1. Introduction
There is great interest worldwide in the pharmacological compounds of the various
Rhodiola species [
1,
2], the extracts of which have been used for centuries in traditional European and Chinese medicine to increase stamina, reduce the effects of ageing, and treat a variety of diseases [
3,
4,
5,
6]. With the implementation of the Nagoya Protocol in 2021, the search for new domestically viable medicinal plant resources has become particularly important for each country [
7]. Due to habitat decline and overexploitation to satisfy the growing commercial demand, the
Rhodiola rosea L. is now an endangered species and included in the Red List of protected plant species in many countries throughout the world [
8]. As a result, a complete investigation of other members of this family is required, in terms of both their biological and pharmacological activity, as well as the numerous methods of adaptation to abiotic stress factors.
Representatives of the Crassulaceae family’s genus
Rhodiola have life cycles driven by similar factors: substantial temperature changes during the day, intense solar radiation, summer snowfalls and vice versa, soil denudation in winter, and deep freezing [
9]. These species’ capacity to adapt make them of special importance in today’s rapidly changing climate.
Despite the well-established significance of
Rhodiola species, our understanding of their distribution, valuable secondary metabolites, adaptive mechanisms, and evolutionary relationships is still limited.
Rhodiola was first distinguished from Sedum by Linnaeus in 1753 due to its dioecious reproductive system. However, further research is needed to fully comprehend the genus and its characteristics [
2].
Rhodiola species are recognized as forming a single phylogenetic group [
8]. At the same time, Ohba classified four
Rhodiola subgenera of the genus Sedum (
Crassulaceae) based on flower features (unisexual or bisexual), basal leaves, and inflorescences, i.e., subgenera of
Rhodiola,
Primuloides, Crassipedes, and
Clementias, including
R. semenovii (Regel and Herder) Boriss [
9,
10].
Rhodiola differs from other subgenera in being dioecious [
8]. Clementsia is a hermaphroditic subgenus [
11]. Ohba, on the other hand, believed the
Rhodiola algida to be part of the subgenus
Rhodiola, which is distinguished by dioecious, unisexual flowers [
12], whereas the flowers in representatives of sect.
Algida are generally hermaphroditic [
13]. The convergent development of dioeciousness in
Rhodiola, on the other hand, shows that dioeciousness should be utilized with caution in the genus’s intrageneric classification [
13]. There is speculation that the explanation for the evolution of dioecious
Rhodiola may lie in abiotic pollination, which leads to increased seed or pollen production and is most functional in high mountain regions [
14].
Previously, we considered certain physiological and phytochemical aspects of the adaptation of individual representatives of the
Crassulaceae family under laboratory conditions of weak and moderate abiotic stresses, such as drought, salinity, and low positive temperatures, which are not lethal for succulents but activate their adaptation mechanisms and stimulate the formation of biologically active secondary antioxidant metabolites [
15,
16], as well as their adaptive mechanisms in the dynamics of vegetation in situ [
17].
The approach related to sexual differentiation in plants is interesting for studying the mechanisms of adaptation to abiotic stressors [
18], but currently, little is known about the processes of plant adaptation and responses to environmental stressors in species belonging to the same genus but differing in mode of reproduction [
19].
Many features of development and adaptation have phytochemical expression, but all changes in the cell and the entire organism are regulated by the structure and function of the genome, its genes, and epigenetic alterations [
20]. Since most indications of stress tolerance in plants are polygenic, it is difficult to interpret them using solely physiological and biochemical techniques. The tolerance of a plant population of abiotic stimuli is primarily determined by the degree of genetic variation in the key genes that govern plant physiological functions [
21]. Thus, one of the most significant tools for understanding genetic processes in wild populations is the investigation of molecular genetic polymorphism, which includes hidden genetic variability in both the coding and noncoding parts of the genome. The study of several protein families and isoenzymes involved in numerous metabolic pathways, as well as research into the involvement of phytohormones in the control of growth, blooming, and daily cycles in plants, is of special interest.
Proteins of the superoxide dismutase (SOD) family, which are found in all plant species and are distinguished by tissue specificity and the ability of these genes to be activated in response to abiotic stresses, are recognized to be key antioxidant enzymes. The auxin response factor (ARF) genes play a crucial role in auxin (one of the most significant groups of phytohormones) signaling and are implicated in plant responses to growth, development, and stress [
22,
23]. The response to abiotic stresses and the activity of ARF genes can be linked in succulent plants of the
Crassulaceae family [
24].
