Responses of photosynthetic properties and chloroplast ultrastructure of Bryum argenteum from a desert biological soil crust to elevated ultraviolet-B radiation

Acclimation of intertidal macroalgae Ulva prolifera to UVB radiation: the important role of alternative oxidase

BACKGROUND

Solar radiation is primarily composed of ultraviolet radiation (UVR, 200 - 400 nm) and photosynthetically active radiation (PAR, 400 - 700 nm). Ultraviolet-B (UVB) radiation accounts for only a small proportion of sunlight, and it is the primary cause of plant photodamage. The use of chlorofluorocarbons (CFCs) as refrigerants caused serious ozone depletion in the 1980s, and this had led to an increase in UVB. Although CFC emissions have significantly decreased in recent years, UVB radiation still remains at a high intensity. UVB radiation increase is an important factor that influences plant physiological processes. Ulva prolifera, a type of macroalga found in the intertidal zone, is intermittently exposed to UVB. Alternative oxidase (AOX) plays an important role in plants under stresses. This research examines the changes in AOX activity and the relationships among AOX, photosynthesis, and reactive oxygen species (ROS) homeostasis in U. prolifera under changes in UVB and photosynthetically active radiation (PAR).

RESULTS

UVB was the main component of solar radiation impacting the typical intertidal green macroalgae U. prolifera. AOX was found to be important during the process of photosynthesis optimization of U. prolifera due to a synergistic effect with non-photochemical quenching (NPQ) under UVB radiation. AOX and glycolate oxidase (GO) worked together to achieve NADPH homeostasis to achieve photosynthesis optimization under changes in PAR + UVB. The synergism of AOX with superoxide dismutase (SOD) and catalase (CAT) was important during the process of ROS homeostasis under PAR + UVB.

CONCLUSIONS

AOX plays an important role in the process of photosynthesis optimization and ROS homeostasis in U. prolifera under UVB radiation. This study provides further insights into the response of intertidal macroalgae to solar light changes.

Development of PEG-mediated genetic transformation and gene editing system of Bryum argenteum as an abiotic stress tolerance model plant

KEY MESSAGE

To establish a sterile culture system and protoplast regeneration system for Bryum argenteum, and to establish and apply CRISPR/Cas9 system in Bryum argenteum. Bryum argenteum is a fascinating, cosmopolitan, and versatile moss species that thrives in various disturbed environments. Because of its comprehensive tolerance to the desiccation, high UV and extreme temperatures, it is emerging as a model moss for studying the molecular mechanisms underlying plant responses to abiotic stresses. However, the lack of basic tools such as gene transformation and targeted genome modification has hindered the understanding of the molecular mechanisms underlying the survival of B. argenteum in different environments. Here, we reported the protonema of B. argenteum can survive up to 95.4% water loss. In addition, the genome size of B. argenteum is approximately 313 Mb by kmer analysis, which is smaller than the previously reported 700 Mb. We also developed a simple method for protonema induction and an efficient protoplast isolation and regeneration protocol for B. argenteum. Furthermore, we established a PEG-mediated protoplast transient transfection and stable transformation system for B. argenteum. Two homologues of ABI3(ABA-INSENSITIVE 3) gene were successfully cloned from B. argenteum. To further investigate the function of the ABI3 gene in B. argenteum, we used the CRISPR/Cas9 genetic editing system to target the BaABI3A and BaABI3B gene in B. argenteum protoplasts. This resulted in mutagenesis at the target in about 2-5% of the regenerated plants. The isolated abi3a and abi3b mutants exhibited increased sensitivity to desiccation, suggesting that BaABI3A and BaABI3B play redundant roles in desiccation stress. Overall, our results provide a rapid and simple approach for molecular genetics in B. argenteum. This study contributes to a better understanding of the molecular mechanisms of plant adaptation to extreme environmental.

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.

