- Introduction & ecology
- Photo gallery
- ... in Art, Medicine & Biology
- Phenotypic plasticity
- Flooding and adventitious roots
- Drought & herbivory
As indicated in the eXtreme mesophytes overview, not all plants with eXtremophyte characteristics have distributions limited to eXtreme ecosystems; local microhabitats can also qualify. On the other hand, the eXtreme characteristics, particularly those manifested at the cellular and subcellular levels, have not been detailed for most of these. Solanum dulcamara is an emerging exception.
S. dulcamara, or bittersweet, bittersweet nightshade, or climbing nightshade*, has a wide natural distribution across Europe, North Africa and East/Central Asia, and a broad naturalized distribution in central and northern North America. For some reason (or by some accident), it has also been introduced to New Zealand, Australia and Peru. It is found in a wide variety of temperate habitats from sea level to 2000 m. It is generally considered a weedy species with high phenotypic variability, and is sometimes considered as invasive but not particularly good at it. A very good introduction to the biology of the species in general, but focussed mainly on its presence in North America, can be found here. The Gallery (link to the left) includes a number of additional habitat photos.
Bittersweet’s occurrence in North America fairly well illustrates its adaptability to different habitats. Most frequently, it occurs in riparian areas, wetlands, and deciduous forests. On the other hand, it is also found sometimes in open habitats, e.g. grasslands. While it is seldom a major component of a community, it may be more significant in riparian areas. Consistent with its status as “introduced” and sometimes “invasive”, bittersweet is often associated with disturbed areas, especially due to human manipulations.
In Britain and Europe, bittersweet occurs mostly in floodplain habitats or marshes, fens, and other types of wetlands. Nevertheless, despite the general association with wetlands, bittersweet is also found on dunes where, instead of very wet conditions, it can experience persistent and significant drought.
The molecular analysis of bittersweet’s genetic structure and environmental responses began with the paper by Golas et al. There was a very practical reason for that investigation: as a known host for the potato pathogen Pseudomonas solanacearum, bittersweet has been linked to the epidemiology of potato late blight. For this reason, quite apart from its invasive and toxic potential, it has been sometimes targeted for local eradication. Golas et al. used AFLP to explore the genetic relationships between physically separated populations and the mechanism of gene flow between them.
Based on genetic analysis of 245 individuals from 79 populations, this study concluded that bittersweet is primarily, but not entirely, an outcrossing species. This was credited to the fact that this is a very common/widespread plant that can reproduce both sexually and vegetatively, and bird and water dispersal both promote gene flow between populations. Moreover it is perennial, enabling crosses between generations which would contribute to apparent uniformity.
At this point, we would be remiss if we did not point out that this species is a woody, rhizomatous perennial. While the woody stem base is persistent between years, the branches are herbaceous and die back each year. The underground stems, the rhizomes, are shallow and can sprout copiously. In most of the pre-21st century literature, it has been noted that adventitious roots arise from woody stems in contact with soil. However, the majority of the research studies considered below – clearly the reason for bittersweet being included on this site – have been done using seedlings. While seedling studies might be useful to answer some questions, they clearly can not explain everything bittersweet “does”. Just keep that in mind.
* A note on common names – perhaps because of its wide distribution and its unique secondary chemistry, S. dulcamara has a large number of common names, at least 25 in English, 2 in French, 7 in German. In Swedish, it can be besksöta (bittersweet), kvesved or vivang; in Finnish (Suomi), punakoiso (red eggplant); in Italian, dulcamara. Contact us if you can help expand this list.
Acknowledgement – This species page was written and produced by John Cheeseman and Maria-Cecilia Costa, the latter of whom suggested it based on its adventitious root characteristics and its broad tolerance of stressful environments, despite the fact that it is a widely distributed mesophyte. We would also like to thank Henrietta Kress for her list of common names of bittersweet in Swedish, Finnish and Italian as well as a photo of it growing over other plants (see the Gallery). Finally, thanks to Drs. Janny Peters and Titti Mariani (Radboud University) for additional photos, including those putting bittersweet into its ecological settings (see the Gallery).
Bittersweet has been a species commanding attention of artists, “medical” practitioners and plant scientists for more than 600 years. In their delightful article in their delightful series on Symbolism in Plants, Kandeler and Ulrich note the traditional origin of the name “bittersweet”, being that when various parts of the plant are chewed, it is first bitter and the sweet. The authors of this site have not verified this (our mothers cautioned us about putting things in our mouths). One German name, Bittersüße Nachtschatten reflects the same characteristics, but interestingly, one of the French names, Morelle douce-amère, puts it backward.
