Seasonal variations in UV-absorbing compounds and physiological characteristics in the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia over a 3-year period

Transcriptome Analysis and Identification of Sesquiterpene Synthases in Liverwort Jungermannia exsertifolia

The liverwort Jungermannia exsertifolia is one of the oldest terrestrial plants and rich in structurally specific sesquiterpenes. There are several sesquiterpene synthases (STSs) with non-classical conserved motifs that have been discovered in recent studies on liverworts; these motifs are rich in aspartate and bind with cofactors. However, more detailed sequence information is needed to clarify the biochemical diversity of these atypical STSs. This study mined J. exsertifolia sesquiterpene synthases (JeSTSs) through transcriptome analysis using BGISEQ-500 sequencing technology. A total of 257,133 unigenes was obtained, and the average length was 933 bp. Among them, a total of 36 unigenes participated in the biosynthesis of sesquiterpenes. In addition, the in vitro enzymatic characterization and heterologous expression in Saccharomyces cerevisiae showed that JeSTS1 and JeSTS2 produced nerolidol as the major product, while JeSTS4 could produce bicyclogermacrene and viridiflorol, suggesting a specificity of J. exsertifolia sesquiterpene profiles. Furthermore, the identified JeSTSs had a phylogenetic relationship with a new branch of plant terpene synthases, the microbial terpene synthase-like (MTPSL) STSs. This work contributes to the understanding of the metabolic mechanism for MTPSL-STSs in J. exsertifolia and could provide an efficient alternative to microbial synthesis of these bioactive sesquiterpenes.

Bryophyte ultraviolet-omics: from genes to the environment

Ultraviolet (UV) radiation has contributed to the evolution of organisms since the origins of life. Bryophytes also have evolutionary importance as the first clearly identified lineage of land plants (embryophytes) colonizing the terrestrial environment, thus facing high UV and water scarcity, among other new challenges. Here we review bryophyte UV-omics, the discipline relating bryophytes and UV, with an integrative perspective from genes to the environment. We consider species and habitats investigated, methodology, response variables, protection mechanisms, environmental interactions, UV biomonitoring, molecular and evolutionary aspects, and applications. Bryophyte UV-omics shows convergences and divergences with the UV-omics of other photosynthetic organisms, from algae to tracheophytes. All these organisms converge in that UV damage may be limited under realistic UV levels, due to structural protection and/or physiological acclimation capacity. Nevertheless, bryophytes diverge because they have a unique combination of vegetative and reproductive characteristics to cope with high UV and other concomitant adverse processes, such as desiccation. This interaction has both evolutionary and ecological implications. In addition, UV effects on bryophytes depend on the species and the evolutionary lineage considered, with mosses more UV-tolerant than liverworts. Thus, bryophytes do not constitute a homogeneous functional type with respect to their UV tolerance.

Antioxidant and Antibacterial Properties of Extracts and Bioactive Compounds in Bryophytes

Today global health problems such as increased risks of oxidative stress-related diseases and antibiotic resistance are issues of serious concern. Oxidative stress is considered to be the underlying cause of many contemporary pathological conditions such as neurological disorders, ischemia, cancer, etc. Antibiotic-resistant bacteria are a concerning issue in clinical practice, causing an increase in deadly infections. Bryophytes synthesize an outstanding number of secondary metabolites that have shown several potential therapeutic and nutraceutical applications. Research in the field has led to the isolation and characterization of several compounds (flavonoids, terpenoids, and bibenzyls). Some of these compounds have shown promising in vitro antibacterial activities and antioxidant potential comparable to known natural antioxidants such as ascorbic acid and α-tocopherol. However, the process of developing new drugs from naturally occurring molecules is often an impervious path. In this paper, the current state of research of bryophytic antioxidant and antibacterial applications is discussed.

