Dodder-transmitted mobile signals prime host plants for enhanced salt tolerance

Long-distance transport of the pear HMGR1 mRNA via the phloem is associated with enhanced salt tolerance

Grafting is the main asexual propagation method for horticultural crops and can enhance their resistance to biotic or abiotic stress. Many mRNAs can be transported over long distances through the graft union, however, the function of mobile mRNAs remains poorly understood. Here, we exploited lists of candidate mobile mRNAs harboring potential 5-methylcytosine (m⁵C) modification in pear (Pyrus betulaefolia). dCAPS RT-PCR and RT-PCR were employed to demonstrate the mobility of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase1 (PbHMGR1) mRNA in grafted plants of both pear and tobacco (Nicotiana tabacum). Overexpressing PbHMGR1 in tobacco plants enhanced salt tolerance during seed germination. In addition, both histochemical staining and GUS expression analysis showed that PbHMGR1 could directly respond to salt stress. Furthermore, it was found that the relative abundance of PbHMGR1 increased in heterografted scion, which avoided serious damage under salt stress. Collectively, these findings established that PbHMGR1 mRNA could act as a salt-responsive signal and move through the graft union to enhance salt tolerance of scion, which might be used as a new plant breeding technique to improve resistance of scion through a stress-tolerant rootstock.

Germinating seedlings and mature shoots of Cuscuta campestris respond differently to light stimuli during parasitism but not during circumnutation

Seedlings of the parasitic plant genus Cuscuta (dodder) locate hosts by circumnutation, coil around the host near soil level and form a haustorium, establishing a primary parasitism beneath the canopy. Mature shoots elongating from the parasitic region parasitize other hosts on the upper surfaces of their canopy. Although parasitism by dodder is stimulated by blue and far-red light, and inhibited by red light, the responses to light signals during the developmental stages are not comprehensively understood. Therefore, we compared the effects of different types of light on both circumnutation and parasitism by germinating seedlings and mature shoots of Cuscuta campestris. Seedlings established parasitism under blue and far-red light, but not under red light, as has been reported repeatedly. By contrast, mature shoots exhibited coiling around the host and haustoria formation even under a red light as well as under blue and far-red light. These findings indicate that C. campestris modified its response to red light during the transition from young seedlings to mature shoots, facilitating parasitism. Light quality did not affect the circumnutation of either seedlings or mature shoots, indicating that circumnutation and the coiling movement that leads to parasitism were regulated by different environmental signals.

A comparative study across the parasitic plants of Cuscuta subgenus Grammica (Convolvulaceae) reveals a possible loss of the plastid genome in its section Subulatae

Most species in Cuscuta subgenus Grammica retain many photosynthesis-related plastid genes, generally under purifying selection. A group of holoparasitic species in section Subulatae may have lost their plastid genomes entirely. The c. 153 species of plants belonging to Cuscuta subgenus Grammica are all obligate stem parasites. However, some have completely lost the ability to conduct photosynthesis while others retain photosynthetic machinery and genes. The plastid genome that primarily encodes key photosynthesis genes functions as a bellwether for how reliant plants are on primary production. This research assembles and analyses 17 plastomes across Cuscuta subgenus Grammica with the aim of characterizing the state of the plastome in each of its sections. By comparing the structure and content of plastid genomes across the subgenus, as well as by quantifying the selection acting upon each gene, we reconstructed the patterns of plastome change within the phylogenetic context for this group. We found that species in 13 of the 15 sections that comprise Grammica retain the bulk of plastid photosynthesis genes and are thus hemiparasitic. The complete loss of photosynthesis can be traced to two clades: the entire section Subulatae and a complex of three species within section Ceratophorae. We were unable to recover any significant plastome sequences from section Subulatae, suggesting that plastomes in these species are either drastically reduced or lost entirely.

