UV-C-Induced alleviation of transcriptional gene silencing through plant-plant communication: Key roles of jasmonic acid and salicylic acid pathways

Flashes of UV-C Light Stimulate Defenses of Vitis vinifera L. 'Chardonnay' Against Erysiphe necator in Greenhouse and Vineyard Conditions

Using detached leaves, UV-C light in the form of 1-s flashes has recently been shown to stimulate defenses of several plants against different pathogens better than 1-min exposures under greenhouse conditions. In the present work, the pathological tests were conducted using undetached leaves under greenhouse and vineyard conditions. In a first trial, two flashes of UV-C light were applied to plants of Vitis vinifera L. 'Chardonnay' grown under greenhouse conditions, at an interval of 10 days. Plants were inoculated with Erysiphe necator 2 days after the last light treatment. After 18 days of inoculation, the symptom severity on leaves was reduced by 60% when compared with the untreated control. In a second trial, flashes of UV-C light were applied to grapevine Chardonnay plants under field conditions in the southeast of France every 10 days from 18 April until 10 July 2019. The symptom severity resulting from natural contaminations by E. necator was reduced by 42% in leaves on 4 July 2019 and by 65% in clusters on 25 July 2019. In a third trial, we observed that UV-C light did not have any effect on net photosynthesis, maximal net photosynthesis, dark respiration, maximal quantum efficiency of photosystem II, the performance index of Strasser, and, generally, any parameter derived from induction curves of maximal chlorophyll fluorescence. It was concluded that flashes of UV-C light have true potential for stimulating plant defenses against E. necator under vineyard conditions and, therefore, help in reducing fungicide use.

Postharvest UV-C Irradiation Influenced Cellular Structure, Jasmonic Acid Accumulation, and Resistance Against Green Mold Decay in Satsuma Mandarin Fruit (Citrus unshiu)

Green mold caused by Penicillium digitatum is an important factor limiting the shelf life of mandarin fruit. In this study, the effect of ultraviolet-C (UV-C) irradiation on cellular structure, endogenous jasmonic acid (JA), and development of P. digitatum in satsuma mandarin fruit was investigated. UV-C treatments included 0 (untreated control), 3, and 10 kJ m−2 or the exposure time of 0, 1.18, and 4.52 min, respectively. The UV-C dose of 10 kJ m−2 significantly reduced the development of P. digitatum both in vitro and in vivo, resulting in the maintenance of the cellular structure of the albedo tissue. The production of malondialdehyde (MDA) was decreased upon UV-C treatment of 10 kJ m−2. The concentration of JA increased in the treatment of 10 kJ m−2 compared to the treatment of 3 kJ m−2 and the control. UV-C irradiation increased total phenolic and total flavonoid concentrations and DPPH radical scavenging capacity. These results suggest that UV-C at 10 kJ m−2 has a potential to control green mold caused by P. digitatum, maintain cellular structure, stimulate the accumulation of JA, and induce biochemical compounds in satsuma mandarin.

Plant volatiles as cues and signals in plant communication

Volatile organic compounds are important mediators of mutualistic interactions between plants and their physical and biological surroundings. Volatiles rapidly indicate competition or potential threat before these can take place, and they regulate and coordinate adaptation responses in neighbouring plants, fine-tuning them to match the exact stress encountered. Ecological specificity and context-dependency of plant-plant communication mediated by volatiles represent important factors that determine plant performance in specific environments. In this review, we synthesise the recent progress made in understanding the role of plant volatiles as mediators of plant interactions at the individual and community levels, highlighting the complexity of the plant receiver response to diverse volatile cues and signals and addressing how specific responses shape plant growth and survival. Finally, we outline the knowledge gaps and provide directions for future research. The complex dialogue between the emitter and receiver based on either volatile cues or signals determines the outcome of information exchange, which shapes the communication pattern between individuals at the community level and determines their ecological implications at other trophic levels.

Flashes of UV-C light: An innovative method for stimulating plant defences

Leaves of lettuce, pepper, tomato and grapevine plants grown in greenhouse conditions were exposed to UV-C light for either 60 s or 1 s, using a specific LEDs-based device, and wavelengths and energy were the same among different light treatments. Doses of UV-C light that both effectively stimulated plant defences and were innocuous were determined beforehand. Tomato plants and lettuce plants were inoculated with Botrytis cinerea, pepper plants with Phytophthora capsici, and grapevine with Plasmopara viticola. In some experiments we investigated the effect of a repetition of treatments over periods of several days. All plants were inoculated 48 h after exposure to the last UV-C treatment. Lesions on surfaces were measured up to 12 days after inoculation, depending on the experiment and the pathogen. The results confirmed that UV-C light stimulates plant resistance; they show that irradiation for one second is more effective than irradiation for 60 s, and that repetition of treatments is more effective than single light treatments. Moreover a systemic effect was observed in unexposed leaves that were close to exposed leaves. The mechanisms of perception and of the signalling and metabolic pathways triggered by flashes of UV-C light vs. 60 s irradiation exposures are briefly discussed, as well as the prospects for field use of UV-C flashes in viticulture and horticulture.

