Angiosperm Plant Desiccation Tolerance: Hints from Transcriptomics and Genome Sequencing

Physiological, transcriptomic and metabolomic insights of three extremophyte woody species living in the multi-stress environment of the Atacama Desert

MAIN CONCLUSIONS

In contrast to Neltuma species, S. tamarugo exhibited higher stress tolerance, maintaining photosynthetic performance through enhanced gene expression and metabolites. Differentially accumulated metabolites include chlorophyll and carotenoids and accumulation of non-nitrogen osmoprotectants. Plant species have developed different adaptive strategies to live under extreme environmental conditions. Hypothetically, extremophyte species present a unique configuration of physiological functions that prioritize stress-tolerance mechanisms while carefully managing resource allocation for photosynthesis. This could be particularly challenging under a multi-stress environment, where the synthesis of multiple and sequential molecular mechanisms is induced. We explored this hypothesis in three phylogenetically related woody species co-occurring in the Atacama Desert, Strombocarpa tamarugo, Neltuma alba, and Neltuma chilensis, by analyzing their leaf dehydration and freezing tolerance and by characterizing their photosynthetic performance under natural growth conditions. Besides, the transcriptomic profiling, biochemical analyses of leaf pigments, and metabolite analysis by untargeted metabolomics were conducted to study gene expression and metabolomic landscape within this challenging multi-stress environment. S. tamarugo showed a higher photosynthetic capacity and leaf stress tolerance than the other species. In this species, a multifactorial response was observed, which involves high photochemical activity associated with a higher content of chlorophylls and β-carotene. The oxidative damage of the photosynthetic apparatus is probably attenuated by the synthesis of complex antioxidant molecules in the three species, but S. tamarugo showed the highest antioxidant capacity. Comparative transcriptomic and metabolomic analyses among the species showed the differential expression of genes involved in the biosynthetic pathways of key stress-related metabolites. Moreover, the synthesis of non-nitrogen osmoprotectant molecules, such as ciceritol and mannitol in S. tamarugo, would allow the nitrogen allocation to support its high photosynthetic capacity without compromising leaf dehydration tolerance and freezing stress avoidance.

Structure and dynamics of microbial communities associated with the resurrection plant Boea hygrometrica in response to drought stress

MAIN CONCLUSION

The resurrection plant Boea hygrometrica selectively recruits and assembles drought-specific microbial communities across the plant-soil compartments, which may benefit plant growth and fitness under extreme drought conditions. Plant-associated microbes are essential for facilitating plant growth and fitness under drought stress. The resurrection plant Boea hygrometrica in natural habitats with seasonal rainfall can survive rapid desiccation, yet their interaction with microbiomes under drought conditions remains unexplored. This study examined the bacterial and fungal microbiome structure and drought response across plant-soil compartments of B. hygrometrica by high-throughput amplicon sequencing of 16S rRNA gene and internal transcribed spacer. Our results demonstrated that the diversity, composition, and functional profile of the microbial community varied considerably across the plant-soil compartments and were strongly affected by drought stress. Bacterial and fungal diversity was significantly reduced from soil to endosphere and belowground to aboveground compartments. The compartment-specific enrichment of the dominant bacteria phylum Cyanobacteriota and genus Methylorubrum in leaf endosphere, genera Pseudonocardia in rhizosphere soil and Actinoplanes in root endosphere, and fungal phylum Ascomycota in the aboveground compartments and genera Knufia in root endosphere and Cladosporium in leaf endosphere composed part of the core microbiota with corresponding enrichment of beneficial functions for plant growth and fitness. Moreover, the recruitment of dominant microbial genera Sphingosinicella and Plectosphaerella, Ceratobasidiaceae mycorrhizal fungi, and numerous plant growth-promoting bacteria involving nutrient supply and auxin regulation was observed in desiccated B. hygrometrica plants. Our results suggest that the stable assembled drought-specific microbial community of B. hygrometrica may contribute to plant survival under extreme environments and provide valuable microbial resources for the microbe-mediated drought tolerance enhancement in crops.

Higher order polyploids exhibit enhanced desiccation tolerance in the grass Microchloa caffra

Desiccation tolerance evolved recurrently across diverse plant lineages to enable survival in water-limited conditions. Many resurrection plants are polyploid, and several groups have hypothesized that polyploidy contributed to the evolution of desiccation tolerance. However, due to the vast phylogenetic distance between resurrection plant lineages, the rarity of desiccation tolerance, and the prevalence of polyploidy in plants, this hypothesis has been difficult to test. Here, we surveyed natural variation in morphological, reproductive, and desiccation tolerance traits across several cytotypes of a single species to test for links between polyploidy and increased resilience. We sampled multiple natural populations of the resurrection grass Microchloa caffra across an environmental gradient ranging from mesic to xeric in South Africa. We describe two distinct ecotypes of M. caffra that occupy different extremes of the environmental gradient and exhibit consistent differences in ploidy, morphological, reproductive, and desiccation tolerance traits in both field and common growth conditions. Interestingly, plants with more polyploid genomes exhibited consistently higher recovery from desiccation, were less reproductive, and were larger than plants with smaller genomes and lower ploidy. These data indicate that selective pressures in increasingly xeric sites may play a role in maintaining and increasing desiccation tolerance and are mediated by changes in ploidy.

Autophagy functions as a cytoprotective mechanism by regulating programmed cell death during desiccation in Syntrichia caninervis

Desiccation is a state of extreme water loss that is lethal to many plant species. Some desert plants have evolved unique strategies to cope with desiccation stress in their natural environment. Here we present the remarkable stress management mechanism of Syntrichia caninervis, a desert moss species which exhibits an 'A' category of desiccation tolerance. Our research demonstrated that desiccation stress triggers autophagy in S. caninervis while inhibiting Programmed Cell Death (PCD). Silencing of two autophagy-related genes, ATG6 and ATG2, in S. caninervis promoted PCD. Desiccation treatment accelerated cell death in ATG6 and ATG2 gene-silenced S. caninervis. Notably, trehalose was not detected during desiccation, and exogenous application of trehalose cannot activate autophagy. These results suggested that S. caninervis is independent of trehalose accumulation to triggered autophagy. Our results showed that autophagy function as prosurvival mechanism to enhance desiccation tolerance of S. caninervis. Our findings enrich the knowledge of the role of autophagy in plant stress response and may provide new insight into understanding of plant desiccation tolerance.

