Dynamics of Endogenous Phytohormones during Desiccation and Recovery of the Resurrection Plant Species Haberlea rhodopensis

Drought is one of the most significant threats to world agriculture and hampers the supply of food and energy. The mechanisms of drought responses can be studied using resurrection plants that are able to survive extreme dehydration. As plant hormones function in an intensive cross-talk, playing important regulatory roles in the perception and response to unfavorable environments, the dynamics of phytohormones was followed in the resurrection plant Haberlea rhodopensis Friv. during desiccation and subsequent recovery. Analysis of both leaves and roots revealed that jasmonic acid, along with and even earlier than abscisic acid, serves as a signal triggering the response of the resurrection plants to desiccation. The steady high levels of salicylic acid could be considered an integral part of the specific set of parameters that prime H. rhodopensis desiccation tolerance. The dynamic changes of cytokinins and auxins suggest that these hormones actively participate in the dehydration response and development of desiccation tolerance in the resurrection plants. Our data contribute to the elucidation of a global complex picture of the resurrection plant’s ability to withstand desiccation, which might be successfully utilized in crop improvement.     

Arabinose-rich polymers as an evolutionary strategy to plasticize resurrection plant cell walls against desiccation

A variety of Southern African resurrection plants were surveyed using high-throughput cell wall profiling tools. Species evaluated were the dicotyledons, Myrothamnus flabellifolia and Craterostigma plantagineum; the monocotyledons, Xerophyta viscosa, Xerophyta schlecterii, Xerophyta humilis and the resurrection grass Eragrostis nindensis, as well as a pteridophyte, the resurrection fern, Mohria caffrorum. Comparisons were made between hydrated and desiccated leaf and frond material, with respect to cell wall composition and polymer abundance, using monosaccharide composition analysis, FT-IR spectroscopy and comprehensive microarray polymer profiling in combination with multivariate data analysis. The data obtained suggest that three main functional strategies appear to have evolved to prepare plant cell walls for desiccation. Arabinan-rich pectin and arabinogalactan proteins are found in the resurrection fern M. caffrorum and the basal angiosperm M. flabellifolia where they appear to act as ‘pectic plasticizers’. Dicotyledons with pectin-rich walls, such as C. plantagineum, seem to use inducible mechanisms which consist of up-regulating wall proteins and osmoprotectants. The hemicellulose-rich walls of the grass-like Xerophyta spp. and the resurrection grass E. nindensis were found to contain highly arabinosylated xylans and arabinogalactan proteins. These data support a general mechanism of ‘plasticising’ the cell walls of resurrection plants to desiccation and implicate arabinose-rich polymers (pectin-arabinans, arabinogalactan proteins and arabinoxylans) as the major contributors in ensuring flexibility is maintained and rehydration is facilitated in these plants.     

更苏被子植物的光合作用

更苏植物是一类在极度干燥条件下组织会迅速脱水后遇水又能很快复苏的植物.极少数被子植物有这种能力,在双子叶植物中尤其罕见,而且脱水时叶绿素含量和叶绿体完整性变化较少,称为叶绿素保持型(HDT).该类植物的复苏机理简单,研究方便,因而得到更广泛注意.更苏被子植物光合作用的最新研究进展说明,光化学活性是研究更苏植物脱水复苏生理状态的灵敏指标.和普通植物一样,在光下,更苏被子植物的光化学活性随着叶片失水而受到抑制,但奇怪的是在失去95%以上的水分后复水仍可迅速复活.在脱水过程中叶黄素循环和抗氧化系统的上调以及光合膜完整性和稳定性的保持,可能对更苏被子植物的耐脱水性起非常重要的作用.磷酸盐对复苏的影响也表现在复水阶段而且与上述两种保护机理关系不大,因此应该加强更苏被子植物复水阶段的研究.     