Biocrust Research in China: Recent Progress and Application in Land Degradation Control

Desert ecosystems are generally considered lifeless habitats characterised by extreme environmental conditions, yet they are successfully colonised by various biocrust nonvascular communities. A biocrust is not only an important ecosystem engineer and a bioindicator of desert ecological restoration but also plays a vital role in linking surficial abiotic and biotic factors. Thus, extensive research has been conducted on biocrusts in critical dryland zones. However, few studies have been conducted in the vast temperate deserts of China prior to the beginning of this century. We reviewed the research on biocrusts conducted in China since 2000, which firstly focused on the eco-physiological responses of biocrusts to species composition, abiotic stresses, and anthropological disturbances. Further, research on the spatial distributions of biocrusts as well as their succession at different spatial scales, and relationships with vascular plants and soil biomes (especially underlying mechanisms of seed retention, germination, establishment and survival of vascular plants during biocrust succession, and creation of suitable niches and food webs for soil animals and microorganisms) was analysed. Additionally, studies emphasising on the contribution of biocrusts to ecological and hydrological processes in deserts as well as their applications in the cultivation and inoculation of nonvascular plants for land degradation control and ecological restoration were assessed. Finally, recent research on biocrusts was evaluated to propose future emerging research themes and new frontiers.

UV-B Radiation Affects Photosynthesis-Related Processes of Two Italian Olea europaea (L.) Varieties Differently

Given the economical importance of the olive tree it is essential to study its responses to stress agents such as excessive UV-B radiation, to understand the defense mechanisms and to identify the varieties that are able to cope with it. In the light of the analysis carried out in this study, we argue that UV-B radiation represents a dangerous source of stress for the olive tree, especially in the current increasingly changing environmental conditions. Both the varieties considered (Giarraffa and Olivastra Seggianese), although resistant to the strong treatment to which they were exposed, showed, albeit in different ways and at different times, evident effects. The two varieties have different response times and the Giarraffa variety seems better suited to prolonged UV-B stress, possible due to a more efficient and quick activation of the antioxidant response (e.g., flavonoids use to counteract reactive oxygen species) and because of its capacity to maintain the photosynthetic efficiency as well as a relatively higher content of mannitol. Moreover, pigments reduction after a long period of UV-B exposure can also be an adaptation mechanism triggered by Giarraffa to reduce energy absorption under UV-B stress. Olivastra Seggianese seems less suited to overcome UV-B stress for a long period (e.g., higher reduction of F_v/F_m) and has a higher requirement for sugars (e.g., glucose) possible to counteract stress and to restore energy.

Comparative physiological responses of Microcoleus vaginatus and Bryum argenteum to enhanced UV-B radiation under field conditions

UV-B radiation is an important environmental factor affecting the composition and function of biological soil crusts (BSCs). The aim of this study was to compare the effects of enhanced UV-B radiation on BSCs from Tengger Desert, north-western China, which are dominated by the cyanobacterium Microcoleus vaginatus Gom. and moss Bryum argenteum Hedw. The BSCs were exposed to four UV-B supplemental treatments, including 2.75 (control), 3.08, 3.25, and 3.41Wm-2, for 40 days under field condition. In both the studied organisms, UV-B radiation significantly affected the physiological properties (total flavonoids, soluble proteins, soluble sugars, and proline contents). While marginally enhanced UV-B radiation for a short period favoured the growth of M. vaginatus and B. argenteum, excessively high and prolonged UV-B radiation suppressed the physiological properties of the two organisms. Moreover, response index revealed that UV-B radiation had more detrimental effects on B. argenteum, suggesting that B. argenteum is more sensitive to UV-B radiation than M. vaginatus. The findings of this study could help to predict and evaluate the possible changes in the structure and function of desert ecosystems, based on the variation in physiological responses of M. vaginatus and B. argenteum to enhanced UV-B radiation.

Identification, Classification, and Functional Analysis of AP2/ERF Family Genes in the Desert Moss Bryum argenteum

