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Citation: Legardón, A.;
García-Plazaola, J.I. Gesneriads, a
Source of Resurrection and
Double-Tolerant Species: Proposal of
New Desiccation- and
Freezing-Tolerant Plants and Their
Physiological Adaptations. Biology
2023,12, 107. https://doi.org/
10.3390/biology12010107
Received: 29 November 2022
Revised: 5 January 2023
Accepted: 6 January 2023
Published: 10 January 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
biology
Review
Gesneriads, a Source of Resurrection and Double-Tolerant
Species: Proposal of New Desiccation- and Freezing-Tolerant
Plants and Their Physiological Adaptations
Ane Legardón * and JoséIgnacio García-Plazaola
Department of Plant Biology and Ecology, University of the Basque Country (UPV/EHU), Barrio Sarriena s/n,
48940 Leioa, Spain
*Correspondence: anelegar.18@gmail.com
Simple Summary:
In the current scenario of climate change, plants need to overcome a great
amplitude of temperatures and increasingly common droughts in the very same space. Gesneriads
are a worldwide family of plants in which many “resurrection” species have arisen: plants with the
ability to withstand long periods of time with very little water content and successfully revive upon
water availability. Due to their rocky and mountainous habitat, many of them have to face a great
temperature variability and freezing temperatures, and indeed, their resurrection ability and freezing
tolerance share similar metabolic responses. Tolerance of gesneriads to different environmental
stresses is thought to be applicable in crop growth improvement, but their difficult indoor cultivation
and outdoor accessibility are major obstacles for their study. Therefore, this review aims to identify
common patterns in the already known resurrection species to propose new tentative resurrection
gesneriads, as well as gather the metabolic responses to desiccation and freezing stress as a way of
making them more reachable to the scientific community.
Abstract:
Gesneriaceae is a pantropical family of plants that, thanks to their lithophytic and epiphytic
growth forms, have developed different strategies for overcoming water scarcity. Desiccation toler-
ance or “resurrection” ability is one of them: a rare phenomenon among angiosperms that involves
surviving with very little relative water content in their tissues until water is again available. Physio-
logical responses of desiccation tolerance are also activated during freezing temperatures, a stress that
many of the resurrection gesneriads suffer due to their mountainous habitat. Therefore, research on
desiccation- and freezing-tolerant gesneriads is a great opportunity for crop improvement, and some
of them have become reference resurrection angiosperms (Dorcoceras hygrometrica, Haberlea rhodopensis
and Ramonda myconi). However, their difficult indoor cultivation and outdoor accessibility are major
obstacles for their study. Therefore, this review aims to identify phylogenetic, geoclimatic, habitat,
and morphological features in order to propose new tentative resurrection gesneriads as a way of
making them more reachable to the scientific community. Additionally, shared and species-specific
physiological responses to desiccation and freezing stress have been gathered as a stress response
metabolic basis of the family.
Keywords:
resurrection species; desiccation tolerance; freezing stress; Gesneriaceae; oxidative stress;
structural damage
1. Introduction
Gesneriads are a family of pantropical middle-sized plants comprising about 150
genera of perennial herbs, shrubs, and small trees. This family has been reported to include
over 3400 species, but the number grows everyday thanks to phylogenetic studies [
1
]. The
richness and dimension of the family have enabled “resurrection” or desiccation-tolerant
(DT) plants to thrive. This characteristic relies on the plant’s ability to withstand very low
Biology 2023,12, 107. https://doi.org/10.3390/biology12010107 https://www.mdpi.com/journal/biology
Biology 2023,12, 107 2 of 21
water contents in their tissues (~10% relative water content (RWC)) and fully recover upon
re-watering, a very rare phenomenon among angiosperms, with less than 0.1% of them
being DT [2].
Most DT gesneriads are relict species that have been sheltered in mountainous habitats,
usually in northern-oriented limestone areas, which means that they have had to overcome
drought and cold stress, which forced double adaptation [
3
]. In fact, tissue desiccation
is the consequence of both drought and freezing stresses; therefore, similar physiological
mechanisms are triggered in response to both, to the point that desiccation tolerance could
entail some kind of cross-tolerance to freezing stress. Moreover, the cross-tolerance that
resurrection characteristics confer has been postulated to have biotechnological applications
in crop growth [4].
The geographical distribution and diversification of gesneriads are exceptionally
advantageous in terms of finding and studying new double-tolerant species, but their
scattered and difficult-to-reach habitats present an accessibility obstacle. Additionally,
their slow growth and complex environmental conditions are disadvantageous for indoor
growth. Consequently, most of the few studies on resurrection gesneriads have been
performed within the native area of the species [
5
]. Identifying common phylogenetic,
geoclimatic, habitat, and morphological characteristics of the already known resurrection
gesneriads should help in finding new species in more convenient locations and widen the
knowledge about their double tolerance.
2. Species Identification
2.1. Phylogeny
The Gesneriaceae family has colonized a great diversity of habitats and developed
specialized plant–animal interactions that have led to a great diversity of floral morpholo-
gies as well (Figure 1). These traits were first used for phylogenetic species classification,
but because they highly converged in different Gesneriaceae lineages, the early taxonomy
of the family was complex and contradictory [6–8].
Biology 2023, 12, x FOR PEER REVIEW 2 of 22
tolerant (DT) plants to thrive. This characteristic relies on the plant’s ability to withstand
very low water contents in their tissues (~10% relative water content (RWC)) and fully
recover upon re-watering, a very rare phenomenon among angiosperms, with less than
0.1% of them being DT [2].
Most DT gesneriads are relict species that have been sheltered in mountainous habi-
tats, usually in northern-oriented limestone areas, which means that they have had to
overcome drought and cold stress, which forced double adaptation [3]. In fact, tissue des-
iccation is the consequence of both drought and freezing stresses; therefore, similar phys-
iological mechanisms are triggered in response to both, to the point that desiccation toler-
ance could entail some kind of cross-tolerance to freezing stress. Moreover, the cross-tol-
erance that resurrection characteristics confer has been postulated to have biotechnologi-
cal applications in crop growth [4].
The geographical distribution and diversification of gesneriads are exceptionally ad-
vantageous in terms of finding and studying new double-tolerant species, but their scat-
tered and difficult-to-reach habitats present an accessibility obstacle. Additionally, their
slow growth and complex environmental conditions are disadvantageous for indoor
growth. Consequently, most of the few studies on resurrection gesneriads have been per-
formed within the native area of the species [5]. Identifying common phylogenetic, geo-
climatic, habitat, and morphological characteristics of the already known resurrection ges-
neriads should help in finding new species in more convenient locations and widen the
knowledge about their double tolerance.
2. Species Identification
2.1. Phylogeny
The Gesneriaceae family has colonized a great diversity of habitats and developed
specialized plant–animal interactions that have led to a great diversity of floral morphol-
ogies as well (Figure 1). These traits were first used for phylogenetic species classification,
but because they highly converged in different Gesneriaceae lineages, the early taxonomy
of the family was complex and contradictory [6–8].
Figure 1. Examples of plants species from Gesnerioideae (A–D) and Didymocarpoideae (E–H):
Achimenes sp. (A), Alloplectus sp. (B), Asteranthera ovata (C), Episcia sp. (D), Streptocarpus spp. (E,F),
Ramonda myconi (G), and Haberlea rhodopensis (H).
In recent years, the family has undergone a deep reconstruction due to advancements
in molecular-phylogenetic studies, which concluded with a new basis for Gesneriaceae
phylogeny in 2013 [1]. Nevertheless, there are limited genetic sites that provide
Figure 1.
Examples of plants species from Gesnerioideae (
A
–
D
) and Didymocarpoideae (
E
–
H
): Achimenes
sp. (
A
), Alloplectus sp. (
B
), Asteranthera ovata (
C
), Episcia sp. (
D
), Streptocarpus spp. (
E
,
F
), Ramonda myconi
(G), and Haberlea rhodopensis (H).
In recent years, the family has undergone a deep reconstruction due to advancements
in molecular-phylogenetic studies, which concluded with a new basis for Gesneriaceae
phylogeny in 2013 [
1
]. Nevertheless, there are limited genetic sites that provide information
Biology 2023,12, 107 3 of 21
for phylogenetic characterization of the family; therefore, phylogenetic relationships in
some genera remain unclear, or there is no consensus regarding them [9].
Nowadays, the Gesneriaceae family contains three subfamilies:
•
Sanangoideae, which did not officially become a subfamily until 2013, when it was
defined as a monotypic family (Sanango racemosum) endemic to South America. It has
a distinctive globose and slightly four-partite ovary with a depression in the internal
structure and on top, from which the style arises surrounded by a large cupular
disc [1].
•
Gesnerioideae, which was initially named New World (NW) Gesneriaceae, since it
was thought to contain only neotropical species. However, nowadays it also contains
gesneriads from Asia and Australia, and it is considered a heterogeneous group with
a well-established phylogeny [
1
,
10
]. It is generally characterized by the presence of
seed endosperm, two equally sized cotyledons with limited growth, a nectary with
separated glands, and an inferior ovary [11].
•
Didymocarpoideae, which has been typically recognized as Old World (OW) Gesne-
riaceae; in this case, with the exception of Rhynchoglossum azureum, species of this
subfamily are indeed found in Asia, Africa, and Europe [
12
,
13
]. The intrinsic mor-
phological characteristics of the subfamily include the lack of endosperm, unequal
cotyledon growth, ring-shaped nectary, and superior ovary [11].
Even though prior phylogenetic classification established nine different tribes for
Gesnerioideae, nowadays it only contains five: Gesnerieae, Titanotricheae, Napeantheae,
Beslerieae, and Coronanthereae. Gesnerieae is the “core tribe” that has included the four
obsolete tribes and has become the largest one. Current classification is strongly supported
by molecular data, and despite the shift in tribe number, Gesnerioideae phylogeny has
remained quite stable throughout the years [1].
Didymocarpoideae is divided into two tribes: Epithemateae and Trichosporeae. Species
in Epithemateae are thought to be relicts from an enormously diversified group that suf-
fered a number of extinctions, which resulted in a few isolated genera with unusual and
marked morphological characteristics. Trichosporeae has typically been the larger and
more complex tribe, giving rise to uneven large groups. This has entailed a deep recon-
struction of the subfamily that is still in process, allowing more homogeneous species
classification [
1
,
13
–
15
]. Some genera have been newly established (Billolivia,Michaelmoel-
leria,Chayamaritia,Glabrella,Microchirita,Middletonia,Rachunia,Somrania) or recovered
(Dorcoceras,Loxocarpus), while others have gained species from other genera (Damrongia,
Oreocharis,Loxostigma,Deinostigma,Paraboea,Primulina,Streptocarpus),lost species by relo-
cation to other genera (Boea), or lost species by synonymization (Acanthonema,Hovanella,
Colpogyne,Linnaeopsis,Nodonema,Schizoboea,Briggsia) [13,16–28].
All documented DT gesneriads are members of the Trichosporeae tribe; thus, efforts
should be focused on untangling its phylogeny. Known resurrection species are Dorcoceras
hygrometricum,Damrongia clarkeana,Boea hygroscopica, Boea resupinata,Paraboea crassifolia,
Paraboea rufescens, Paraboea neurophylla, Streptocarpus revivescens,Jancaea heldreichii,Ramonda
serbica,Ramonda nathaliae,Ramonda myconi,Haberlea rhodopensis,Oreocharis billburttii,Ore-
ocharis primuloides, and Oreocharis mileensis [
3
,
29
–
39
]. Even if Henckelia and Corallodiscus
still have no known resurrection species, it has been proposed that they potentially contain
them [
1
]. When it comes to freezing tolerance, Ramonda myconi and Haberlea rhodopensis
are the only resurrection gesneriads that have been tested for freezing stress, but due to
geographic, habitat, and morphologic similarities, many of them may indeed be double-
tolerant [
40
,
41
]. On top of that, the homogeneity of the latest phylogenetic classification
implies that Trichosporeae genera already containing DT species could be sources of new
resurrection plants (Figure 2).
