Biochemical and molecular responses to water stress in resurrection plants

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.

Novel flavin-containing monooxygenase protein FMO1 interacts with CAT2 to negatively regulate drought tolerance through ROS homeostasis and ABA signaling pathway in tomato

Drought stress is the major abiotic factor that can seriously affect plant growth and crop production. The functions of flavin-containing monooxygenases (FMOs) are known in animals. They add molecular oxygen to lipophilic compounds or produce reactive oxygen species (ROS). However, little information on FMOs in plants is available. Here, we characterized a tomato drought-responsive gene that showed homology to FMO, and it was designated as FMO1. FMO1 was downregulated promptly by drought and ABA treatments. Transgenic functional analysis indicated that RNAi suppression of the expression of FMO1 (FMO1-Ri) improved drought tolerance relative to wild-type (WT) plants, whereas overexpression of FMO1 (FMO1-OE) reduced drought tolerance. The FMO1-Ri plants exhibited lower ABA accumulation, higher levels of antioxidant enzyme activities, and less ROS generation compared with the WT and FMO1-OE plants under drought stress. RNA-seq transcriptional analysis revealed the differential expression levels of many drought-responsive genes that were co-expressed with FMO1, including PP2Cs, PYLs, WRKY, and LEA. Using Y2H screening, we found that FMO1 physically interacted with catalase 2 (CAT2), which is an antioxidant enzyme and confers drought resistance. Our findings suggest that tomato FMO1 negatively regulates tomato drought tolerance in the ABA-dependent pathway and modulates ROS homeostasis by directly binding to SlCAT2.

Reactivation of the Photosynthetic Apparatus of Resurrection Plant Haberlea rhodopensis during the Early Phase of Recovery from Drought- and Freezing-Induced Desiccation

Haberlea rhodopensis is a unique desiccation-tolerant angiosperm that also survives winter frost. As, upon freezing temperatures, H. rhodopensis desiccates, the taxon is proposed to survive low temperature stress using its desiccation tolerance mechanisms. To reveal the validity of this hypothesis, we analyzed the structural alterations and organization of photosynthetic apparatus during the first hours of recovery after drought- and freezing-induced desiccation. The dynamics of the ultrastructure remodeling in the mesophyll cells and the restoration of the thylakoid membranes shared similarities independent of the reason for desiccation. Among the most obvious changes in thylakoid complexes, the proportion of the PSI-LHCII complex strongly increased around 70% relative water content (RWC), whereas the proportion of Lhc monomers decreased from the beginning of rehydration. We identified enhanced levels of cyt b₆f complex proteins that contributed to the enhanced electron flow. The high abundance of proteins related to excitation energy dissipation, PsbS, Lhcb5, Lhcb6 and ELIPs, together with the increased content of dehydrins contributed to the preservation of cellular integrity. ELIP expression was maintained at high levels up to 9 h into recovery. Although the recovery processes from drought- and freezing-induced desiccation were found to be similar in progress and time scale, slight variations indicate that they are not identical.

The Native Arbuscular Mycorrhizal Fungi and Vermicompost-Based Organic Amendments Enhance Soil Fertility, Growth Performance, and the Drought Stress Tolerance of Quinoa

The present study aimed to determine the effects of biostimulants on the physicochemical parameters of the agricultural soil of quinoa under two water regimes and to understand the mode of action of the biostimulants on quinoa for drought adaptation. We investigated the impact of two doses of vermicompost (5 and 10 t/ha) and arbuscular mycorrhizal fungi applied individually, or in joint application, on attenuating the negative impacts of water shortage and improving the agro-physiological and biochemical traits of quinoa, as well as soil fertility, under two water regimes (well-watered and drought stress) in open field conditions. Exposure to drought decreased biomass, leaf water potential, and stomatal conductance, and increased malondialdehyde and hydrogen peroxide content. Mycorrhiza and/or vermicompost promoted plant growth by activating photosynthesis machinery and nutrient assimilation, leading to increased total soluble sugars, proteins, and antioxidant enzyme activities in the leaf and root. After the experiment, the soil's total organic matter, phosphorus, nitrogen, calcium, and soil glomalin content improved by the single or combined application of mycorrhiza and vermicompost. This knowledge suggests that the combination of mycorrhiza and vermicompost regulates the physiological and biochemical processes employed by quinoa in coping with drought and improves the understanding of soil-plant interaction.