Photodamage repair pathways contribute to the accurate maintenance of the DNA methylome landscape upon UV exposure

Plants are exposed to the damaging effect of sunlight that induces DNA photolesions. In order to maintain genome integrity, specific DNA repair pathways are mobilized. Upon removal of UV-induced DNA lesions, the accurate re-establishment of epigenome landscape is expected to be a prominent step of these DNA repair pathways. However, it remains poorly documented whether DNA methylation is accurately maintained at photodamaged sites and how photodamage repair pathways contribute to the maintenance of genome/methylome integrities. Using genome wide approaches, we report that UV-C irradiation leads to CHH DNA methylation changes. We identified that the specific DNA repair pathways involved in the repair of UV-induced DNA lesions, Direct Repair (DR), Global Genome Repair (GGR) and small RNA-mediated GGR prevent the excessive alterations of DNA methylation landscape. Moreover, we identified that UV-C irradiation induced chromocenter reorganization and that photodamage repair factors control this dynamics. The methylome changes rely on misregulation of maintenance, de novo and active DNA demethylation pathways highlighting that molecular processes related to genome and methylome integrities are closely interconnected. Importantly, we identified that photolesions are sources of DNA methylation changes in repressive chromatin. This study unveils that DNA repair factors, together with small RNA, act to accurately maintain both genome and methylome integrities at photodamaged silent genomic regions, strengthening the idea that plants have evolved sophisticated interplays between DNA methylation dynamics and DNA repair.

Negative Modulation of Bystander DNA Repair Potential by X-Ray Targeted Tissue Volume in Arabidopsis thaliana

Radiation-induced bystander effects (RIBE) entail a cascade of bystander signals produced by the hit cells to the neighboring cells to regulate various biological processes including DNA damage repair. However, there is little clarity regarding the effect of radiation-targeted volume (hit cell amount) on the DNA repair potential of the bystander cells. This is especially important to understand in the context of the whole organism, where the target usually consists of multiple types of cells/tissues. To address this question, model plant Arabidopsis thaliana was locally irradiated, and the DNA repair potential of bystander root-tip cells was assessed based on their radioresistance to subsequent high-dose radiation, i.e. radioadaptive responses (RAR). We found that X-ray irradiation of the aerial parts (AP) of A. thaliana seedlings (5 Gy) initiated RAR in the root-tip cells, which exhibited an alleviated repression of root growth and root cell division, and reduced amount of DNA strand breaks. We also observed an improvement in the repair efficiency of the homologous recombination (HR) and non-homologous end joining (NHEJ) pathways in the bystander root tip cells. We further expanded the X-ray targeted volume to include the aerial parts with upper parts of the primary root and compared it with X-ray irradiated aerial parts alone. Comparative analysis revealed that RAR for these end points either disappeared or decreased; specifically, the repair efficiency of HR was significantly reduced, indicating that radiation-targeted volume negatively modulates the bystander DNA repair potential. In contrast, X-ray irradiation of upper part of the primary root alone did not induce RAR of the root tip cells. Thus, we propose that additional X-ray irradiation of upper part of the primary root reduces the bystander DNA repair potential, possibly by selectively disturbing the transport of bystander signals responsible for HR repair.

A potential involvement of plant systemic response in initiating genotoxicity of Ag-nanoparticles in Arabidopsis thaliana