Biology 2023,12, 107 4 of 21
Biology 2023, 12, x FOR PEER REVIEW 4 of 22
Figure 2.
Current phylogenetic classification of Didymocarpoideae with resurrection (grey squares)
and double-tolerant (blue squares) species.
Biology 2023,12, 107 5 of 21
2.2. Geographic Distribution and Habitat
2.2.1. Origin and Geographic Evolution of Gesneriads
The most supported hypothesis regarding the origin and evolution of gesneriads
proposes that ancestors of Gesneriaceae probably originated in the neotropics in South
America during the Late Cretaceous, with possible points of origin in the temperate Andes
and Amazonian rainforest. Indeed, the disjunctive geographic distribution of the Coronan-
thereae tribe on the Pacific coasts of South America, Australia, New Zealand, and some
islands in between and its current placement in the Gesnerioideae subfamily imply a NW
origin of Gesneriaceae and posterior dispersal to the OW. The split between Didymocar-
poideae and Gesnerioideae dates from 44.7 Mya in the Late Palaeocene/Early Eocene and
would have occurred though multiple independent long-distance oceanic dispersals from
South America to the Indian Plate via Antarctica until 45 Mya, when it collided with the
Eurasian Plate [
42
–
44
]. The basal taxa of the OW are considered to be Jerdonia (mountains
of southwest India), Corallodiscus (Himalayas and China), and Tetraphyllum, Leptoboea,
and Boeica (Himalayas and adjacent areas) [8,10].
Dispersal to Europe apparently happened quite rapidly after the collision between
the two tectonic plates [
10
]. Quaternary glaciations led to changes in the elevation and
latitudinal distribution of species, and three glacial refugia were defined in the Iberian,
Italian, and Balkan Peninsulas [
45
,
46
]. Other hypotheses suggest that such species already
existed in the refugia in the Tertiary, prior to the Quaternary oscillations [
47
]. However,
such hypotheses would need phylogenetic clarification regarding the age estimation of the
subtribe [
45
]. Independent of their origin, just five species (all of them DT) inhabit Europe:
four in the Balkan Peninsula (Jancaea heldreichii, Ramonda serbica, Ramonda, nathaliae,
and Haberlea rhodopensis) and a single species in the Pyrenees (Ramonda myconi) [
48
].
Transgression of the species to Africa from Eurasia followed that to Europe, and they could
have first colonized Madagascar, from where individuals were dropped to Africa via the
land bridge that existed 25–35 Mya [10].
2.2.2. Current Distribution and Habitat
Currently, gesneriads are mainly distributed in tropical and subtropical regions of
Asia, Africa, and South and Central America, and they predominantly grow on rocks or
trees of mountain forests; meanwhile, they are scarce on lowlands [1,43].
In the NW, conformed by Sanangoideae and Gesnerioideae, they primarily became
diverse in the northern Andes and Central America and then in the West Indies and the
Brazilian Atlantic forest [
49
–
51
]. Tropical and subtropical Asia are the main distribution
areas of Didymocarpoideae, containing 85% of the genera and 90% of the species.
In addition, the Indo-China Peninsula, Southwest China, and nearby limestone regions
are diversity centers of the subfamily [52].
Most DT and freezing-tolerant (FT) gesneriads described so far occur in the northern
hemisphere [
38
]. They are found sheltered in the Pyrenees, the Balkan Peninsula, and
central and northern China [
10
,
13
]. Resurrection gesneriads in temperate areas in the
northern hemisphere principally inhabit rocky surfaces with very little soil coverage that
have served as ecological shelters for these species [
53
,
54
]. They are usually limestone karst
with high porosity and salinity in fragmented landscapes [12,55].
Epiphytism and lithophytism have arisen extensively in gesneriads. Epithytic growth
is more extensively spread in NW gesneriads, especially in the Columneinae clade, and
lithophytism in OW temperate zone gesneriads, although it is prevalent in both areas.
Epiphytism and lithophytism do not directly trigger speciation in gesneriads, but they
do promote a lower extinction rate and a greater diversification rate [
1
,
8
]. These habitats
force species to survive with low nutrient and water availability, high irradiance, and wide
temperature ranges. The rapid water flow does not allow the plant to absorb water and
generates hydric stress, even in wetter climates [
56
]. The fluctuating environmental condi-
tions of these habitats, such as water availability, have enabled a great diversity of plants to
thrive. Furthermore, the adverse environmental conditions of rocky habitats are favorable
Biology 2023,12, 107 6 of 21
for vascular resurrection plants, which become the dominant species in this habitat. The
scarce water availability coupled with the low temperatures of the mountainous habitat
make these suitable places to find double-tolerant species [42,43].
Species that no longer undergo desiccation stress might have lost this characteristic
in favor of greater growth rate and plant size. However, in some species such as Boea
hygroscopica, which is endemic to wet forest understories and never experiences seasonal
drought, this characteristic has been maintained, implying that a greater range of habitats
could be considered when looking for resurrection species [2].
Nevertheless, DT Didymocarpoideae species have arisen in arid areas that are mostly
subjected to freezing temperatures in mountains of the OW. These habitats can also be
found in the NW Andes, where hypothetically the Gesnerioideae subfamily arose and
is maintained. However, no DT Gesnerioideae has been found so far. There have been
some examples of independently developed rhizomatous species in the OW, but they have
been mainly developed in NW Gesnerioideae, and scaly rhizomes and tubers can only
be found there [
3
]. General climatic areas seem to have partly modelled the appearance
of DT species, since they dominate temperate areas of the northern hemisphere and even
some subtropical areas in Africa and China. Therefore, aridity and freezing temperatures
themselves do not seem to be enough to trigger them (Figure 3).
Biology 2023, 12, x FOR PEER REVIEW 6 of 22
temperature ranges. The rapid water flow does not allow the plant to absorb water and
generates hydric stress, even in wetter climates [56]. The fluctuating environmental con-
ditions of these habitats, such as water availability, have enabled a great diversity of plants
to thrive. Furthermore, the adverse environmental conditions of rocky habitats are favor-
able for vascular resurrection plants, which become the dominant species in this habitat.
The scarce water availability coupled with the low temperatures of the mountainous hab-
itat make these suitable places to find double-tolerant species [42,43].
Species that no longer undergo desiccation stress might have lost this characteristic
in favor of greater growth rate and plant size. However, in some species such as Boea
hygroscopica, which is endemic to wet forest understories and never experiences seasonal
drought, this characteristic has been maintained, implying that a greater range of habitats
could be considered when looking for resurrection species [2].
Nevertheless, DT Didymocarpoideae species have arisen in arid areas that are mostly
subjected to freezing temperatures in mountains of the OW. These habitats can also be
found in the NW Andes, where hypothetically the Gesnerioideae subfamily arose and is
maintained. However, no DT Gesnerioideae has been found so far. There have been some
examples of independently developed rhizomatous species in the OW, but they have been
mainly developed in NW Gesnerioideae, and scaly rhizomes and tubers can only be found
there [3]. General climatic areas seem to have partly modelled the appearance of DT spe-
cies, since they dominate temperate areas of the northern hemisphere and even some sub-
tropical areas in Africa and China. Therefore, aridity and freezing temperatures them-
selves do not seem to be enough to trigger them (Figure 3).
Figure 3. World climatic areas and distribution of the three Gesneriaceae subfamilies.
Figure 3. World climatic areas and distribution of the three Gesneriaceae subfamilies.
2.3. Morphological Characterization
2.3.1. Adaptation Pressures
Different speciation pressures that influence niche occupation have been identified
during recent decades. Climatic conditions and oscillations play a major role in this,
but morphology and growth forms have been proposed as the main drivers of habitat
colonization, with their influence also being notable in gesneriads [57,58].
Biology 2023,12, 107 7 of 21
Plant morphology is also shaped by the life habits of the plant. To a large extent,
gesneriads are small-sized plants; therefore, they may have needed to develop epiphytic
and lithophytic growth forms to occupy sunnier niches. This adaptation would have
required drought-resistant leaf morphology, organs, or strategies even in wetter environ-
ments. Indeed, most gesneriads (even shrubs) are either epiphytic or lithophytic, and
even non-resurrection gesneriads show drought adaptations in their leaf morphology and
arrangement. Thus, we hypothesize that morphological plasticity and preadaptation to
drought were developed early in the family, making it possible for plants to conquer
different habitats and climates worldwide.
The process of leaf curling and folding has obvious ecophysiological advantages for
resurrection species, but at the same time it generates enormous tension that has to be
efficiently channeled to avoid the generation of mechanical and structural damage. At both
the macro and micro scale, leaves have to fold and unfold efficiently following cycles of
hydration and dehydration. This mechanism has been studied in detail in the spikemoss
Selaginella lepidophylla [
59
]. In the case of angiosperms, several papers individually link
specific morphological traits to desiccation and cold tolerance, but an integrated view of
the morphological response as a whole is still missing.
The habitats of DT gesneriads in temperate zones tend to be rocky surfaces in cold
mountainous areas, where many times they are often exposed to direct sunlight. Therefore,
they have to overcome great temperature variation, high irradiance, and drought. Thermal
tolerance, both cold and hot, is greatly influenced by the height, leaf morphology, and
growth form of the plant [60,61].
2.3.2. Morphological Desiccation and Freezing Tolerance Traits
Annual life habit is a drought avoidance strategy, as plants have a high growth inten-
sity that enables them to complete their life cycle during optimal environmental conditions.
Therefore, desiccation tolerance is a mechanism that has been developed in perennial plants,
as they thoroughly invest multiple metabolic and physiological resources into growing
resistant aboveground organs [
62
,
63
]. Annual species are rare among gesneriads, with
some species scattered in mainly perennial genera in subtropical and tropical areas, while
perennial herbs are the predominant growth form [3].
As desiccation tolerance has been mainly found in slow-growing small plants, it
has been suggested that it trades off with growth and reproduction [
64
]. In parallel,
these characteristics have been related to the resilience they offer: the slow growth and
small size give the plant control over its metabolic reactions with more precision than
non-resurrection plants, so they can use resources in a more efficient way according to
environmental fluctuations [
65
]. European gesneriads (Ramonda spp., Haberlea rhodopensis,
and Jancaea heldreichii) have been reported to have long-lived leaves and slow growth [
36
].
These characteristics can surely be extrapolated to other temperate zone DT gesneriads that
are potentially FT due to their habitat (Jancaea heldreichii,Ramonda serbica,Ramonda nathaliae,
and Dorcoceras hygrometricum, among others).
Leaf morphology and disposition are also important. Many species exposed to freezing
temperatures are small, acaulescent, and flat and form compact rosettes, while taller plants
are usually associated with freeze avoidance mechanisms [
66
]. Smaller plants use tolerance
strategies, which confer resistance to colder temperatures [
67
]. Tolerance of rosette growth
is associated with the insulation that the rosette structure itself provides, as it reduces the
loss of heat from the leaves [
68
]. Furthermore, flat rosettes placed at ground level can
take advantage of the ground heat output during cold nights. In addition to height, leaf
area also decreases with elevation, which minimizes plant exposure to cold air [
67
,
69
].
Meanwhile, leaf dry matter content and thickness increase, which allows better resource
conservation in the plant and the creation of densely packed rosettes with coarse leaves.
