses highlight the unique evolution and complex regulations of the desiccation response in S. tamariscina, including species-specific expansion of the oleosin and pentatricopeptide repeat gene families, unique genes and pathways for reactive oxygen species generation and scavenging, and enhanced abscisic acid (ABA) biosynthesis and potentially distinct regulation of ABA signaling and response. Comparative analysis of chloroplast genomes of several Selaginella species revealed a unique structural rearrangement and the complete loss of chloroplast NAD(P)H dehydrogenase (NDH) genes in S. tamariscina, suggesting a link between the absence of the NDH complex and desiccation tolerance. Taken together, our comparative genomic and transcriptomic analyses reveal common and species-specific desiccation tolerance strategies in S. tamariscina, providing significant insights into the desiccation tolerance mechanism and the evolution of resurrection plants.

Genome Analysis of the Ancient Tracheophyte Selaginella tamariscina Reveals Evolutionary Features Relevant to the Acquisition of Desiccation Tolerance

Resurrection plants, which are the "gifts" of natural evolution, are ideal models for studying the genetic basis of plant desiccation tolerance. Here, we report a high-quality genome assembly of 301 Mb for the diploid spike moss Selaginella tamariscina, a primitive vascular resurrection plant. We predicated 27 761 protein-coding genes from the assembled S. tamariscina genome, 11.38% (2363) of which showed significant expression changes in response to desiccation. Approximately 60.58% of the S. tamariscina genome was annotated as repetitive DNA, which is an almost 2-fold increase of that in the genome of desiccation-sensitive Selaginella moellendorffii. Genomic and transcriptomic analyses highlight the unique evolution and complex regulations of the desiccation response in S. tamariscina, including species-specific expansion of the oleosin and pentatricopeptide repeat gene families, unique genes and pathways for reactive oxygen species generation and scavenging, and enhanced abscisic acid (ABA) biosynthesis and potentially distinct regulation of ABA signaling and response. Comparative analysis of chloroplast genomes of several Selaginella species revealed a unique structural rearrangement and the complete loss of chloroplast NAD(P)H dehydrogenase (NDH) genes in S. tamariscina, suggesting a link between the absence of the NDH complex and desiccation tolerance. Taken together, our comparative genomic and transcriptomic analyses reveal common and species-specific desiccation tolerance strategies in S. tamariscina, providing significant insights into the desiccation tolerance mechanism and the evolution of resurrection plants.

Analysis of pcC13-62 promoters predicts a link between cis-element variations and desiccation tolerance in Linderniaceae

Reproductive structures of plants (e.g. seeds) and vegetative tissues of resurrection plants can tolerate desiccation. Many genes encoding desiccation-related proteins (DRPs) have been identified in the resurrection plant Craterostigma plantagineum, but the function of these genes remains mainly hypothetical. Here, the importance of the DRP gene pcC13-62 for desiccation tolerance is evaluated by analysing its expression in C. plantagineum and in the closely related desiccation-tolerant species Lindernia brevidens and the desiccation-sensitive species Lindernia subracemosa. Quantitative analysis revealed that pcC13-62 transcripts accumulate at a much lower level in desiccation-sensitive species than in desiccation-tolerant species. The study of pcC13-62 promoters from these species demonstrated a correlation between promoter activity and gene expression levels, suggesting transcriptional regulation of gene expression. Comparison of promoter sequences identified a dehydration-responsive element motif in the promoters of tolerant species that is required for dehydration-induced β-glucuronidase (GUS) accumulation. We hypothesize that variations in the regulatory sequences of the pcC13-62 gene occurred to establish pcC13-62 expression in vegetative tissues, which might be required for desiccation tolerance. The pcC13-62 promoters could also be activated by salt stress in Arabidopsis thaliana plants stably transformed with promoter::GUS constructs.