Acclimation of Bryophytes to Sun Conditions, in Comparison to Shade Conditions, Is Influenced by Both Photosynthetic and Ultraviolet Radiations

We studied the acclimation modalities of bryophytes to sun and shade under ambient or close-to-ambient conditions, measuring variables usually influenced by photosynthetically active (PAR) and ultraviolet (UV) radiations. Our aim was to elucidate to what extent the responses to changing radiations were influenced by PAR and UV wavelengths. For this aim, we used three taxonomically and structurally different species: the thalloid liverwort Marchantia polymorpha subsp. polymorpha, the leafy liverwort Jungermannia exsertifolia subsp. cordifolia, and the moss Fontinalis antipyretica. In the field, liverworts were more radiation-responsive than the moss, and the thalloid liverwort was more responsive than the leafy liverwort. Sun plants of M. polymorpha showed, in comparison to shade plants, higher sclerophylly, lower Chl a + b contents, higher Chl a/b ratios, higher (antheraxanthin + zeaxanthin)/(violaxanthin + antheraxanthin + zeaxanthin) ratios (xanthophyll index), lower F v/F m values, higher contents of methanol-soluble vacuolar UV-absorbing compounds (soluble UVACs), higher values of the ratio between the contents of methanol-insoluble cell wall-bound UVACs (insoluble UVACs) and soluble UVACs, higher contents of soluble luteolin and apigenin derivatives and riccionidin A, and higher contents of insoluble p-coumaric and ferulic acids. Overall, these responses reduced light absorption, alleviated overexcitation, increased photoprotection through non-photochemical energy dissipation, increased UV protection through UV screening and antioxidant capacity, and denoted photoinhibition. J. exsertifolia showed moderate differences between sun and shade plants, while responses of F. antipyretica were rather diffuse. The increase in the xanthophyll index was the most consistent response to sun conditions, occurring in the three species studied. The responses of soluble UVACs were generally clearer than those of insoluble UVACs, probably because insoluble UVACs are relatively immobilized in the cell wall. These modalities of radiation acclimation were reliably summarized by principal components analysis. Using the most radiation-responsive species in the field (M. polymorpha), we found, under close-to-ambient greenhouse conditions, that sclerophylly and Chl a + b content were only influenced by PAR, F v/F m, and luteolin and apigenin derivatives were only determined by UV, and xanthophyll index was influenced by both radiation types. Thus, responses of bryophytes to radiation can be better interpreted considering the influence of both PAR and UV radiation.

Photosynthetically-active radiation, UV-A and UV-B, causes both common and specific damage and photoprotective responses in the model liverwort Marchantia polymorpha subsp. ruderalis

We studied the effects of different radiation treatments on the physiology and UV-absorbing compounds of the model liverwort Marchantia polymorpha subsp. ruderalis. Starting from gemmae, samples were exposed to five radiation treatments: low photosynthetically active radiation (PAR), low PAR+ UV-A, low PAR + UV-B, low PAR + UV-A + UV-B, and high PAR. After 35 days, the maximum quantum yield of photosystem II was similar between treatments, which suggested comparable photoinhibition and physiological vitality, also supported by results showing an unchanged chlorophyll a/b ratio and only slight changes in growth. However, the total contents of both chlorophylls and carotenoids decreased in the UV radiation treatments and, more strongly, in the high-PAR samples, suggesting mainly PAR-dependent damage to the photosynthetic pigments. The xanthophyll index (antheraxanthin + zeaxanthin)/(violaxanthin + antheraxanthin + zeaxanthin) was only increased in the high-PAR samples, indicating an increase in photoprotection through nonphotochemical dissipation of the excess energy. The sclerophylly index (the ratio between the thallus dry mass and surface area) was increased in the UV-B-exposed samples, suggesting a UV-induced structural protection. Only the UV-B-exposed samples showed DNA damage. Several apigenin and luteolin derivatives were found in the methanol-soluble vacuolar fraction of the liverwort and p-coumaric and ferulic acids in the methanol-insoluble cell wall-bound fraction. Most individual soluble compounds, the bulk level of soluble compounds, and chalcone synthase expression increased in UV-B-exposed samples, whereas individual insoluble compounds increased in the samples exposed to only PAR. Principal components analysis summarized these responses, showing the strong influence of both UV-B and PAR levels on the physiology and UV protection of the samples.