Apart from genes coding for isoenzymes, the noncoding component of the genome is of special importance, where eukaryotes have abundant transposable or mobile elements that can replicate parasitically in the host genome [
25,
26]. These multiple transposable elements play a vital role in all eukaryotic genomes, including the plant genome, and are the primary source of epigenetic alterations under stressful circumstances, as well as probably being the primary cause of species evolution. Studies of the noncoding part of the genome can reveal particular signs of hidden genetic diversity and, more broadly, indicators of the genetic potential and evolutionary changes happening in a specific species [
26,
27,
28,
29]. Mobile elements are important in the distribution of cis-regulatory elements, which contribute to genetic control in both the short term (adaptation to environmental changes) and the long term (evolutionary changes) [
26,
30,
31].
PCR methods for the identification of hidden genetic variation, such as a system of genome-profiling molecular markers, were created based on sequences for multiple families of complex interspersed genomic repeats [
32]. These genome-profiling PCR techniques for investigating genetic variation in eukaryotes, using a multicopy and genomic abundance of transposable elements and endogenous viruses, can extend knowledge of phylogenetic relationships and estimate the genetic diversity of particular species [
33,
34].
Interspersed repeats-based genome-profiling applications offer a simple and cost-effective PCR method to study individual genetic polymorphisms [
33,
35]. This genome profiling is based on the fact that transposable elements (in particular, long terminal tepeat (LTR) retrotransposons) are distributed throughout the genome and are involved in recombination events that occur during meiosis. PCR with a single primer corresponding to conserved sequences in LTR retrotransposons is one of the most prominent examples of interspersed repeats-based genome-profiling applications. Specifically, PCR methods based on the detection of transposable element insertion site polymorphisms include inter-retrotransposon amplified polymorphism (IRAP) and retrotransposon–microsatellite amplified polymorphism (REMAP) [
33]. The inter-primer binding site (iPBS) amplification technique also proved to be a powerful genome fingerprinting method that does not require information about retrotransposon sequences [
36]. Furthermore, new enhanced methods for high-throughput targeted gene characterization and transposon display were added to current methodologies, which may be modified to adopt high-throughput sequencing technologies, among other things. For example, palindromic sequence-targeted (PST) PCR utilizes a pair of primers, one of which is complementary to 6 bp long palindromic sequences (PST sites) and the other of which conserves transposable element sequences [
37,
38]. The PST-PCR technique allows genome walking and profiling that can be used in initial descriptions of intraspecific and interspecies genetic variability and to track lines and genotypes [
37].
The study of the genetic diversity of the gene families of SOD and ARF in Rhodiola species differing in the mode of reproduction in conjunction with genetic analysis based on genome-profiling approaches can be informative from a variety of perspectives, including evolutionary. As a result, the purpose of this work was to look at the genetic variation in the context of resilience against abiotic stressors in several Rhodiola species with different sexual systems.
3. Discussion
The study of molecular mechanisms of plant adaptation to abiotic stress factors is a timely topic in the context of global climate change [
41]. Plants that grow in adverse or unstable climatic conditions, such as
Rhodiola species studied in our work in high mountains, have accumulated unique combinations of latent variability as well as allelic variations of genes that affect the ability to tolerate environmental stress factors with which they are met during the growth season. The species’ adaptive traits have repeatedly evolved under strong selection pressure [
45,
46]. It is well understood that intraspecific genetic variability is the most important factor that ensures both the survival of species under adverse environmental conditions and the exploitation of their reproductive potential, allowing them to successfully resist natural selection pressures at the population level, as it is an indicator of a species’ potential ability to expand its range [
47]. Species studied exhibit a high percentage of intrapopulation polymorphism compared to others, making them an important source of genetic diversity.
The mechanism of action of proteins encoding superoxide dismutase genes comprises the successive reduction and oxidation of the metal of the active center of the enzyme by superoxide anion radicals, avoiding the conversion of the superoxide anion radical into a harmful hydroxyl radical and therefore having a favorable effect on plant growth and development and their response to abiotic stress [
42,
48,
49]. At the same time, the regulatory regions, intron sequences, and coding portions of the SOD genes had the most variability. A complicated polymorphism was discovered in the intron region of beta-amylase genes, presenting itself as prolonged insertions and deletions that can be identified by EPIC-PCR profiling [
39].
The high amount of genetic polymorphism reported in this study implies the presence of several allelic forms of the SOD genes, which may be connected with the ability to adjust to unfavorable environmental circumstances. Based on the results, it can be expected that genetic variation of the SOD loci can contribute to the increase in the adaptive capacity of plants of the examined species without the creation of new forms. It can also be assumed that the examined Rhodiola species lacked uncommon alleles in the SOD gene families at the time of the experiment, which could lead to a dramatic decrease in their numbers.