The genome of Haberlea rhodopensis provides insights into the mechanisms for tolerance to multiple extreme environments

Haberlea rhodopensis, a resurrection species, is the only plant known to be able to survive multiple extreme environments, including desiccation, freezing temperatures, and long-term darkness. However, the molecular mechanisms underlying tolerance to these stresses are poorly studied. Here, we present a high-quality genome of Haberlea and found that ~ 23.55% of the 44,306 genes are orphan. Comparative genomics analysis identified 89 significantly expanded gene families, of which 25 were specific to Haberlea. Moreover, we demonstrated that Haberlea preserves its resurrection potential even in prolonged complete darkness. Transcriptome profiling of plants subjected to desiccation, darkness, and low temperatures revealed both common and specific footprints of these stresses, and their combinations. For example, PROTEIN PHOSPHATASE 2C (PP2C) genes were substantially induced in all stress combinations, while PHYTOCHROME INTERACTING FACTOR 1 (PIF1) and GROWTH RESPONSE FACTOR 4 (GRF4) were induced only in darkness. Additionally, 733 genes with unknown functions and three genes encoding transcription factors specific to Haberlea were specifically induced/repressed upon combination of stresses, rendering them attractive targets for future functional studies. The study provides a comprehensive understanding of the genomic architecture and reports details of the mechanisms of multi-stress tolerance of this resurrection species that will aid in developing strategies that allow crops to survive extreme and multiple abiotic stresses.

Analysis and Identification of Genes Associated with the Desiccation Sensitivity of Panax notoginseng Seeds

Panax notoginseng (Burk.) F. H. Chen, a species of the genus Panax, radix has been traditionally used to deal with various hematological diseases and cardiovascular diseases since ancient times in East Asia. P. notoginseng produces recalcitrant seeds which are sensitive to desiccation and difficult to store for a long time. However, few data are available on the mechanism of the desiccation sensitivity of P. notoginseng seeds. To gain a comprehensive perspective of the genes associated with desiccation sensitivity, cDNA libraries from seeds under control and desiccation processes were prepared independently for Illumina sequencing. The data generated a total of 70,189,896 reads that were integrated and assembled into 55,097 unigenes with a mean length of 783 bp. In total, 12,025 differentially expressed genes (DEGs) were identified during the desiccation process. Among these DEGs, a number of central metabolism, hormonal network-, fatty acid-, and ascorbate-glutathione-related genes were included. Our data provide a comprehensive resource for identifying the genes associated with the desiccation sensitivity of P. notoginseng seeds.

Antioxidative Defense, Suppressed Nitric Oxide Accumulation, and Synthesis of Protective Proteins in Roots and Leaves Contribute to the Desiccation Tolerance of the Resurrection Plant Haberlea rhodopensis

The desiccation tolerance of plants relies on defense mechanisms that enable the protection of macromolecules, biological structures, and metabolism. Although the defense of leaf tissues exposed to solar irradiation is challenging, mechanisms that protect the viability of the roots, yet largely unexplored, are equally important for survival. Although the photosynthetic apparatus in leaves contributes to the generation of oxidative stress under drought stress, we hypothesized that oxidative stress and thus antioxidative defense is also predominant in the roots. Thus, we aimed for a comparative analysis of the protective mechanisms in leaves and roots during the desiccation of Haberlea rhodopensis. Consequently, a high content of non-enzymatic antioxidants and high activity of antioxidant enzymes together with the activation of specific isoenzymes were found in both leaves and roots during the final stages of desiccation of H. rhodopensis. Among others, catalase and glutathione reductase activity showed a similar tendency of changes in roots and leaves, whereas, unlike that in the leaves, superoxide dismutase activity was enhanced under severe but not under medium desiccation in roots. Nitric oxide accumulation in the root tips was found to be sensitive to water restriction but suppressed under severe desiccation. In addition to the antioxidative defense, desiccation induced an enhanced abundance of dehydrins, ELIPs, and sHSP 17.7 in leaves, but this was significantly better in roots. In contrast to leaf cells, starch remained in the cells of the central cylinder of desiccated roots. Taken together, protective compounds and antioxidative defense mechanisms are equally important in protecting the roots to survive desiccation. Since drought-induced damage to the root system fundamentally affects the survival of plants, a better understanding of root desiccation tolerance mechanisms is essential to compensate for the challenges of prolonged dry periods.

Overexpression of the Selaginella lepidophylla bHLH transcription factor enhances water-use efficiency, growth, and development in Arabidopsis

Abiotic stresses have the greatest impact on the growth and productivity of crops, especially under current and future extreme weather events due to climate change. Thus, it is vital to explore novel strategies to improve crop plant abiotic stress tolerance to feed an ever-growing world population. Selaginella lepidophylla is a desiccation-tolerant spike moss with specialized adaptations that allow it to tolerate water loss down to 4% relative water content. A candidate basic helix-loop-helix (bHLH) transcription factor was highly expressed at 4% relative water content in S. lepidophylla (SlbHLH). This SlbHLH gene was codon-optimized (SlbHLH_opt) and overexpressed in Arabidopsis for functional characterization. Overexpression of the SlbHLH_opt gene not only significantly increased plant growth, development, and integrated water-use efficiency, but also significantly increased seed germination and green cotyledon emergence rates under water-deficit stress and salt stress conditions. Under a 150 mM NaCl salt stress condition, SlbHLH_opt-overexpressing lines increased primary root length, the number of lateral roots, and fresh and dry biomass at the seedling stage compared to control lines. Interestingly, SlbHLH_opt-overexpressing lines also have significantly higher flavonoid content. Altogether, these results suggest that SlbHLH functions as an important regulator of plant growth, development, abiotic stress tolerance, and water-use efficiency.