Genomic and Epigenomic Mechanisms of the Interaction between Parasitic and Host Plants

Parasitic plants extract nutrients from the other plants to finish their life cycle and reproduce. The control of parasitic weeds is notoriously difficult due to their tight physical association and their close biological relationship to their hosts. Parasitic plants differ in their susceptible host ranges, and the host species differ in their susceptibility to parasitic plants. Current data show that adaptations of parasitic plants to various hosts are largely genetically determined. However, multiple cases of rapid adaptation in genetically homogenous parasitic weed populations to new hosts strongly suggest the involvement of epigenetic mechanisms. Recent progress in genome-wide analyses of gene expression and epigenetic features revealed many new molecular details of the parasitic plants' interactions with their host plants. The experimental data obtained in the last several years show that multiple common features have independently evolved in different lines of the parasitic plants. In this review we discuss the most interesting new details in the interaction between parasitic and host plants.

Assembly, annotation, and comparative analysis of Ipomoea chloroplast genomes provide insights into the parasitic characteristics of Cuscuta species

In the Convolvulaceae family, around 1650 species belonging to 60 genera are widely distributed globally, mainly in the tropical and subtropical regions of America and Asia. Although a series of chloroplast genomes in Convolvulaceae were reported and investigated, the evolutionary and genetic relationships among the chloroplast genomes of the Convolvulaceae family have not been extensively elucidated till now. In this study, we first reported the complete chloroplast genome sequence of Ipomoea pes-caprae, a widely distributed coastal plant with medical values. The chloroplast genome of I. pes-caprae is 161667 bp in length, and the GC content is 37.56%. The chloroplastic DNA molecule of I. pes-caprae is a circular structure composed of LSC (large-single-copy), SSC (small-single-copy), and IR (inverted repeat) regions, with the size of the three regions being 88210 bp, 12117 bp, and 30670 bp, respectively. The chloroplast genome of I. pes-caprae contains 141 genes, and 35 SSRs are identified in the chloroplast genome. Our research results provide important genomic information for the molecular phylogeny of I. pes-caprae. The Phylogenetic analysis of 28 Convolvulaceae chloroplast genomes showed that the relationship of I. pes-caprae with I. involucrata or I. obscura was much closer than that with other Convolvulaccae species. Further comparative analyses between the Ipomoea species and Cuscuta species revealed the mechanism underlying the formation of parasitic characteristics of Cuscuta species from the perspective of the chloroplast genome.

Dodder-transmitted mobile systemic signals activate a salt-stress response characterized by a transcriptome change in Citrus sinensis

Citrus is an essential horticultural fruit whose yield and quality are affected by salinity all over the world. The recognition and adaptive regulation of citrus against salt stress are important areas for cultivar improvement, but the vascular system signal transduction mechanism of the plant response to salt stress remains elusive. In this study, we constructed a dodder (Cuscuta spp.) linked Hamlin sweet orange (Citrus sinensis) plant community in which deliver a vascular signal through the dodder in response to salt stress. RNA-seq technology was used to analyze the gene expression profile of citrus leaves after salt treatment. The results showed that a vascular signal was transmitted to a dodder-linked host plant, triggering a transcriptional response to salt stress. However, the phenotypic and transudative ability of the dodder changed after 24 h. The salt treatment group (Group S) and the dodder-linked group (Group D) respectively contained 1,472 and 557 differentially expressed genes (DEGs). 454 of which were common to both groups. The results of our analysis revealed that the gene expression categories in Group D represented a highly consistent trend compared to the group S plants, indicating that the dodder-bridged vascular signals activated the stress-response of citrus leaves for transcriptomic reconfiguration. The KEGG pathway database and an analysis of key drivers revealed that phenylpropanoid biosynthesis, photosynthesis-antenna proteins, starch and sucrose metabolism, plant hormone signal transduction, circadian rhythm, and MAPK signaling pathways were significantly enriched as the critical genes during salt stress. A systemic signal in the dodder-bridged host significantly regulated abiotic stress-related secondary metabolic pathways, including those for phenylpropanoids, lignin, and lignans. The physiological indexes of photosynthetic intensity, respiration, and attractiveness among communities supported the transcriptional changes. Thus, our results indicate that salt stress-induced vascular system signals can be transmitted through the vascular system of a dodder linking citrus plants, revealing the genetic regulation and physiological changes of citrus leaves responding to plant stress signal transmission.