We observed that the genetic polymorphism of the SOD genes is conserved without the creation of new morphophysiological adaptations or the production of new forms, but rather by enhancing the adaptive capacity of existing populations. This might potentially point to a recent fall in population size, implying that only genotypes descended from forms that survived an unfavorable time remained. Furthermore, because the impact of the geographical factors on the distribution of the examined species in the coordinate system was not noticed, their distribution may be related to both physiological traits and sexuality systems.
Auxin, like all the major phytohormones found in higher plants, functions as a mediator for cellular activity transmission, coordinating numerous signal transduction pathways during a stress response and regulating the effect of both external and internal stimuli [
50,
51]. Genes involved in diverse auxin-associated pathways express differentially under different environmental pressures, indicating the importance of auxin under various types of abiotic stresses [
22,
23]. ARF genes may be the components that impart specificity to the auxin response via target gene selection as transcription factors [
24]. ARF gene promoters include a substantial number of cis-acting regions related to abiotic stress, indicating that ARF genes are engaged in stress protection [
52,
53]. Presumably, the ARF family of genes can provide plants from
Rhodiola species with the ability to adapt to adverse climatic conditions such as a substantial temperature amplitude during the day and excessive solar radiation [
22]. According to our results, there is a significant amount of genetic variation in ARF genes in studied
Rhodiola species representatives. In contrast to the examination of polymorphism in SOD genes [
48], it can be stated in this situation that the appearance of novel polymorphic forms within populations ensures the successful existence of populations, which explains the relatively high level of the Shannon index (
I), as well as the effective
Ne alleles number.
Both SOD family genes and ARF genes had a great variety of EPIC-PCR amplicons in all Rhodiola species studied. This indicates the latent diversity linked with adaptation processes that exist in all the images studied for Rhodiola species, allowing them to survive under demanding conditions. It may be inferred that the analyzed populations of samples for Rhodiola species, like related succulent plants, respond mainly similarly to stress. The high intraspecific variability values for the examined gene families might be attributed to species’ geographic isolation, which is common in many natural species, including those living in mountain environments.
Most plants’ dioecious reproduction mechanisms permit them to avoid the harmful effects of inbreeding, but under stressful situations (for example, in highlands where pollinating insects are few), their reproductive capacity can be considerably diminished [
53]. Numerous studies suggest that the development of sexual differentiation under the impact of stressful situations is primarily due to conflict between the functions of selecting the reproductive strategy and the plant organism’s defensive measures under conditions of limited resources [
18,
19,
54]. The development of species based on syngenesis is a potential adaptive approach for maintaining individual viability under adverse situations.
The activation of mobile elements and endogenous viruses, particularly LTR retrotransposons, is a critical factor in the creation of latent variability and the origin of genetic polymorphism linked with various sections of the genome [
26,
55]. Endogenous viruses and LTR retrotransposons play a unique role in the instability and evolution of the genome, as well as the body’s adaptability to aberrant situations [
55]. They make up a considerable portion of the genome, include cis-regulatory regions comparable to protective transcription factors, and can regulate nearby genes by adjusting their expression in response to stress [
56]. Some LTR retrotransposons are probably deployed directly into the genes that are most susceptible to environmental changes as a result of evolutionary adaptations [
57].
In general, our data revealed significant variations in latent genomic variability for iPBS profiles amongst Rhodiola species, possibly due to evolutionary distinctiveness and the modest effect of genetic drift. At the same time, intrapopulation variability accounted for 74% of genetic variation.
The results of the iPBS profiling analysis for the studied samples of Rhodiola species revealed the level of genetic differentiation of the studied species’ populations and demonstrated the lowest average number of alleles per locus in R. algida and R. rosea, demonstrating for the same species low values of all indicators of genetic diversity. At the same time, the values of Ne, a marker of a species’ genetic variety, surpassed these indicators in all species, suggesting that these plants possess a reservoir of latent variability in the noncoding region of the genome, allowing it to respond to stresses that produce major genetic changes throughout evolution. According to the results of the iPBS study, the hermaphrodite species R. semenovii and R. algida are placed independently from other species. In the future, it will undoubtedly be important to examine the control of mobile elements and endogenous viruses, as well as their impact on the adaptive genome of succulent plants, including representatives of Rhodiola species. These succulent plants are one-of-a-kind not only phenotypically but also genetically, and they serve as distinctive representatives of the plant world for acquiring better knowledge of plant adaptation to and tolerance of abiotic conditions.