更苏被子植物的光合作用

更苏植物是一类在极度干燥条件下组织会迅速脱水后遇水又能很快复苏的植物。极少数被子植物有这种能力,在双子叶植物中尤其罕见,而且脱水时叶绿素含量和叶绿体完整性变化较少,称为叶绿素保持型(HDT)。该类植物的复苏机理简单,研究方便,因而得到更广泛注意。更苏被子植物光合作用的最新研究进展说明,光化学活性是研究更苏植物脱水复苏生理状态的灵敏指标。和普通植物一样,在光下,更苏被子植物的光化学活性随着叶片失水而受到抑制,但奇怪的是在失去95％以上的水分后复水仍可迅速复活。在脱水过程中叶黄素循环和抗氧化系统的上调以及光合膜完整性和稳定性的保持,可能对更苏被子植物的耐脱水性起非常重要的作用。磷酸盐对复苏的影响也表现在复水阶段而且与上述两种保护机理关系不大,因此应该加强更苏被子植物复水阶段的研究。     

Desiccation tolerance of the resurrection plant Ramonda serbica is associated with dehydration-dependent changes in levels of proteolytic activities

The unique response of desiccation-tolerant, or resurrection plants, to extreme drought is accompanied by major changes in the protein pool, raising the possibility of the involvement of proteases. We detected and characterized proteases present in their active state in leaf extracts of desiccated Ramonda serbica Pan?., a resurrection plant from the Balkan Peninsula. Plants desiccated under laboratory conditions and maintained in anhydrobiosis for 4 and 14 months revived upon rehydration. Protease activities were determined spectrophotometrically in solution and by zymography on gels. Several endo- and aminopeptidases were detected and characterized by their pH profiles. Their enzyme class was determined using specific inhibitors. Those with higher activities were a serine endopeptidase active against Bz-Arg-pNA with a pH optimum around 9, and aminopeptidases optimally active at pHs from 7 to 9 against Leu-pNA, Met-pNA, Phe-pNA, Pro-pNA and Ala-pNA. The levels of their activities in leaf extracts from desiccated plants were significantly higher than those from rehydrated plants and from regularly watered plants, implying their involvement in the recovery of vegetative tissues from desiccation.     

Solute leakage resulting from leaf desiccation

The leakage of solutes from foliar tissue is utilized as a dynamic measure of apparent changes in membrane integrity in response to desiccation. It is found that rehydrating leaf discs of cowpea (Vigna sinensis [L.] Endl.) show increasing leakiness in proportion to the extent of prior desiccation, whereas Selaginella lepidophylla Spring., a resurrection plant, does not. The elevated leakage rate of cowpea after desiccation recovers with time, and the passage of time in the stressed condition results in reduced subsequent leakiness. These characteristics are interpreted as suggesting that the leakage of solute reflects the condition of cellular membranes, and that desiccation stress leads to lesions in the membranes. The kinetics of solute leakage is suggested as a simple means of following changes in membrane lesions and associated features of membrane repair and hardening.     

Sugar ratios, glutathione redox status and phenols in the resurrection species Haberlea rhodopensis and the closely related non-resurrection species Chirita eberhardtii

Because of their unique tolerance to desiccation, the so‐called resurrection plants can be considered as excellent models for extensive research on plant reactions to environmental stresses. The vegetative tissues of these species are able to withstand long dry periods and to recover very rapidly upon re‐watering. This study follows the dynamics of key components involved in leaf tissue antioxidant systems under desiccation in the resurrection plant Haberlea rhodopensis and the related non‐resurrection species Chirita eberhardtii. In H. rhodopensis these parameters were also followed during recovery after full drying. A well‐defined test system was developed to characterise the different responses of the two species under drought stress. Results show that levels of H₂O₂ decreased significantly both in H. rhodopensis and C. eberhardtii, but that accumulation of malondialdehyde was much more pronounced in the desiccation‐tolerant H. rhodopensis than in the non‐resurrection C. eberhardtii. A putative protective role could be attributed to accumulation of total phenols in H. rhodopensis during the late stages of drying. The total glutathione concentration and GSSG/GSH ratio increased upon complete dehydration of H. rhodopensis. Our data on soluble sugars suggest that sugar ratios might be important for plant desiccation tolerance. An array of different adaptations could thus be responsible for the resurrection phenotype of H. rhodopensis.     

