Insights into the cellular mechanisms of desiccation tolerance among angiosperm resurrection plant species

Water is a major limiting factor in growth and reproduction in plants. The ability of tissues to survive desiccation is commonly found in seeds or pollen but rarely present in vegetative tissues. Resurrection plants are remarkable as they can tolerate almost complete water loss from their vegetative tissues such as leaves and roots. Metabolism is shut down as they dehydrate and the plants become apparently lifeless. Upon rehydration these plants recover full metabolic competence and ‘resurrect’. In order to cope with desiccation, resurrection plants have to overcome a number of stresses as water is lost from the cells, among them oxidative stress, destabilization or loss of membrane integrity and mechanical stress. This review will mainly focus on the effect of dehydration in angiosperm resurrection plants and some of the strategies developed by these plants to tolerate desiccation. Resurrection plants are important experimental models and understanding the physiological and molecular aspects of their desiccation tolerance is of great interest for developing drought‐tolerant crop species adapted to semi‐arid areas.     

An investigation into the role of light during desiccation of three angiosperm resurrection plants

Under water‐limiting conditions excitation energy harnessed by chlorophyll can lead to the formation of reactive oxygen species (ROS). Resurrection plants minimize their formation by preventing the opportunity for light–chlorophyll interaction but also quench them via antioxidants. Poikilochlorohyllous species such as Xerophyta humilis break down chlorophyll to avoid ROS formation. Homoiochlorophyllous types retain chlorophyll. We proposed that leaf folding during drying of Craterostigma wilmsii and Myrothamnus flabellifolius shades chlorophyll to avoid ROS (Farrant, Plant Ecology 151, 29–39, 2000). This was tested by preventing leaf folding during drying in light. As controls, plants were dried without light, and X. humilis was included. Craterostigma wilmsii did not survive drying in light if the leaves were prevented from folding, despite protection from increased anthocyanin and sucrose and elevated antioxidant enzyme activity. Membranes were damaged, electrolyte leakage was elevated and plastoglobuli (evidence of light stress) accumulated in chloroplasts. Restrained leaves of M. flabellifolius survived drying in light. Leaf folding allows less shading, but the extent of chemical protection (anthocyanin content and antioxidant activity) is considerably higher in this species compared with C. wilmsii. Chemical protection appears to be light regulated in M. flabellifolius but not in C. wilmsii. Drying in the dark resulted in loss of viability in the homoiochlorophyllous but not the poikilochlorophyllous species. It is hypothesized that some of the genes required for protection are light regulated in the former.     

A comparison of desiccation tolerance among 12 species of chironomid larvae

Tolerance to desiccation was compared among 12 Japanese species of chironomid larvae under the condition of 60% in relative humidity at 25.5?°C. Three parameters were assessed: time to 50% survival (T ₅₀), water loss at 50% survival (WL₅₀) and water loss rate (WLR). T ₅₀, WL₅₀ and WLR were determined as measures of desiccation tolerance, dehydration tolerance, and dehydration resistance, respectively. T ₅₀ was 64.4–142 min for most species, except Propsilocerus akamusi (Tokunaga) which took 872 min. WL₅₀ was 60.6–82.4% for all species. WLR was only 0.0664% per minute for Pr. akamusi, while it was 0.629–1.50% for the other species. These results showed that Pr. akamusi had a high desiccation tolerance due to a high preventive ability of evaporation from body surface. T ₅₀ showed no significant relationships to WL₅₀ or WLR among the 12 species, while there was a significant positive relationship between WL₅₀ and WLR. These results suggest that chironomid species have a trade-off tendency that a species has a high tolerance – low resistance or a high resistance – low tolerance for dehydration.     