The Orthodox Dry Seeds Are Alive: A Clear Example of Desiccation Tolerance

To survive in the dry state, orthodox seeds acquire desiccation tolerance. As maturation progresses, the seeds gradually acquire longevity, which is the total timespan during which the dry seeds remain viable. The desiccation-tolerance mechanism(s) allow seeds to remain dry without losing their ability to germinate. This adaptive trait has played a key role in the evolution of land plants. Understanding the mechanisms for seed survival after desiccation is one of the central goals still unsolved. That is, the cellular protection during dry state and cell repair during rewatering involves a not entirely known molecular network(s). Although desiccation tolerance is retained in seeds of higher plants, resurrection plants belonging to different plant lineages keep the ability to survive desiccation in vegetative tissue. Abscisic acid (ABA) is involved in desiccation tolerance through tight control of the synthesis of unstructured late embryogenesis abundant (LEA) proteins, heat shock thermostable proteins (sHSPs), and non-reducing oligosaccharides. During seed maturation, the progressive loss of water induces the formation of a so-called cellular "glass state". This glassy matrix consists of soluble sugars, which immobilize macromolecules offering protection to membranes and proteins. In this way, the secondary structure of proteins in dry viable seeds is very stable and remains preserved. ABA insensitive-3 (ABI3), highly conserved from bryophytes to Angiosperms, is essential for seed maturation and is the only transcription factor (TF) required for the acquisition of desiccation tolerance and its re-induction in germinated seeds. It is noteworthy that chlorophyll breakdown during the last step of seed maturation is controlled by ABI3. This update contains some current results directly related to the physiological, genetic, and molecular mechanisms involved in survival to desiccation in orthodox seeds. In other words, the mechanisms that facilitate that an orthodox dry seed is a living entity.

Crops for dry environments

Climate change necessitates increased stress resilience of food crops. We describe four potential solutions, with emphasis on a relatively novel approach aiming at true tolerance of drought rather than improved water-holding capacity of crops, which is a common approach in current breeding and genome editing efforts. Some Angiosperms are known to tolerate loss of 95% of their cellular water, without dying, not dissimilar to seeds. The molecular mechanisms and their regulation underlying this remarkable ability are potentially useful to design tolerant crops. Since most crops produce desiccation tolerant seeds, genomic information for this attribute is present but inactive in vegetative parts of the plant. Based on recent evidence from both seeds and desiccation tolerant Angiosperms we address possible routes to 'flipping the switch' to vegetative desiccation tolerance in major crops.

Pseudocrossidium replicatum (Taylor) R.H. Zander is a fully desiccation-tolerant moss that expresses an inducible molecular mechanism in response to severe abiotic stress

KEY MESSAGE

The moss Pseudocrossidium replicatum is a desiccation-tolerant species that uses an inducible system to withstand severe abiotic stress in both protonemal and gametophore tissues. Desiccation tolerance (DT) is the ability of cells to recover from an air-dried state. Here, the moss Pseudocrossidium replicatum was identified as a fully desiccation-tolerant (FDT) species. Its gametophores rapidly lost more than 90% of their water content when exposed to a low-humidity atmosphere [23% relative humidity (RH)], but abscisic acid (ABA) pretreatment diminished the final water loss after equilibrium was reached. P. replicatum gametophores maintained good maximum photosystem II (PSII) efficiency (Fv/Fm) for up to two hours during slow dehydration; however, ABA pretreatment induced a faster decrease in the Fv/Fm. ABA also induced a faster recovery of the Fv/Fm after rehydration. Protein synthesis inhibitor treatment before dehydration hampered the recovery of the Fv/Fm when the gametophores were rehydrated after desiccation, suggesting the presence of an inducible protective mechanism that is activated in response to abiotic stress. This observation was also supported by accumulation of soluble sugars in gametophores exposed to ABA or NaCl. Exogenous ABA treatment delayed the germination of P. replicatum spores and induced morphological changes in protonemal cells that resembled brachycytes. Transcriptome analyses revealed the presence of an inducible molecular mechanism in P. replicatum protonemata that was activated in response to dehydration. This study is the first RNA-Seq study of the protonemal tissues of an FDT moss. Our results suggest that P. replicatum is an FDT moss equipped with an inducible molecular response that prepares this species for severe abiotic stress and that ABA plays an important role in this response.

Convergent evolution of gene regulatory networks underlying plant adaptations to dry environments

Plants transitioned from an aquatic to a terrestrial lifestyle during their evolution. On land, fluctuations on water availability in the environment became one of the major problems they encountered. The appearance of morpho-physiological adaptations to cope with and tolerate water loss from the cells was undeniably useful to survive on dry land. Some of these adaptations, such as carbon concentrating mechanisms (CCMs), desiccation tolerance (DT) and root impermeabilization, appeared in multiple plant lineages. Despite being crucial for evolution on land, it has been unclear how these adaptations convergently evolved in the various plant lineages. Recent advances on whole genome and transcriptome sequencing are revealing that co-option of genes and gene regulatory networks (GRNs) is a common feature underlying the convergent evolution of these adaptations. In this review, we address how the study of CCMs and DT has provided insight into convergent evolution of GRNs underlying plant adaptation to dry environments, and how these insights could be applied to currently emerging understanding of evolution of root impermeabilization through different barrier cell types. We discuss examples of co-option, conservation and innovation of genes and GRNs at the cell, tissue and organ levels revealed by recent phylogenomic (comparative genomic) and comparative transcriptomic studies.

Comparative Transcriptome Analysis Revealed Candidate Genes Potentially Related to Desiccation Sensitivity of Recalcitrant Quercus variabilis Seeds

Chinese cork oak (Quercus variabilis) is a widely distributed and highly valuable deciduous broadleaf tree from both ecological and economic perspectives. Seeds of this species are recalcitrant, i.e., sensitive to desiccation, which affects their storage and long-term preservation of germplasm. However, little is known about the underlying molecular mechanism of desiccation sensitivity of Q. variabilis seeds. In this study, the seeds were desiccated with silica gel for certain days as different treatments from 0 (Control) to 15 days (T15) with a gradient of 1 day. According to the seed germination percentage, four key stages (Control, T2, T4, and T11) were found. Then the transcriptomic profiles of these four stages were compared. A total of 4,405, 4,441, and 5,907 differentially expressed genes (DEGs) were identified in T2 vs. Control, T4 vs. Control, and T11 vs. Control, respectively. Among them, 2,219 DEGs were overlapped in the three comparison groups. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that these DEGs were enriched into 124 pathways, such as "Plant hormone signal transduction" and "Glycerophospholipid metabolism". DEGs related to hormone biosynthesis and signal transduction (ZEP, YUC, PYR, ABI5, ERF1B, etc.), stress response proteins (LEA D-29, HSP70, etc.), and phospholipase D (PLD1) were detected during desiccation. These genes and their interactions may determine the desiccation sensitivity of seeds. In addition, group specific DEGs were also identified in T2 vs. Control (PP2C62, UNE12, etc.), T4 vs. Control (WRKY1-like, WAK10, etc.), and T11 vs. Control (IBH1, bZIP44, etc.), respectively. Finally, a possible work model was proposed to show the molecular regulation mechanism of desiccation sensitivity in Q. variabilis seeds. This is the first report on the molecular regulation mechanism of desiccation sensitivity of Q. variabilis seeds using RNA-Seq. The findings could make a great contribution to seed storage and long-term conservation of recalcitrant seeds in the future.