Combining yield potential and drought resilience in a spring wheat diversity panel

Pressures of population growth and climate change require the development of resilient higher yielding crops, particularly to drought. A spring wheat diversity panel was developed to combine high-yield potential with resilience. To assess performance under drought, which in many environments is intermittent and dependent on plant development, 150 lines were grown with drought imposed for 10 days either at jointing or at anthesis stages in Obregon, Mexico. Both drought treatments strongly reduced grain numbers compared with the fully irrigated check. Best performers under drought at jointing had more grain than poor performers, while best performers under drought at anthesis had larger grain than poor performers. Most high-yielding lines were high yielding in one drought environment only. However, some of the best-performing lines displayed yield potential and resilience across two environments (28 lines), particularly for yield under well-watered and drought at jointing, where yield was most related to grain numbers. Strikingly, only three lines were high yielding across all three environments, and interestingly, these lines had high grain numbers. Among parameters measured in leaves and grain, leaf relative water content did not correlate with yield, and proline was negatively correlated with yield; there were small but significant relationships between leaf sugars and yield. This study provides a valuable resource for further crosses and for elucidating genes and mechanisms that may contribute to grain number and grain filling conservation to combine yield potential and drought resilience.

Gene expression profiles of Pyropia yezoensis in response to dehydration and rehydration stresses

Pyropia yezoensis is an economically important marine macroalgae, naturally distributed in the upper intertidal zone. Owing to the nature of its habitat, the thallus will periodically be exposed to seawater or atmosphere, and can lose up to 95% of its cellular water content. This makes the alga an ideal research model to investigate the mechanisms of desiccation tolerance. In this study, we investigated the response mechanisms to dehydration and rehydration stresses at the transcription level in Pyropia yezoensis. The differently expressed genes were analyzed based on the different functions of encoding proteins: effector proteins (chloroplast proteins, macromolecular protective substances, and toxicity degradation enzymes) and regulatory proteins (protein kinases and phosphatases). Under osmotic stress, the unigenes related to photosynthesis were down-regulated significantly while those encoding glutathione transferase, superoxide dismutase and heat shock proteins were up-regulated significantly. We inferred that the photosynthetic activity was reduced to prevent damage caused by photosynthetic by-products and that the expression of antioxidant enzyme was increased to prevent the damage associated with reactive oxygen species. Additionally, unigenes encoding serine/threonine kinases and phospholipases were up-regulated in response to osmotic stress, indicating that these kinases play an important role in osmotolerance. Our work will serve as an essential foundation for the understanding of desiccation tolerance mechanisms in Pyropia yezoensis in the upper intertidal zones of rocky coasts.

Simultaneous expression of regulatory genes associated with specific drought-adaptive traits improves drought adaptation in peanut

Adaptation of crops to drought-prone rain-fed conditions can be achieved by improving plant traits such as efficient water mining (by superior root characters) and cellular-level tolerance mechanisms. Pyramiding these drought-adaptive traits by simultaneous expression of genes regulating drought-adaptive mechanisms has phenomenal relevance in improving stress tolerance. In this study, we provide evidence that peanut transgenic plants expressing Alfalfa zinc finger 1 (Alfin1), a root growth-associated transcription factor gene, Pennisetum glaucum heat-shock factor (PgHSF4) and Pea DNA helicase (PDH45) involved in protein turnover and protection showed improved tolerance, higher growth and productivity under drought stress conditions. Stable integration of all the transgenes was noticed in transgenic lines. The transgenic lines showed higher root growth, cooler crop canopy air temperature difference (less CCATD) and higher relative water content (RWC) under drought stress. Low proline levels in transgenic lines substantiate the maintenance of higher water status. The survival and recovery of transgenic lines was significantly higher under gradual moisture stress conditions with higher biomass. Transgenic lines also showed significant tolerance to ethrel-induced senescence and methyl viologen-induced oxidative stress. Several stress-responsive genes such as heat-shock proteins (HSPs), RING box protein-1 (RBX1), Aldose reductase, late embryogenesis abundant-5 (LEA5) and proline-rich protein-2 (PRP2), a gene involved in root growth, showed enhanced expression under stress in transgenic lines. Thus, the simultaneous expression of regulatory genes contributing for drought-adaptive traits can improve crop adaptation and productivity under water-limited conditions.