Desiccation increases sucrose levels in Ramonda and Haberlea, two genera of resurrection plants in the Gesneriaceae

A few genera of angiosperms are known as 'resurrection plants' since their leaves withstand complete desiccation. In many organisms, including some resurrection plants, desiccation tolerance is associated with the accumulation of special carbohydrates. We examined whether this is also true for the two European angiosperm genera of resurrection plants, Ramonda and Haberlea in the Gesneriaceae. Using gas chromatography, non-structural carbohydrates were determined as a percentage of the dry weight in leaves of Ramonda nathaliae subjected to various desiccation regimes. Sucrose was the predominant soluble carbohydrate in all samples, and its level steadily increased from 2 to 10% during desiccation. Starch amounted to ca 2% in control leaves and disappeared completely within 8 days of desiccation. Considerable amounts (1–2.5%) of raffinose and smaller amounts of its precursor galactinol (1-a-galactosyl- myo -inositol) were present in control leaves; these carbohydrates showed only minor changes upon desiccation. Similar results were obtained when excised leaves of Ramonda nathaliae, Ramonda myconi and Haberlea rhodopensis were subjected to desiccation. These data indicate that sucrose accumulation is connected to desiccation tolerance in Gesneriaceae; the presence of raffinose may be a pre-adaptation since this sugar prevents crystallization of sucrose during drying.     

The effect of dehydration and rehydration on the nitrogen content of various fractions from resurrection plants

Nitrogen contents were determined in 20 species of “resurrection plants”,i.e. plants with leaves which are able to revive from an air-dry state (viz. Boea hygroscopica, Borya nitida, Cheilanthes sieberi, Coleochloa pallidior, C. setifera, Craterostigma plantagineum, Myrothamnus flabellifolia, Oropetium capense, Pellaea calomelanos, P. falcata, P, viridis, Polypodium polypodioides, Ramondia pyrenaica, Selaginella lepidophylla, Sporobolus stapfianus, Talbotia elegans,Tripogon loliiformis, Xerophyta retinervis, X. villosa, X. viscosa), and in three desiccation sensitive species (Eragrostis tenuifolia, Selaginella kraussiana andSporobolus pyramidalis). In a preponderance of resurrection plants insoluble nitrogen content fell during dehydration of intact plants and soluble non-protein N rose. Both changes were particularly marked in species which lose chlorophyll and thylakoid structure during drying. These trends were usually only partially reversed after 24 h rehydration. Recovery of¹⁴C-leucine incorporation in rehydrating leaves was slow. Leaves of desiccation sensitive vascular plants tended on the average to lose soluble protein rather than insoluble N during drying, and tended to have higher soluble non-protein N contents than tolerant plants. However, similarity in the changes in N-contents inXerophyta villosa leaves killed by airdrying compared to leaves surviving air-drying, opposes the view that death was due to excessive loss of protein.     

The role of antioxidant defense in freezing tolerance of resurrection plant Haberlea rhodopensis

Haberlea rhodopensis Friv. is unique with its ability to survive two extreme environmental stresses—desiccation to air-dry state and subzero temperatures. In contrast to desiccation tolerance, the mechanisms of freezing tolerance of resurrection plants are scarcely investigated. In the present study, the role of antioxidant defense in the acquisition of cold acclimation and freezing tolerance in this resurrection plant was investigated comparing the results of two sets of experiments—short term freezing stress after cold acclimation in controlled conditions and long term freezing stress as a part of seasonal temperature fluctuations in an outdoor ex situ experiment. Significant enhancement in flavonoids and anthocyanin content was observed only as a result of freezing-induced desiccation. The total amount of polyphenols increased upon cold acclimation and it was similar to the control in post freezing stress and freezing-induced desiccation. The main role of phenylethanoid glucoside, myconoside and hispidulin 8-C-(2-O-syringoyl-b-glucopyranoside) in cold acclimation and freezing tolerance was elucidated. The treatments under controlled conditions in a growth chamber showed enhancement in antioxidant enzymes activity upon cold acclimation but it declined after subsequent exposure to −10 °C. Although it varied under ex situ conditions, the activity of antioxidant enzymes was high, indicating their important role in overcoming oxidative stress under all treatments. In addition, the activity of specific isoenzymes was upregulated as compared to the control plants, which could be more useful for stress counteraction compared to changes in the total enzyme activity, due to the action of these isoforms in the specific cellular compartments.Supplementary informationThe online version contains supplementary material available at 10.1007/s12298-021-00998-0.     