A prion-like protein regulator of seed germination undergoes hydration-dependent phase separation

Many organisms evolved strategies to survive desiccation. Plant seeds protect dehydrated embryos from various stressors and can lay dormant for millennia. Hydration is the key trigger to initiate germination, but the mechanism by which seeds sense water remains unresolved. We identified an uncharacterized Arabidopsis thaliana prion-like protein we named FLOE1, which phase separates upon hydration and allows the embryo to sense water stress. We demonstrate that biophysical states of FLOE1 condensates modulate its biological function in vivo in suppressing seed germination under unfavorable environments. We find intragenic, intraspecific, and interspecific natural variation in FLOE1 expression and phase separation and show that intragenic variation is associated with adaptive germination strategies in natural populations. This combination of molecular, organismal, and ecological studies uncovers FLOE1 as a tunable environmental sensor with direct implications for the design of drought-resistant crops, in the face of climate change.

Differences in biochemical, gas exchange and hydraulic response to water stress in desiccation tolerant and sensitive fronds of the fern Anemia caffrorum

Desiccation tolerant plants can survive extreme water loss in their vegetative tissues. The fern Anemia caffrorum produces desiccation tolerant (DT) fronds in the dry season and desiccation sensitive (DS) fronds in the wet season, providing a unique opportunity to explore the physiological mechanisms associated with desiccation tolerance. Anemia caffrorum plants with either DT or DS fronds were acclimated in growth chambers. Photosynthesis, frond structure and anatomy, water relations and minimum conductance to water vapour were measured under well-watered conditions. Photosynthesis, hydraulics, frond pigments, antioxidants and abscisic acid contents were monitored under water deficit. A comparison between DT and DS fronds under well-watered conditions showed that the former presented higher leaf mass per area, minimum conductance, tissue elasticity and lower CO₂ assimilation. Water deficit resulted in a similar induction of abscisic acid in both frond types, but DT fronds maintained higher stomatal conductance and upregulated more prominently lipophilic antioxidants. The seasonal alternation in production of DT and DS fronds in A. caffrorum represents a mechanism by which carbon gain can be maximized during the rainy season, and a greater investment in protective mechanisms occurs during the hot dry season, enabling the exploitation of episodic water availability.

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.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

Resurrection plants optimize photosynthesis despite very thick cell walls by means of chloroplast distribution

Resurrection plants are vascular species able to sustain extreme desiccation in their vegetative tissues. Despite its potential interest, the role of leaf anatomy in CO2 diffusion and photosynthesis under non-stressed conditions has not been explored in these species. Net CO2 assimilation (An) and its underlying diffusive, biochemical, and anatomical determinants were assessed in 10 resurrection species from diverse locations, including ferns, and homoiochlorophyllous and poikilochlorophyllous angiosperms. Data obtained were compared with previously published results in desiccation-sensitive ferns and angiosperms. An in resurrection plants was mostly driven by mesophyll conductance to CO2 (gm) and limited by CO2 diffusion. Resurrection species had a greater cell wall thickness (Tcw) than desiccation-sensitive plants, a feature associated with limited CO2 diffusion in the mesophyll, but also greater chloroplast exposure to intercellular spaces (Sc), which usually leads to higher gm. This combination enabled a higher An per Tcw compared with desiccation-sensitive species. Resurrection species possess unusual anatomical features that could confer stress tolerance (thick cell walls) without compromising the photosynthetic capacity (high chloroplast exposure). This mechanism is particularly successful in resurrection ferns, which display higher photosynthesis than their desiccation-sensitive counterparts.

Integration of MsrB1 and MsrB2 in the Redox Network during the Development of Orthodox and Recalcitrant Acer Seeds

Two related tree species, Norway maple (Acer platanoides L.) and sycamore (Acer pseudoplatanus L.), produce desiccation-tolerant (orthodox) and desiccation-sensitive (recalcitrant) seeds, respectively. We compared the seeds of these two species to characterize the developmentally driven changes in the levels of peptide-bound methionine sulfoxide (MetO) and the abundance of methionine sulfoxide reductases (Msrs) B1 and B2, with respect to the cellular redox environment. Protein oxidation at the Met level was dynamic only in Norway maple seeds, and the reduced MsrB2 form was detected only in this species. Cell redox status, characterized by the levels of reduced and oxidized ascorbate, glutathione, and nicotinamide adenine dinucleotide (NAD)/phosphate (NADP), was clearly more reduced in the Norway maple seeds than in the sycamore seeds. Clear correlations between MetO levels, changes in water content and redox status were reported in orthodox Acer seeds. The abundance of Msrs was correlated in both species with redox determinants, mainly ascorbate and glutathione. Our data suggest that MsrB2 is associated with the acquisition of desiccation tolerance and that ascorbate might be involved in the redox pathway enabling the regeneration of Msr via intermediates that are not known yet.

Comparative transcriptome analysis suggests convergent evolution of desiccation tolerance in Selaginella species

BACKGROUND

Desiccation tolerant Selaginella species evolved to survive extreme environmental conditions. Studies to determine the mechanisms involved in the acquisition of desiccation tolerance (DT) have focused on only a few Selaginella species. Due to the large diversity in morphology and the wide range of responses to desiccation within the genus, the understanding of the molecular basis of DT in Selaginella species is still limited.