These traits are appropriate for cold acclimation, but also for dry environments and high
temperature variation. This seems to be true for temperate and artic alpine zones, but
in Afroalpine, Andean, and Hawaiian areas, plants can develop very different heights
Biology 2023,12, 107 8 of 21
with freezing tolerance [
67
,
68
,
70
,
71
]. Nevertheless, gesneriads do seem to follow this
trend, and flat and dense rosettes with small leaves are found in all European species,
many Chinese and Himalayan genera (e.g., Corallodiscus), and African Streptocarpus, among
others [
3
]. Furthermore, work on Streptocarpus shows that the rosette growth form can
increase diversification because of the deep shade adaptation it provides [17,45].
Small leaf area is usually related to denser leaf venation [
72
]. Leaf veins are responsible
for mechanical support, molecule transport, and water flux due to transpiration [
73
–
75
].
Thus, greater leaf vein density has been postulated to confer drought resistance in leaves
and whole plants [
72
]. Plants with small leaves and considerable vein density would
typically inhabit drier and more exposed places, contrary to plants with bigger leaves and
lower vein density, which would occupy wet and shady areas [
76
,
77
]. Vessel diameter
also influences freezing and desiccation tolerance. However, a balance is needed between
obtaining vascular efficiency with wide conduits and preventing freezing- and desiccation-
induced embolism by using narrow veins [
78
]. In the case of drought-induced embolism,
smaller pore size of the pit membrane is an important trait to consider. In the case of
freezing stress, narrow veins are the most essential feature [
79
]. It is also important to
note that although leaves with higher primary vein density are less prone to major vein
cavitation, they are more susceptible to minor vein cavitation. Thus, high primary vein
density and low minor vein density would be the most suitable combination [
80
]. High
major vein density and small leaves are redundant mechanisms that contribute to more
efficient water transport, which would aid in more easily resolving blockages caused by
xylem embolisms under drought and freezing stress and help protect the vein system from
damage [
81
]. Venation typology and density in gesneriads have not been researched in
terms of function in counteracting environmental stressors, and there seems to be no record
of venation patterns of different genera. However, it has been found that temperate zone
DT gesneriads exposed to freezing temperatures mostly share a dense reticulate/cross-
venulate venation pattern with conspicuous veins. These traits seem to be less pronounced
in DT gesneriads in other habitats and tend to show more longitudinal patterns.
Trichomes, which make the leaves even coarser, also play an important role, as they
participate in the protection against biotic and abiotic stresses, such as extreme temperature
variations (both low and high), light intensity, drought, and high ultraviolet radiation [
82
].
At high temperatures, trichomes accelerate heat loss by reflecting sunlight and increasing
the convection and thermal conductivity of leaves [
83
]. Lower leaf temperatures also
help minimize transpiration rates for better water use efficiency in dry conditions [
84
].
Accumulated phenolic compounds in non-glandular trichomes are capable of filtering UV
and high visible radiation [
85
]. They can even store glutathione against oxidative stress [
86
].
On the other hand, glandular trichomes are capable of synthesizing and excreting secondary
metabolites such as phenolics. These trichomes can serve as morphological adaptations
under low temperatures, and they play a part in decreasing photoinhibition [
87
,
88
]. Some
plants, such as Solanum habrochaites, increase their trichome density when exposed to high
temperatures; Caragana korshinskii, when exposed to scarce precipitation; Arabidopsis thaliana
mutants, when exposed to UV-B; and Betula pendula, under freezing stress [
88
–
91
]. This
supposes not only a constitutive but also an inducible origin of trichomes. Trichomes are
widespread in gesneriads, which gives a hint about the preadaptation of the family to
overcoming environmental stressors and the tendency to be denser in DT species from
temperate zones. They are commonly simple and uniseriate, and their characterization is
often used for species identification [3].
When it comes to leaf margins, dentated and serrated edges have higher photosyn-
thetic activity. It has been hypothesized that greater vein density and free-ending veins that
go from the central vein to the margin, which is a characteristic of toothed leaves, increase
hydration, transpiration, and gas diffusion and decrease boundary layer resistance [
92
,
93
].
Therefore, they allow better photosynthetic performance under dry conditions. Indeed,
vein patterns associated with dentated margins seem to be the decisive characteristic that
indicates greater performance [
93
]. Studies with both serrated and non-serrated leaves
Biology 2023,12, 107 9 of 21
from Juniperus spp. have associated toothed-margin species with drier habitats, narrower
veins, and lower risk to the xylem. Moreover, smooth margins are usually farther from
secondary veins and thus are more hydraulically vulnerable [94].
Additionally, dentated and serrated edges, widely developed features in gesneriads,
have been proposed to facilitate the folding of leaves and the rapid rehydration and shape
recovery [
95
]. In the same way, tissues from the petioles are arranged in semi-circular
cross-sections, which, along with the dense leaf venation, are highly suitable for directing
internal forces during the critical points of dehydration and rehydration; in some cases,
this morphological trait is visible in the external form of the petiole. Indeed, Kampowski
et al. describe the contraction of the petiole as one of the first noticeable morphological
signs of drought stress in R. myconi [96].
As a result, gesneriads growing in mountainous regions need to overcome a great
variety of extreme conditions, which leads to a complex relationship among morphological
features. Thus, temperate zone DT and FT gesneriads have been found to be perennial,
slow-growing, small acaulescent plants with flat compact rosettes. Plant compaction
involves a small leaf area with great dry matter content per area and thickness, which,
along with trichomes, gives the plant a coarse touch. High vein density and narrow
veins are also indispensable for hydraulic and mechanical support, the latter aided by
a petiole in semi-circular cross-section and dentated margins (Figure 4). However, this
pattern does not entirely adjust for species in areas susceptible to drought that lack freezing
stress, as they exhibit more subtle morphological characteristics: they tend to have less
compact acaulescent rosettes with longer and less coarse leaves (possibly due to lower dry
matter content) and more longitudinal vein patterns. Therefore, differentiation between
resurrection and non-resurrection morphology is not always clear, and the morphology
described for double tolerance will be mainly considered for species proposal.
Biology 2023, 12, x FOR PEER REVIEW 10 of 22
Figure 4. Morphological preadaptations of gesneriads to desiccation. A foldable umbrella is used as
an analogy of the structural requirements for a successful process of leaf curling.
When it comes to the Gesnerioideae–Didymocarpoideae dichotomy, morphological
differences primarily signify different flower colors and subsequent pollination syn-
drome. Gesnerioideae are dominated by the transition of any other color to red, and Did-
ymocarpoideae are based on bidirectional transitions between white and purple, which
has positively influenced Gesnerioideae diversification, but not extinction rates. This di-
versification is directly related to the hummingbird diversification that occurred in the
NW [8]. Apart from that, no clear morphological difference has been documented between
the two major subfamilies. Nevertheless, it seems that intrinsic evolutionary, climatic, or
metabolic drivers make Didymocarpoideae more prone to developing desiccation toler-
ance.
Nevertheless, desiccation tolerance is not the only growth form that gesneriads have
evolved to overcome drought stress. Alternative strategies would comprise leaf abscission
and geophytism. Leaf abscission, which is confined to Streptocarpus genera, consists of
an ever-growing large leaf that, under unfavorable climatic conditions, creates an abscis-
sion line from which the dry part of the leaf is detached. Geophytism is present in both
Indo-China and America, with a much greater prevalence in Gesnerioideae. In fact, rhi-
zomes can be found in both main subfamilies, but scaly rhizomes and tubers only in Ges-
nerioideae [3]. Rhizomes and tubers have been developed in tropical and subtropical ar-
eas, presumably with a marked seasonality that confers environmental adverse conditions
in certain periods of the year. In contrast, resurrection gesneriads are mainly found in
temperate or subtropical mountainous areas where adverse conditions frequently occur.
Both strategies perfectly fit the environmental conditions they face: it would be biologi-
cally inefficient to constantly sprout new plants from rhizomes in temperate mountainous
areas, and the resurrection strategy seems to be too expensive for a limited period of the
year. However, desiccation tolerance and geophytism are not mutually exclusive strate-
gies and have been found in the same species of pteridophytes and some angiosperms,
such as Pitcairnia burchellii, Chamaegigas intrepidus, and Eragrostis invalida [38,97–99].
In Pitcairnia burchellii, the resurrection strategy is activated and the rhizomes act as
Figure 4.
Morphological preadaptations of gesneriads to desiccation. A foldable umbrella is used as
an analogy of the structural requirements for a successful process of leaf curling.
When it comes to the Gesnerioideae–Didymocarpoideae dichotomy, morphological
differences primarily signify different flower colors and subsequent pollination syndrome.
Biology 2023,12, 107 10 of 21
Gesnerioideae are dominated by the transition of any other color to red, and Didymo-
carpoideae are based on bidirectional transitions between white and purple, which has
positively influenced Gesnerioideae diversification, but not extinction rates. This diversifi-
cation is directly related to the hummingbird diversification that occurred in the NW [
8
].
Apart from that, no clear morphological difference has been documented between the two
major subfamilies. Nevertheless, it seems that intrinsic evolutionary, climatic, or metabolic
drivers make Didymocarpoideae more prone to developing desiccation tolerance.
Nevertheless, desiccation tolerance is not the only growth form that gesneriads have
evolved to overcome drought stress. Alternative strategies would comprise leaf abscission
and geophytism. Leaf abscission, which is confined to Streptocarpus genera, consists of an
ever-growing large leaf that, under unfavorable climatic conditions, creates an abscission
line from which the dry part of the leaf is detached. Geophytism is present in both Indo-
China and America, with a much greater prevalence in Gesnerioideae. In fact, rhizomes can
be found in both main subfamilies, but scaly rhizomes and tubers only in Gesnerioideae [
3
].
Rhizomes and tubers have been developed in tropical and subtropical areas, presumably
with a marked seasonality that confers environmental adverse conditions in certain periods
of the year. In contrast, resurrection gesneriads are mainly found in temperate or subtropical
mountainous areas where adverse conditions frequently occur. Both strategies perfectly fit
the environmental conditions they face: it would be biologically inefficient to constantly
sprout new plants from rhizomes in temperate mountainous areas, and the resurrection
strategy seems to be too expensive for a limited period of the year. However, desiccation
tolerance and geophytism are not mutually exclusive strategies and have been found in
the same species of pteridophytes and some angiosperms, such as Pitcairnia burchellii,
Chamaegigas intrepidus, and Eragrostis invalida [
38
,
97
–
99
]. In Pitcairnia burchellii, the
resurrection strategy is activated and the rhizomes act as storage organs during water stress.
When the plant suffers a very long period in the desiccated state, starch is mobilized from
the rhizomes, and the production of new tissue is prioritized instead of the recovery of
the desiccated tissue, which indicates that intermediate mechanisms could also exist in
gesneriads [97].
2.4. Tentative Desiccation/Double-Tolerant Species
The identified phylogenetic, geographic, and morphological patterns can serve as
support for identifying tentative resurrection and FT species prior to physiological studies.
Here, we propose tentative double-tolerant/DT gesneriads, so that greater availability of
these species could help expand our understanding of the DT and FT characteristics in
angiosperms (Table 1).
Table 1.
Tentative new double-tolerant/DT species proposed according to the phylogenetic, geo-
graphical, and morphological patterns already described.
Subtribe Documented DT Species
Tentative
Double-Tolerant/DT
Species
Streptocarpinae Streptocarpus revivescens
Streptocarpus rexii
Streptocarpus meyeri
Streptocarpus baudertii
Streptocarpus montigena
Streptocarpus rhodesianus
Corallodiscinae -
Corallodiscus kingianus
Corallodiscus cooperi
Corallodiscus bhutanicus
Biology 2023,12, 107 11 of 21
Table 1. Cont.