Bryum argenteum is a desert moss which shows tolerance to the desert environment and is emerging as a good plant material for identification of stress-related genes. AP2/ERF transcription factor family plays important roles in plant responses to biotic and abiotic stresses. AP2/ERF genes have been identified and extensively studied in many plants, while they are rarely studied in moss. In the present study, we identified 83 AP2/ERF genes based on the comprehensive dehydrationrehydration transcriptomic atlas of B. argenteum. BaAP2/ERF genes can be classified into five families, including 11 AP2s, 43 DREBs, 26 ERFs, 1 RAV, and 2 Soloists. RNA-seq data showed that 83 BaAP2/ERFs exhibited elevated transcript abundances during dehydration⁻rehydration process. We used RT-qPCR to validate the expression profiles of 12 representative BaAP2/ERFs and confirmed the expression trends using RNA-seq data. Eight out of 12 BaAP2/ERFs demonstrated transactivation activities. Seven BaAP2/ERFs enhanced salt and osmotic stress tolerances of yeast. This is the first study to provide detailed information on the identification, classification, and functional analysis of the AP2/ERFs in B. argenteum. This study will lay the foundation for the further functional analysis of these genes in plants, as well as provide greater insights into the molecular mechanisms of abiotic stress tolerance of B. argenteum.

Ultrastructure variations in Sphagnum denticulatum ecotypes in response to desiccation stress matter to conservation

Global warming and peat bogs drying are having a strong negative effect on the survival of endangered peat mosses. Here, we aimed to identify ultrastructural and physiological trait variation during dehydration and rehydration in the (sub-)meristematic cells of buds among clonally propagated individuals of Sphagnum denticulatum in relation to their ecological origin. We cultivated five clones in common garden conditions (CGCs) to exclude a carryover effect and we subsequently water-stressed (-40 MPa) and rehydrated (7 days) them. For the ultrastructure analysis, over 1280 measurements were recorded for 34 traits. Compared with the control, the treatment led to alterations in organelles that appeared to be ecotype- and genotype-dependent and characteristic for desiccation-sensitive mosses. Also, the recovery of chloroplasts, as measured by the initial and maximal fluorescence yield, were incomplete for all studied plants indicating desiccation sensitivity. Terrestrial genotypes possessed better recovery capability than did aquatic genotypes, suggesting an adaptation of the former to tolerate unpredictable terrestrial conditions in time and space. Genotype-specific requirements of water availability in the original environments should be considered before transplanting gametophytes for peatland restoration programs.

Spores potentially dispersed to longer distances are more tolerant to ultraviolet radiation: A case study in the moss genus Orthotrichum

PREMISE OF THE STUDY

Ultraviolet (UV) radiation influences the viability of algal spores and seed-plant pollen depending on the species, the dose, and the wavelength. In bryophytes, one of the dominant groups of plants in many habitats, UV radiation could determine their spore dispersal strategy, and such data are critical for reconstructing the ancestral state in plants and for determining the distribution range and persistence of bryophyte species.

METHODS

Spores of four bryophyte species of the moss genus Orthotrichum that were either hygrochastic or xerochastic (spores dispersed under wet or dry conditions, respectively) were exposed to realistic doses of UV radiation under laboratory conditions. Spore viability was evaluated through germination experiments and, for the first time in bryophytes, ultrastructural observations. Given that the UV-B doses used were relatively higher than the UV-A doses, the UV effect was probably due more to UV-B than UV-A wavelengths.

KEY RESULTS

All four species reduced their spore germination capacity in a UV dose-dependent manner, concomitantly increasing spore ultrastructural damage (cytoplasmic and plastid alterations). Most spores eventually died when exposed to the highest UV dose. Interestingly, spores of hygrochastic species were much more UV-sensitive than those of xerochastic species.

CONCLUSIONS

UV tolerance determines moss spore viability, as indicated by germination capacity and ultrastructural damage, and differs between spores of species with different dispersal strategies. Specifically, the higher UV tolerance of xerochastic spores may enable them to be dispersed to longer distances than hygrochastic spores, thus extending more efficiently the distribution range of the corresponding species.

Effects of enhanced ultraviolet-B radiation, water deficit, and their combination on UV-absorbing compounds and osmotic adjustment substances in two different moss species

