The competence to acquire cellular desiccation tolerance is independent of seed morphological development

Quantitative Imaging Reveals Distinct Contributions of SnRK2 and ABI3 in Plasmodesmatal Permeability in Physcomitrella patens

Cell-to-cell communication is tightly regulated in response to environmental stimuli in plants. We previously used a photoconvertible fluorescent protein Dendra2 as a model reporter to study this process. This experiment revealed that macromolecular trafficking between protonemal cells in Physcomitrella patens is suppressed in response to abscisic acid (ABA). However, it remains unknown which ABA signaling components contribute to this suppression and how. Here, we show that ABA signaling components SUCROSE NON-FERMENTING 1-RELATED PROTEIN KINASE 2 (PpSnRK2) and ABA INSENSITIVE 3 (PpABI3) play roles as an essential and promotive factor, respectively, in regulating ABA-induced suppression of Dendra2 diffusion between cells (ASD). Our quantitative imaging analysis revealed that disruption of PpSnRK2 resulted in defective ASD onset itself, whereas disruption of PpABI3 caused an 81-min delay in the initiation of ASD. Live-cell imaging of callose deposition using aniline blue staining showed that, despite this onset delay, callose deposition on cross walls remained constant in the PpABI3 disruptant, suggesting that PpABI3 facilitates ASD in a callose-independent manner. Given that ABA is an important phytohormone to cope with abiotic stresses, we further explored cellular physiological responses. We found that the acquisition of salt stress tolerance is promoted by PpABI3 in a quantitative manner similar to ASD. Our results suggest that PpABI3-mediated ABA signaling may effectively coordinate cell-to-cell communication during the acquisition of salt stress tolerance. This study will accelerate the quantitative study for ABA signaling mechanism and function in response to various abiotic stresses.

Spectroscopy and SEM imaging reveal endosymbiont-dependent components changes in germinating kernel through direct and indirect coleorhiza-fungus interactions under stress

In the present study, FTIR spectroscopy and hyperspectral imaging was introduced as a non-destructive, sensitive-reliable tool for assessing the tripartite kernel-fungal endophyte environment interaction. Composition of coleorhizae of Triticum durum was studied under ambient and drought stress conditions. The OH-stretch IR absorption spectrum suggests that the water-deficit was possibly improved or moderated by kernel's endophytic partner. The OH-stretch frequency pattern coincides with other (growth and stress) related molecular changes. Analysis of lipid (3100-2800 cm-1) and protein (1700-1550 cm-1) regions seems to demonstrate that drought has a positive impact on lipids. The fungal endosymbiont direct contact with kernel during germination had highest effect on both lipid and protein (Amide I and II) groups, indicating an increased stress resistance in inoculated kernel. Compared to the indirect kernel-fungus interaction and to non-treated kernels (control), direct interaction produced highest effect on lipids. Among treatments, the fingerprint region (1800-800 cm-1) and SEM images indicated an important shift in glucose oligosaccharides, possibly linked to coleorhiza-polymer layer disappearance. Acquired differentiation in coleorhiza composition of T. durum, between ambient and drought conditions, suggests that FTIR spectroscopy could be a promising tool for studying endosymbiont-plant interactions within a changing environment.

A decrease in bulk water and mannitol and accumulation of trehalose and trehalose-based oligosaccharides define a two-stage maturation process towards extreme stress resistance in ascospores of Neosartorya fischeri (Aspergillus fischeri)

Fungal propagules survive stresses better than vegetative cells. Neosartorya fischeri, an Aspergillus teleomorph, forms ascospores that survive high temperatures or drying followed by heat. Not much is known about maturation and development of extreme stress resistance in fungal cells. This study provides a novel two-step model for the acquisition of extreme stress resistance and entry into dormancy. Ascospores of 11- and 15-day-old cultures exhibited heat resistance, physiological activity, accumulation of compatible solutes and a steep increase in cytoplasmic viscosity. Electron spin resonance spectroscopy indicated that this stage is associated with the removal of bulk water and an increase of chemical stability. Older ascospores from 15- to 50-day-old cultures showed no changes in compatible solute content and cytoplasmic viscosity, but did exhibit a further increase of heat resistance and redox stability with age. This stage was also characterized by changes in the composition of the mixture of compatible solutes. Mannitol levels decreased and the relative quantities of trehalose and trehalose-based oligosaccharides increased. Dormant ascospores of N. fischeri survive in low-water habitats. After activation of the germination process, the stress resistance decreases, compatible solutes are degraded and the cellular viscosity drops. After 5 h, the hydrated cells enter the vegetative stage and redox stability has decreased notably.