Molecular responses to dehydration and desiccation in desiccation-tolerant angiosperm plants

Due to the ability to tolerate extreme dehydration, desiccation-tolerant plants have been widely investigated to find potential approaches for improving water use efficiency or developing new crop varieties. The studies of desiccation-tolerant plants have identified sugar accumulation, specific protein synthesis, cell structure changes, and increased anti-oxidative reactions as part of the mechanisms of desiccation tolerance. However, plants respond differently according to the severity of water loss, and the process of water loss affects desiccation tolerance. A detailed analysis within the dehydration process is important for understanding the process of desiccation tolerance. This review defines dehydration and desiccation, finds the boundary for the relative water content between dehydration and desiccation, compares the molecular responses to dehydration and desiccation, compares signaling differences between dehydration and desiccation, and finally summarizes the strategies launched in desiccation-tolerant plants for dehydration and desiccation, respectively. The roles of abscisic acid (ABA) and reactive oxygen species (ROS) in sensing and signaling during dehydration are discussed. We outline how this knowledge can be exploited to generate drought-tolerant crop plants.

Sugar metabolism in the desiccation tolerant grass Oropetium thomaeum in response to environmental stresses

Oropetium thomaeum is a desiccation tolerant grass and acquisition of desiccation tolerance is correlated with changes in carbohydrate metabolism. Here we address the question whether the changes in carbohydrate metabolism are specific to the dehydration process or whether other environmental factors such as high temperature, low temperature, hypoxia, salinity or exogenous ABA application trigger the same or different changes in the sugar metabolism. Fifteen different sugar metabolites were identified by GC/MS, including erythritol, arabinose, fructose, galactose, glucose, myo-inositol, sedoheptulose, sucrose, trehalose, galactinol, maltose, raffinose, manninotriose and stachyose. Together with starch, these sugars were placed into the pathways of sucrose metabolism and raffinose family oligosaccharides (RFOs) metabolism, as well as into the group of rare sugars. By comparing the changes of sugars under various stresses, we concluded that the changes in the sugar metabolism are both convergent and divergent in response to different stresses. Except for the general response to stress, such as starch degradation, the changes of specific sugar metabolites reflect a stress-specific response of O. thomaeum. Erythritol seems to be specific for dehydration, myo-inositol for salt stress and trehalose for hypoxia stress. Similar as dehydration, low temperature, salt stress and ABA application resulted in the accumulation of sucrose and RFOs in O. thomaeum, which indicates that these stresses share high similarity with dehydration. Thus it is proposed that sucrose and RFOs have a general protective role under these stresses. In contrast sucrose and RFOs did not accumulate in response to high temperature or hypoxia whose effects tend to be consumptive and destructive. The accumulation of galactose, melibiose and manninotriose demonstrate that RFOs are degraded under stress. The accumulation of these sugar metabolites might result from the reaction of RFOs and stress-produced hydroxyl radicals, which supports a possible role of RFOs in stress defense. In addition, ABA application led to substantial synthesis of stachyose which occurs only in response to dehydration, indicating that stachyose synthesis is possibly closely related to ABA in O. thomaeum.

Aldehyde Dehydrogenases Function in the Homeostasis of Pyridine Nucleotides in Arabidopsis thaliana

Aldehyde dehydrogenase enzymes (ALDHs) catalyze the oxidation of aliphatic and aromatic aldehydes to their corresponding carboxylic acids using NAD⁺ or NADP⁺ as cofactors and generating NADH or NADPH. Previous studies mainly focused on the ALDH role in detoxifying toxic aldehydes but their effect on the cellular NAD(P)H contents has so far been overlooked. Here, we investigated whether the ALDHs influence the cellular redox homeostasis. We used a double T-DNA insertion mutant that is defective in representative members of Arabidopsis thaliana ALDH families 3 (ALDH3I1) and 7 (ALDH7B4), and we examined the pyridine nucleotide pools, glutathione content, and the photosynthetic capacity of the aldh mutants in comparison with the wild type. The loss of function of ALDH3I1 and ALDH7B4 led to a decrease of NAD(P)H, NAD(P)H/NAD(P) ratio, and an alteration of the glutathione pools. The aldh double mutant had higher glucose-6-phosphate dehydrogenase activity than the wild type, indicating a high demand for reduced pyridine nucleotides. Moreover, the mutant had a reduced quantum yield of photosystem II and photosynthetic capacity at relatively high light intensities compared to the wild type. Altogether, our data revealed a role of ALDHs as major contributors to the homeostasis of pyridine nucleotides in plants.