Nonetheless, assuming bitter comes first, a brave soul munching on the plant, if s/he is faithful enough, will be rewarded for their persistence. For that reason, in the language of flowers, bittersweet indicates fidelity, or sometimes, truth. Kandeler and Ulrich provided two fine illustrations of the appearance of bittersweet in art, one from the ceiling of St Thomas Church in Leipzig, dating from the late 15th century, and one from a tapestry from Strasbourg dating to the beginning of the 1500s. It’s a short article and free… go read it.
In the 17th century, in the astrological era of medicine, Nicholas Culpeper declared bittersweet to be under the planet Mercury. It was good to remove witchcraft both in men and beasts, and all sudden diseases whatsoever. In more “modern” times, it was considered a narcotic, diuretic, expectorant and depurative, with the minor side effect of potential central nervous system paralysis and death.
Beginning in the late 19th century, physiological, biochemical and morphological research using Solanum dulcamara began. Major initial studies, interestingly, used the species for the investigation of the origin and nature of adventitious roots; bittersweet was central to the discovery that adventitious roots were not, in fact, outgrowths of lenticels. [9,10]After a rather long period with very little attention, from the 1960s to mid-1990s the focus was mainly on secondary compounds, particularly glycoalkaloids and steroids. The most famous (but definitely not the only) of these is solanine, a substance mildly toxic to humans, livestock, dogs and various model species (e.g. mice and rabbits). A PubMed list of applicable references is here. While some reports have described the amount of toxin as not dangerous, other reports have attributed poisonous effects of its berries, especially in children. “Average” adult humans will reportedly experience mild symptoms after eating 10 or more unripe berries (which are more toxic than the pretty, red, ripe ones). A fatal dose would require about 200 berries. On the other hand, bittersweet provides an important fall and winter food source for birds, who quite happily eat the fruits and spread the seeds.
As noted earlier, the modern (i.e. molecular genetic) era of studies involving Solanum dulcamara began with the publication in 2010 of an analysis of its genetic structure by Golas et al. and the subsequent NGS sequencing of the transcriptome. It is “arguable” if for no other reason than that the authors of this page are not part of the bittersweet research community, and if you care to argue differently, feel free to contact us. The B’sweet Project at Radboud University in The Netherlands has been at the forefront of the recent resurgence of S. dulcamara research, producing a number of articles pertaining to the species’ ecology and its molecular characteristics that establish its eXtreme credentials. The present discussion will rely heavily on the work by that group.
Phenotypic plasticity and genetic variability
As noted earlier, Solanum dulcamara is widely distributed across a spectrum of habitats, from perpetually flooded to typically “mesophytic” to chronically droughted. In general, contrasting habitats can be expected to lead to local adaptation, i.e. directional selection producing genotypes particularly fit to their habitat. Indeed, in this species, this appears to have happened with respect to sun vs. shade adaptation.
Zhang et al. addressed the question, what underpins performance if a species has adapted to a very wide range of hydrological habitats, i.e. if it has a wide ecological amplitude? What enables such species to occupy contrasting niches at the far ends of gradients? In the case of Solanum dulcamara under contrasting hydrological gradients, is local adaptation still involved?
Based on common garden studies, what they found was that while there was genetic differentiation between flooded and droughted populations, locally adapted populations had not evolved. Instead, the species showed high levels of adaptive phenotypic plasticity, i.e. all of the tested genotypes could produce multiple phenotypes morphologically and physiologically adapted to multiple local environments. As Dawood et al. say it, “Plasticity, or environmental responsiveness, is a universal property of life needed to optimize fitness under local circumstances”. With respect to traits increasing flooding and drought tolerance, for example, large leaves, enhanced shoot elongation, shallow root systems, adventitious root development and aerenchyma formation were found in all flooded plants, regardless of seed source. In contrast, deep rooting, large root systems, small leaves, strong stomatal regulation and high water-use efficiency were found with drought, again regardless of seed source.
At both the genome and transcriptome levels, this clearly raises a number of interesting questions and testable hypotheses concerning the genetic and physiological/developmental bases for life at the hydrological extremes. To date, these have only been partially explored with respect to herbivory and pathogen responses, and particularly with respect to flooding.