Subtribe Documented DT Species
Tentative
Double-Tolerant/DT
Species
Didymocarpinae
Oreocharis billburttii
Oreocharis primuloides
Oreocharis mileensis
Oreocharis pankaiyuae
Oreocharis mairei
Oreocharis ovatilobata
Oreocharis flavovirens
Oreocharis muscicola
Oreocharis blepharophylla
Oreocharis delavayi
Oreocharis ninglangensis
Oreocharis crispata
Oreocharis magnidens
Oreocharis stewardii
Didymocarpinae -
Henckelia incana
Henckelia gambleana
Henckelia fischeri
Henckelia bracteata
Henckelia wayanadensis
Henckelia innominata
3. Physiological Adaptations for Desiccation and Freezing Tolerance
3.1. Desiccation Tolerance Strategies among Gesneriads
Desiccation tolerance has been described in at least nine Gesneriaceae genera, implying
that this is the angiosperm family with the largest number of DT genera [
38
]. So far, mechanisms
have been characterized among a significant number of studies, and in fact two gesneriads
(D. hyrgrometricum and H. rhodopensis) are among the top five most studied resurrection an-
giosperms [
5
]. Both species, together with R. serbica and, to a lesser extent, others such as
P. rufencens, P. crassifolia,O. mileensis,R. myconi, and R. nathaliae, have allowed researchers to
unravel the mechanisms of desiccation tolerance in gesneriads [
34
,
39
,
100
–
102
]. Even though
more studies are needed, currently available information suggests that most resurrection gesner-
iads share similar protective mechanisms, although minor exceptions to this general rule were
reported in a comparison between D. hygrometricum and H. rhodopensis [103].
In response to desiccation, all resurrection gesneriads utilize a strategy known as
homoiochlorophylly; that is, they retain chlorophyll (Chl) in the desiccated state. The
opposite strategy, shown by some monocots, is poikilochlorophylly, which involves a
complete degradation of the photosynthetic apparatus. However, in gesneriads, Chl
retention is not always complete; for example, it has been reported to decrease by 20 to 70%
in the desiccated state in Ramonda species [
104
]. Parallel to Chl loss, there is a reduction
in associated genes, such as those of Chl-a/b-binding antenna proteins [
105
]. Most other
photosynthetic proteins are largely maintained during desiccation, so there is no need to
resynthesize them upon rehydration.
The rate of CO
2
assimilation is remarkably low in R. myconi, and it decreases concomi-
tantly with the loss of water content in H. rhodopensis. This process is first due to stomatal
closure and afterwards to reduced photochemical activity [
101
,
106
]. In parallel, there is a
shift from linear electron transport to alternative pathways such as PSI-dependent cyclic
electron flux. However, ATP has not been detected in H. rhodopensis in the full dry state,
which could be due to its massive use for energy-dependent metabolic reactions. This
switch between linear electron flow (LEF) and cyclic electron flow (CEF) may contribute to
the absence of a drastic decrease in the NADP/NADPH ratio, although a slight increase in
NADPH could be observed at 38% water content [
107
]. Once rehydration starts, the activity
of PSI is more rapidly recovered than that of PSII, while the PSI/PSII ratio, D2 protein,
and LHCI and LHCII apoproteins (Lhca1 and Lhcb2, respectively) remain stable [
106
].
These observations suggest that the partial loss of Chl does not indicate the occurrence
Biology 2023,12, 107 12 of 21
of damage in the photosynthetic apparatus. Furthermore, given that Chl molecules are a
major source of reactive oxygen species (ROS) when photosynthesis is impeded, partial
Chl degradation represents the activation of a photoprotective mechanism [
108
]. As a
consequence of the potential ROS generation within the photosynthetic apparatus, oxida-
tive stress is one of the main challenges associated with the process of desiccation and
subsequent rehydration [109].
At the same time, the physiological responses conferred by the resurrection strategy
have been favorable for colonizing cold mountainous habitats, and what started as a
cross-tolerance to freezing stress has evolved into a double tolerance.
3.2. Avoiding Reactive Oxygen Species Formation
To counteract ROS production, resurrection gesneriads constitutively express large
pools of antioxidant molecules, including ascorbate and glutathione [
102
,
110
]. Other low-
molecular-weight antioxidants are phenolic compounds (phenolic acids, polyphenols),
whose biosynthetic route (Shikimate pathway) is overexpressed during desiccation in H.
rhodopensis [
105
]. A similar polyphenol composition has been described in all gesneriad
species studied so far: H. rhodopensis,R. myconi,R. serbica, and D. hygrometricum [
111
–
113
].
Thus, Georgieva et al. proposed that the high polyphenol content is a characteristic feature
of gesneriads [114].
The activity of all of these antioxidants is complemented by the action of antioxi-
dant enzymes. For example, Gechev et al. found more genes in H. rhodopensis encoding
superoxide dismutases, monodehydroascorbate reductases (MDHARs), and glutathione
reductases than in most plant species with sequenced genomes, which gives a hint about
its constitutive tolerance against desiccation [
115
]. Mladenov et al. also reported on the
accumulation of ascorbate peroxidase and glutathione peroxidase in response to desicca-
tion, as was shown in R. Nathalie. The antioxidant machinery that accumulates during
desiccation is also maintained in the desiccated state to be used during the first stages of
subsequent rehydration. Thus, during the critical first hours of rehydration, superoxide
dismutase, catalase, glutathione reductase, and glutathione S-transferase activity remains
high to overcome oxidative stress [102–116].
One of the main targets of ROS is polyunsaturated fatty acids, whose oxidation gives
rise to the formation of lipid peroxides that propagate in membranes through peroxidation
chains. Paradoxically, polyunsaturated fatty acids enhance membrane fluidity in desiccated
tissues. This is probably why the response of the unsaturation ratio varies so widely
among the gesneriads studied: in H. rhodopensis, it was not affected by desiccation, while
it increased in B. hygroscopica and decreased in R. serbica [
117
–
119
]. The main antioxidant
involved in avoiding the propagation of such peroxidation chains is tocopherol, and in fact
the enhancement of this antioxidant in response to desiccation is a general trait observed in
most (if not all) DT plants, including gesneriads such as D. hygrometricum,R. myconi, and H.
rhodopensis [101,109,118,120].
The production of ROS within the photosynthetic apparatus can be prevented by
simply reducing light absorption within the photosystem antennae. To reduce photon
absorption by Chl, H. rhodopensis constitutively expresses high levels of red anthocyanins
in the abaxial side of the leaves [
41
]. When the leaves curl, the anthocyanic layer becomes
exposed to light, causing a decrease in light reaching the mesophyll. Another strategy
to reduce ROS formation is to enhance the dissipation of energy absorbed by Chl as
heat, so-called non-photochemical quenching (NPQ). This mechanism is linked to the
accumulation of certain proteins, such as PsbS, and the presence of zeaxanthin [
121
]. PsbS
is specifically induced by desiccation in H. rhodopensis, while zeaxanthin has been shown to
be synthesized in response to desiccation in R. myconi [101,122].
3.3. Avoiding Structural Damage
A second challenge linked to desiccation is mechanical/structural damage caused by
cell shrinkage, which is an obvious consequence of desiccation and leaf curling. First of all,
Biology 2023,12, 107 13 of 21
cell walls have to be flexible enough to allow correct folding and to follow such volume
alterations [
123
]. Changes in cell wall permeability and plasticity have been reported in H.
rhodopensis [
124
]. Indeed, Mladenov et al. reported downregulated levels of genes involved
in lignin and cellulose synthesis and increased levels of enzymes involved in cell wall
remodeling. The plasticity of the cell wall mainly depends on the relationship between
the cellulose–xyloglucan network and pectin polysaccharides, whereby changes in their
composition and connection lead to changes in cell flexibility [116].
In H. rhodopensis, the primary central vacuole disappears when the cell desiccates,
and smaller secondary vacuoles emerge in the vicinity of the cell wall. At the same time,
organelles take the place of the central primary vacuole [
110
]. The increase in the number of
smaller vacuoles, which have a greater area/volume ratio, makes it possible to maintain the
membrane surface area during the volume reduction caused by water loss [
125
]. Desiccated
chloroplasts (termed desiccoplasts) adopt a rounded shape, but grana are kept intact
during desiccation, although their repeat distance is diminished due to the shrinkage. The
most noticeable change in desiccoplasts is the enhancement in the number and size of
plastoglobules [110].
Maintaining membrane integrity is the main objective of desiccation tolerance strate-
gies. This is accomplished by profound lipid remodeling, and membrane stabilization
is further strengthened by the interaction with proteins, sugars, and remaining water
molecules [109]. Consequently, the degradation of lipids is a limited phenomenon among
resurrection plants, and membranes are highly preserved. In chloroplasts of R. myconi,
membrane stability is enhanced by a partial conversion of monogalactosyldiacylglycerol
(MGDG) to digalactosyldiacylglycerol (DGDG), a bilayer-forming lipid [
101
]. Stability is
reinforced in H. rhodopensis by the presence of a dense luminal substance (DLS), most likely
a phenolic compound that prevents conformational changes of thylakoids [
126
]. Addi-
tionally, phospholipids, such as phosphatidylethanolamine and phosphatidylcholine, are
degraded during desiccation, and there is an increase in phospholipase D [105]. The accu-
mulation of other lipids, such as sitosterol, is also a species-specific response to desiccation
in H. rhodopensis [118].
The accumulation of compatible solutes, acting as osmoprotectants, is a general re-
sponse to water loss. In D. hygrometricum,R. myconi, and H. rhodopensis, sucrose is the main
compound accumulated in response to desiccation [
37
]. Sucrose can be produced after
starch degradation or can come from gluconeogenesis, as there is consumption of glycolytic
intermediates directly related to the accumulation of sucrose [
110
,
118
]. In D. hygrometricum,
raffinose family compounds also increase during desiccation [
103
]. At the later stages of
desiccation, the massive accumulation of sucrose is more directly related to membrane
protection by preventing non-bilayer phase separation and membrane fusion [
109
]. Co-
incident with their protective functions, both sucrose and raffinose sharply decline after
rehydration [110,118].
Once the water potential surpasses a certain threshold (
−
100 MPa), there is a process of
vitrification, and the cytoplasm reaches the so-called glassy state, an amorphous metastable
state [
109
]. In this situation, most metabolic activities cease, and chemical reactivity is
inhibited. Transition to the glass state implies positive aspects for DT organisms, as the
diffusion of oxygen is greatly reduced, decreasing ROS generation and preventing further
water loss. Vitrification was studied in R. myconi by Fernández-Marín et al., who showed
that this stage can occur in nature, as it can be reached in desiccated leaves at 20 ◦C [40].
3.4. Cellular Protection
Desiccation induces the expression of genes encoding several sets of protective proteins.
This is the case of early light-induced proteins (ELIPs) and PsbS, which are involved in
the maintenance of chloroplasts and the regulation of photosynthesis. Their expression is
maintained during the rehydration process, and some of them are still present 7 days after
the onset of rehydration. Desiccation also induces massive expression of late embryogenesis
abundant (LEA) proteins in H. rhodopensis,R. serbica, and D. hygrometricum [
115
,
116
]. These
Biology 2023,12, 107 14 of 21
proteins act as water replacement molecules to maintain the structure of the membranes
and organelles, as they have little possibility of interacting with other molecules due to
their low capacity for forming hydrogen bonds [127].