RESULTS

Here we present a reference transcriptome for the desiccation tolerant species S. sellowii and the desiccation sensitive species S. denticulata. The analysis also included transcriptome data for the well-studied S. lepidophylla (desiccation tolerant), in order to identify DT mechanisms that are independent of morphological adaptations. We used a comparative approach to discriminate between DT responses and the common water loss response in Selaginella species. Predicted proteomes show strong homology, but most of the desiccation responsive genes differ between species. Despite such differences, functional analysis revealed that tolerant species with different morphologies employ similar mechanisms to survive desiccation. Significant functions involved in DT and shared by both tolerant species included induction of antioxidant systems, amino acid and secondary metabolism, whereas species-specific responses included cell wall modification and carbohydrate metabolism.

CONCLUSIONS

Reference transcriptomes generated in this work represent a valuable resource to study Selaginella biology and plant evolution in relation to DT. Our results provide evidence of convergent evolution of S. sellowii and S. lepidophylla due to the different gene sets that underwent selection to acquire DT.

Transcriptome analysis of Paris polyphylla var. yunnanensis illuminates the biosynthesis and accumulation of steroidal saponins in rhizomes and leaves

Paris polyphylla var. yunnanensis can synthesize Paris saponins with multiple effective therapies, and its rhizome has become an indispensable ingredient in many patented drugs. However, how Paris saponin content changes in tissues at different stages and the molecular mechanisms underlying the production and accumulation of the bioactive compounds are unclear. This study aimed to uncover the mechanisms underlying the biosynthesis and accumulation by integrating transcriptome sequencing and phytochemical investigation of the leaves and rhizomes at different growth stages. Paris saponin content in leaves was lower during the fruiting stage than the vegetative stage, whereas the content in rhizomes increased during the fruiting stage. The candidate genes related to Paris saponin biosynthesis were determined by transcriptome analyses. Most biosynthetic genes were found to be abundantly expressed in the leaves during the vegetative stage in the light of expression profiles and functional enrichment results. The expression patterns of the differentially expressed genes related to the biosynthesis were positively correlated with the accumulation of saponins in tissues. These findings suggest that both leaves and rhizomes are capable of biosynthesizing Paris saponins, and that aerial plant parts can be used to extract them. The different patterns of biosynthesis and accumulation in the leaves and rhizomes were also determined here. This study will help improve our understanding of the mechanisms underlying the biosynthesis and accumulation of Paris saponins, and aid in the comprehensive development and utilization of this medicinal plant.

Shared up-regulation and contrasting down-regulation of gene expression distinguish desiccation-tolerant from intolerant green algae

Among green plants, desiccation tolerance is common in seeds and spores but rare in leaves and other vegetative green tissues. Over the last two decades, genes have been identified whose expression is induced by desiccation in diverse, desiccation-tolerant (DT) taxa, including, e.g., late embryogenesis abundant proteins (LEA) and reactive oxygen species scavengers. This up-regulation is observed in DT resurrection plants, mosses, and green algae most closely related to these Embryophytes. Here we test whether this same suite of protective genes is up-regulated during desiccation in even more distantly related DT green algae, and, importantly, whether that up-regulation is unique to DT algae or also occurs in a desiccation-intolerant relative. We used three closely related aquatic and desert-derived green microalgae in the family Scenedesmaceae and capitalized on extraordinary desiccation tolerance in two of the species, contrasting with desiccation intolerance in the third. We found that during desiccation, all three species increased expression of common protective genes. The feature distinguishing gene expression in DT algae, however, was extensive down-regulation of gene expression associated with diverse metabolic processes during the desiccation time course, suggesting a switch from active growth to energy-saving metabolism. This widespread downshift did not occur in the desiccation-intolerant taxon. These results show that desiccation-induced up-regulation of expression of protective genes may be necessary but is not sufficient to confer desiccation tolerance. The data also suggest that desiccation tolerance may require induced protective mechanisms operating in concert with massive down-regulation of gene expression controlling numerous other aspects of metabolism.

Desiccation does not drastically increase the accessibility of exogenous DNA to nuclear genomes: evidence from the frequency of endosymbiotic DNA transfer

Background

Although horizontal gene transfer (HGT) is a widely accepted force in the evolution of prokaryotic genomes, its role in the evolution of eukaryotic genomes remains hotly debated. Some bdelloid rotifers that are resistant to extreme desiccation and radiation undergo a very high level of HGT, whereas in another desiccation-resistant invertebrate, the tardigrade, the pattern does not exist. Overall, the DNA double-strand breaks (DSBs) induced by prolonged desiccation have been postulated to open a gateway to the nuclear genome for exogenous DNA integration and thus to facilitate the HGT process, thereby enhancing the rate of endosymbiotic DNA transfer (EDT).

Results

We first surveyed the abundance of nuclear mitochondrial DNAs (NUMTs) and nuclear plastid DNAs (NUPTs) in five eukaryotes that are highly resistant to desiccation: the bdelloid rotifers Adineta vaga and Adineta ricciae, the tardigrade Ramazzottius varieornatus, and the resurrection plants Dorcoceras hygrometricum and Selaginella tamariscina. Excessive NUMTs or NUPTs were not detected. Furthermore, we compared 24 groups of desiccation-tolerant organisms with their relatively less desiccation-tolerant relatives but did not find a significant difference in NUMT/NUPT contents.

Conclusions

Desiccation may induce DSBs, but it is unlikely to dramatically increase the frequency of exogenous sequence integration in most eukaryotes. The capture of exogenous DNA sequences is possible only when DSBs are repaired through a subtype of non-homologous end joining, named alternative end joining (alt-EJ). Due to the deleterious effects of the resulting insertion mutations, alt-EJ is less frequently initiated than other mechanisms.

Decoding Gene Networks Modules That Explain the Recovery of Hymenoglossum cruentum Cav. After Extreme Desiccation

Hymenoglossum cruentum (Hymenophyllaceae) is a poikilohydric, homoiochlorophyllous desiccation-tolerant (DT) epiphyte fern. It can undergo fast and frequent dehydration-rehydration cycles. This fern is highly abundant at high-humidity/low-light microenvironments within the canopy, although rapid changes in humidity and light intensity are frequent. The objective of this research is to identify genes associated to desiccation-rehydration cycle in the transcriptome of H. cruentum to better understand the genetic dynamics behind its desiccation tolerance mechanism. H. cruentum plants were subjected to a 7 days long desiccation-rehydration process and then used to identify key expressed genes associated to its capacity to dehydrate and rehydrate. The relative water content (RWC) and maximum quantum efficiency (F v/F m) of H. cruentum fronds decayed to 6% and 0.04, respectively, at the end of the desiccation stage. After re-watering, the fern showed a rapid recovery of RWC and F v/F m (ca. 73% and 0.8, respectively). Based on clustering and network analysis, our results reveal key genes, such as UBA/TS-N, DYNLL, and LHC, orchestrating intracellular motility and photosynthetic metabolism; strong balance between avoiding cell death and defense (CAT3, AP2/ERF) when dehydrated, and detoxifying pathways and stabilization of photosystems (GST, CAB2, and ELIP9) during rehydration. Here we provide novel insights into the genetic dynamics behind the desiccation tolerance mechanism of H. cruentum.