Induction of the PDH bypass and upregulation of the ALDH7B4 in plants treated with herbicides inhibiting amino acid biosynthesis

Imazamox and glyphosate represent two classes of herbicides that inhibit the activity of acetohydroxyacid synthase in the branched-chain amino acid biosynthesis pathway and the activity of 5-enolpyruvylshikimate-3-phosphate synthase in the aromatic amino acid biosynthesis pathway, respectively. However, it is still unclear how imazamox and glyphosate lead to plant death. Both herbicides inhibit amino-acid biosynthesis and were found to induce ethanol fermentation in plants, but an Arabidopsis mutant deficient in alcohol dehydrogenase 1 was neither more susceptible nor more resistant than the wild-type to the herbicides. In this study, we investigated the effects of the amino acid biosynthesis inhibitors, imazamox and glyphosate, on the pyruvate dehydrogenase bypass reaction and fatty acid metabolism in A. thaliana. We found that the pyruvate dehydrogenase bypass was upregulated following the treatment by the two herbicides. Our results suggest that the Arabidopsis aldehyde dehydrogenase 7B4 gene might be participating in the pyruvate dehydrogenase bypass reaction. We evaluated the potential role of the aldehyde dehydrogenase 7B4 upon herbicide treatment in the plant defence mechanism. Plants that overexpressed the ALDH7B4 gene accumulated less soluble sugars, starch, and fatty acids and grew better than the wild-type after herbicide treatment. We discuss how the upregulation of the ALDH7B4 alleviates the effects of the herbicides, potentially through the detoxification of the metabolites produced in the pyruvate dehydrogenase bypass.

Identification and characterization of the phosphatidic acid-binding A. thaliana phosphoprotein PLDrp1 that is regulated by PLDα1 in a stress-dependent manner

Phospholipase D (PLD) and its cleavage product phosphatidic acid (PA) are crucial in plant stress-signalling. Although some targets of PLD and PA have been identified, the signalling pathway is still enigmatic. This study demonstrates that the phosphoprotein At5g39570, now called PLD-regulated protein1 (PLDrp1), from Arabidopsis thaliana is directly regulated by PLDα1. The protein PLDrp1 can be divided into two regions with distinct properties. The conserved N-terminal region specifically binds PA, while the repeat-rich C-terminal domain suggests interactions with RNAs. The expression of PLDrp1 depends on PLDα1 and the plant water status. Water stress triggers a pldα1-like phenotype in PLDrp1 mutants and induces the expression of PLDrp1 in pldα1 mutants. The regulation of PLDrp1 by PLDα1 and environmental stressors contributes to the understanding of the complex PLD regulatory network and presents a new member of the PA-signalling chain in plants.

Seed desiccation mechanisms co-opted for vegetative desiccation in the resurrection grass Oropetium thomaeum

Resurrection plants desiccate during periods of prolonged drought stress, then resume normal cellular metabolism upon water availability. Desiccation tolerance has multiple origins in flowering plants, and it likely evolved through rewiring seed desiccation pathways. Oropetium thomaeum is an emerging model for extreme drought tolerance, and its genome, which is the smallest among surveyed grasses, was recently sequenced. Combining RNA-seq, targeted metabolite analysis and comparative genomics, we show evidence for co-option of seed-specific pathways during vegetative desiccation. Desiccation-related gene co-expression clusters are enriched in functions related to seed development including several seed-specific transcription factors. Across the metabolic network, pathways involved in programmed cell death inhibition, ABA signalling and others are activated during dehydration. Oleosins and oil bodies that typically function in seed storage are highly abundant in desiccated leaves and may function for membrane stability and storage. Orthologs to seed-specific LEA proteins from rice and maize have neofunctionalized in Oropetium with high expression during desiccation. Accumulation of sucrose, raffinose and stachyose in drying leaves mirrors sugar accumulation patterns in maturing seeds. Together, these results connect vegetative desiccation with existing seed desiccation and drought responsive pathways and provide some key candidate genes for engineering improved drought tolerance in crop plants.