Damaged DNA is another key aspect of maintaining genome integrity. It can be
repaired by several processes, including nucleotide excision repair, in which genes have
been reported to be specifically activated during dehydration in H. rhodopensis. In case
cellular damage occurs, autophagy is also an option [
105
]. By inducing autophagy, cell
death can be inhibited in order to recycle damaged structures to create new structures
needed for the protective response of the organism against water deficiency stress, as has
been observed in D. hygrometricum [
120
]. The promotion of autophagy is in accordance with
the decreasing transcription and protein accumulation of AMC4 in H. rhodopensis during
desiccation and works as a promoter of programmed cell death under stress [
116
,
128
]. This
is how DT plants suppress senescence when dehydrated [109].
Overall, with the limited information currently available, it is known that most DT
mechanisms are common to all resurrection gesneriads. However, there are some others that
seem to be species-specific. Thus, specific features such as ascorbic acid and glutaric acid
accumulation during desiccation were found only in D. hygrometricum, while DNA repair
and increased
β
-aminoisobutyric acid and
β
-sitosterol were found in H. rhodopensis. When
DT mechanisms in gesneriads are compared with those in other homoiochlorophyllous
angiosperms, such as Craterostigma, the same conclusion is reached: in essence, the tolerance
strategies are similar, with only minor species-specific differences [103].
3.5. Freezing-Induced Desiccation
More than half a century ago, Kappen demonstrated that desiccated leaves of R. myconi
were able to successfully recover after immersion in liquid nitrogen [
129
]. Recently, the
same phenomenon of recovery from freezing conditions was re-examined in resurrection
gesneriads in both field and laboratory studies [
40
,
101
,
106
,
130
]. Cross-tolerance to freezing
and desiccation is somewhat logical, since dehydration prevents the formation of ice
crystals inside cells. In fact, it has been documented that H. rhodopensis has the ability to
dehydrate rapidly under freezing temperatures, and this could be interpreted as a freezing
tolerance mechanism [
130
]. Such rapid desiccation might be mediated by the presence of
narrow epidermal channels of the leaves [
131
]. Consequently, most responses are shared
between drought-induced desiccation (DID) and freezing-induced desiccation (FID) [
132
].
However, there are slight differences between these two processes; for example, FID is
characterized by faster recovery of PSII compared to DID [
133
]. In addition, during FID,
secondary vacuoles are reported to appear at 60% RWC. This process is similar to the one
that happens after DID, but it occurs more rapidly after FID, most likely because of the
RWC recovery rate enabled by the environment and the influence of freezing [132].
In addition to FID, it has been documented in manipulative experiments that hydrated
leaves of R. myconi freeze at a relatively high temperature (
−
2
◦
C) [
40
]. Tissue freezing
involves an abrupt reduction in photochemical efficiency (Fv/Fm), which correlates with
the enzymatic formation of zeaxanthin. Interestingly, the glass transition in hydrated leaves
occurs at
−
15
◦
C, meaning that enzyme activity is possible in frozen leaves of R. myconi [
40
].
This justifies the induction of antioxidant enzymes in frozen leaves of H. rhodopensis [
130
].
Recovery of Fv/Fm is initially fast upon transfer to warm conditions and is completely
restored in 6 h. This means that even in winter, R. myconi is photosynthetically active
whenever the temperature is above the freezing point [101].
Apart from the obvious fact that desiccation protects the plant from ice formation, in
order to fully withstand winter stress, resurrection gesneriads have to develop a profound
metabolic reconfiguration through a process of seasonal acclimation. Only a few studies
have addressed the specific mechanisms that H. rhodopensis and R. myconi employ to
cope with low-temperature stress. These include substantial induction of thermal energy
dissipation, which is in agreement with a high accumulation of zeaxanthin and the PsbS
protein [
101
,
106
]. Other stress proteins that accumulate in H. rhodopensis in response to low
Biology 2023,12, 107 15 of 21
temperature are Lhcb5, Lhcb6, dehydrins, and ELIPs [
132
]. Genes encoding lipocalins are
also upregulated [
41
]. Lipocalins are small ligand-binding proteins that can be found in both
the cell and the chloroplast membrane. In winter, there is also a substantial accumulation
of low-molecular-weight metabolites, such as polyphenols [
130
]; sugars, such as trehalose,
maltose, raffinose, sucrose, and glucose; amino acids, such as proline, glycine, serine,
alanine, asparagine, and aspartate; polyamines, such as putrescine and ornithine; and
antioxidants of the glutathione–ascorbate system [
41
,
101
,
122
,
130
]. The joint action of all of
these mechanisms decreases photosynthesis, lowers osmotic potential, and keeps plants in
a state primed for freezing protection [41].
Overall, most physiological mechanisms are shared in terms of the responses to
desiccation and low-temperature stresses (Figure 5). This is perhaps the reason why
tertiary paleotropical relict gesneriads were able to survive in sheltered habitats of southern
Europe during the quaternary glaciations. The question of whether freezing tolerance is a
constitutive trait in resurrection gesneriads or evolved in temperate species as a result of
climate cooling deserves further studies.
Biology 2023, 12, x FOR PEER REVIEW 16 of 22
Figure 5. A summary of the main mechanisms of tolerance to desiccation and freezing in gesneriads.
Brown box includes the responses to desiccation while blue box contains the mechanisms activated
in response to freezing. The intersection between both boxes contains the mechanisms which are
common to both stresses. Constitutive traits of gesneriads that might favor cross-tolerance are
shown out of both boxes. The foldable umbrella analogy is depicted in the background.
4. Conclusions
Full understanding of the desiccation tolerance physiological response is still miss-
ing. The bulk of scientific studies on DT gesneriads have mainly researched leaf tissues,
while the role of roots and root-associated soil microbiota has not been taken into consid-
eration. Moreover, resurrection plants have usually been researched using physiological,
transcriptomic, and metabolomic methods, mainly focusing on protection mechanisms
during the desiccation phase, and the scarce studies on the rehydration phase are usually
centered on the later stages of rehydration [41]. There is no record of whether morpholog-
ical leaf preadaptations spread in both resurrection and non-resurrection members of the
family are reflected in higher basal levels or greater numbers of coding domains of mole-
cules that intervene in the different adaptation strategies. Similarly, the critical point at
which rehydration starts has not been fully documented [106]. Indeed, even though water
triggers the metabolic adaptation, changes in the dynamic state of water’s molecular struc-
ture and aquaphotomics are still emerging fields of study concerning resurrection species
[134]. Equally, data on protein and DNA integrity maintenance and repair and mitochon-
drial functioning are also scarce despite their importance [105,135]. There is little infor-
mation on the response of DT plants to other abiotic stresses, such as low temperatures,
despite their similar physiological responses.
The descriptions of the physiological, geographical, and morphological patterns pro-
vide evidence that habitat, growth form, morphology, and phylogenetic origin have
jointly shaped gesneriads at the beginning of their evolution, making them more resilient
Figure 5.
A summary of the main mechanisms of tolerance to desiccation and freezing in gesneriads.
Brown box includes the responses to desiccation while blue box contains the mechanisms activated
in response to freezing. The intersection between both boxes contains the mechanisms which are
common to both stresses. Constitutive traits of gesneriads that might favor cross-tolerance are shown
out of both boxes. The foldable umbrella analogy is depicted in the background.
4. Conclusions
Full understanding of the desiccation tolerance physiological response is still missing.
The bulk of scientific studies on DT gesneriads have mainly researched leaf tissues, while
the role of roots and root-associated soil microbiota has not been taken into consideration.
Moreover, resurrection plants have usually been researched using physiological, transcrip-
Biology 2023,12, 107 16 of 21
tomic, and metabolomic methods, mainly focusing on protection mechanisms during the
desiccation phase, and the scarce studies on the rehydration phase are usually centered
on the later stages of rehydration [
41
]. There is no record of whether morphological leaf
preadaptations spread in both resurrection and non-resurrection members of the family
are reflected in higher basal levels or greater numbers of coding domains of molecules
that intervene in the different adaptation strategies. Similarly, the critical point at which
rehydration starts has not been fully documented [
106
]. Indeed, even though water triggers
the metabolic adaptation, changes in the dynamic state of water’s molecular structure
and aquaphotomics are still emerging fields of study concerning resurrection species [
134
].
Equally, data on protein and DNA integrity maintenance and repair and mitochondrial
functioning are also scarce despite their importance [
105
,
135
]. There is little information on
the response of DT plants to other abiotic stresses, such as low temperatures, despite their
similar physiological responses.
The descriptions of the physiological, geographical, and morphological patterns pro-
vide evidence that habitat, growth form, morphology, and phylogenetic origin have jointly
shaped gesneriads at the beginning of their evolution, making them more resilient to
drought stress. In fact, apart from the DT strategy, other resistance growth forms, such
as leaf abscission and geophytism, have also successfully developed. On the whole, the
plasticity and adaptability of gesneriads are indisputable, and the whole Gesneriaceae
family seems predisposed to endure dry circumstances in a great variety of habitats and
climates. Thus, they could be the source of resurrection and double-tolerant species world-
wide. In fact, the identified common traits have allowed us to propose tentative new
double-tolerant/DT gesneriads that can make DT species more easily accessible and help
broaden the knowledge of the mechanisms listed above.
Author Contributions:
All authors contributed equally to the conception and revision of this work.
All authors have read and agreed to the published version of the manuscript.
Funding:
This work was supported by the project PGC2018-093824-B-C44 from the Ministerio de
Ciencia, Innovación y Universidades (MCIU, Spain) and grant UPV/EHU IT-1648-22 from the Basque
Government (Spain).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
This work was supported by the project PGC2018-093824-B-C44 from the Minis-
terio de Ciencia, Innovación y Universidades (MCIU, Spain) and grant UPV/EHU IT-1648-22 from the
Basque Government (Spain). A.L. was the recipient of a pre-doctoral grant of the Basque Government.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Weber, A.; Clark, J.L.; Möller, M. A New Formal Classification of Gesneriaceae. Selbyana 2013,31, 68–94.
2.
Marks, R.A.; Farrant, J.M.; Nicholas McLetchie, D.; VanBuren, R. Unexplored Dimensions of Variability in Vegetative Desiccation
Tolerance. Am. J. Bot. 2021,108, 346–358. [CrossRef] [PubMed]
3.
Weber, A. Gesneriaceae. In The Families and Genera of Vascular Plants; Kulbitzki, K., Ed.; Springer: Berlin, Germany, 2004; Volume
7, pp. 63–158.
4.
Verhoeven, A.; García-Plazaola, J.I.; Fernández-Marín, B. Shared Mechanisms of Photoprotection in Photosynthetic Organisms
Tolerant to Desiccation or to Low Temperature. Environ. Exp. Bot. 2018,154, 66–79. [CrossRef]
5.
Tebele, S.M.; Marks, R.A.; Farrant, J.M. Two Decades of Desiccation Biology: A Systematic Review of the Best Studied Angiosperm
Resurrection Plants. Plants 2021,10, 2784. [CrossRef]
6.
Clark, J.L.; Funke, M.M.; Duffy, A.M.; Smith, J.F. Phylogeny of a Neotropical Clade in the Gesneriaceae: More Tales of Convergent
Evolution. Int. J. Plant Sci. 2012,173, 894–916. [CrossRef]
7.
Serrano-Serrano, M.L.; Rolland, J.; Clark, J.L.; Salamin, N.; Perret, M. Hummingbird Pollination and the Diversification of
Angiosperms: An Old and Successful Association in Gesneriaceae. Proc. R. Soc. B Biol. Sci. 2017,284, 20162816. [CrossRef]
8.
Roalson, E.H.; Roberts, W.R. Distinct Processes Drive Diversification in Different Clades of Gesneriaceae. Syst. Biol.
2016
,
65, 662–684. [CrossRef]
Biology 2023,12, 107 17 of 21
9.