Phenolic compounds from different bryophyte species and cell compartments respond specifically to ultraviolet radiation, but not particularly quickly

To study the potential quick responses to ultraviolet (UV) radiation of bryophyte phenolic compounds, we cultivated two thalloid liverworts, two leafy liverworts, and two mosses under three moderate realistic UV levels in the laboratory for 22 days. At the end of the daylight period on the first and last culture days, we measured the bulk levels and individual contents of phenolic UV-absorbing compounds (UVACs) of each species, differentiating in both cases the UVACs located in the methanol-soluble (mainly vacuolar) and -insoluble (cell wall-bound) fractions (SUVACs and IUVACs, respectively). The bulk levels of SUVACs and IUVACs mostly showed linear or hyperbolic relationships with the UV dose applied. Thirteen flavones (apigenin and luteolin derivatives) and two hydroxycinnamic acids (p-coumaric and ferulic acids) were identified in the soluble and insoluble fractions, respectively. Only two compounds (p-coumaric and ferulic acids) from the insoluble fraction of the leafy liverwort Plagiochila asplenioides showed a significant quick accumulation in response to UV radiation in the first day of culture, whereas six UVACs (mainly soluble apigenin and luteolin derivatives) from different species (mainly liverworts) were significantly accumulated at the end of the culture. In conclusion, the responses of bryophyte UVACs to UV radiation were influenced by the specific compound considered, the fraction in which each UVAC was located, the global or individual way of UVACs quantification, the bryophyte species and evolutionary lineage, and the experimental conditions used. Particularly, SUVACs were more UV-responsive than IUVACs and liverworts than mosses, and responses were not especially quick.

Spatial variability of ultraviolet-absorbing compounds in an aquatic liverwort and their usefulness as biomarkers of current and past UV radiation: a case study in the Atlantic-Mediterranean transition

The spatial variability of ultraviolet-absorbing compounds (UVACs) in the freshwater liverwort Jungermannia exsertifolia subsp. cordifolia was studied in mid-latitudes (the Atlantic-Mediterranean transition) across a wide lati-altitudinal gradient, with the aim of testing the usefulness of UVACs as biomarkers of current ambient levels of UV radiation. We analysed 17 samples from streams located in the main mountain ranges of the Iberian Peninsula, differentiating methanol-soluble (SUVACs, mainly located in the vacuoles) and methanol-insoluble (IUVACs, bound to cell walls) compounds, since they represent different manners to cope with UV radiation. In both fractions, the bulk level of UVACs and the concentrations of several individual compounds were measured. In addition, we measured Fv/Fm, DNA damage and sclerophylly index (SI) as possible additional UV biomarkers. UVACs showed a high variability, probably due not only to the gradients of macroenvironmental factors (UV radiation, PAR, and water temperature), but also to microenvironmental factors inherent to the dynamic nature of mountain streams. Two soluble coumarins were positively correlated with UV levels and could be used for ambient UV biomonitoring in the spatial scale. In contrast to the variability in UVACs, the relatively homogeneous values of Fv/Fm and the lack of any DNA damage made these variables useless for ambient UV biomonitoring, but suggested a strong acclimation capacity of this liverwort to changing environmental conditions (in particular, to UV levels). Finally, UVACs of fresh samples of the liverwort were compared to those of herbarium samples collected in the same lati-altitudinal gradient. SUVACs were significantly higher in fresh samples, whereas IUVACs generally showed the contrary. Thus, IUVACs were more stable than SUVACs and hence more adequate for retrospective UV biomonitoring. In conclusion, UVAC compartmentation should be taken into account for bryophyte-based UV biomonitoring in future studies.