Predominantly Cytoplasmic Localization in Yeast of ASR1, a Non-Receptor Transcription Factor from Plants

The Asr gene family (named after abscisic acid, stress and ripening), currently classified as a novel group of the LEA superfamily, is exclusively present in the genomes of seed plants, except for the Brassicaceae family. It is associated with water-deficit stress and is involved in adaptation to dry climates. Motivated by separate reports depicting ASR proteins as either transcription factors or chaperones, we decided to determine the intracellular localization of ASR proteins. For that purpose, we employed an in vivo eukaryotic expression system, the heterologous model Saccharomyces cerevisiae, including wild type strains as well as mutants in which the variant ASR1 previously proved to be functionally protective against osmotic stress. Our methodology involved immunofluorescence-based confocal microscopy, without artificially altering the native structure of the protein under study. Results show that, in both normal and osmotic stress conditions, recombinant ASR1 turned out to localize mainly to the cytoplasm, irrespective of the genotype used, revealing a scattered distribution in the form of dots or granules. The results are discussed in terms of a plausible dual (cytoplasmic and nuclear) role of ASR proteins.

Reactive oxygen species scavenging enzymes and down-adjustment of metabolism level in mitochondria associated with desiccation-tolerance acquisition of maize embryo

It is a well-known fact that a mature seed can survive losing most of its water, yet how seeds acquire desiccation-tolerance is not well understood. Through sampling maize embryos of different developmental stages and comparatively studying the integrity, oxygen consumption rate and activities of antioxidant enzymes in the mitochondria, the main origin site of reactive oxygen species (ROS) production in seed cells, we found that before an embryo achieves desiccation-tolerance, its mitochondria shows a more active metabolism, and might produce more ROS and therefore need a more effective ROS scavenging system. However, embryo dehydration in this developmental stage declined the activities of most main antioxidant enzymes and accumulated thiobarbituric acid-reactive products in mitochondria, and then destroyed the structure and functional integrity of mitochondria. In physiologically-matured embryos (dehydration-tolerant), mitochondria showed lower metabolism levels, and no decline in ROS scavenging enzyme activities and less accumulation of thiobarbituric acid-reactive products after embryo dehydration. These data indicate that seed desiccation-tolerance acquisition might be associated with down-adjustment of the metabolism level in the late development stage, resulting in less ROS production, and ROS scavenging enzymes becoming desiccation-tolerant and then ensuring the structure and functional integrity of mitochondria.

Identification and functional validation of a unique set of drought induced genes preferentially expressed in response to gradual water stress in peanut

Peanut, found to be relatively drought tolerant crop, has been the choice of study to characterize the genes expressed under gradual water deficit stress. Nearly 700 genes were identified to be enriched in subtractive cDNA library from gradual process of drought stress adaptation. Further, expression of the drought inducible genes related to various signaling components and gene sets involved in protecting cellular function has been described based on dot blot experiments. Fifty genes (25 regulators and 25 functional related genes) selected based on dot blot experiments were tested for their stress responsiveness using northern blot analysis and confirmed their nature of differential regulation under different field capacity of drought stress treatments. ESTs generated from this subtracted cDNA library offered a rich source of stress-related genes including signaling components. Additional 50% uncharacterized sequences are noteworthy. Insights gained from this study would provide the foundation for further studies to understand the question of how peanut plants are able to adapt to naturally occurring harsh drought conditions. At present functional validation cannot be deemed in peanut, hence as a proof of concept seven orthologues of drought induced genes of peanut have been silenced in heterologous N. benthamiana system, using virus induced gene silencing method. These results point out the functional importance for HSP70 gene and key regulators such as Jumonji in drought stress response.