Variation in grazing tolerance among three tallgrass prairie plant species

Three tallgrass prairie plant species, two common perennial forbs (Artemisia ludoviciana and Aster ericoides [Asteraceae]) and a dominant C(4) perennial grass (Sorghastrum nutans) were studied under field and greenhouse conditions to evaluate interspecific variation in grazing tolerance (compensatory growth capacity). Adaptation to ungulate grazing was also assessed by comparing defoliation responses of plants from populations with a 25-yr history of no grazing or moderate ungulate grazing. Under field conditions, all three species showed significant reductions in shoot relative growth rates (RGR), biomass, and reproduction with defoliation. In the two forbs, clipping resulted in negative shoot RGR and reductions in both number and length of shoot branches per ramet. Sorghastrum nutans maintained positive RGR under defoliation due to a compensatory increase in leaf production. Defoliation reduced rhizome production in A. ericoides and S. nutans, but not in A. ludoviciana. Clipping significantly reduced sexual reproductive allocation in all three species, although S. nutans showed a smaller reduction than the forbs. All three species showed similar responses to defoliation in burned and unburned sites. Under greenhouse conditions, a similar clipping regimen resulted in smaller reductions in growth and reproduction than those observed in the field. For all three species, the grazing tolerance indices calculated under natural field conditions were significantly lower than those estimated from greenhouse-grown plants, and the interspecific patterns of grazing tolerance were different. Aster ericoides exhibited the highest overall defoliation tolerance under greenhouse conditions, followed by S. nutans. Artemisia ludoviciana, the only study species that is typically not grazed by ungulates in the field, showed the lowest grazing tolerance. In the field experiment S. nutans showed the highest grazing tolerance and the two forbs had similar low tolerance indices. These patterns indicate that, despite high compensatory growth potential, limited resource availability and competition in the field significantly reduce the degree of compensation and alter interspecific differences in grazing tolerance among prairie plants. In all three species, defoliation suppressed sexual reproduction more than growth or vegetative reproduction. Significant interactions between plant responses to defoliation and site of origin (historically grazed or ungrazed sites) for some response variables (root/shoot ratios, rhizome bud initiation, and reproductive allocation) indicated some degree of population differentiation and genetic adaptation in response to a relatively short history of ungulate grazing pressure. The results of this study indicate that patterns of grazing tolerance in tallgrass prairie are both genetically based and also environmentally dependent.     

Investigating the molecular basis of desiccation tolerance using the resurrection plant Craterostigma plantagineum as an experimental system

Conclusion The array of genes isolated and characterised from Craterostigma to date allow many insights to be made into the protection mechanisms underlying the desiccation tolerance phenomenon and drought-induced changes in gene expression. However, more work is necessary to define which gene products positively contribute to stress tolerance and which may be considered as secondary stress metabolites or as gene regulators. It is apparent that the gene regulatory mechanisms involved are complex, but future work with more refined biochemical and genetic approaches, together with the analysis of defined mutants will greatly help to dissect the important area of desiccation perception, perhaps faciliate manipulation of desiccation tolerance in a genetic engineering context, and undoubtedly bring much progress in the coming years.     

A combination of five mechanisms confers a high tolerance for aluminum to a wild species of Poaceae,Andropogon virginicus L.

How can high tolerance against aluminum (Al) toxicity be obtained in plants? To address this question, tolerant mechanisms were characterized in a highly Al tolerant wild species of Poaceae, Andropogon virginicus L. A. virginicus showed an Al-stress-induced synthesis and secretion of citrate and malate in roots. This mechanism may help to suppress an increase of toxic Al ions in the root region. Microscopic observation of the morin-stained leaves indicated that the Al transferred to shoots was specifically accumulated in the trichomes and spikes of the leaves and that some portion of the accumulated Al was furthermore secreted as sap from the tips of trichomes. Al-induced synthesis of poly-phenolic compounds including anthocyanin also occurred in roots as a long term response to Al toxicity and anthocyanin production did not co-localize with either Al accumulation, nitric oxide (NO) production or lipid peroxides production in the roots. It was suggested that oxidative damage caused by Al stress was suppressed in these areas where anthocyanin was localized. Moreover, induction of NO production occurred in roots within 24 h of Al treatment. Our results suggested that NO could not efficiently ameliorate the Al-dependent nuclei deformation and DNA fragmentation, but could function as a trigger to stimulate anti-peroxidation enzymes under Al stress. Collectively the results suggested that A. virginicus manifests its high Al tolerance by a unique combination of effective mechanisms.     