A field portable method for the semi-quantitative estimation of dehydration tolerance of photosynthetic tissues across distantly related land plants

Desiccation tolerant (DT) plants withstand complete cellular dehydration, reaching relative water contents (RWC) below 30% in their photosynthetic tissues. Desiccation sensitive (DS) plants exhibit different degrees of dehydration tolerance (DHT), never surviving water loss >70%. To date, no procedure for the quantitative evaluation of DHT extent exists that is able to discriminate DS species with differing degrees of DHT from truly DT plants. We developed a simple, feasible and portable protocol to differentiate between DT and different degrees of DHT in the photosynthetic tissues of seed plants and between fast desiccation (< 24 h) tolerant (FDT) and sensitive (FDS) bryophytes. The protocol is based on (1) controlled desiccation inside Falcon tubes equilibrated at three different relative humidities that, consequently, induce three different speeds and extents of dehydration and (2) an evaluation of the average percentage of maximal photochemical efficiency of PSII (F_v /fm) recovery after rehydration. Applying the method to 10 bryophytes and 28 tracheophytes from various locations, we found that (1) imbibition of absorbent material with concentrated salt-solutions inside the tubes provides stable relative humidity and avoids direct contact with samples; (2) for 50 ml capacity tubes, the optimal plant amount is 50-200 mg fresh weight; (3) the method is useful in remote locations due to minimal instrumental requirements; and (4) a threshold of 30% recovery of the initial F_v /fm upon reaching RWC ≤ 30% correctly categorises DT species, with three exceptions: two poikilochlorophyllous species and one gymnosperm. The protocol provides a semi-quantitative expression of DHT that facilitates comparisons of species with different morpho-physiological traits and/or ecological attributes.

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.

Metabolic Changes on the Acquisition of Desiccation Tolerance in Seeds of the Brazilian Native Tree Erythrina speciosa

Erythrina speciosa Andrews (Fabaceae) is a native tree of Atlantic forest from Southern and Southeastern Brazil. Although this species is found in flooded areas, it produces highly desiccation tolerant seeds. Here, we investigated the physiological and metabolic events occurring during seed maturation of E. speciosa aiming to better understand of its desiccation tolerance acquisition. Seeds were separated into six stages of maturation by the pigmentation of the seed coat. Water potential (WP) and water content (WC) decreased gradually from the first stage to the last stage of maturation (VI), in which seeds reached the highest accumulation of dry mass and seed coat acquired water impermeability. At stage III (71% WC), although seeds were intolerant to desiccation, they were able to germinate (about 15%). Desiccation tolerance was first observed at stage IV (67% WC), in which 40% of seeds were tolerant. At stage V (24% WC), all seeds were tolerant to desiccation and at stage VI all seeds germinated. Increased deposition of the arabinose-containing polysaccharides, which are known as cell wall plasticizers polymers, was observed up to stage IV of seed maturation. Raffinose and stachyose gradually increased in axes and cotyledons with greater increment in the fourth stage. Metabolic profile analysis showed that levels of sugars, organic, and amino acids decrease drastically in embryonic axes, in agreement with lower respiratory rates during maturation. Moreover, a non-aqueous fractionation revealed a change on the proportions of sugar accumulation among cytosol, plastid, and vacuoles between the active metabolism (stage I) and the dormant seeds (stage VI). The results indicate that the physiological maturity of the seeds of E. speciosa is reached at stage V and that the accumulation of raffinose can be a result of the change in the use of carbon, reducing metabolic activity during maturation. This work confirms that raffinose is involved in desiccation tolerance in seeds of E. speciosa, especially considering the different subcellular compartments and suggests even that the acquisition of desiccation tolerance in this species occurs in stages prior to the major changes in WC.

Common and Specific Mechanisms of Desiccation Tolerance in Two Gesneriaceae Resurrection Plants. Multiomics Evidences

Environmental stress, especially water deficiency, seriously limits plant distribution and crop production worldwide. A small group of vascular angiosperm plants termed "resurrection plants," possess desiccation tolerance (DT) to withstand dehydration and to recover fully upon rehydration. In recent years, with the rapid development of life science in plants different omics technologies have been widely applied in resurrection plants to study DT. Boea hygrometrica is native in East and Southeast Asia, and Haberlea rhodopensis is endemic to the Balkans in Europe. They are both resurrection pants from Gesneriaceae family. This paper reviews recent advances in transcriptome and metabolome, and discusses the differences and similarities of DT features between both species. Finally, we believe we provide novel insights into understanding the mechanisms underlying the acquisition and evolution of desiccation tolerance of the resurrection plants that could substantially contribute to develop new approaches for agriculture to overcome water deficiency in future.

Characterization of novel small RNAs (sRNAs) contributing to the desiccation response of Salmonella enterica serovar Typhimurium

Noncoding RNA (ncRNA) modulation of gene expression has now been ubiquitously observed across all domains of life. An increasingly apparent role of ncRNAs is to coordinate changes in gene expressions in response to environmental stress. Salmonella enterica, a common food-born pathogen, is known for its striking ability to survive, adapt, and thrive in various unfavourable environments which makes it a particularly difficult pathogen to eliminate as well as an interesting model in which to study ncRNA contributions to cellular stress response. Mounting evidence now suggests that small RNAs (sRNAs) represent key regulators of Salmonella stress adaptation. Approximately 50-500 nucleotides in length, sRNAs regulate gene expression through complementary base pairing with molecular targets and have recently been suggested to outnumber protein-coding genes in bacteria. In this work, we employ small RNA transcriptome sequencing to characterize changes in the sRNA profiles of Salmonella in response to desiccation. In all, we identify 102 previously annotated sRNAs significantly differentially expressed during desiccation; and excitingly, 71 novel sRNAs likewise differentially expressed. Small transcript northern blotting and qRT-PCRs confirm the identities and expressions of several of our novel sRNAs, and computational analyses indicate the majority are highly conserved and structurally related to characterized sRNAs. Predicted sRNA targets include several proteins necessary for desiccation survival and this, in part, suggests a role for desiccation-regulated sRNAs in this stress response. Furthermore, we find individual knock-outs of two of the novel sRNAs identified herein, either sRNA1320429 or sRNA3981754, significantly impairs the ability of Salmonella to survive desiccation, confirming their involvements (and suggesting the potential involvements of other sRNAs we identify in this work) in the Salmonella response to desiccation.