Plant responses to drought stress and exogenous ABA application are modulated differently by mycorrhization in tomato and an ABA-deficient mutant (sitiens)

The aims of the present study are to find out whether the effects of arbuscular mycorrhizal (AM) symbiosis on plant resistance to water deficit are mediated by the endogenous abscisic acid (ABA) content of the host plant and whether the exogenous ABA application modifies such effects. The ABA-deficient tomato mutant sitiens and its near-isogenic wild-type parental line were used. Plant development, physiology, and expression of plant genes expected to be modulated by AM symbiosis, drought, and ABA were studied. Results showed that only wild-type tomato plants responded positively to mycorrhizal inoculation, while AM symbiosis was not observed to have any effect on plant development in sitiens plants grown under well-watered conditions. The application of ABA to sitiens plants enhanced plant growth both under well-watered and drought stress conditions. In respect to sitiens plants subjected to drought stress, the addition of ABA had a cumulative effect in relation to that of inoculation with G. intraradices. Most of the genes analyzed in this study showed different regulation patterns in wild-type and sitiens plants, suggesting that their gene expression is modulated by the plant ABA phenotype. In the same way, the colonization of roots with the AM fungus G. intraradices differently regulated the expression of these genes in wild-type and in sitiens plants, which could explain the distinctive effect of the symbiosis on each plant ABA phenotype. This also suggests that the effects of the AM symbiosis on plant responses and resistance to water deficit are mediated by the plant ABA phenotype.

When cells lose water: Lessons from biophysics and molecular biology

Organisms living in deserts and anhydrobiotic species are useful models for unraveling mechanisms used to overcome water loss. In this context, late embryogenesis abundant (LEA) proteins and sugars have been extensively studied for protection against desiccation stress and desiccation tolerance. This article aims to reappraise the current understanding of these molecules by focusing on converging contributions from biochemistry, molecular biology, and the use of biophysical tools. Such tools have greatly advanced the field by uncovering intriguing aspects of protein 3-D structure, such as folding upon stress. We summarize the current research on cellular responses against water deficit at the molecular level, considering both plausible water loss-sensing mechanisms and genes governing signal transduction pathways. Finally, we propose models that could guide future experimentation, for example, by concentrating on the behavior of selected proteins in living cells.

In situ localization of glucose and sucrose in dehydrating leaves of Sporobolus stapfianus

Accumulation of soluble carbohydrates during dehydration stress is thought to be a very important mechanism for the acquisition of desiccation tolerance. Despite the proposed importance of soluble carbohydrate accumulation (especially sucrose), nothing is known about the cellular localization of carbohydrates in desiccation-tolerant plants. The present study proposes a novel and selective method for the in situ localization of sucrose and glucose in the desiccation-tolerant plant Sporobolus stapfianus. The detection of sucrose and glucose is based on a series of coupled enzymatic reactions leading to the formation of NADH. Iodonitrotetrazolium (INT) reacts with NADH, thereby providing the red-colored insoluble INT-formazan. Stained tissue sections were immediately visualized using light microscopy. Localization of the respective sugars was site specific. Sucrose was visualized in all leaf cell types during dehydration: vascular bundles, bundle sheath cells, mesophyll cells and epidermal cells. Similarly, glucose was shown to be localized in the same leaf compartments as reported for sucrose. This is the first report that describes sucrose localization in dehydrating leaf tissues of a "resurrection" plant. We conclude that, during dehydration stress, sucrose accumulates in all viable tissues; these results are in agreement with the previously proposed theories about its function as a cellular protectant.

