4.1 Cell type adaptation to drought: Root endodermis and
exodermis
The diversification of biosynthetic pathways leading to the production
of impermeabilizing hydrophobic compounds has also contributed to the
evolution of distinct cell types important for adaptation to terrestrial
life and for resilience to drought. For example, the roots of all
vascular plants contain an endodermis surrounding the vascular tissues
(Doblas, Geldner, & Barberon, 2017; Enstone et al., 2002). The
endodermis cell layer forms a diffusion barrier for water, gases and
nutrients due to the presence of two cell wall modifications: the
Casparian strip and suberin lamella (Barberon et al., 2016; Doblas et
al., 2017; Seago & Fernando, 2013; Vishwanath, Delude, Domergue, &
Rowland, 2015). Casparian strip is composed of lignin, deposited in the
walls of endodermal cells at their junctions, dividing the layer into
outward and inward polarities and forming an effective barrier to the
apoplastic movement of molecules into the stele and preventing their
backflow (Barberon, 2017; Enstone et al., 2002; Roppolo et al., 2014;
Roppolo et al., 2011). The suberin
lamella is a secondary cell wall modification deposited in the inner
surface of the primary cell walls, usually after the Casparian strip is
formed in the mature endodermis (Barberon, 2017; Enstone et al., 2002).
Different from Casparian strip, the suberin lamella may not form in
every root nor in every endodermal cell (the so-called passage cells)
(Andersen et al., 2018; Barberon et al., 2016; Enstone et al., 2002).
Despite debate in the past years, the role of suberin lamella as an
apoplastic barrier for water and nutrient uptake from the apoplast to
the endodermis cytoplasm has been demonstrated (Barberon et al., 2016;
Ranathunge & Schreiber, 2011; Peng Wang et al., 2019).
The roots of several species also develop an exodermis below the
epidermis, which is a specialized type of hypodermis with Casparian
bands and suberin lamellae depositions (Enstone et al., 2002; Perumalla
et al., 1990). The exodermis function as a dynamic barrier not only
against water loss under drought and salinity, but also against loss of
oxygen under anoxic conditions, against penetration of ions and heavy
metals, and against pathogen infections (Aloni, Enstone, & Peterson,
1998; Damus, Peterson, Enstone, & Peterson, 1997; Ejiri & Shiono,
2019; Enstone et al., 2002; Líška et al., 2016; Namyslov, Bauriedlová,
Janoušková, Soukup, & Tylová, 2020; Ranathunge, Lin, Steudle, &
Schreiber, 2011; Tylová, Pecková, Blascheová, & Soukup, 2017). At the
same time, the development of exodermis barriers has its downside as it
may impair the uptake of nutrients and interaction with beneficial
microbes (Kamula, Peterson, & Mayfield, 1994). To cope with this
problem, many plant species developed the ability to induce an exodermis
dynamically in response to abiotic stresses, such as drought (Enstone et
al., 2002; Kreszies et al., 2020; Líška et al., 2016; Reinhardt & Rost,
1995; Taleisnik et al., 1999). Interestingly, the development of the
exodermis may vary among closely related species displaying distinct
stress response phenotypes (Ejiri & Shiono, 2019), indicating that this
cell type contributes to plant plasticity and acclimation and may also
help plants to adapt and colonize dry environments.
Regardless of its adaptive role, the evolution of exodermis in plants
still remains untangled. Perumalla et al. (1990) surveyed 181 species
from 53 families of plants from different ecological groups
(hydrophytic, mesophytic, and xerophytic) to determine the presence of
hypodermis with Casparian bands (exodermis). As the majority (156) of
the species assessed presented an exodermis with suberin only
(hypodermis) or with both suberin and lignin, the authors hypothesized
that the presence of a modified hypodermis is ancestral to flowering
plants, and has been retained in many species (Perumalla et al., 1990).
Furthermore, the authors found that festucoid grasses lack Casparian
bands despite presenting cells with similar shape and packing as species
with hypodermal Casparian bands, leading to the hypothesis that their
recent ancestor may have lost the trait. Interestingly in seminal roots
of modern cultivars of barley (a festucoid species) the exodermis fails
to develop even upon severe osmotic stress (Kreszies et al., 2019),
while in wild barley the exodermis is induced in response to osmotic
stress (Kreszies et al., 2020). On the other hand, in other crop grasses
(non-festucoid), such as rice and maize, an exodermis is present and
develops faster in response to stress (Ranathunge, Schreiber, Bi, &
Rothstein, 2016; Schreiber, Franke, Hartmann, Ranathunge, & Steudle,
2005).