The extensive availability of engineered nanomaterials in global markets has led to the release of substantial amounts of nanoparticles (NP) into atmosphere, water body and soil, yielding both beneficial and harmful effects in plant systems. The NP are mainly aggregated onto the surface of plant roots and leaves exposed and only slightly transported into other tissues with a low rate of internalization. This raises a question of whether plant systemic response is involved in the induction of biological effects of NP. To address this, model plant Arabidopsis thaliana were root exposed to low concentrations of Ag-NP of two particle sizes (10-nm and 60-nm), and expressions of homologous recombination (HR)-related genes and the alleviation of transcriptional gene silencing (TGS) in aerial leafy tissues were examined as genotoxic endpoints. Results showed that exposure of roots to two sizes of Ag-NP up-regulated expressions of HR genes, and reactivated TGS-silenced repetitive elements in aerial tissues. These effects were blocked by the impairment in the salicylic acid signal pathway, indicating a potential involvement of plant systemic response in the induction of Ag-NP genotoxicity. This is further supported by ICP-MS analysis, in which the Ag content in aerial tissues was not significantly changed by root exposure to 10-nm Ag-NP. Although a significant increase in the Ag content in aerial tissues was observed after root exposure to 60-nm Ag-NP, its genotoxic effects had no obvious difference from that by 10-nm Ag-NP exposure, also suggesting that the genotoxicity might be mainly induced via plant systemic response, at least in the experiments of root exposure to Ag-NP.

The moss jasmonate ZIM-domain protein PnJAZ1 confers salinity tolerance via crosstalk with the abscisic acid signalling pathway

Abscisic acid (ABA) and jasmonates (JAs) are the primary plant hormones involved in mediating salt tolerance. In addition, these two plant hormones exert a synergistic effect to inhibit seed germination. However, the molecular mechanism of the interaction between ABA signalling and JA signalling is still not well documented. Here, a moss jasmonate ZIM-domain gene (PnJAZ1), which encodes a nucleus-localized protein with conserved ZIM and Jas domains, was cloned from Pohlia nutans. PnJAZ1 expression was rapidly induced by various abiotic stresses. The PnJAZ1 protein physically interacted with MYC2 and was degraded by exogenous 12-oxo-phytodienoic acid (OPDA) treatment, implying that the JAZ protein-mediated signalling pathway is conserved in plants. Transgenic Arabidopsis and Physcomitrella plants overexpressing PnJAZ1 showed increased tolerance to salt stress and decreased ABA sensitivity during seed germination and early development. The overexpression of PnJAZ1 inhibited the expression of ABA pathway genes related to seed germination and seedling growth. Moreover, the transgenic Arabidopsis lines exhibited enhanced tolerance to auxin (IAA) and glucose, mimicking the phenotypes of abi4 or abi5 mutants. These results suggest that PnJAZ1 acts as a repressor, mediates JA-ABA synergistic crosstalk and enhances plant growth under salt stress.

Intercellular communication of DNA damage and oxidative status underpin bystander effects

PURPOSE

A well-known phenomenon in the field of radiation biology is that cells exposed to ionizing radiation (IR) (targeted cells) can induce in non-irradiated (non-targeted), bystander cells effects reminiscent of DNA damage responses (DDR) normally expected, exclusively in targeted cells. These phenomena are collectively referred to as radiation-induced bystander effects (RIBE) and have different manifestations depending on the endpoint studied. Although it is now recognized that RIBE reflects to a considerable extent communication by the targeted cells to undamaged cells of their damaged status, the molecular underpinnings of this communication and its significance for the organism are only partly understood. In particular, it remains unknown why and how targeted cells induce DNA damage in non-targeted, bystander cells threatening their genomic stability and risking thus their transformation to cancer cells. Here, we outline observations hinting to possible sources of artifacts in experiments designed to detect RIBE and summarize a model according to which targeted cells modulate their redox status as part of their overall response to IR and use this modified redox status as a source to generate signals that are transmitted to non-irradiated cells of the organism.

MATERIAL AND METHODS

A synthesis of published evidence is presented.

RESULTS

Depending on type, RIBE signals may be transmitted through various forms of direct intercellular contact, through molecules acting locally in a paracrine fashion, or through molecules acting remotely in an endocrine fashion. We reason that DNA damage generated in bystander cells is unlikely to manifest the clustered character exhibited in directly exposed cells and postulate that RIBE will depend on complications generated when simpler forms of damage encounter the DNA replication fork.

CONCLUSIONS

We suggest that RIBE result from intercellular communication mechanisms designed to spread within tissues, or the organism, alarm signals of DNA damage inflicted in subsets of the constituent cells. This response likely evolved to protect organisms by appropriately modulating stress response, repair or apoptosis, and may in some instances also cause adverse effects, e.g. as collateral damage.