Retrotransposons and siRNA have a role in the evolution of desiccation tolerance leading to resurrection of the plant Craterostigma plantagineum

* Craterostigma plantagineum can lose up to 96% of its water content but fully recover within hours after rehydration. The callus tissue of the plant becomes desiccation tolerant upon pre-incubation with abscisic acid (ABA). In callus and vegetative organs, ABA addition and water depletion induce a set of dehydration-responsive genes. * Previously, activation tagging led to the isolation of Craterostigma desiccation tolerant (CDT-1), a dehydration-related ABA-inducible gene which renders callus desiccation tolerant without ABA pre-treatment. This gene belongs to a family of retroelements, members of which are inducible by dehydration. * Craterostigma plantagineum transformation with mutated versions of CDT-1 indicated that protein is not required for the induction of callus desiccation tolerance. Northern analysis and protoplast transfection indicated that CDT-1 directs the synthesis of a double-stranded 21-bp short interfering RNA (siRNA), which opens the metabolic pathway for desiccation tolerance. * Via transposition, these retroelements have progressively increased the capacity of the species to synthesize siRNA and thus recover after dehydration. This may be a case of evolution towards the acquisition of a new trait, stimulated by the environment acting directly on intra-genomic DNA replication.     

Moisture content of the dried leaf is critical to desiccation tolerance in detached leaves of the resurrection plant Boea hygroscopica

In our experimental conditions detached leaves of the resurrection plant Boea hygroscopica survived equilibration to 65–80% RH (Relative Humidity), but not to very low RH (close to 0%). The first aim of our research was to determine whether sensitivity to equilibration to very low RH depends on the rate of the drying process or on the very low final MC (Moisture Content) attained. The second aim of our research was to determine ABA content of leaves exposed to the two drying processes: a first step towards understanding whether ABA is involved in the tolerance mechanism of Boea hygroscopica.Detached leaves were equilibrated either to 1.4 or to 60–70% RH or to various temporal combinations of these two RH. ABA content was monitored during drying. Dehydrated leaves were imbibed in liquid water either directly or after a slow rehydration at 98% RH. Tolerance was assessed after 48 h imbibition in liquid water.The low final MC attained (about 3%) and not the rate of drying was responsible of the sensitivity of leaves equilibrated to 1.4% RH. Slow rehydration attained better recovery, but it was not able to allow full resurrection thus suggesting that a plain biophysical liquid-crystalline to gel phase transition of the membrane lipid bilayer could not fully account for the lethal damage of the very low MC.The conclusions relative to the first part of our research was of primary importance in interpreting results concerning ABA variations during the two drying treatments. ABA showed a very similar transient increase when excised leaves were dried at either 1.4% RH (sensitive leaves) or at 60–70% RH (tolerant leaves). However we cannot exclude that the transient increase of the hormone is a necessary component of the desiccation tolerance mechanisms in detached leaves of Boea hygroscopica: the extremely low MC reached by equilibration to 1.4% RH may impair the mechanism itself.     

Sugar-induced tolerance to the herbicide atrazine in Arabidopsis seedlings involves activation of oxidative and xenobiotic stress responses

Exogenous sucrose confers to Arabidopsis seedlings a very high level of tolerance to the herbicide atrazine that cannot be ascribed to photoheterotrophic growth. Important differences of atrazine tolerance between sucrose and glucose treatments showed that activation of chloroplast biogenesis per se could not account for induced tolerance. Sucrose-induced acquisition of defence mechanisms was shown by the gene expression pattern of a chloroplastic iron superoxide dismutase and by enhancement of whole-cell glucose-6-phosphate dehydrogenase activity. Activation of these defence mechanisms depended on both soluble sugar and atrazine. Moreover, acquisition of sucrose protection was shown to unmask atrazine-induced gene expression, such as that of a cytosolic glutathione-S-transferase, which remained otherwise cryptic because of the lethal effects of atrazine in the absence of soluble sugars.     

三种冬青属树种的耐涝性和耐旱性评价

通过致死性干旱和致死性水涝处理,用生理生态方法,对冬青(Ilexchinensis)、绿冬青(I.viridis)和无刺枸骨(I.cornatavar.fortunei)进行抗逆性研究。耐涝性结果表明随淹水时间延长,3种受淹冬青体内的游离脯氨酸和丙二醛含量增加,净光合速率下降;比较而言,绿冬青上述受淹反应出现早,无刺枸骨出现迟,而冬青介于二者之间;绿冬青耐涝约1周,无刺枸骨耐涝2周以上,冬青耐涝介于二者之间,在江南水乡推广利用,耐涝方面不会成为限制因素。耐旱结果表明随干旱的逐渐加重,3种冬青体内的游离脯氨酸含量呈上升趋势,比较而言,绿冬青上升的峰值出现早,冬青和无刺枸骨的上升峰值出现迟;绿冬青耐旱约15d,无刺枸骨耐旱约25d,冬青介于二者之间。3种冬青均有一定的抗逆性,其中无刺枸骨对水胁迫的适应能力最强,冬青次之,而绿冬青相对较弱。     