Cyanobacteria in the Australian northern savannah detect the difference between intermittent dry season and wet season rain

This research was conducted in the northern Australian savannah at Boodjamulla National Park where cyanobacterial crusts dominate the soil and rock surfaces in between tussock grasses. It is widely accepted that terrestrial cyanobacteria are drought tolerant and rapidly recommence photosynthesis once moisture is available. Initial tests at the research site indicated that cyanobacteria did not respond to rehydration during the dry season, even after several days. We hypothesised that resurrection had not taken place and new growth from survival cells had to take place during the follow-up wet season. To further understand the desiccation–resurrection processes we tested photosystem II (PSII) responses both during the dry and wet seasons. In the 2009 dry season after 125 days without rain, crust samples were regularly rehydrated. Over the 10 day trial cyanobacteria did not recover PSII activity or CO₂-uptake. Although new colonies of Nostoc grew other cyanobacteria remained inactive, even though liverworts and lichens in the same crusts had responded within 24 h. Dry season cyanobacterial crusts were collected in 2010 then reintroduced into their natural environment and exposed to rainfall during the 2011 wet season. Within 24 h PSII in cyanobacteria from a range of crust types had resurrected and CO₂-uptake was verified, although different crust types responded at significantly different rates. These are the first studies that have demonstrated that PSII does not respond to rainfall during the dry season and cyanobacterial function appears controlled by other environmental conditions. It is likely that mass extracellular polysaccharide (EPS) production during the wet season, once dry, protects cyanobacteria from premature resurrection in the dry season. We propose that EPS regulates moisture penetration, thus the resurrection of PSII at the onset of the wet season, at which time moisture and humidity alters the rheological properties of EPS permitting rehydration.     

Molecular characterization of two alanine-rich Lea genes abundantly expressed in the resurrection plant C. plantagineum in response to osmotic stress and ABA

Expression of many genes is induced during dehydration in vegetative tissues of the desiccation tolerant resurrection plantCraterostigma plantagineum. The most abundant group of desiccation-related gene products belong to the LEA (= Late Embryogenesis Abundant) proteins. Here we describe structures and expression patterns of members of group 3 and group 4Lea genes fromC. plantagineum. The most intriguing observation is the strong conservation of repeat motifs inLea genes found across divers plant species includingC. plantagineum and non-desiccation tolerant plants. This conservation of structural elements leads to speculations about evolution of desiccation tolerance in the resurrection plant.     

Maintenance or Collapse: Responses of Extraplastidic Membrane Lipid Composition to Desiccation in the Resurrection Plant Paraisometrum mileense

Resurrection plants usually grow in specific or extreme habitats and have the capacity to survive almost complete water loss. We characterized the physiological and biochemical responses of Paraisometrum mileense to extreme desiccation and found that it is a resurrection plant. We profiled the changes in lipid molecular species during dehydration and rehydration in P. mileense, and compared these with corresponding changes in the desiccation-sensitive plant Arabidopsis thaliana. One day of desiccation was lethal for A. thaliana but not for P. mileense. After desiccation and subsequent rewatering, A. thaliana showed dramatic lipid degradation accompanied by large increases in levels of phosphatidic acid (PA) and diacylglycerol (DAG). In contrast, desiccation and rewatering of P. mileense significantly decreased the level of monogalactosyldiacylglycerol and increased the unsaturation of membrane lipids, without changing the level of extraplastidic lipids. Lethal desiccation in P. mileense caused massive lipid degradation, whereas the PA content remained at a low level similar to that of fresh leaves. Neither damage nor repair processes, nor increases in PA, occurred during non-lethal desiccation in P. mileense. The activity of phospholipase D, the main source of PA, was much lower in P. mileense than in A. thaliana under control conditions, or after either dehydration or rehydration. It was demonstrated that low rates of phospholipase D-mediated PA formation in P. mileense might limit its ability to degrade lipids to PA, thereby maintaining membrane integrity following desiccation.     