Altitudinal and seasonal changes of phenolic compounds in Buxus sempervirens leaves and cuticles

The variation in the leaf content of phenolic compounds has been related to the UV-B changes of the environment in which plants grow. In this context, we aimed to investigate: a) whether the seasonal and altitudinal changes in the content of phenolic compounds of Buxus sempervirens L. leaves and cuticles could be related to the natural fluctuations in UV-B levels and b) the possible use of specific phenolic compounds as biomarkers of ambient UV-B levels. To achieve these goals we sampled, every three months during one year, leaves of B. sempervirens along an altitudinal gradient. At the lowest and the highest altitudes, we also conducted a UV-exclusion experiment to discern whether the observed changes could be attributed to the natural variation in UV-B. Results show that total phenolic content of leaves was lower in June than in the other sampling dates, which suggests a leaf ontogenic rather than a UV-B effect on the leaf content of these compounds. Regarding the elevational gradient, the overall amount of phenolic acids and neolignan of entire leaves increased with altitude while the total amount of flavonoids in leaf cuticles decreased. However, the lack of a significant effect of our UV-exclusion treatment on the content of these compounds suggests that the observed variations along the altitudinal gradient would respond to other factors rather than to UV-B. Concomitantly, we did not find any phenolic compound in leaves or cuticles of B. sempervirens that could be considered as a biomarker of ambient UV-B levels.

Dynamic response of UV-absorbing compounds, quantum yield and the xanthophyll cycle to diel changes in UV-B and photosynthetic radiations in an aquatic liverwort

We studied the diel responses of the liverwort Jungermannia exsertifolia subsp. cordifolia to radiation changes under laboratory conditions. The samples were exposed to three radiation regimes: P (only PAR), PA (PAR+UV-A), and PAB (PAR+UV-A+UV-B). The day was divided in four periods: darkness, a first low-PAR period, the high-PAR plus UV period, and a second low-PAR period. After 15 days of culture, we measured photosynthetic pigments, chlorophyll fluorescence and UV-absorbing compounds in the four periods of the day on two consecutive days. With respect to UV-absorbing compounds, we analyzed their global amount (as the bulk UV absorbance of methanolic extracts) and the concentration of seven hydroxycinnamic acid derivatives, both in the soluble (mainly vacuolar) and insoluble (cell wall-bound) fractions of the plant extracts. PAB samples increased the bulk UV absorbance of the soluble and insoluble fractions, and the concentrations of p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall. Most of these variables showed significant diel changes and responded within a few hours to radiation changes (more strongly to UV-B), increasing at the end of the period of high-PAR plus UV. F(v)/F(m), Φ(PSII), NPQ and the components of the xanthophyll cycle showed significant and quick diel changes in response to high PAR, UV-A and UV-B radiation, indicating dynamic photoinhibition and protection of PSII from excess radiation through the xanthophyll cycle. Thus, the liverwort showed a dynamic protection and acclimation capacity to the irradiance level and spectral characteristics of the radiation received.

Effects of enhanced UV-B radiation on hydroxycinnamic acid derivatives extracted from different cell compartments in the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia

We examined the responses of ultraviolet-absorbing compounds (UVAC) to enhanced UV-B radiation in the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia for 31 days under laboratory conditions. Samples were exposed to three radiation regimes: P (only photosynthetic radiation), PA (photosynthetic + UV-A radiation) and PAB (photosynthetic + UV-A + UV-B radiation). We measured both the bulk UV absorbance of the methanolic extracts and the levels of individual UVAC. In both cases, the methanol-soluble and the methanol-insoluble, alkali-extractable cell wall-bound fractions were analyzed. The bulk UV absorbance of the soluble fraction was higher than that of the cell wall-bound fraction. The bulk UV absorbances of both fractions increased under enhanced UV-B (PAB regime). Five different hydroxycinnamic acid (HCA) derivatives were found in the soluble fraction and two additional ones in the cell wall-bound fraction, among which only p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall-bound fraction increased under enhanced UV-B. The maximum quantum yield of PSII (F(v) /F(m)) decreased and DNA damage (amount of thymine dimers) strongly increased under enhanced UV-B, showing UV-B-induced damage. We conclude that methanol-soluble and cell wall-bound fractions of the liverwort studied have different UVAC, and each individual compound may respond in a different way to UV-B radiation. Thus, the analysis of individual UVAC in both the methanol-soluble and cell wall-bound fractions is advisable to better evaluate the protection mechanisms of liverworts against UV-B radiation. In particular, p-coumaric acid and p-coumaroylmalic acid seem to be especially UV-B responsive and merit further investigation.