Integration of the Physiology, Transcriptome and Proteome Reveals the Molecular Mechanism of Drought Tolerance in Cupressus gigantea

Drought stress can dramatically impair woody plant growth and restrict the geographical distribution of many tree species. To better understand the dynamics between the response and mechanism of Cupressus gigantea to drought and post-drought recovery, a comparative analysis was performed, relying on physiological measurements, RNA sequencing (RNA-Seq) and two-dimensional gel electrophoresis (2-DE) proteins. In this study, the analyses revealed that photosynthesis was seriously inhibited, while osmolyte contents, antioxidant enzyme activity and non-enzymatic antioxidant contents were all increased under drought stress in seedlings. Re-watering led to a recovery in most of the parameters analyzed, mainly the photosynthetic parameters and osmolyte contents. Transcriptomic and proteomic profiling suggested that most of the differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) were specifically altered, and a few were consistently altered. Drought induced a common reduction in the level of DEGs and DEPs associated with photosynthesis. Notably, DEGs and DEPs involved in reactive oxygen species (ROS) scavenging, such as ascorbate oxidase and superoxide dismutase (SOD), showed an inverse pattern under desiccation. This study may improve our understanding of the underlying molecular mechanisms of drought resistance in C. gigantea and paves the way for more detailed molecular analysis of the candidate genes.

Bidirectional lncRNA Transfer between Cuscuta Parasites and Their Host Plant

Dodder species (Cuscuta spp.) are holoparasites that have extensive material exchange with their host plants through vascular connections. Recent studies on cross-species transfer have provided breakthrough insights, but little is known about the interaction mechanisms of the inter-plant mobile substances in parasitic systems. We sequenced the transcriptomes of dodder growing on soybean hosts to characterize the long non-coding RNA (lncRNA) transfer between the two species, and found that lncRNAs can move in high numbers (365 dodder lncRNAs and 14 soybean lncRNAs) in a bidirectional manner. Reverse transcription-polymerase chain reaction further confirmed that individual lncRNAs were trafficked in the dodder-soybean parasitic system. To reveal the potential functions of mobile transcripts, the Gene Ontology terms of mobile lncRNA target genes were predicted, and mobile dodder target genes were found to be mainly enriched in "metabolic process", "catalytic activity", "signaling", and "response to stimulus" categories, whereas mobile soybean target genes were enriched in organelle-related categories, indicating that specific mobile lncRNAs may be important in regulating dodder parasitism. Our findings reveal that lncRNAs are transferred between dodder and its host soybean plants, which may act as critical regulators to coordinate the host-dodder interaction at the whole parasitic level.

Inter-species mRNA transfer among green peach aphids, dodder parasites, and cucumber host plants

mRNAs are transported within a plant through phloem. Aphids are phloem feeders and dodders (Cuscuta spp.) are parasites which establish phloem connections with host plants. When aphids feed on dodders, whether there is trafficking of mRNAs among aphids, dodders, and host plants and if aphid feeding affects the mRNA transfer between dodders and hosts are unclear. We constructed a green peach aphid (GPA, Myzus persicae)-dodder (Cuscuta australis)-cucumber (Cucumis sativus) tritrophic system by infesting GPAs on C. australis, which parasitized cucumber hosts. We found that GPA feeding activated defense-related phytohormonal and transcriptomic responses in both C. australis and cucumbers and large numbers of mRNAs were found to be transferred between C. australis and cucumbers and between C. australis and GPAs; importantly, GPA feeding on C. australis greatly altered inter-species mobile mRNA profiles. Furthermore, three cucumber mRNAs and three GPA mRNAs could be respectively detected in GPAs and cucumbers. Moreover, our statistical analysis indicated that mRNAs with high abundances and long transcript lengths are likely to be mobile. This study reveals the existence of inter-species and even inter-kingdom mRNA movement among insects, parasitic plants, and parasite hosts, and suggests complex regulation of mRNA trafficking.