Ogutcen, E.; Christe, C.; Nishii, K.; Salamin, N.; Möller, M.; Perret, M. Phylogenomics of Gesneriaceae Using Targeted Capture of
Nuclear Genes. Mol. Phylogenet. Evol. 2021,157, 107068. [CrossRef]
10.
Möller, M.; Pfosser, M.; Jang, C.G.; Mayer, V.; Clark, A.; Hollingsworth, M.L.; Barfuss, M.H.J.; Wang, Y.Z.; Kiehn, M.; Weber, A. A
Preliminary Phylogeny of the “didymocarpoid Gesneriaceae” Based on Three Molecular Data Sets: Incongruence with Available
Tribal Classifications. Am. J. Bot. 2009,96, 989–1010. [CrossRef]
11. Burtt, B.L.; Wiehler, H. Classification of the Family Gesneriaceae. Gesneriana 1995,1, 1–4.
12.
Xu, W.; Guo, J.; Pan, B.; Zhang, Q.; Liu, Y. Diversity and Distribution of Gesneriaceae in China. Guihaia
2017
,37, 1219–1226.
[CrossRef]
13.
Möller, M.; Wei, Y.G.; Wen, F.; Clark, J.L.; Weber, A. You Win Some You Lose Some: Updated Generic Delimitations and
Classification of Gesneriaceae-Implications for the Family in China. Guihaia 2016,35, 44–60. [CrossRef]
14.
Weber, A.; Middleton, D.J.; Clark, J.L.; Möller, M. Keys to the Infrafamilial Taxa and Genera of Gesneriaceae. J. Indian Assoc.
Angiosperm Taxon. 2020,30, 5–47. [CrossRef]
15.
Tan, K.; Lu, T.; Ren, M.X. Biogeography and Evolution of Asian Gesneriaceae Based on Updated Taxonomy. PhytoKeys
2020
,157,
26. [CrossRef] [PubMed]
16.
Middleton, D.J.; Atkins, H.; Truong, L.H.; Nishii, K.; Möller, M. Billolivia, a New Genus of Gesneriaceae from Vietnam with Five
New Species. Phytotaxa 2014,161, 241–269. [CrossRef]
17.
Wen, F.; Xin, Z.B.; Fu, L.F.; Li, S.; Su, L.Y.; Maciejewski, S.; Huang, Z.J.; Van Do, T.; Wei, Y.G. Michaelmoelleria (Gesneriaceae),
a New Lithophilous Dwelling Genus and Species with Zigzag Corolla Tube from Southern Vietnam. PhytoKeys
2020
,146, 107.
[CrossRef] [PubMed]
18.
Middleton, D.J.; Nishii, K.; Puglisi, C.; Forrest, L.L.; Möller, M. Chayamaritia (Gesneriaceae: Didymocarpoideae), a New Genus
from Southeast Asia on JSTOR. Plant Syst. Evol. 2015,301, 1947–1966. [CrossRef]
19.
Möller, M.; Chen, W.-H.; Shui, Y.-M.; Atkins, H.; Middleton, D.J. A New Genus of Gesneriaceae in China and the Transfer of
Briggsia Species to Other Genera. Gard. Bull. Singap. 2014,66, 195–205.
20.
Puglisi, C.; Middleton, D.J. A Revision of Microchirita (Gesneriaceae) in Thailand. Gard. Bull. Singap.
2017
,69, 211–284. [CrossRef]
21.
Puglisi, C.; Yao, T.L.; Milne, R.; Möller, M.; Middleton, D.J. Generic Recircumscription in the Loxocarpinae (Gesneriaceae), as
Inferred by Phylogenetic and Morphological Data. Taxon 2016,65, 277–292. [CrossRef]
22.
Middleton, D.J.; Khew, G.S.; Poopath, M.; Möller, M.; Puglisi, C. Rachunia cymbiformis, a New Genus and Species of Gesneriaceae
from Thailand. Nord. J. Bot. 2018,36, e01992. [CrossRef]
23. Middleton, D.J.; Triboun, P. A New Species of Somrania (Gesneriaceae) from Thailand. Bull. Singap. 2013,65, 181–184.
24. Puglisi, C.; Middleton, D.J. A Revision of Damrongia (Gesneriaceae) in Thailand. Thai For. Bull. 2017,45, 79–93. [CrossRef]
25.
Möller, M.; Nishii, K.; Atkins, H.J.; Kong, H.H.; Kang, M.; Wei, Y.G.; Wen, F.; Hong, X.; Middleton, D.J. An Expansion of the
Genus Deinostigma (Gesneriaceae). Gard. Bull. Singap. 2016,68, 145–172. [CrossRef]
26.
Wen, F.; Li, S.; Xin, Z.; Fu, L.F.; Hong, X.; Cai, L.; Qin, J.Q.; Pan, B.; Pan, F.Z.; Wei, Y.G. The Latest List of Gesneriaceae in China
under the New Chinese Nomenclature. Guangxi Sci. 2019,26, 37–63.
27.
Nishii, K.; Hughes, M.; Briggs, M.; Haston, E.; Christie, F.; DeVilliers, M.J.; Hanekom, T.; Roos, W.G.; Bellstedt, D.U.; Möller, M.
Streptocarpus Redefined to Include All Afro-Malagasy Gesneriaceae: Molecular Phylogenies Prove Congruent with Geographical
Distribution and Basic Chromosome Numbers and Uncover Remarkable Morphological Homoplasies. Taxon
2015
,64, 1243–1274.
[CrossRef]
28.
Middleton, D.J.; Weber, A.; Yao, T.L.; Sontag, S.; Möller, M. The Current Status of the Species Hitherto Assigned to Henckelia
(Gesneriaceae). Edinb. J. Bot. 2013,70, 385–404. [CrossRef]
29.
Xin, D.; Zhiang, H.; Hongxin, W.; Xiaogang, W.; Tingyun, K. Effects of Dehydration and Rehydration on Photosynthesis of
Detached Leaves of the Resurrective Plant Boea hygrometrica.Acta Bot. Sin. 2000,42, 321–323.
30.
Wang, Y.; Liu, K.; Bi, D.; Zhou, S.; Shao, J. Molecular Phylogeography of East Asian Boea clarkeana (Gesneriaceae) in Relation to
Habitat Restriction. PLoS ONE 2018,13, e0199780. [CrossRef] [PubMed]
31.
Maria Sgherri, C.L.; Loggini, B.; Bochicchio, A.; Navari-Izzo, F. Antioxidant System in Boea hygroscopica: Changes in Response to
Desiccation and Rehydration. Phytochemistry 1994,37, 377–381. [CrossRef]
32.
Gray, B.; Tropical Herbarium, A.; Cook University, J.; Qld, S. Boea resupinata Zich & B.Gray (Gesneriaceae), a New Species from
Cape York Peninsula, Queensland, Australia. Austrobaileya 2021,11, 56–66.
33.
Zhao, H.; Liu, H.; Yu, H.; Hu, Y.; Gao, Y.; Li, Z.; Lin, Z. Cloning and Expression Pattern of a Dehydrin-LikeBDN1 Gene from
Drought-Tolerant Boea crassifolia Hemsl. Chin. Sci. Bull. 2000,45, 2072–2077. [CrossRef]
34.
Huang, W.; Yang, S.J.; Zhang, S.B.; Zhang, J.L.; Cao, K.F. Cyclic Electron Flow Plays an Important Role in Photoprotection for the
Resurrection Plant Paraboea rufescens under Drought Stress. Planta 2012,235, 819–828. [CrossRef]
35.
Djilianov, D.L.; Moyankova, D.P.; Mladenov, P.V. The Mediterranean: A Cradle of the Resurrection Plants in Europe. Phytol. Balc.
2016,22, 141–147.
36.
Raki´c, T.; Lazarevi´c, M.; Jovanovi´c, Ž.S.; Radovi´c, S.; Siljak-Yakovlev, S.; Stevanovi´c, B.; Stevanovi´c, V. Resurrection Plants of the
Genus Ramonda: Prospective Survival Strategies-Unlock Further Capacity of Adaptation, or Embark on the Path of Evolution?
Front. Plant Sci. 2014,4, 550. [CrossRef]
Biology 2023,12, 107 18 of 21
37.
Muller, J.; Sprenger, N.; Bortlik, K.; Boiler, T.; Wiemken Muller, A.; Bortlik, N.; Wiemken, T.; Muller, J.; Sprenger, N.; Bortlik, K.;
et al. Desiccation Increases Sucrose Levels in Ramonda and Haberlea, Two Genera of Resurrection Plants in the Gesneriaceae.
Physiol. Plant 1997,100, 153–158. [CrossRef]
38.
Gaff, D.F.; Oliver, M. The Evolution of Desiccation Tolerance in Angiosperm Plants: A rare yet common phenomenon. Funct.
Plant Biol. 2013,40, 315–328. [CrossRef] [PubMed]
39.
Li, A.; Wang, D.; Yu, B.; Yu, X.; Li, W. Maintenance or Collapse: Responses of Extraplastidic Membrane Lipid Composition to
Desiccation in the Resurrection Plant Paraisometrum mileense.PLoS ONE 2014,9, e103430. [CrossRef]
40.
Fernández-Marín, B.; Neuner, G.; Kuprian, E.; Laza, J.M.; García-Plazaola, J.I.; Verhoeven, A. First Evidence of Freezing Tolerance
in a Resurrection Plant: Insights into Molecular Mobility and Zeaxanthin Synthesis in the Dark. Physiol. Plant
2018
,163, 472–489.
[CrossRef] [PubMed]
41.
Benina, M.; Obata, T.; Mehterov, N.; Ivanov, I.; Petrov, V.; Toneva, V.; Fernie, A.R.; Gechev, T.S. Comparative Metabolic Profiling of
Haberlea rhodopensis,Thellungiella halophyla, and Arabidopsis thaliana Exposed to Low Temperature. Front. Plant Sci.
2013
,4, 499.
[CrossRef]
42. Morley, R.J. Interplate Dispersal Paths for Megathermal Angiosperms. Perspect. Plant Ecol. Evol. Syst. 2003,6, 5–20. [CrossRef]
43.
Perret, M.; Chautems, A.; de Araujo, A.O.; Salamin, N. Temporal and Spatial Origin of Gesneriaceae in the New World Inferred
from Plastid DNA Sequences. Bot. J. Linn. Soc. 2013,171, 61–79. [CrossRef]
44.
Woo, V.L.; Funke, M.M.; Smith, J.F.; Lockhart, P.J.; Garnock-Jones, P.J. New World Origins of Southwest Pacific Gesneriaceae:
Multiple Movements Across and Within the South Pacific. Int. J. Plant Sci. 2011,172, 434–457. [CrossRef]
45.
Petrova, G.; Moyankova, D.; Nishii, K.; Forrest, L.; Tsiripidis, I.; Drouzas, A.D.; Djilianov, D.; Möller, M. The European
Paleoendemic Haberlea rhodopensis (Gesneriaceae) Has an Oligocene Origin and a Pleistocene Diversification and Occurs in a
Long-Persisting Refugial Area in Southeastern Europe. Int. J. Plant Sci. 2015,176, 499–514. [CrossRef]
46.
Hultén, E.; Fries, M. Atlas of North European Vascular Plants: North of the Tropic of Cancer; Koeltz Scientific Books: Königstein,
Germany, 1986.
47.
Bettin, O.; Cornejo, C.; Edwards, P.J.; Holderegger, R. Phylogeography of the High Alpine Plant Senecio halleri (Asteraceae) in the
European Alps: In Situ Glacial Survival with Postglacial Stepwise Dispersal into Peripheral Areas. Mol. Ecol.