Loss of desiccation tolerance in Copaifera langsdorffii Desf. seeds during germination

This study evaluated the loss of desiccation tolerance in C. langsdorffii seeds during the germination process. Seeds were imbibed for 24, 48, 72, 96, 120 and 144 hours and dried to the initial moisture content, kept in this state for 3 days after which they were submitted to pre-humidification and rehydration. Ultraestructural evaluations were done aiming to observe the cell damage caused by the dry process. Desiccation tolerance was evaluated in terms of the percentage of normal seedlings. Seeds not submitted to the drying process presented 61% of normal seedlings, and after 24 hours of imbibition, followed by drying, the seeds presented the same percentage of survival. However, after 48 hours of imbibition, seeds started to lose the desiccation tolerance. There was twenty six percent of normal seedlings formed from seeds imbibed for 96 hours and later dried and rehydrated. Only 5% of seeds imbibed for 144 hours, dried and rehydrated formed normal seedlings. At 144 hours of imbibition followed the dry process, there was damage into the cell structure, indicating that the seeds were unable to keep the cell structure during the drying process. Copaifera langsdorffii seeds loses the desiccation tolerance at the start of Phase 2 of imbibition.

The effects of drying following heat shock exposure of the desert moss Syntrichia caninervis

Desert mosses are components of biological soil crusts (BSCs) and their ecological functions make assessment and protection of these mosses a high-ranking management priority in desert regions. Drying is thought to be useful for desert mosses surviving heat shock. In this study, we investigated the role of drying by monitoring the responses of physiological characters and asexual reproduction in the typical desert moss Syntrichia caninervis. Heat significantly decreased chlorophyll content and weakened rapid recovery of photochemical activity, and increased carotenoid content and membrane permeability. Lethal temperatures significantly destroyed shoot regeneration potential. In comparison with heat alone, drying significantly increased protonema emergence time and depressed protonema emergence area. Drying combined with heat accelerated water loss, followed by a decrease of photosynthetic activity. Drying had different influences on membrane permeability at different temperatures. When moss leaves were subjected to a combined stress of drying and heat shock, photosynthesis was maintained mainly due to the effects of drying on physiological activity although the cellular morphological integrity was affected. Drying caused opposing effects on moss physiological and reproductive characteristics. On the one hand, drying caused a positive synergistic effect with heat shock when the temperature was below 40 degrees C. On the other hand, drying showed antagonism with heat shock when the moss was subjected to temperatures higher than 40 degrees C. These findings may help in understanding the survival mechanism of dessert mosses under heat shock stress which will be helpful for the artificial reconstruction of BSCs.

Invited Review: Cryo-scanning electron microscopy (CSEM) in the advancement of functional plant biology. Morphological and anatomical applications

Cryo-scanning electron microscopy (CSEM) is reviewed by exploring how the images obtained have changed paradigms of plant functions and interactions with their environment. Its power to arrest and stabilise plant parts in milliseconds, and to preserve them at full hydration for examination at micrometre resolution has changed many views of plant function. For example, it provides the only feasible way of accurately measuring stomatal aperture during active transpiration, and volume and shape changes in guard cells, or examining the contents of laticifers. It has revealed that many xylem conduits contain gas, not liquid, during the day, and that they can be refilled with sap and resume water transport. It has elucidated the management of ice to prevent cell damage in frost tolerant plants and has revealed for the first time inherent biological and physical features of root/soil interactions in the field. CSEM is increasingly used to reveal complementary structural information in studies of metabolism, fungal infection and symbiosis, molecular and genetic analysis.

Quantitative imaging of oil storage in developing crop seeds

In this article, we present a tool which allows the rapid and non-invasive detection and quantitative visualization of lipid in living seeds at a variety of stages using frequency-selected magnetic resonance imaging. The method provides quantitative lipid maps with a resolution close to the cellular level (in-plane 31 microm x 31 microm). The reliability of the method was demonstrated using two contrasting subjects: the barley grain (monocot, 2% oil, highly compartmentalized) and the soybean grain (dicot, 20% oil, economically important oilseed). Steep gradients in local oil storage were defined at the organ- and tissue-specific scales. These gradients were closely coordinated with tissue differentiation and seed maturation, as revealed by electron microscopy and biochemical and gene expression analysis. The method can be used to elucidate similar oil accumulation processes in different tissues/organs, as well as to follow the fate of storage lipids during deposition and subsequent mobilization.

Transcriptome profiling uncovers metabolic and regulatory processes occurring during the transition from desiccation-sensitive to desiccation-tolerant stages in Medicago truncatula seeds

To investigate regulatory processes and protective mechanisms leading to desiccation tolerance (DT) in seeds, 16086-element microarrays were used to monitor changes in the transcriptome of desiccation-sensitive 3-mm-long radicles of Medicago truncatula seeds at different time points during incubation in a polyethylene glycol (PEG) solution at -1.7 MPa, resulting in a gradual re-establishment of DT. Gene profiling was also performed on embryos before and after the acquisition of DT during maturation. More than 1300 genes were differentially expressed during the PEG incubation. A large number of genes involved in C metabolism are expressed during the re-establishment of DT. Quantification of C reserves confirms that lipids, starch and oligosaccharides were mobilised, coinciding with the production of sucrose during the early osmotic adjustment. Several clusters of gene profiles were identified with different time-scales. Genes expressed early during the PEG incubation belonged to classes involved in early stress and adaptation responses. Interestingly, several regulatory genes typically expressed during abiotic/drought stresses were also upregulated during maturation, arguing for the partial overlap of ABA-dependent and -independent regulatory pathways involved in both drought and DT. At later time points, in parallel to the re-establishment of DT, upregulated genes are comparable with those involved in late seed maturation. Concomitantly, a massive repression of genes belonging to numerous classes occurred, including cell cycle, biogenesis, primary and energy metabolism. The re-establishment of DT in the germinated radicles appears to concur with a partial return to the quiescent state prior to germination.