Understanding how the exodermis evolved in plants can help in the
identification of the underlying regulatory networks responsible for its
induction in response to drought. To obtain more knowledge about the
evolution of exodermis, we compiled the current information about their
presence in plant species based on literature search (Figure 2) (Bani,
Pérez-De-Luque, Rubiales, & Rispail, 2018; Barrios-Masias, Knipfer, &
McElrone, 2015; Barykina & Kramina, 2006; Brundrett, Murase, &
Kendrick, 1990; Calvo-Polanco, Sánchez-Romera, & Aroca, 2014; Damus et
al., 1997; Demchenko, Winzer, Stougaard, Parniske, & Pawlowski, 2004;
Eissenstat & Achor, 1999; Ejiri & Shiono, 2019; Enstone et al., 2002;
Ghanati, Morita, & Yokota, 2005; Kosma, Rice, & Pollard, 2015; Liu et
al., 2019; Perumalla et al., 1990; Ranathunge et al., 2017; Reinhardt &
Rost, 1995; Ron et al., 2013; Schreiber, Franke, & Hartmann, 2005;
Schreiber, Hartmann, Skrabs, & Zeier, 1999; Shiono & Yamada, 2014;
Thomas et al., 2007; Zhang, Yang, & Seago Jr, 2018).
Based on this analysis, the exodermis with suberin first appeared in
early land plants (lycophytes) but it is missing from other seedless
vascular plants and all but one gymnosperms (Damus et al., 1997).
Interestingly, four species in the lycophyte genus Selaginella contain
exodermal with lignified Casparian strips (Damus et al., 1997). Most
flowering plants contain an exodermis with suberization only
(hypodermis), while a lignified exodermis appears in about a third of
the species. The scattered appearances of the exodermal lignification
indicates that it has evolved independently multiple times, suggesting a
high evolutionary pressure and pre-conditioning for the characteristic
to arise. Species with no exodermis have been identified in seven clades
(in purple, Figure 2), and the most parsimonious explanation for
presence/absence of exodermis is the loss of the cell type in these
lineages. However, the evolutionary hypotheses are restricted by the
sparse sampling in families of interest. This is highlighted by the
relevant literature containing contradictions (e.g. pea Bani et al.
(2018); Perumalla et al. (1990); Taleisnik et al. (1999)), likely due to
the dynamic nature of the exodermis.
Evolutionary studies focused on characterizing the exodermis, e.g. by
staining suberin and lignin or using barrier property assays
(Supplementary Table 1) will contribute with important information about
how this cell type has appeared or disappeared multiple times across the
plant lineages. Coupling that with comparative genomics and
transcriptomics of phylogenetically close species (e.g. from the same
family) but with different phenotypes (e.g. non-exodermal, constitutive
and stress-inducible exodermis) will be key to identify the origin of
the regulatory networks and how master regulators underlying exodermis
development and suberization in response to drought evolved. It is
possible that the the sporadic appearance of exodermis during plant
evolution was possible through rewiring regulatory networks of Casparian
strips and suberin lamellae formation in endodermis or similar lignin
and suberin biosynthetic pathways from other cell types. Recent studies
are showing the importance of distinct clades of the MYB transcription
factor family as conserved regulators of suberin deposition in response
to osmotic stress in different cell-types and in phylogenetically
distant plants, linking their evolution with colonization of dry
terrestrial environments by early land plants (Capote et al., 2018;
Cohen, Fedyuk, Wang, Wu, & Aharoni, 2020; Gou et al., 2017; Kajala et
al., 2020; Kosma et al., 2014; Lashbrooke et al., 2016; Legay et al.,
2016; To et al., 2020; Wei et al., 2020). Two of the possible scenarios
are: (1) osmotic stress-inducible regulation of suberization diversified
from pre-existing developmental pathways, (2) the regulation of
suberization in response to drought was re-activated as plants colonized
drier environments. Taking that into account, further
evolutionary/phylogenetic study of exodermis is needed and selection of
a good clade(s) to dissect the gain/loss events is key for understanding
exodermis development and how it evolves so readily.