2007
,16, 2517–2524.
[CrossRef] [PubMed]
48. Milne, R.; Abbott, R.J. The Origin and Evolution of Tertiary Relict Floras. Adv. Bot. Res. 2002,38, 281–314. [CrossRef]
49.
Skog, L.E. A Study of the Tribe Gesneriaceae, with a Revision of Gesneria (Gesneriaceae-Gesnerioideae). Smithson. Contrib. Bot.
1976,29, 1–182. [CrossRef]
50.
Martén-Rodríguez, S.; Fenster, C.B.; Agnarsson, I.; Skog, L.E.; Zimmer, E.A. Evolutionary Breakdown of Pollination Specialization
in a Caribbean Plant Radiation. New Phytol. 2010,188, 403–417. [CrossRef] [PubMed]
51.
Perret, M.; Chautems, A.; Spichiger, R. Dispersal-Vicariance Analyses in the Tribe Sinningiaeae (Gesneriaceae): A Clue to
Understanding Biogeographical History of the Brazilian Atlanric Forest. Ann. Mo. Bot. Gard. 2006,93, 340–358. [CrossRef]
52.
Clements, R.; Sodhi, N.S.; Schilthuizen, M.; Hg, P.K.L. Limestone Karsts of Southeast Asia: Imperiled Arks of Biodiversity.
Bioscience 2006,56, 733–742. [CrossRef]
53.
Porembski, S.; Barthlott, W. Granitic and Gneissic Outcrops (Inselbergs) as Centers of Diversity for Desiccation-Tolerant Vascular
Plants. Plant Ecol. 2000,151, 19–28. [CrossRef]
54.
Alejo-Jacuinde, G.; Herrera-Estrella, L. Exploring the High Variability of Vegetative Desiccation Tolerance in Pteridophytes. Plants
2022,11, 1222. [CrossRef] [PubMed]
55.
Liu, C.; Huang, Y.; Wu, F.; Liu, W.; Ning, Y.; Huang, Z.; Tang, S.; Liang, Y. Plant Adaptability in Karst Regions. J. Plant Res.
2021
,
134, 889–906. [CrossRef] [PubMed]
56.
Burke, A. Island–Matrix Relationships in Nama Karoo Inselberg Landscapes Part II: Are Some Inselbergs Better Sources than
Others? Plant Ecol. 2002,158, 41–48. [CrossRef]
57.
Smith, S.A.; Beaulieu, J.M. Life History Influences Rates of Climatic Niche Evolution in Flowering Plants. Proc. R. Soc. B Biol. Sci.
2009,276, 4352. [CrossRef] [PubMed]
58.
Möller, M.; Cronk, Q.C.B. Evolution of Morphological Novelty: A Phylogenetic Analysis of Growth Patterns in Streptocarpus
(Gesneriaceae). Evolution 2001,55, 918–929. [CrossRef] [PubMed]
59.
Rafsanjani, A.; Brulé, V.; Western, T.L.; Pasini, D. Hydro-Responsive Curling of the Resurrection Plant Selaginella lepidophylla.Sci.
Rep. 2015,5, srep08064. [CrossRef] [PubMed]
60. Körner, C. Alpine Plant Life: Functional Plant Ecology of High Mountain Ecosystems; Springer: Heidelberg, Germany, 2003.
61.
Sklenáˇr, P.; Kuˇcerová, A.; Macková, J.; Romoleroux, K. Temperature Microclimates of Plants in a Tropical Alpine Environment:
How Much Does Growth Form Matter? Arct. Antarct. Alp. Res. 2018,48, 61–78. [CrossRef]
62.
Werk, K.S.; Ehleringer, J.; Forseth, I.N.; Cook, C.S. Photosynthetic Characteristics of Sonoran Desert Winter Annuals. Oecologia
1983,59, 101–105. [CrossRef] [PubMed]
63. Sakai, A.; Larcher, W. Frost Survival of Plants: Responses and Adaptation to Freezing Stress; Springer: Berlin, Germany, 1987.
64.
Alpert, P. Constraints of Tolerance: Why Are Desiccation-Tolerant Organisms so Small or Rare? J. Exp. Biol.
2006
,209, 1575–1584.
[CrossRef]
65.
Asami, P.; Mundree, S.; Williams, B. Saving for a Rainy Day: Control of Energy Needs in Resurrection Plants. Plant Sci.
2018
,
271, 62–66. [CrossRef]
Biology 2023,12, 107 19 of 21
66.
Squeo, F.A.; Rada, F.; Azocar, A.; Goldstein, G. Freezing Tolerance and Avoidance in High Tropical Andean Plants: Is It Equally
Represented in Species with Different Plant Height? Oecologia 1991,86, 378–382. [CrossRef]
67.
Cruz-Maldonado, N.; Weemstra, M.; Jiménez, L.; Roumet, C.; Angeles, G.; Barois, I.; de los Santos, M.; Morales-Martinez, M.A.;
Palestina, R.A.; Rey, H.; et al. Aboveground-Trait Variations in 11 (Sub)Alpine Plants along a 1000-m Elevation Gradient in
Tropical Mexico. Alp. Bot. 2021,131, 187–200. [CrossRef]
68.
Leon-Garcia, I.V.; Lasso, E. High Heat Tolerance in Plants from the Andean Highlands: Implications for Paramos in a Warmer
World. PLoS ONE 2019,14, e0224218. [CrossRef]
69.
Scheepens, J.F.; Frei, E.S.; Stöcklin, J. Genotypic and Environmental Variation in Specific Leaf Area in a Widespread Alpine Plant
after Transplantation to Different Altitudes. Oecologia 2010,164, 141–150. [CrossRef]
70.
Beck, E.; Schulze, E.D.; Senser, M.; Scheibe, R. Equilibrium Freezing of Leaf Water and Extracellular Ice Formation in Afroalpine
‘Giant Rosette’ Plants. Planta 1984,162, 276–282. [CrossRef] [PubMed]
71.
Melcher, P.J.; Goldstein, G.; Meinzer, F.C.; Minyard, B.; Giambelluca, T.W.; Loope, L.L. Determinants of Thermal Balance in the
Hawaiian Giant Rosette Plant, Argyroxiphium sandwicense.Oecologia 1994,98, 412–418. [CrossRef]
72.
McKown, A.D.; Cochard, H.; Sack, L. Decoding Leaf Hydraulics with a Spatially Explicit Model: Principles of Venation
Architecture and Implications for Its Evolution. Am. Nat. 2010,175, 447–460. [CrossRef] [PubMed]
73. Niklas, K.J. A Mechanical Perspective on Foliage Leaf Form and Function. New Phytol. 1999,143, 19–31. [CrossRef]
74.
Kehr, J.; Buhtz, A. Long Distance Transport and Movement of RNA through the Phloem. J. Exp. Bot.
2008
,59, 85–92. [CrossRef]
[PubMed]
75.
Sack, L.; Frole, K. Leaf Structural Diversity Is Related to Hydraulic Capacity in Tropical Rain Forest Trees. Ecology
2006
,87, 483–491.
[CrossRef]
76.
Givnish, T. Comparative Studies of Leaf from: Assessing the Relative Roles of Selective Preassures and Phylogenetic Constraints.
New Phytol. 1987,106, 131–160. [CrossRef]
77.
Fonseca, C.R.; Overton, J.M.C.; Collins, B.; Westoby, M. Shifts in Trait-Combinations along Rainfall and Phosphorus Gradients. J.
Ecol. 2000,88, 964–977. [CrossRef]
78.
Davis, S.D.; Sperry, J.S.; Hacke, U.G. The Relationship between Xylem Conduit Diameter and Cavitation Caused by Freezing. Am.
J. Bot. 1999,86, 1367–1372. [CrossRef]
79.
Baas, P.; Ewers, F.W.; Davis, S.D.; Wheeler, E.A. Evolution of Xylem Physiology. In The Evolution of Plant Physiology; Hemsley,
A.R., Poole, I., Eds.; Academic Press: Oxford, UK, 2004; pp. 273–295. ISBN 9780123395528.
80.
Scoffoni, C.; Rawls, M.; Mckown, A.; Cochard, H.; Sack, L. Decline of Leaf Hydraulic Conductance with Dehydration: Relationship
to Leaf Size and Venation Architecture. Plant Physiol. 2011,156, 843. [CrossRef]
81.
Sack, L.; Dietrich, E.M.; Streeter, C.M.; Sánchez-Gómez, D.; Holbrook, N.M. Leaf Palmate Venation and Vascular Redundancy
Confer Tolerance of Hydraulic Disruption. Proc. Natl. Acad. Sci. USA 2008,105, 1567–1572. [CrossRef]
82.
Ichie, T.; Inoue, Y.; Takahashi, N.; Kamiya, K.; Kenzo, T. Ecological Distribution of Leaf Stomata and Trichomes among Tree
Species in a Malaysian Lowland Tropical Rain Forest. J. Plant Res. 2016,129, 625–635. [CrossRef]
83.
Dahlin, R.M.; Brick, M.A.; Barry, J.; Dahlin, O.R.M.; Brick, M.A.; Ogg, J.B. Characterization and Density of Trichomes on Three
Common Bean Cultivars. Econ. Bot. 1992,46, 299–304. [CrossRef]
84.
Benz, B.W.; Martin, C.E. Foliar Trichomes, Boundary Layers, and Gas Exchange in 12 Species of Epiphytic Tillandsia (Bromeliaceae).
J. Plant Physiol. 2006,163, 648–656. [CrossRef] [PubMed]
85.
Agati, G.; Tattini, M. Multiple Functional Roles of Flavonoids in Photoprotection on JSTOR. New Phytol.
2010
,186, 786–793.
[CrossRef] [PubMed]
86.
Gutiérrez-Alcalá, G.; Gotor, C.; Meyer, A.J.; Fricker, M.; Vega, J.M.; Romero, L.C. Glutathione Biosynthesis in Arabidopsis Trichome
Cells. Proc. Natl. Acad. Sci. USA 2000,97, 11108–11113. [CrossRef] [PubMed]
87.
Hauser, M.T. Molecular Basis of Natural Variation and Environmental Control of Trichome Patterning. Front. Plant Sci.
2014
,
5, 320. [CrossRef] [PubMed]
88.
Prozherina, N.; Freiwald, V.; Rousi, M.; Oksanen, E. Interactive Effect of Springtime Frost and Elevated Ozone on Early Growth,
Foliar Injuries and Leaf Structure of Birch (Betula pendula). New Phytol. 2003,159, 623–636. [CrossRef] [PubMed]
89.
Tian, D.; Peiffer, M.; de Moraes, C.M.; Felton, G.W. Roles of Ethylene and Jasmonic Acid in Systemic Induced Defense in Tomato
(Solanum lycopersicum) against Helicoverpa zea.Planta 2014,239, 577–589. [CrossRef]
90.
Ning, P.; Wang, J.; Zhou, Y.; Gao, L.; Wang, J.; Gong, C. Adaptional Evolution of Trichome in Caragana korshinskii to Natural
Drought Stress on the Loess Plateau, China. Ecol. Evol. 2016,6, 3786–3795. [CrossRef]
91.
Yan, A.; Pan, J.; An, L.; Gan, Y.; Feng, H. The Responses of Trichome Mutants to Enhanced Ultraviolet-B Radiation in Arabidopsis
thaliana.J. Photochem. Photobiol. B 2012,113, 29–35. [CrossRef]
92. Schuepp, P.H. Leaf Boundary Layers. New Phytol. 1993,125, 477–507. [CrossRef]
93.
Lacey, M. From Fossils to Physiology: Testing the Functional Significance of Leaf Shape; University of Washington: Washington, DC,
USA, 2019.