Adventitious roots ameliorate flooding stress
Flooding is one of the more agronomically significant abiotic stresses experienced by terrestrial plants, and tolerant species, including Solanum dulcamara, have evolved various physiological and developmental solutions to the problem. At the gross morphological level, bittersweet’s adaptations include large leaves, enhanced shoot elongation, shallow root systems, adventitious root (AR) development and aerenchyma formation. The adventitious root feature is particularly interesting and significant for plants, especially seedlings, which developed in seasonally dry floodplains or upland farm fields.
With flooding, the diffusion of oxygen to roots is seriously curtailed; anoxic conditions develop within hours. Deep roots, or roots without extensive aerenchyma, are rapidly and adversely affected by this. These need to be replaced rapidly by shallow, better adapted, adventitious roots. In plants where de novo initiation of root primordia is required, adventitious roots basically develop too slowly to allow plants to adjust without extensive metabolic damage.
In natural floodplains, bittersweet’s main stem and branches carry an important adaptation to flooded soil, i.e. pre-formed AR primordia. They are formed even in the dry conditions, as in sand dunes, and stay dormant until stimulated to grow out by flooding. Then they develop rapidly. Within hours rather than days, aerenchyma connections link root apices with the shoot.
Rice has a very similar strategy, where preformed primordia are the source of nodal crown roots that grow out during flooding. Other species, such as sunflower and tomato form adventitious root primordia de novo upon flooding and subsequently continue their development into roots. Clearly, this makes their adjustment considerably slower.
Not all bittersweet plants are flood-ready, however. In particular, very young plants lack AR primordia. Zhang et al., for example reported that juveniles were much less capable of, and much slower in responding to shallow flooding with AR formation. Complete submergence of seedlings suppressed AR formation until even 2 weeks after the shoots had broken the surface again. AR formation is thus an escape strategy whose availability changes with development and severity of flooding.
In bittersweet, upon flooding, changes in gene expression happen quickly, within 2h. After 6h, a metabolic adjustment to flooding stress via ethylene signaling is induced. And by 24h, the abundance of ca. 15% of all detected transcripts is modulated even though AR growth is not yet visible. Both cell division and elongation of primordial cells is well established and by 48h, root cell types start to differentiate (delineating a vascular cylinder and forming aerenchyma) and the cortex of the surrounding stem ruptures, giving way to an elongating root tip. Three days after flooding, a fully differentiated short root emerges from the stem.
About a quarter of all observed flooding-induced transcriptomic changes occur in both the root primordia and stem. For example, genes involved in hypoxia acclimation accumulate transcripts in both tissues. In contrast, the regulation of genes involved in cell division and cell growth occurs differentially.
A number of steps in this process involving various plant hormones have now been elucidated. During flooding, the ethylene concentration in the submerged stem parts increases and ethylene signaling is necessary and sufficient for emergence of adventitious roots. Subsequently, a drop in abscisic acid (ABA) level occurs. Exogenous ABA treatment however, prevents flooding related primordia activation. And conversely, inhibition of ABA synthesis is sufficient to activate them in absence of flooding. Although auxin concentration does not differ between complete and partial submerged stems, proper auxin biosynthesis, polar transport, perception and signaling are all required for normal adventitious root formation; application of auxin doesn’t substitute.
Although emergence of pre-formed AR in flooded rice also involves ethylene and ABA responses, there are important differences between the two species.. In rice, for example, ethylene accumulation induces H2O2 formation, leading in turn to cell death in the epidermal layer that covers the AR primordia. Increasing ethylene production with ethephon increased the number of AR emerging. In bittersweet, in contrast, the AR are located much deeper in the root, covered by several cortical cell layers and the epidermis already has cracks in these areas (surrounded by dead cells) before flooding happens. No ROS/cell death response is needed.
Other stress responses
Several recent studies have concentrated on aspects of response to herbivores, as bittersweet, a nightshade (Solanaceae), has also long been recognized as one of the alternative hosts for pathogens and pests responsible for some important diseases in tuberous nightshade (a.k.a potato). As might be expected, there is some overlap between studies of herbivory and studies of alkaloids (e.g. with respect to slug resistance). Similarly, there is overlap between studies of response mechanisms to herbivory and to abiotic stresses such as drought.