Long-distance communication through systemic macromolecular signaling mediates stress defense responses in plants

Land plants have a unique vascular bundle system that ranges in length from a few centimeters to hundreds of meters. These systems integrate the various organs of the whole plant, perform material exchange between different plant tissues and mediate the transmission of signals between cells or over long distances. Grafting and parasitism can reshape the vascular tissues of different ecotypes or species and represent two important systems for studying plant systemic signaling. In recent years, with the advancement of genomics and sequencing technology, the transportation, identification, and function of systemic plant macromolecules have been extensively studied. Here, we review the current body of knowledge of the transport pathways and regulatory mechanisms of macromolecules in plants and assess systemic, long-distance signal trafficking that mediates stress responses, and plant-environment or plant-insect community interactions. Additionally, we propose several methods for identifying mobile mRNAs and proteins. Finally, we discuss the challenges facing systemic signaling research and put forth the most urgent questions that need to be answered to advance our understanding of plant systemic signaling.

Involvement of NAC transcription factor NaNAC29 in Alternaria alternata resistance and leaf senescence in Nicotiana attenuata

NAC-LIKE, ACTIVATED BY AP3/PI (NAP) is a NAC transcription factor regulating leaf senescence in Arabidopsis thaliana. In wild tobacco Nicotiana attenuata, a nuclear localized NAC transcription factor NaNAC29 was identified to be highly elicited after inoculation of Alternaria alternata, a notorious necrotic fungus on tobacco species. The NaNAC29 possesses similar tertiary structure to NAP with 60% amino acid identity. However, it remains unknown the role of NaNAC29 in plant defense responses to A. alternata and leaf senescence in N. attenuata. In this paper, Defensin-like protein 1 (NaDLP1) was highly induced in N. attenuata after A. alternata inoculation and bigger lesions were developed in NaDLP1-silenced plants. Interestingly, A. alternata-induced NaDLP1 was reduced by 76% in VIGS NaNAC29 plants and by 61% in JA deficient irAOC plants at 3 days post inoculation. The regulation of NaDLP1 expression by NaNAC29 was clearly independent on JA pathway, since exogenous methyl jasmonate treatment could not complement the induction levels of NaDLP1 in NaNAC29-silenced plants to the levels in WT plants. Otherwise, the expression of NaNAC29 was low expressed in young leaves but highly in senescent leaves and dark-treated leaves. NaNAC29-silenced plants, which were generated by virus-induced gene silencing (VIGS NaNAC29), showed delayed senescence phenotype. In addition, constitutive over-expression of NaNAC29 in A. thaliana could rescue the delayed-senescence phenotype of nap and caused precocious leaf senescence of wild-type Col-0 plants. All the data above demonstrate that NaNAC29 is a NAP homolog in N. attenuata participating in the defense responses to A. alternata by regulation of a defensin protein NaDLP1 and promoting leaf senescence.

Herbivory-induced systemic signals are likely to be evolutionarily conserved in euphyllophytes

Herbivory-induced systemic signaling has been demonstrated in monocots and dicots, and is essential for plant defense against insects. However, the nature and evolution of herbivory-induced systemic signals remain unclear. Grafting is widely used for studying systemic signaling; however, grafting between dicot plants from different families is difficult, and grafting is impossible for monocots. In this study, we took advantage of dodder's extraordinary capability of parasitizing various plant species. Field dodder (Cuscuta campestris) was employed to connect pairs of species that are phylogenetically very distant, ranging from fern to monocot and dicot plants, and so determine whether interplant signaling occurs after simulated herbivory. It was found that simulated herbivory-induced systemic signals can be transferred by dodder between a monocot and a dicot plant and even between a fern and a dicot plant, and the plants that received the systemic signals all exhibited elevated defenses. Thus, we inferred that the herbivory-induced systemic signals are likely to be evolutionarily well conserved among vascular plants. Importantly, we also demonstrate that the jasmonate pathway is probably an ancient regulator of the biosynthesis and/or transport of systemic signals in vascular plants. These findings provide new insight into the nature and evolution of systemic signaling.