94.
Willson, C.J.; Manos, P.S.; Jackson, R.B. Hydraulic Traits Are Influenced by Phylogenetic History in the Drought-Resistant,
Invasive Genus Juniperus (Cupressaceae). Am. J. Bot. 2008,95, 299–314. [CrossRef]
95.
Moore, J.P.; Lindsey, G.G.; Farrant, J.M.; Brandt, W.F. An Overview of the Biology of the Desiccation-Tolerant Resurrection Plant
Myrothamnus flabellifolia.Ann. Bot. 2007,99, 211–217. [CrossRef]
Biology 2023,12, 107 20 of 21
96.
Kampowski, T.; Demandt, S.; Poppinga, S.; Speck, T. Kinematical, Structural and Mechanical Adaptations to Desiccation in
Poikilohydric Ramonda myconi (Gesneriaceae). Front. Plant Sci. 2018,9, 1701. [CrossRef]
97.
Vieira, E.A.; Silva, K.R.; Oriani, A.; Moro, C.F.; Braga, M.R. Mechanisms of Desiccation Tolerance in the Bromeliad Pitcairnia
burchellii Mez: Biochemical Adjustments and Structural Changes. Plant Physiol. Biochem. 2017,121, 21–30. [CrossRef] [PubMed]
98.
Heilmeier, H.; Hartung, W. Chamaegigas Intrepidus DINTER: An Aquatic Poikilohydric Angiosperm That Is Perfectly Adapted
to Its Complex and Extreme Environmental Conditions. In Plant Desiccation Tolerance. Ecological Studies; Lüttge, U., Beck, E.,
Bartel, D., Eds.; Springer: Berlin/Heidelberg, Germany, 2011; Volume 215, pp. 233–254.
99.
Gaff, D.F. Desiccation Tolerant ‘Resurrection’ Grasses from Kenya and West Africa. Oecologia
1986
,70, 118–120. [CrossRef]
[PubMed]
100.
Shen, Y.; Tang, M.J.; Hu, Y.L.; Lin, Z.P. Isolation and Characterization of a Dehydrin-like Gene from Drought-Tolerant Boea
crassifolia.Plant Sci. 2004,166, 1167–1175. [CrossRef]
101.
Fernández-Marín, B.; Nadal, M.; Gago, J.; Fernie, A.R.; López-Pozo, M.; Artetxe, U.; García-Plazaola, J.I.; Verhoeven, A. Born to
Revive: Molecular and Physiological Mechanisms of Double Tolerance in a Paleotropical and Resurrection Plant. New Phytol.
2020,226, 741–759. [CrossRef] [PubMed]
102.
Jovanovi´c, Ž.; Raki´c, T.; Stevanovi´c, B.; Radovi´c, S. Characterization of Oxidative and Antioxidative Events during Dehydration
and Rehydration of Resurrection Plant Ramonda nathaliae.Plant Growth Regul. 2011,64, 231–240. [CrossRef]
103.
Liu, J.; Moyankova, D.; Djilianov, D.; Deng, X. Common and Specific Mechanisms of Desiccation Tolerance in Two Gesneriaceae
Resurrection Plants. Multiomics Evidences. Front. Plant Sci. 2019,10, 1067. [CrossRef] [PubMed]
104.
Drazic, G.; Mihailovic, N.; Stevanovic, B. Chlorophyll Metabolism in Leaves of Higher Poikilohydric Plants Ramonda serbica Panˇc,
and Ramonda nathaliae Panˇc, et Petrov. during Dehydration and Rehydration. J. Plant Physiol. 1999,154, 379–384. [CrossRef]
105.
Liu, J.; Moyankova, D.; Lin, C.T.; Mladenov, P.; Sun, R.Z.; Djilianov, D.; Deng, X. Transcriptome Reprogramming during Severe
Dehydration Contributes to Physiological and Metabolic Changes in the Resurrection Plant Haberlea rhodopensis.BMC Plant Biol.
2018,18, 351. [CrossRef]
106.
Georgieva, K.; Mihailova, G.; Velitchkova, M.; Popova, A. Recovery of Photosynthetic Activity of Resurrection Plant Haberlea
rhodopensis from Drought-and Freezing-Induced Desiccation. Photosynthetica 2020,58, 911–921. [CrossRef]
107.
Mladenov, P.; Finazzi, G.; Bligny, R.; Moyankova, D.; Zasheva, D.; Boisson, A.M.; Brugière, S.; Krasteva, V.; Alipieva, K.; Simova,
S.; et al. In Vivo Spectroscopy and NMR Metabolite Fingerprinting Approaches to Connect the Dynamics of Photosynthetic and
Metabolic Phenotypes in Resurrection Plant Haberlea rhodopensis during Desiccation and Recovery. Front. Plant Sci.
2015
,6, 564.
[CrossRef]
108.
Asada, K. Production and Scavenging of Reactive Oxygen Species in Chloroplasts and Their Functions. Plant Physiol.
2006
,141,
391–396. [CrossRef]
109.
Oliver, M.J.; Farrant, J.M.; Hilhorst, H.W.M.; Mundree, S.; Williams, B.; Bewley, J.D. Desiccation Tolerance: Avoiding Cellular
Damage during Drying and Rehydration. Annu. Rev. Plant Biol. 2020,71, 435–460. [CrossRef] [PubMed]
110.
Georgieva, K.; Rapparini, F.; Bertazza, G.; Mihailova, G.; Sárvári, É.; Solti, Á.; Keresztes, Á. Alterations in the Sugar Metabolism
and in the Vacuolar System of Mesophyll Cells Contribute to the Desiccation Tolerance of Haberlea rhodopensis Ecotypes. Protoplasma
2017,254, 193–201. [CrossRef]
111.
Cañigueral, S.; Salvía, M.J.; Vila, R.; Iglesias, J.; Virgili, A.; Parella, T. New Polyphenol Glycosides from Ramonda myconi.J. Nat.
Prod. 1996,59, 419–422. [CrossRef]
112.
Jensen, S.R. Caffeoyl Phenylethanoid Glycosides in Sanango racemosum and in the Gesneriaceae. Phytochemistry
1996
,43, 777–783.
[CrossRef]
113.
Feng, W.S.; Li, Y.J.; Zheng, X.K.; Wang, Y.Z.; Su, F.Y.; Pei, Y.Y. Two New C-Glycosylflavones from Boea hygrometrica.J. Asian Nat.
Prod. Res. 2011,13, 618–623. [CrossRef]
114.
Georgieva, K.; Dagnon, S.; Gesheva, E.; Bojilov, D.; Mihailova, G.; Doncheva, S. Antioxidant Defense during Desiccation of the
Resurrection Plant Haberlea rhodopensis.Plant Physiol. Biochem. 2017,114, 51–59. [CrossRef]
115.
Gechev, T.S.; Benina, M.; Obata, T.; Tohge, T.; Sujeeth, N.; Minkov, I.; Hille, J.; Temanni, M.R.; Marriott, A.S.; Bergström, E.; et al.
Molecular Mechanisms of Desiccation Tolerance in the Resurrection Glacial Relic Haberlea rhodopensis.Cell. Mol. Life Sci.
2013
,70,
689–709. [CrossRef]
116.
Mladenov, P.V.; Zasheva, D.; Planchon, S.; Leclercq, C.; Falconet, D.; Moyet, L.; Brugière, S.; Moyankova, D.; Tchorbadjieva, M.;
Ferro, M.; et al. Proteomics Evidence of a Systemic Response to Desiccation in the Resurrection Plant Haberlea rhodopensis.SSRN
Electron. J. 2022,23, 8520. [CrossRef]
117.
Quartacci, M.F.; Gliši´c, O.; Stevanovi´c, B.; Navari-Izzo, F. Plasma Membrane Lipids in the Resurrection Plant Ramonda serbica
Following Dehydration and Rehydration. J. Exp. Bot. 2002,53, 2159–2166. [CrossRef]
118.
Moyankova, D.; Mladenov, P.; Berkov, S.; Peshev, D.; Georgieva, D.; Djilianov, D. Metabolic Profiling of the Resurrection Plant
Haberlea rhodopensis during Desiccation and Recovery. Physiol. Plant 2014,152, 675–687. [CrossRef]
119.
Navari-Izzo, F.; Ricci, F.; Vazzana, C.; Quartacci, M.F. Unusual Composition of Thylakoid Membranes of the Resurrection Plant
Boea hygroscopica: Changes in Lipids upon Dehydration and Rehydration. Physiol. Plant 1995,94, 135–142. [CrossRef]
120.
Zhu, Y.; Wang, B.; Phillips, J.; Zhang, Z.N.; Du, H.; Xu, T.; Huang, L.C.; Zhang, X.F.; Xu, G.H.; Li, W.L.; et al. Global Transcriptome
Analysis Reveals Acclimation-Primed Processes Involved in the Acquisition of Desiccation Tolerance in Boea hygrometrica.Plant
Cell Physiol. 2015,56, 1429–1441. [CrossRef]
Biology 2023,12, 107 21 of 21
121.
Pinnola, A.; Bassi, R. Molecular Mechanisms Involved in Plant Photoprotection. Biochem. Soc. Trans.
2018
,46, 467–482. [CrossRef]
[PubMed]
122.
Mihailova, G.; Vasileva, I.; Gigova, L.; Gesheva, E.; Simova-Stoilova, L.; Georgieva, K. Antioxidant Defense during Recovery of
Resurrection Haberlea rhodopensis from Drought- and Freezing-Induced Desiccation. Plants 2022,11, 175. [CrossRef] [PubMed]
123.
Mitra, J.; Xu, G.; Wang, B.; Li, M.; Deng, X. Understanding Desiccation Tolerance Using the Resurrection Plant Boea hygrometrica
as a Model System. Front. Plant Sci. 2013,4, 446. [CrossRef]
124.
Mihailova, G.; Kocheva, K.; Goltsev, V.; Kalaji, H.M.; Georgieva, K. Application of a Diffusion Model to Measure Ion Leakage of
Resurrection Plant Leaves Undergoing Desiccation. Plant Physiol. Biochem. 2018,125, 185–192. [CrossRef]
125.
Farrant, J.M.; Cooper, K.; Nell, H. Desiccation Tolerance. In Plant Stress Physiology; CABI: Boston, MA, USA, 2000; pp. 248–265.
ISBN 9781845939953.
126.
Georgieva, K.; Sárvári, É.; Keresztes, Á. Protection of Thylakoids against Combined Light and Drought by a Lumenal Substance
in the Resurrection Plant Haberlea rhodopensis.Ann. Bot. 2010,105, 117–126. [CrossRef]
127.
Djilianov, D.; Ivanov, S.; Moyankova, D.; Miteva, L.; Kirova, E.; Alexieva, V.; Joudi, M.; Peshev, D.; van den Ende, W. Sugar Ratios,
Glutathione Redox Status and Phenols in the Resurrection Species Haberlea rhodopensis and the Closely Related Non-Resurrection
Species Chirita eberhardtii.Plant Biol. 2011,13, 767–776. [CrossRef] [PubMed]
128.
Petrov, V.; Hille, J.; Mueller-Roeber, B.; Gechev, T.S. ROS-Mediated Abiotic Stress-Induced Programmed Cell Death in Plants.
Front. Plant Sci. 2015,6, 69. [CrossRef]
129. Kappen, V.L. Sucht an Blättern Einiger Farne Und von Ramonda myconi.Flora 1966,156, 427.
130.
Georgieva, K.; Mihailova, G.; Gigova, L.; Dagnon, S.; Simova-Stoilova, L.; Velitchkova, M. The Role of Antioxidant Defense in