Root Plasticity in the Pursuit of Water

One of the greatest challenges of terrestrial vegetation is to acquire water through soil-grown roots. Owing to the scarcity of high-quality water in the soil and the environment's spatial heterogeneity and temporal variability, ranging from extreme flooding to drought, roots have evolutionarily acquired tremendous plasticity regarding their geometric arrangement of individual roots and their three-dimensional organization within the soil. Water deficiency has also become an increasing threat to agriculture and dryland ecosystems due to climate change. As a result, roots have become important targets for genetic selection and modification in an effort to improve crop resilience under water-limiting conditions. This review addresses root plasticity from different angles: Their structures and geometry in response to the environment, potential genetic control of root traits suitable for water-limiting conditions, and contemporary and future studies of the principles underlying root plasticity post-Darwin's 'root-brain' hypothesis. Our increasing knowledge of different disciplines of plant sciences and agriculture should contribute to a sustainable management of natural and agricultural ecosystems for the future of mankind.

Genome-level responses to the environment: plant desiccation tolerance

Plants being sessile organisms are well equipped genomically to respond to environmental stressors peculiar to their habitat. Evolution of plants onto land was enabled by the ability to tolerate extreme water loss (desiccation), a feature that has been retained within genomes but not universally expressed in most land plants today. In the majority of higher plants, desiccation tolerance (DT) is expressed only in reproductive tissues (seeds and pollen), but some 135 angiosperms display vegetative DT. Here, we review genome-level responses associated with DT, pointing out common and yet sometimes discrepant features, the latter relating to evolutionary adaptations to particular niches. Understanding DT can lead to the ultimate production of crops with greater tolerance of drought than is currently realized.

On the origins of osmotically driven stomatal movements

Contents Summary 84 I. Introduction 84 II. Stomatal form and biomechanics 85 III. Stomatal function 86 IV. Evolution of guard cell ion channels 87 V. Conclusions 88 Acknowledgements 88 Author contributions 88 References 88 SUMMARY: Stomatal pores with apertures that can be adjusted by changes in guard cell turgor have facilitated plant success in dry environments. We explore their evolutionary origins, considering recent findings from bryophytes. Unlike vascular plant stomata, which close to prevent water loss, bryophyte stomata become locked open to promote spore desiccation. We find that the families of ion channels, known to control stomatal movements in angiosperms, are ancient and represented across extant land plants. However, although angiosperm guard cells express specific ion channel genes, none appear specifically expressed in stomata-bearing moss tissues. Given the evolutionary shift in stomatal function from promotion to prevention of water loss, we postulate that ion channels adopted guard cell-specific functions after the divergence of bryophytes.

Combinatorial Interactions of Biotic and Abiotic Stresses in Plants and Their Molecular Mechanisms: Systems Biology Approach

Plants are continually facing biotic and abiotic stresses, and hence, they need to respond and adapt to survive. Plant response during multiple and combined biotic and abiotic stresses is highly complex and varied than the individual stress. These stresses resulted alteration of plant behavior through regulating the levels of microRNA, heat shock proteins, epigenetic variations. These variations can cause many adverse effects on the growth and development of the plant. Further, in natural conditions, several abiotic stresses causing factors make the plant more susceptible to pathogens infections and vice-versa. A very intricate and multifaceted interactions of various biomolecules are involved in metabolic pathways that can direct towards a cross-tolerance and improvement of plant's defence system. Systems biology approach plays a significant role in the investigation of these molecular interactions. The valuable information obtained by systems biology will help to develop stress-resistant plant varieties against multiple stresses. Thus, this review aims to decipher various multilevel interactions at the molecular level under combinatorial biotic and abiotic stresses and the role of systems biology to understand these molecular interactions.

The drnf1 Gene from the Drought-Adapted Cyanobacterium Nostoc flagelliforme Improved Salt Tolerance in Transgenic Synechocystis and Arabidopsis Plant

Environmental abiotic stresses are limiting factors for less tolerant organisms, including soil plants. Abiotic stress tolerance-associated genes from prokaryotic organisms are supposed to have a bright prospect for transgenic application. The drought-adapted cyanobacterium Nostoc flagelliforme is arising as a valuable prokaryotic biotic resource for gene excavation. In this study, we evaluated the salt-tolerant function and application potential of a candidate gene drnf1 from N. flagelliforme, which contains a P-loop NTPase (nucleoside-triphosphatase) domain, through heterologous expression in two model organisms Synechocystis sp. PCC 6803 and Arabidopsis thaliana. It was found that DRNF1 could confer significant salt tolerance in both transgenic organisms. In salt-stressed transgenic Synechocystis, DRNF1 could enhance the respiration rate; slow-down the accumulation of exopolysaccharides; up-regulate the expression of salt tolerance-related genes at a higher level, such as those related to glucosylglycerol synthesis, Na⁺/H⁺ antiport, and sugar metabolism; and maintain a better K⁺/Na⁺ homeostasis, as compared to the wild-type strain. These results imply that DRNF1 could facilitate salt tolerance by affecting the respiration metabolism and indirectly regulating the expression of important salt-tolerant genes. Arabidopsis was employed to evaluate the salt tolerance-conferring potential of DRNF1 in plants. The results show that it could enhance the seed germination and shoot growth of transgenic plants under saline conditions. In general, a novel prokaryotic salt-tolerant gene from N. flagelliforme was identified and characterized in this study, enriching the candidate gene pool for genetic engineering in plants.