As would be expected, of course, the molecular and physiological responses to flooding and to other stresses differ. Clearly, the adjustments a plant needs to respond to flooding are considerably different than those required to withstand drought. On the other hand, it is interesting that drought and herbivory induced many similar biological processes, ones that were repressed by flooding.
Also as would be expected, the responses to herbivores and their relationship to drought responses are determined by the species of herbivore and complex plant metabolic responses to drought. Nguyen et al. evaluated this using a generalist herbivore, Spodoptera exigua (beet armyworm), and a specialist, Leptinotarsa decemlineata (Colorado potato beetle). Performance of the generalist, but not the specialist, was adversely affected by drought. Feeding by the specialist induced jasmonic acid and salicylic acid accumulation and ethylene emissions, and these were enhanced by drought. Generalist feeding combined with drought, on the other hand, showed enhanced cell-wall remodeling and altered metabolism of carbohydrates, lipids, and secondary metabolites. Specialist herbivory combined with drought showed a variety of enhanced photosynthesis-related and pathogen responses. Overall, the group concluded that while the army worm was adversely affected by drought-enhanced defenses, the potato beetle benefited from responses to enhanced SA and reduced ET signalling, and that its interactions were more finely tuned and sustained under drought.
Other studies involve using bittersweet nightshade to study the chemical and genetic processes behind resistance to slug feeding and examining nightshade resistance to the dreaded late potato blight, Phytophthora infestans.
With respect to genomic and transcriptomic sequencing studies, there are two main projects. The first, in 2013, by the B’sweet project at Radboud University, was a next generation sequencing of the transcriptome using a combination of 454 GS FLX and Illumina HiSeq 2000 techniques. The associated files can be downloaded either from the European Nucleotide Archive or NCBI.
A reference genome project is also registered at NCBI since 2017 although no results have been submitted (as of February 2019). The link below will report results as they come in.
The following links will lead to the most current listings for Solanum dulcamara sequencing at NCBI:
The following is a partial list of publications on Solanum dulcamara focussed on studies emphasizing a molecular aspect. The list was generated by a search of PubMed; additional publications not indexed there are, of course, not included. Abstracts and/or full text versions can be accessed by clicking doi, PMID or PMC numbers.
- Calf, OW, Lortzing, T, Weinhold, A, Poeschl, Y, Peters, JL, Huber, H et al. (2020) Slug Feeding Triggers Dynamic Metabolomic and Transcriptomic Responses Leading to Induced Resistance in Solanum dulcamara. Front Plant Sci 11:803. doi: 10.3389/fpls.2020.00803. PubMed PMID:32625224 PubMed Central PMC7314995.
- Geuss, D, Lortzing, T, Schwachtje, J, Kopka, J, Steppuhn, A (2018) Oviposition by Spodoptera exigua on Solanum dulcamara Alters the Plant's Response to Herbivory and Impairs Larval Performance. Int J Mol Sci 19:. doi: 10.3390/ijms19124008. PubMed PMID:30545097 PubMed Central PMC6321313.
- Amiryousefi, A, Hyvönen, J, Poczai, P (2018) The chloroplast genome sequence of bittersweet (Solanum dulcamara): Plastid genome structure evolution in Solanaceae. PLoS ONE 13:e0196069. doi: 10.1371/journal.pone.0196069. PubMed PMID:29694416 PubMed Central PMC5919006.
- Dawood, T, Yang, X, Visser, EJ, Te Beek, TA, Kensche, PR, Cristescu, SM et al. (2016) A Co-Opted Hormonal Cascade Activates Dormant Adventitious Root Primordia upon Flooding in Solanum dulcamara. Plant Physiol. 170:2351-64. doi: 10.1104/pp.15.00773. PubMed PMID:26850278 PubMed Central PMC4825138.
- Perry, KL, McLane, H (2011) Potato virus M in Bittersweet Nightshade (Solanum dulcamara) in New York State. Plant Dis. 95:619. doi: 10.1094/PDIS-10-10-0768. PubMed PMID:30731971 .
- Golas, TM, Sikkema, A, Gros, J, Feron, RM, van den Berg, RG, van der Weerden, GM et al. (2010) Identification of a resistance gene Rpi-dlc1 to Phytophthora infestans in European accessions of Solanum dulcamara. Theor. Appl. Genet. 120:797-808. doi: 10.1007/s00122-009-1202-3. PubMed PMID:19936699 PubMed Central PMC2812418.