Angiosperm Plant Desiccation Tolerance: Hints from Transcriptomics and Genome Sequencing

Desiccation tolerance (DT) in angiosperms is present in the small group of resurrection plants and in seeds. DT requires the presence of protective proteins, specific carbohydrates, restructuring of membrane lipids, and regulatory mechanisms directing a dedicated gene expression program. Many components are common to resurrection plants and seeds; however, some are specific for resurrection plants. Understanding how each component contributes to DT is challenging. Recent transcriptome analyses and genome sequencing indicate that increased expression is essential of genes encoding protective components, recently evolved, species-specific genes and non-protein-coding RNAs. Modification and reshuffling of existing cis-regulatory promoter elements seems to play a role in the rewiring of regulatory networks required for increased expression of DT-related genes in resurrection species.

The role of Arabidopsis aldehyde dehydrogenase genes in response to high temperature and stress combinations

Aldehyde dehydrogenases (ALDH) are a family of enzymes that are involved in plant metabolism and contribute to aldehyde homeostasis to eliminate toxic aldehydes. The ALDH enzymes produce NADPH and NADH in their enzymatic reactions and thus contribute to balancing redox equivalents. Previous studies showed that Arabidopsis ALDH genes are expressed in response to high salinity, dehydration, oxidative stress, or heavy metals, suggesting important roles in environmental adaptation. However, the role of ALDH genes in high temperature and stress combinations (heat stress combined with dehydration, wounding, or salt stress) is unclear. Here, we analysed expression patterns of selected ALDH genes on the transcript and protein level at different time points of heat stress, basal and acquired thermotolerance, and stress combination treatments. Our results indicate that ALDH3I1 and ALDH7B4 are strongly induced by heat stress. Higher levels of ALDH7B4 accumulated in response to dehydration-heat, heat-salt and wounding-heat combination stress than in response to single stressors. The comparison of physiological and biological parameters in T-DNA double mutants of ALDH genes and wild-type plants demonstrated that mutant lines are more sensitive to heat stress and stress combinations than wild-type plants.

LEA gene expression, RNA stability and pigment accumulation in three closely related Linderniaceae species differing in desiccation tolerance

Desiccation-tolerant plants (Craterostigma plantagineum and Lindernia brevidens) evolved a highly efficient strategies to prevent dehydration-induced irreversible damage. The protection system involves synthesis of LEA proteins, decrease of photosynthetic activity and activation of antioxidant systems. The regulation of these processes requires joint action of multiple proteins. Here, we present comparative analyses of accumulation of transcripts encoding components of the protection machinery, such as selected LEA proteins, enzymes of the chlorophyll degradation pathway and anthocyanin biosynthesis enzymes in total and polysomal RNA pools. The analyses revealed that desiccation-tolerant plants recruit mRNAs to ribosomes with higher efficiency than the desiccation-sensitive species L. subracemosa. Desiccation-tolerant species accumulated high amounts of LEA transcripts during dehydration and precisely controlled the amounts of chlorophyll keeping it at a level sufficient to activate photosynthesis after rehydration. In contrast, mRNA of L. subracemosa was prone to dehydration-induced degradation, decomposition of the photosynthetic apparatus and degradation of free chlorophyll. Thus, the results of the studies point to differences in the control of gene expression and degradation of chlorophyll in desiccation-tolerant versus desiccation-sensitive species when the plants were subjected to dehydration.