A simulation experiment was conducted to explore the influence of enhanced ultraviolet-B (UV-B) radiation, water deficit, and their combination on UV-absorbing compounds and osmotic adjustment substances of mosses Bryum argenteum and Didymodon vinealis isolated from biological soil crusts (BSCs) growing in a revegetated area of the Tengger Desert, China. Four levels of UV-B radiation and two gradients of water regime were employed. Compared with their controls, amounts of total flavonoids, chlorophyll, carotenoids, soluble sugars, and soluble proteins significantly decreased (p < 0.05), but proline content significantly increased (p < 0.05), when exposed to either enhanced UV-B or water deficit. The negative effects of enhanced UV-B were alleviated when water deficit was applied. There were increases in UV-absorbing compounds and osmotic adjustment substances when exposed to a combination of enhanced UV-B and water deficit compared with single stresses, except for the proline content in D. vinealis. In addition, our results also indicated interspecific differences in response to enhanced UV-B, water deficit, and their combination. Compared with B. argenteum, D. vinealis was more resistant to enhanced UV-B and water deficit singly and in combination. These results suggest that the damage of enhanced UV-B on both species might be alleviated by water deficit. This alleviation is important for understanding the response of BSCs to UV-B radiation in future global climate change. This also provides novel insights into assessment damages of UV-B to BSC stability in arid and semiarid regions.

UV-B radiation modulates physiology and lipophilic metabolite profile in Olea europaea

Ultraviolet-B (UV-B) radiation plays an important role in plant photomorphogenesis. Whilst the morpho-functional disorders induced by excessive UV irradiation are well-known, it remains unclear how this irradiation modulates the metabolome, and which metabolic shifts improve plants' tolerance to UV-B. In this study, we use an important Mediterranean crop, Olea europaea, to decipher the impacts of enhanced UV-B radiation on the physiological performance and lipophilic metabolite profile. Young olive plants (cv. 'Galega Vulgar') were exposed for five days to UV-B biologically effective doses of 6.5 kJ m^-2 d^-1 and 12.4 kJ m^-2 d^-1. Cell cycle/ploidy, photosynthesis and oxidative stress, as well as GC-MS metabolites were assessed. Both UV-B treatments impaired net CO₂ assimilation rate, transpiration rate, photosynthetic pigments, and RuBisCO activity, but 12.4 kJ m^-2 d^-1 also decreased the photochemical quenching (qP) and the effective efficiency of PSII (Φ_PSII). UV-B treatments promoted mono/triperpene pathways, while only 12.4 kJ m^-2 d^-1 increased fatty acids and alkanes, and decreased geranylgeranyl-diphosphate. The interplay between physiology and metabolomics suggests some innate ability of these plants to tolerate moderate UV-B doses (6.5 kJ m^-2 d^-1). Also their tolerance to higher doses (12.4 kJ m^-2 d^-1) relies on plants' metabolic adjustments, where the accumulation of specific compounds such as long-chain alkanes, palmitic acid, oleic acid and particularly oleamide (which is described for the first time in olive leaves) play an important protective role. This is the first study demonstrating photosynthetic changes and lipophilic metabolite adjustments in olive leaves under moderate and high UV-B doses.

Distinct physiological and metabolic reprogramming by highbush blueberry (Vaccinium corymbosum) cultivars revealed during long-term UV-B radiation

Despite the Montreal protocol and the eventual recovery of the ozone layer over Antarctica, there are still concerns about increased levels of ultraviolet-B (UV-B) radiation in the Southern Hemisphere. UV-B induces physiological, biochemical and morphological stress responses in plants, which are species-specific and different even for closely related cultivars. In woody plant species, understanding of long-term mechanisms to cope with UV-B-induced stress is limited. Therefore, a greenhouse UV-B daily course simulation was performed for 21 days with two blueberry cultivars (Legacy and Bluegold) under UV-B_BE irradiance doses of 0, 0.07 and 0.19 W m^-2 . Morphological changes, photosynthetic performance, antioxidants, lipid peroxidation and metabolic features were evaluated. We found that both cultivars behaved differently under UV-B exposure, with Legacy being a UV-B-resistant cultivar. Interestingly, Legacy used a combined strategy: initially, in the first week of exposure its photoprotective compounds increased, coping with the intake of UV-B radiation (avoidance strategy), and then, increasing its antioxidant capacity. These strategies proved to be UV-B radiation dose dependent. The avoidance strategy is triggered early under high UV-B radiation in Legacy. Moreover, the rapid metabolic reprogramming capacity of this cultivar, in contrast to Bluegold, seems to be the most relevant contribution to its UV-B stress-coping strategy.