Cryopreservation of immature seeds of Bletilla striata by vitrification

An efficient protocol was established for the cryopreservation of immature seeds of a terrestrial orchid, Bletilla striata. Immature seeds collected 2-4 months after pollination (MAP) were treated using three different cryogenic procedures: (1) direct plunging into liquid nitrogen, (2) vitrification, and (3) vitrification with preculture. When immature seeds collected 3 MAP and 4 MAP were precultured for 3 days on New Dogashima medium supplemented with 0.3 M sucrose and cryopreserved by vitrification, the survival rate after preservation, as assessed by staining with 2,3,5-triphenyltetrazolium chloride, was 92% and 81%, respectively. Immature seeds thus treated showed no decrease in germination rate relative to untreated immature seeds, and they developed into normal plantlets in vitro.

Membrane behavior as influenced by partitioning of amphiphiles during drying: a comparative study in anhydrobiotic plant systems

During cellular desiccation, reduction in volume can in principle cause amphiphilic compounds to partition from the cytoplasm into membranes, with structural perturbance as the result. Here, we studied the effect of partitioning of endogenous amphiphiles on membrane surface dynamics in desiccation-tolerant and -intolerant, higher and lower plant systems, using electron paramagnetic resonance (EPR) spin probe techniques. Labeling cells with the amphiphilic spin probe perdeuterated TEMPONE (PDT) enabled partitioning into the various phases to be followed. During drying, PDT molecules preferentially partitioned from the aqueous cytoplasm into the membrane surface and, at advanced stages of water loss, also into oil bodies. There was no specific partition behavior that could be correlated with lower/higher plants or with desiccation-tolerance. In vivo labeling with 5-doxylstearate (5-DS) enabled membrane surface fluidity to be characterized. In hydrated plants, the 5-DS spectra contained an immobile and a fluid component. The characteristics of the immobile component could not be specifically correlated with either lower or higher plants, or with desiccation tolerance. The relative contribution of the fluid component to the 5-DS spectra was higher in lower plants than in higher plants, but considerably decreased with drying in all desiccation-tolerant organisms. In contrast, the proportion of the fluid component in desiccation-sensitive wheat seedling root was higher than that in desiccation-tolerant wheat axis and considerably increased at the onset of water loss. We suggest that partitioning of amphipaths fluidize the membrane surface, but that in desiccation-tolerant systems the membranes are protected from excessive fluidization.

Induction of desiccation tolerance in plant somatic embryos: how exclusive is the protective role of sugars?

Plant somatic embryos usually lack desiccation tolerance. They may acquire such a tolerance upon preculture in the presence of abscisic acid (ABA), followed by slow drying, but not fast drying. ABA causes torpedo-shaped somatic embryos to lose their chlorophyll, suspend growth, exhibit low rates of respiration, and maintain elevated sucrose contents. The subsequent slow drying leads to a partial conversion of sucrose into oligosaccharides and the expression of dehydrin transcripts. Slow-dried, desiccation-tolerant somatic embryos have stable membranes, retain their native protein secondary structure, and have a densely packed cytoplasmic glassy matrix. Fast-dried, desiccation-sensitive somatic embryos experience some loss of phospholipids and an increase in free fatty acids. Their proteins show signs of denaturation and aggregation, and the glassy matrix has reduced hydrogen bonding. The reduced conversion of sucrose into oligosaccharides appears not to underlie dehydration injury. Proteins in slow-dried somatic embryos, not pretreated with ABA, also show signs of denaturation, which might be attributed to low sugar contents. We conclude that by reducing cellular metabolism, ABA maintains high sugar contents. These sugars contribute to the stability of membranes, proteins, and the cytoplasmic glassy matrix, whereas slow drying permits a further fine tuning of this stability. Partitioning of endogenous amphiphiles from the cytoplasm into membranes during drying may cause membrane perturbance, although it might confer protection to membranes in the case of amphiphilic antioxidants. The perturbance appears to be effectively controlled in desiccation-tolerant systems but not in sensitive systems, for which we suggest dehydrins are responsible. In this context, the low desiccation tolerance in the presence of ample sugars is discussed.

Mechanisms of plant desiccation tolerance

Anhydrobiosis ("life without water") is the remarkable ability of certain organisms to survive almost total dehydration. It requires a coordinated series of events during dehydration that are associated with preventing oxidative damage and maintaining the native structure of macromolecules and membranes. The preferential hydration of macromolecules is essential when there is still bulk water present, but replacement by sugars becomes important upon further drying. Recent advances in our understanding of the mechanism of anhydrobiosis include the downregulation of metabolism, dehydration-induced partitioning of amphiphilic compounds into membranes and immobilization of the cytoplasm in a stable multicomponent glassy matrix.