Plant Parasites under Pressure: Effects of Abiotic Stress on the Interactions between Parasitic Plants and Their Hosts

Parasitic angiosperms, comprising a diverse group of flowering plants, are partially or fully dependent on their hosts to acquire water, mineral nutrients and organic compounds. Some have detrimental effects on agriculturally important crop plants. They are also intriguing model systems to study adaptive mechanisms required for the transition from an autotrophic to a heterotrophic metabolism. No less than any other plant, parasitic plants are affected by abiotic stress factors such as drought and changes in temperature, saline soils or contamination with metals or herbicides. These effects may be attributed to the direct influence of the stress, but also to diminished host availability and suitability. Although several studies on abiotic stress response of parasitic plants are available, still little is known about how abiotic factors affect host preferences, defense mechanisms of both hosts and parasites and the effects of combinations of abiotic and biotic stress experienced by the host plants. The latter effects are of specific interest as parasitic plants pose additional pressure on contemporary agriculture in times of climate change. This review summarizes the existing literature on abiotic stress response of parasitic plants, highlighting knowledge gaps and discussing perspectives for future research and potential agricultural applications.

Large-scale sequencing paves the way for genomic and genetic analyses in parasitic plants

Parasitic plants pose a serious agricultural threat, but are also precious resources for valuable metabolites. The heterotrophic nature of these plants has resulted in the development of several morphological and physiological features that are of evolutionary significance. Recent advances in large-scale sequencing technology have provided insights into the evolutionary and molecular mechanisms of plant parasitism. Genome sequencing has revealed gene losses and horizontal gene transfers in parasitic plants. Mobile signals traveling between the parasite and host may have contributed to the increased fitness of parasitic life styles. Transcriptome analyses implicate shared processes among various parasitic species and the establishment of functional analysis is beginning to reveal molecular mechanisms during host and parasite interactions.

Parasite dodder enables transfer of bidirectional systemic nitrogen signals between host plants

Dodder (Cuscuta spp., Convolvulaceae) is a genus of parasitic plants with worldwide distribution. Dodders are able to simultaneously parasitize two or more adjacent hosts, forming dodder-connected plant clusters. Nitrogen (N) deficiency is a common challenge to plants. To date, it has been unclear whether dodder transfers N-systemic signals between hosts grown in N-heterogeneous soil. Transcriptome and methylome analyses were carried out to investigate whether dodder (Cuscuta campestris) transfers N-systemic signals between N-replete and N-depleted cucumber (Cucumis sativus) hosts, and it was found that N-systemic signals from the N-deficient cucumber plants were rapidly translocated through C. campestris to the N-replete cucumber plants. Unexpectedly, certain systemic signals were also transferred from the N-replete to N-depleted cucumber hosts. We demonstrate that these systemic signals are able to regulate large transcriptome and DNA methylome changes in the recipient hosts. Importantly, N stress also induced many long-distance mobile mRNA transfers between C. campestris and hosts, and the bilateral N-systemic signaling between N-replete and N-depleted hosts had a strong impact on the inter-plant mobile mRNAs. Our 15N labeling experiment indicated that under N-heterogeneous conditions, N-systemic signals from the N-deficient cucumber hosts did not obviously change the N-uptake activity of the N-replete cucumber hosts; however, in plant clusters comprising C. campestris-connected cucumber and soybean (Glycine max) plants, if the soybean plants were N-starved, the cucumber plants exhibited increased N-uptake activity. This study reveals that C. campestris facilitates plant-plant communications under N-stress conditions by enabling extensive bilateral N-systemic signaling between different hosts.