Proteomic analysis of leaf proteins during dehydration of the resurrection plant Xerophyta viscosa

The desiccation-tolerant phenotype of angiosperm resurrection plants is thought to rely on the induction of protective mechanisms that maintain cellular integrity during water loss. Two-dimensional (2D) sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the Xerophyta viscosa Baker proteome was carried out during dehydration to identify proteins that may play a role in such mechanisms. Quantitative analysis revealed a greater number of changes in protein expression levels at 35% than at 65% relative water content (RWC) compared to fully hydrated plants, and 17 dehydration-responsive proteins were identified by tandem mass spectrometry (MS). Proteins showing increased abundance during drying included an RNA-binding protein, chloroplast FtsH protease, glycolytic enzymes and antioxidants. A number of photosynthetic proteins declined sharply in abundance in X. viscosa at RWC below 65%, including four components of photosystem II (PSII), and Western blot analysis confirmed that two of these (psbP and Lhcb2) were not detectable at 30% RWC. These data confirm that poikilochlorophylly in X. viscosa involves the breakdown of photosynthetic proteins during dismantling of the thylakoid membranes. In contrast, levels of these photosynthetic proteins were largely maintained during dehydration in the homoiochlorophyllous species Craterostigma plantagineum Hochst, which does not dismantle thylakoid membranes on drying.

Constraints of tolerance: why are desiccation-tolerant organisms so small or rare?

Drying to equilibrium with the air kills nearly all animals and flowering plants, including livestock and crops. This makes drought a key ecological problem for terrestrial life and a major cause of human famine. However, the ability to tolerate complete desiccation is widespread in organisms that are either <5 mm long or found mainly where desiccation-sensitive organisms are scarce. This suggests that there is a trade-off between desiccation tolerance and growth. Recent molecular and biochemical research shows that organisms tolerate desiccation through a set of mechanisms, including sugars that replace water and form glasses, proteins that stabilize macromolecules and membranes, and anti-oxidants that counter damage by reactive oxygen species. These protections are often induced by drying, and some of the genes involved may be homologous in microbes, plants and animals. Understanding how mechanisms of desiccation tolerance may constrain growth might show how to undo the constraint in some economically important macroorganisms and elucidate the much-studied but elusive relationship between tolerance of stress and productivity.

Seasonal adaptations of the tuberous roots of Ranunculus asiaticus to desiccation and resurrection by changes in cell structure and protein content

The annual developmental cycle of tuberous roots of Ranunculus asiaticus was studied with respect to structure and content of their cells, to understand how these roots are adapted to desiccation, high temperature and rehydration. Light microscopy, histochemical analysis, and protein analyses by SDS-PAGE were employed at eight stages of annual root development. During growth and maturation of the roots, cortical cells increased in size and their cell walls accumulated pectin materials in a distinct layer to the inside of the primary walls, with pits between adjoining cells. The number of starch granules and protein bodies also increased within the cells. Several discrete proteins accumulated. Following quiescence and rehydration of the roots there was a loss of starch and proteins from the cells, and cell walls decreased in thickness. The resurrection geophyte R. asiaticus possesses desiccation-tolerant annual roots. They store carbon and nitrogen reserves within their cells, and pectin within the walls to support growth of the plant following summer quiescence and rehydration.

An LEA group 3 family member is involved in survival of C. elegans during exposure to stress

In order to establish a functional role for late embryogenesis abundant (LEA) proteins in response to stress conditions in Caenorhabditis elegans, we silenced the expression of an LEA (Ce-lea-1) gene and determined the survival of worms under stress conditions. Ce-lea-1 transcription was induced during dehydration of C. elegans dauer juveniles. Following partial silencing of Ce-lea-1 transcription, we demonstrated a specific and significant reduction in worm survival during induction of desiccation, osmotic and heat stress. Together, these results establish a functional role for Ce-lea-1 in stress survival of C. elegans and suggest that Ce-lea-1 may function as a component that is common to the responses to the examined stress conditions.