The signature of seeds in resurrection plants: a molecular and physiological comparison of desiccation tolerance in seeds and vegetative tissues

Desiccation-tolerance in vegetative tissues of angiosperms hasa polyphyletic origin and could be due to 1) appropriation ofthe seed-specific program of gene expression that protects orthodoxseeds against desiccation, and/or 2) a sustainable version ofthe abiotic stress response. We tested these hypotheses by comparingmolecular and physiological data from the development of orthodoxseeds, the response of desiccation-sensitive plants to abioticstress, and the response of desiccation-tolerant plants to extremewater loss. Analysis of publicly-available gene expression dataof 35 LEA proteins and 68 anti-oxidant enzymes in the desiccation-sensitiveArabidopsis thaliana identified 13 LEAs and 4 anti-oxidantsexclusively expressed in seeds. Two (a LEA6 and 1-cys-peroxiredoxin)are not expressed in vegetative tissues in A. thaliana, buthave orthologues that are specifically activated in desiccatingleaves of Xerophyta humilis. A comparison of antioxidant enzymeactivity in two desiccation-sensitive species of Eragrostiswith the desiccation-tolerant E. nindensis showed equivalentresponses upon initial dehydration, but activity was retainedat low water content in E. nindensis only. We propose that theseantioxidants are housekeeping enzymes and that they are protectedfrom damage in the desiccation-tolerant species. Sucrose isconsidered an important protectant against desiccation in orthodoxseeds, and we show that sucrose accumulates in drying leavesof E. nindensis, but not in the desiccation-sensitive Eragrostisspecies. The activation of "seed-specific" desiccation protectionmechanisms (sucrose accumulation and expression of LEA6 and1-cys-peroxiredoxin genes) in the vegetative tissues of desiccation-tolerantplants points towards acquisition of desiccation tolerance fromseeds.     

Differential effects of abscisic acid on desiccation tolerance and carbohydrates in three species of liverworts

Tissues of three species of in vitro grown liverworts, Riccia fluitans, Pallavicinia lyellii, and Marchantia polymorpha, were subjected to rapid drying with and without preculture for 1 week on medium containing 10 μM ABA. ABA preculture initiated total desiccation tolerance in R. fluitans, whereas control tissues were killed after 30 min of drying. Survival was also improved in P. lyellii, whereas ABA did not affect survival of M. polymorpha after rapid drying. ABA treatment did, however, reduce the rate of water loss in M. polymorpha. Total soluble carbohydrates were increased in ABA-treated R. fluitans and P. lyellii, but not in M. polymorpha, although there was no correlation between survival and changes in the percentage of these carbohydrates as reducing sugars. These differences in response to ABA and desiccation likely reflect different adaptations of these three species to conditions in situ.     

The bryophyte paradox: tolerance of desiccation,evasion of drought

Vascular plants represent one strategy of adaptation to the uneven and erratic supply of water on land. Desiccation-tolerant (DT) bryophytes represent an alternative, photosynthesising and growing when water is freely available, and suspending metabolism when it is not. By contrast with vascular plants, DT bryophytes are typically ectohydric, carrying external capillary water which can vary widely in quantity without affecting the water status of the cells. External water is important in water conduction, and results in bryophyte leaf cells functioning for most of the time at full turgor; water stress is a relatively brief transient phase before full desiccation. All bryophytes are C3 plants, and their cells are essentially mesophytic in important physiological respects. Their carbohydrate content shows parallels with that of maturing embryos of DT seeds. Initial recovery from moderate periods of desiccation is very rapid, and substantial elements of it appear to be independent of protein synthesis. Desiccation tolerance in effect acts as a device that evades the problems of drought, and in various adaptive features DT bryophytes are more comparable with (mesic) desert ephemerals or temperate winter annuals (but on a shorter time scale, with DT vegetative tissues substituting for DT seeds) than with drought-tolerant vascular plants.