Effect of modeled microgravity on UV-C-induced interplant communication of Arabidopsis thaliana

Controlled ecological life support systems (CELSS) will be an important feature of long-duration space missions of which higher plants are one of the indispensable components. Because of its pivotal role in enabling plants to cope with environmental stress, interplant communication might have important implications for the ecological stability of such CELSS. However, the manifestations of interplant communication in microgravity conditions have yet to be fully elucidated. To address this, a well-established Arabidopsis thaliana co-culture experimental system, in which UV-C-induced airborne interplant communication is evaluated by the alleviation of transcriptional gene silencing (TGS) in bystander plants, was placed in microgravity modeled by a two-dimensional rotating clinostat. Compared with plants under normal gravity, TGS alleviation in bystander plants was inhibited in microgravity. Moreover, TGS alleviation was also prevented when plants of the pgm-1 line, which are impaired in gravity sensing, were used in either the UV-C-irradiated or bystander group. In addition to the specific TGS-loci, interplant communication-shaped genome-wide DNA methylation in bystander plants was altered under microgravity conditions. These results indicate that interplant communications might be modified in microgravity. Time course analysis showed that microgravity interfered with both the production of communicative signals in UV-C-irradiated plants and the induction of epigenetic responses in bystander plants. This was further confirmed by the experimental finding that microgravity also prevented the response of bystander plants to exogenous methyl jasmonate (JA) and methyl salicylate (SA), two well-known airborne signaling molecules, and down-regulated JA and SA biosynthesis in UV-C-irradiated plants.

Effect of modeled microgravity on radiation-induced adaptive response of root growth in Arabidopsis thaliana

Space particles have an inevitable impact on organisms during space missions; radio-adaptive response (RAR) is a critical radiation effect due to both low-dose background and sudden high-dose radiation exposure during solar storms. Although it is relevant to consider RAR within the context of microgravity, another major space environmental factor, there is no existing evidence as to its effects on RAR. In the present study, we established an experimental method for detecting the effects of gamma-irradiation on the primary root growth of Arabidopsis thaliana, in which RAR of root growth was significantly induced by several dose combinations. Microgravity was simulated using a two-dimensional rotation clinostat. It was shown that RAR of root growth was significantly inhibited under the modeled microgravity condition, and was absent in pgm-1 plants that had impaired gravity sensing in root tips. These results suggest that RAR could be modulated in microgravity. Time course analysis showed that microgravity affected either the development of radio-resistance induced by priming irradiation, or the responses of plants to challenging irradiation. After treatment with the modeled microgravity, attenuation in priming irradiation-induced expressions of DNA repair genes (AtKu70 and AtRAD54), and reduced DNA repair efficiency in response to challenging irradiation were observed. In plant roots, the polar transportation of the phytohormone auxin is regulated by gravity, and treatment with an exogenous auxin (indole-3-acetic acid) prevented the induction of RAR of root growth, suggesting that auxin might play a regulatory role in the interaction between microgravity and RAR of root growth.

A pivotal role of the jasmonic acid signal pathway in mediating radiation-induced bystander effects in Arabidopsis thaliana

Although radiation-induced bystander effects (RIBE) in Arabidopsis thaliana have been well demonstrated in vivo, little is known about their underlying mechanisms, particularly with regard to the participating signaling molecules and signaling pathways. In higher plants, jasmonic acid (JA) and its bioactive derivatives are well accepted as systemic signal transducers that are produced in response to various environmental stresses. It is therefore speculated that the JA signal pathway might play a potential role in mediating radiation-induced bystander signaling of root-to-shoot. In the present study, pretreatment of seedlings with Salicylhydroxamic acid, an inhibitor of lipoxigenase (LOX) in JA biosynthesis, significantly suppressed RIBE-mediated expression of the AtRAD54 gene. After root irradiation, the aerial parts of A. thaliana mutants deficient in JA biosynthesis (aos) and signaling cascades (jar1-1) showed suppressed induction of the AtRAD54 and AtRAD51 genes and TSI and 180-bp repeats, which have been extensively used as endpoints of bystander genetic and epigenetic effects in plants. These results suggest an involvement of the JA signal pathway in the RIBE of plants. Using the root micro-grafting technique, the JA signal pathway was shown to participate in both the generation of bystander signals in irradiated root cells and radiation responses in the bystander aerial parts of plants. The over-accumulation of endogenous JA in mutant fatty acid oxygenation up-regulated 2 (fou2), in which mutation of the Two Pore Channel 1 (TPC1) gene up-regulates expression of the LOX and allene oxide synthase (AOS) genes, inhibited RIBE-mediated expression of the AtRAD54 gene, but up-regulated expression of the AtKU70 and AtLIG4 genes in the non-homologous end joining (NHEJ) pathway. Considering that NHEJ is employed by plants with increased DNA damage, the switch from HR to NHEJ suggests that over-accumulation of endogenous JA might enhance the radiosensitivity of plants in terms of RIBE.