Enzymes and Metabolites in Carbohydrate Metabolism of Desiccation Tolerant Plants

Resurrection plants can tolerate extreme water loss. Substantial sugar accumulation is a phenomenon in resurrection plants during dehydration. Sugars have been identified as one important factor contributing to desiccation tolerance. Phylogenetic diversity of resurrection plants reflects the diversity of sugar metabolism in response to dehydration. Sugars, which accumulate during dehydration, have been shown to protect macromolecules and membranes and to scavenge reactive oxygen species. This review focuses on the performance of enzymes participating in sugar metabolism during dehydration stress. The relation between sugar metabolism and other biochemical activities is discussed and open questions as well as potential experimental approaches are proposed.

Active Sites of Reduced Epidermal Fluorescence1 (REF1) Isoforms Contain Amino Acid Substitutions That Are Different between Monocots and Dicots

Plant aldehyde dehydrogenases (ALDHs) play important roles in cell wall biosynthesis, growth, development, and tolerance to biotic and abiotic stresses. The Reduced Epidermal Fluorescence1 is encoded by the subfamily 2C of ALDHs and was shown to oxidise coniferaldehyde and sinapaldehyde to ferulic acid and sinapic acid in the phenylpropanoid pathway, respectively. This knowledge has been gained from works in the dicotyledon model species Arabidopsis thaliana then used to functionally annotate ALDH2C isoforms in other species, based on the orthology principle. However, the extent to which the ALDH isoforms differ between monocotyledons and dicotyledons has rarely been accessed side-by-side. In this study, we used a phylogenetic approach to address this question. We have analysed the ALDH genes in Brachypodium distachyon, alongside those of other sequenced monocotyledon and dicotyledon species to examine traits supporting either a convergent or divergent evolution of the ALDH2C/REF1-type proteins. We found that B. distachyon, like other grasses, contains more ALDH2C/REF1 isoforms than A. thaliana and other dicotyledon species. Some amino acid residues in ALDH2C/REF1 isoforms were found as being conserved in dicotyledons but substituted by non-equivalent residues in monocotyledons. One example of those substitutions concerns a conserved phenylalanine and a conserved tyrosine in monocotyledons and dicotyledons, respectively. Protein structure modelling suggests that the presence of tyrosine would widen the substrate-binding pocket in the dicotyledons, and thereby influence substrate specificity. We discussed the importance of these findings as new hints to investigate why ferulic acid contents and cell wall digestibility differ between the dicotyledon and monocotyledon species.

The Role of Phospholipase D and MAPK Signaling Cascades in the Adaption of Lichen Microalgae to Desiccation: Changes in Membrane Lipids and Phosphoproteome

Classically, lichen phycobionts are described as poikilohydric organisms able to undergo desiccation due to the constitutive presence of molecular protection mechanisms. However, little is known about the induction of cellular responses in lichen phycobionts during drying. The analysis of the lipid composition of the desiccated lichen microalga Asterochloris erici revealed the unusual accumulation of highly polar lipids (oligogalactolipids and phosphatidylinositol), which prevents the fusion of membranes during stress, but also the active degradation of cone-shaped lipids (monogalactosyldiacylglycerol and phosphatidylethanolamine) to stabilize membranes in desiccated cells. The level of phosphatidic acid increased 7-fold during desiccation, implicating a possible role for phospholipase D (PLD) in the response to osmotic stress. Inhibition of PLD with 1-butanol markedly impaired the recovery of photosynthesis activity in A. erici upon desiccation and salt stress (2 M NaCl). These two hyperosmotic stresses caused the phosphorylation of c-Jun N-terminal kinase (JNK) and p38-like mitogen-activated protein kinase (MAPK) and the dephosphorylation of extracellular signal-regulated kinase (ERK). The incubation with 1-butanol reduced the phosphorylation of JNK-like proteins and increased the dephosphorylation of ERK-like proteins, which indicates an upstream control of MAPK cascades by PLD. The phosphoproteome showed that desiccation caused the phosphorylation of several proteins in A. erici, most of them involved in protein turnover. The results demonstrate that lichen phycobionts possess both constitutive and inducible protective mechanisms to acquire desiccation tolerance. Among others, these responses are controlled by the PLD pathway through the activation of MAPK cascades.