Plant Desiccation Tolerance and its Regulation in the Foliage of Resurrection “Flowering-Plant” Species

The majority of flowering-plant species can survive complete air-dryness in their seed and/or pollen. Relatively few species (‘resurrection plants’) express this desiccation tolerance in their foliage. Knowledge of the regulation of desiccation tolerance in resurrection plant foliage is reviewed. Elucidation of the regulatory mechanism in resurrection grasses may lead to identification of genes that can improve stress tolerance and yield of major crop species. Well-hydrated leaves of resurrection plants are desiccation-sensitive and the leaves become desiccation tolerant as they are drying. Such drought-induction of desiccation tolerance involves changes in gene-expression causing extensive changes in the complement of proteins and the transition to a highly-stable quiescent state lasting months to years. These changes in gene-expression are regulated by several interacting phytohormones, of which drought-induced abscisic acid (ABA) is particularly important in some species. Treatment with only ABA induces desiccation tolerance in vegetative tissue of Borya constricta Churchill. and Craterostigma plantagineum Hochstetter. but not in the resurrection grass Sporobolus stapfianus Gandoger. Suppression of drought-induced senescence is also important for survival of drying. Further research is needed on the triggering of the induction of desiccation tolerance, on the transition between phases of protein synthesis and on the role of the phytohormone, strigolactone and other potential xylem-messengers during drying and rehydration.

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.

The Multiplanetary Future of Plant Synthetic Biology

The interest in human space journeys to distant planets and moons has been re-ignited in recent times and there are ongoing plans for sending the first manned missions to Mars in the near future. In addition to generating oxygen, fixing carbon, and recycling waste and water, plants could play a critical role in producing food and biomass feedstock for the microbial manufacture of materials, chemicals, and medicines in long-term interplanetary outposts. However, because life on Earth evolved under the conditions of the terrestrial biosphere, plants will not perform optimally in different planetary habitats. The construction or transportation of plant growth facilities and the availability of resources, such as sunlight and liquid water, may also be limiting factors, and would thus impose additional challenges to efficient farming in an extraterrestrial destination. Using the framework of the forthcoming human missions to Mars, here we discuss a series of bioengineering endeavors that will enable us to take full advantage of plants in the context of a Martian greenhouse. We also propose a roadmap for research on adapting life to Mars and outline our opinion that synthetic biology efforts towards this goal will contribute to solving some of the main agricultural and industrial challenges here on Earth.

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.

Perspectives on Structural, Physiological, Cellular, and Molecular Responses to Desiccation in Resurrection Plants

Resurrection plants possess a unique ability to counteract desiccation stress. Desiccation tolerance (DT) is a very complex multigenic and multifactorial process comprising a combination of physiological, morphological, cellular, genomic, transcriptomic, proteomic, and metabolic processes. Modification in the sugar composition of the hemicellulosic fraction of the cell wall is detected during dehydration. An important change is a decrease of glucose in the hemicellulosic fraction during dehydration that can reflect a modification of the xyloglucan structure. The expansins might also be involved in cell wall flexibility during drying and disrupt hydrogen bonds between polymers during rehydration of the cell wall. Cleavages by xyloglucan-modifying enzymes release the tightly bound xyloglucan-cellulose network, thus increasing cell wall flexibility required for cell wall folding upon desiccation. Changes in hydroxyproline-rich glycoproteins (HRGPs) such as arabinogalactan proteins (AGPs) are also observed during desiccation and rehydration processes. It has also been observed that significant alterations in the process of photosynthesis and photosystem (PS) II activity along with changes in the antioxidant enzyme system also increased the cell wall and membrane fluidity resulting in DT. Similarly, recent data show a major role of ABA, LEA proteins, and small regulatory RNA in regulating DT responses. Current progress in "-omic" technologies has enabled quantitative monitoring of the plethora of biological molecules in a high throughput routine, making it possible to compare their levels between desiccation-sensitive and DT species. In this review, we present a comprehensive overview of structural, physiological, cellular, molecular, and global responses involved in desiccation tolerance.

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.

Plant Life in Extreme Environments: How Do You Improve Drought Tolerance?

Systems studies of drought stress in resurrection plants and other xerophytes are rapidly identifying a large number of genes, proteins and metabolites that respond to severe drought stress or desiccation. This has provided insight into drought resistance mechanisms, which allow xerophytes to persist under such extreme environmental conditions. Some of the mechanisms that ensure cellular protection during severe dehydration appear to be unique to desert species, while many other stress signaling pathways are in common with well-studied model and crop species. However, despite the identification of many desiccation inducible genes, there are few "gene-to-field" examples that have led to improved drought tolerance and yield stability derived from resurrection plants, and only few examples have emerged from model species. This has led to many critical reviews on the merit of the experimental approaches and the type of plants used to study drought resistance mechanisms. This article discusses the long-standing arguments between the ecophysiology and molecular biology communities, on how to "drought-proof" future crop varieties. It concludes that a more positive and inclusive dialogue between the different disciplines is needed, to allow us to move forward in a much more constructive way.

Dehydration survival of crop plants and its measurement

Dehydration survival under drought stress is defined in this review as the transition from plant activity into a quiescent state of life preservation, which will be terminated by either recovery or death, depending on the stress regime and the plant's resilience. Dehydration survival is a popular phenotype by which functional genomics attempts to test gene function in drought resistance and survival. The available reports on phenotyping and genotyping of dehydration survival in genomic studies indicate that the measurement of this trait is often biased to the extent that misguided interpretations are likely to occur. This review briefly discusses the physiological basis of dehydration survival in resurrection plants and crop plants, and concludes that in phenotyping dehydration survival there is a need to distinguish between dehydration avoidance and dehydration tolerance (also termed desiccation tolerance) in affecting survival and recovery. Without this distinction, functional genomics studies of the trait might be biased. Survival due to dehydration avoidance is expressed by the capacity to maintain a relatively high plant water status as the plant is desiccated. Survival due to dehydration tolerance is expressed by delayed mortality (mortality at a relatively low plant water status) as affected by the resilience of plant metabolism. The common test of dehydration survival, using the relative recovery after a given number of stress days, is therefore insufficient because it is mainly driven by dehydration avoidance and so ignores a possible role for dehydration tolerance. Conceivable methods for more accurate phenotyping of the two components of dehydration survival are proposed and discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.