The bright side of parasitic plants: what are they good for?

Parasitic plants are mostly viewed as pests. This is caused by several species causing serious damage to agriculture and forestry. There is however much more to parasitic plants than presumed weeds. Many parasitic plans exert even positive effects on natural ecosystems and human society, which we review in this paper. Plant parasitism generally reduces the growth and fitness of the hosts. The network created by a parasitic plant attached to multiple host plant individuals may however trigger transferring systemic signals among these. Parasitic plants have repeatedly been documented to play the role of keystone species in the ecosystems. Harmful effects on community dominants, including invasive species, may facilitate species coexistence and thus increase biodiversity. Many parasitic plants enhance nutrient cycling and provide resources to other organisms like herbivores or pollinators, which contributes to facilitation cascades in the ecosystems. There is also a long tradition of human use of parasitic plants for medicinal and cultural purposes worldwide. Few species provide edible fruits. Several parasitic plants are even cultivated by agriculture/forestry for efficient harvesting of their products. Horticultural use of some parasitic plant species has also been considered. While providing multiple benefits, parasitic plants should always be used with care. In particular, parasitic plant species should not be cultivated outside their native geographical range to avoid the risk of their uncontrolled spread and the resulting damage to ecosystems.

Social networking in crop plants: Wired and wireless cross-plant communications

The plant-associated microbial community (microbiome) has an important role in plant-plant communications. Plants decipher their complex habitat situations by sensing the environmental stimuli and molecular patterns and associated with microbes, herbivores and dangers. Perception of these cues generates inter/intracellular signals that induce modifications of plant metabolism and physiology. Signals can also be transferred between plants via different mechanisms, which we classify as wired- and wireless communications. Wired communications involve direct signal transfers between plants mediated by mycorrhizal hyphae and parasitic plant stems. Wireless communications involve plant volatile emissions and root exudates elicited by microbes/insects, which enable inter-plant signalling without physical contact. These producer-plant signals induce microbiome adaptation in receiver plants via facilitative or competitive mechanisms. Receiver plants eavesdrop to anticipate responses to improve fitness against stresses. An emerging body of information in plant-plant communication can be leveraged to improve integrated crop management under field conditions.

Parasitism induces negative effects of physiological integration in a clonal plant

Clonal integration often increases fitness of clonal plants, but it may decrease it when some but not all connected plants (ramets) within a clone are parasitized. This hypothesis was synthesized in a conceptual model and tested by growing pairs of connected ramets of two congeneric clonal plants, Sphagneticola trilobata and Sphagneticola calendulacea, with and without parasitizing one ramet with Cuscuta australis and with and without severing the connection (allowing or preventing integration). Consistent with the model, integration in S. calendulacea did not affect biomass of the parasitized ramet, decreased biomass of its connected, unparasitized ramet by 60% and of the clone by 40%, and increased biomass of the parasite by 50%. By contrast, integration in S. trilobata did not affect biomass of the clone or the parasite. The parasite increased export of nitrogen-15 from the connected, unparasitized ramet seven-fold in S. calendulacea but did not affect export in S. trilobata. Parasitism can cause clonal integration to negatively affect fitness in clonal plants because parasites can import resources from connected, unparasitized ramets, possibly partly through signaling. This is the first experimental demonstration that clonal integration can decrease fitness in plants induced by parasitism and may help explain community-level effects of parasites.

Inter-plant communication via parasitic bridging

This article comments on:

Li S, Zhang J, Liu H, Liu N, Shen G, Zhuang H, Wu J. 2020. Dodder-transmitted mobile signals prime host plants for enhanced salt tolerance. Journal of Experimental Botany 71, 1171–1184.