Assessment of leaf micromorphology after full desiccation of resurrection plants

Resurrection plants are unique among higher plants because of their ability to withstand long periods of dehydration without damages. In this study, leaf epidermis and palisade mesophyll of three resurrection species, Haberlea rodopensis, Ramonda serbica and Ramonda myconi, grown under full desiccation and benign conditions, were analyzed by differential interference contrast microscopy. Detailed investigation of adaxial and abaxial leaf surfaces revealed species-specific differences in the size and number of epidermal cells and stomatal density. The applied full desiccation did not cause any significant deviations of these parameters from the controls. There were no changes in the size and number of mesophyll cells as well. Analysis of stomatal patterning displayed essentially hypostomatic leaves, having stomata mainly abaxially positioned. The most significant change detected in the leaves of dehydration-treated plants was the increased formation of adaxially positioned trichomes. This increase was very high in R. myconi, where the adaxial leaf surface was fully covered by trichomes. Despite the existence of small species-specific differences, the results showed uniform desiccation-related responses of the studied resurrection species. The quantified leaf epidermal and mesophyll features are discussed with respect to their possible contribution to the desiccation tolerance of resurrection species.     

Quantitative genetic analysis suggests causal association between cuticular hydrocarbon composition and desiccation survival in Drosophila melanogaster

Survival to low relative humidity is a complex adaptation, and many repeated instances of evolution to desiccation have been observed among Drosophila populations and species. One general mechanism for desiccation resistance is Cuticular Hydrocarbon (CHC) melting point. We performed the first Quantitative Trait Locus (QTL) map of population level genetic variation in desiccation resistance in D. melanogaster. Using a panel of Recombinant Inbred Lines (RILs) derived from a single natural population, we mapped QTL in both sexes throughout the genome. We found that in both sexes, CHCs correlated strongly with desiccation resistance. At most desiccation resistance loci there was a significant association between CHCs and desiccation resistance of the sort predicted from clinal patterns of CHC variation and biochemical properties of lipids. This association was much stronger in females than males, perhaps because of greater overall abundance of CHCs in females, or due to correlations between CHCs used for waterproofing and sexual signalling in males. CHC evolution may be a common mechanism for desiccation resistance in D. melanogaster. It will be interesting to compare patterns of CHC variation and desiccation resistance in species which adapt to desiccation, and rainforest restricted species which cannot.     

Resurrection Plants and the Secrets of Eternal Leaf

Most higher plants possess a phase in their life cycle in whichtissues can survive desiccation. However, this is restrictedto specialized tissues such as seeds and pollen. Resurrectionplants are remarkable in that they can tolerate almost completewater loss in their vegetative tissues. The desiccated plantcan remain alive in the dried state for several years. However,upon watering the plants rehydrate and are fully functionalwithin 48 h. Underpinning this amazing ability is the capacityto accumulate large amounts of sucrose in the tissues. Thissugar has the property of stabilizing enzymes and cellular structuresin the absence of water. The sources of carbon that fuel sucrosesynthesis are not known, but temporary carbohydrate stores andphotosynthesis are the most likely candidates. On rewatering,the sucrose is metabolized rapidly as the tissues rehydrate.Increased expression of a number of genes in response to droughtstress have been noted. A number of these are associated withmetabolic pathways linked with primary carbohydrate metabolism.However, some genes related to LEA (Late Embryogenic Abundant)proteins have been isolated which suggests they too may playa role in maintaining tissue integrity during desiccation. Howthese mechanisms are integrated to enable resurrection plantsto survive desiccation is discussed. Copyright 2000 Annals ofBotany Company ABA, Craterostigma, desiccation tolerance, poikilohydric, resurrection.     

Novel chloroplasts and unusual cellular ultrastructure in the "resurrection" plant Myrothamnus flabellifolia Welw. (Myrothamnaceae)

The vacuoles of three "resurrection" plants, Myrothamnus flabellifolia, Anastatica hierochuntica and Selaginella dregei were found to contain large quantities of osmiophilic material which may be part of the "resurrection" mechanism. Myrothamnus differed from the others by having mitochondria, and possibly plastids, which are separated from the remainder of the cytoplasm by sheaths or membranes during desiccation. Upon "resurrection" these barriers appear to be perforated and explain in part the faster rate of "resurrection" in Myrothamnus than in other "resurrection" plants. The chloroplasts of Myrothamnus are remarkable in that they possess "staircase" granum stacks of a type not previously described in any other plant tissue.