Polychromatic Supplemental Lighting from underneath Canopy Is More Effective to Enhance Tomato Plant Development by Improving Leaf Photosynthesis and Stomatal Regulation

Light insufficient stress caused by canopy interception and mutual shading is a major factor limiting plant growth and development in intensive crop cultivation. Supplemental lighting can be used to give light to the lower canopy leaves and is considered to be an effective method to cope with low irradiation stress. Leaf photosynthesis, stomatal regulation, and plant growth and development of young tomato plants were examined to estimate the effects of supplemental lighting with various composite spectra and different light orientations. Light-emitting diodes (LEDs) of polychromatic light quality, red + blue (R/B), white + red + blue (W/R/B), white + red + far-red (W/R/FR), and white + blue (W/B) were assembled from the underneath canopy or from the inner canopy as supplemental lighting resources. The results showed that the use of supplemental lighting significantly increased the photosynthetic efficiency, and reduced stomatal closure while promoting plant growth. Among all supplemental lighting treatments, the W/R/B and W/B from the underneath canopy had best performance. The different photosynthetic performances among the supplemental lighting treatments are resulted from variations in CO2 utilization. The enhanced blue light fraction in the W/R/B and W/B could better stimulate stomatal opening and promote photosynthetic electron transport activity, thus better improving photosynthetic rate. Compared with the inner canopy treatment, the supplemental lighting from the underneath canopy could better enhance the carbon dioxide assimilation efficiency and excessive energy dissipation, leading to an improved photosynthetic performance. Stomatal morphology was highly correlated to leaf photosynthesis and plant development, and should thus be an important determinant for the photosynthesis and the growth of greenhouse tomatoes.

Root Zone Cooling and Exogenous Spermidine Root-Pretreatment Promoting Lactuca sativa L. Growth and Photosynthesis in the High-temperature Season

Root zone high-temperature stress is a major factor limiting hydroponic plant growth during the high-temperature season. The effects of root zone cooling (RZC; at 25°C) and exogenous spermidine (Spd) root-pretreatment (SRP, 0.1 mM) on growth, leaf photosynthetic traits, and chlorophyll fluorescence characteristics of hydroponic Lactuca sativa L. grown in a high-temperature season (average temperature > 30°C) were examined. Both treatments significantly promoted plant growth and photosynthesis in the high-temperature season, but the mechanisms of photosynthesis improvement in the hydroponic grown lettuce plants were different between the RZC and SRP treatments. The former improved plant photosynthesis by increasing stoma conductance (G s) to enhance CO2 supply, thus promoting photosynthetic electron transport activity and phosphorylation, which improved the level of the photochemical efficiency of photosystem II (PSII), rather than enhancing CO2 assimilation efficiency. The latter improved plant photosynthesis by enhancing CO2 assimilation efficiency, rather than stomatal regulation. Combination of RZC and SRP significantly improved P N of lettuce plants in a high-temperature season by both improvement of G s to enhance CO2 supply and enhancement of CO2 assimilation. The enhancement of photosynthetic efficiency in both treatments was independent of altering light-harvesting or excessive energy dissipation.

Responses of photosynthetic properties and antioxidant enzymes in high-yield rice flag leaves to supplemental UV-B radiation during senescence stage

Despite the increasing occurrence of ultraviolet-B (UV-B) radiation, its molecular mechanism is poorly documented in higher plants compared to other environmental stress. In present study, the influence of supplemental UV-B radiation on photosynthetic performance and antioxidant enzymes in rice (Oryza sativa L.) was investigated. Supplemental UV-B radiation reduced net photosynthetic rate in rice flag leaves during senescence stage. By means of the JIP-test, it was found that the potential of processing light energy through the photosynthetic machinery was slightly inhibited by the increased thermal dissipation. Furthermore, 18 thylakoid membrane protein spots were differentially expressed (5-fold or greater variation compared to the control) in supplemental UV-B-treated rice. These identified proteins were involved in various cellular responses and metabolic processes including photosynthesis, stress defense, Calvin cycle, and others of unknown functions. Taken together, these results suggested that physiological changes that resulted from supplemental UV-B radiation were linked to the light reaction, carbon metabolism, and antioxidant enzymes in rice leaves during senescence stage.