Physiological factors determine the accumulation of D-glycero-D-ido-octulose (D-g-D-i-oct) in the desiccation tolerant resurrection plant Craterostigma plantagineum

The relationship between the accumulation of D-glycero-D-ido-octulose (D-g-D-i-oct) and sucrose and desiccation tolerance was analysed in leaves of Craterostigma plantagineum Hochst. in various conditions. The D-g-D-i-oct level is strictly controlled in C. plantagienum. Light is an important factor enhancing D-g-D-i-oct synthesis when exogenous sucrose is supplied. Desiccation tolerance is lost during natural senescence and during sugar starvation that leads to senescence. The differences in expression patterns of senescence-related genes and the carbohydrate status between vigorous and senescent plants indicate that desiccation tolerance and accumulation of octulose in C. plantagineum is dependent on the developmental stage. Sucrose synthesis is affected more by dehydration than by senescence. D-g-D-i-oct has superior hydroxyl scavenging ability to other common sugars accumulating in C. plantagineum. In the presence of reactive oxygen species (ROS) D-g-D-i-oct levels decreased, probably as a defence reaction.

Differences in LEA-like 11-24 gene expression in desiccation tolerant and sensitive species of Linderniaceae are due to variations in gene promoter sequences

Many desiccation induced late embryogenesis abundant (LEA) protein encoding genes have been identified from Craterostigma plantagineum Hochst. In the desiccation tolerant plants C. plantagineum (Cp) and Lindernia brevidens Skan (Lb) transcripts encoding LEA-like 11-24 protein are abundantly expressed during desiccation whereas in Lindernia subracemosa De Wild. (Ls), a desiccation sensitive plant, the LEA-like 11-24 transcripts are expressed at a low level. Since promoters determine gene expression, a comparative promoter analysis was carried out to decipher the underlying mechanisms of differential gene expression. Two transient transformation methods (particle bombardment and optimised Agrobacterium co-cultivation) were used to analyse the promoter activities of the Cp, Lb and Ls LEA-like 11-24 gene in homologous and heterologous systems. Minimal promoters were isolated from all three species and their promoter activities were assessed in response to mannitol or ABA. Particle bombardment or Agrobacterium co-cultivation yielded similar results. Site-directed mutagenesis was used to identify which cis-acting elements in the LEA-like 11-24 promoter fragments are crucial during mannitol and ABA treatments. The presence of these promoter cis-elements explains the differences in transcript abundance in the desiccation tolerant and desiccation sensitive species. Results indicated the importance of the drought responsive elements (DRE) element for promoter activity.

Nicotinamide Cofactors Suppress Active-Site Labeling of Aldehyde Dehydrogenases

Active site labeling by (re)activity-based probes is a powerful chemical proteomic tool to globally map active sites in native proteomes without using substrates. Active site labeling is usually taken as a readout for the active state of the enzyme because labeling reflects the availability and reactivity of active sites, which are hallmarks for enzyme activities. Here, we show that this relationship holds tightly, but we also reveal an important exception to this rule. Labeling of Arabidopsis ALDH3H1 with a chloroacetamide probe occurs at the catalytic Cys, and labeling is suppressed upon nitrosylation and oxidation, and upon treatment with other Cys modifiers. These experiments display a consistent and strong correlation between active site labeling and enzymatic activity. Surprisingly, however, labeling is suppressed by the cofactor NAD(+), and this property is shared with other members of the ALDH superfamily and also detected for unrelated GAPDH enzymes with an unrelated hydantoin-based probe in crude extracts of plant cell cultures. Suppression requires cofactor binding to its binding pocket. Labeling is also suppressed by ALDH modulators that bind at the substrate entrance tunnel, confirming that labeling occurs through the substrate-binding cavity. Our data indicate that cofactor binding adjusts the catalytic Cys into a conformation that reduces the reactivity toward chloroacetamide probes.