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ScienceDirect Plant Biology

Illuminating the role of the calyptra in sporophyte

development
Jessica M. Budke

Abstract covered by a waxy cuticle which is impervious to water

The study of moss calyptra form and function began almost uptake and thus they acquire water and mineral nutri-
250 years ago, but calyptra research has remained a niche ents from their parental gametophyte [2]. While bryo-
endeavor focusing on only a small number of species. Recent phyte sporophytes can photosynthesize, most taxa also
advances have focused on calyptra cuticular waxes, which acquire the majority of their photosynthates from their
function in dehydration protection of the immature sporophyte parental gametophyte [3]. All of these resources are
apex. The physical presence of the calyptra also plays a role in transferred through the foot, which is located at the base
sporophyte development, potentially via its influence on auxin of the sporophyte [4]. Thus the survival and develop-
transport. Progress developing genomic resources for mosses ment of bryophyte sporophytes are intimately connec-
beyond the model Physcomitrium patens, specifically for spe- ted with the parental gametophyte.
cies with larger calyptrae and taller sporophytes, in combina-
tion with advances in CRISPR-Cas9 genome editing will In mosses, the apical region of the sporophyte also in-
enable the influence of the calyptra on gene expression and teracts with the gametophyte. During the early stages of
the production of RNAs and proteins that coordinate sporo- development, the sporophyte apex is covered by a cap of
phyte development to be explored. gametophyte tissue, which is called the calyptra
(plural = calyptrae; Figure 1a; [5]). The calyptra forms
Addresses early during development from the archegonium and, in
Department of Ecology and Evolutionary Biology, University of
some taxa, also from the subtending gametophyte stem
Tennessee, Knoxville, TN 37996, USA
(Figure 2; [6]). In some species, the calyptra separates
Corresponding author: Budke, Jessica M. (jbudke@utk.edu) from the leafy gametophyte below via a ring of dehiscent
cells (e.g., Funaria in Ref. [7]). Once disconnected from
the rest of the gametophyte the calyptra persists atop
Current Opinion in Plant Biology 2024, 81:102565
the sporophyte apex during capsule expansion
This review comes from a themed issue on Growth and development (Figure 1b) and can remain alive for a time, but does not
2024 continue to grow and ultimately dies [8,9].
Edited by Madelaine Bartlett and Annis Richardson
For a complete overview see the Issue and the Editorial Sporophyte development and the calyptra
Available online xxx The calyptra plays a critical role in moss sporophyte
https://doi.org/10.1016/j.pbi.2024.102565 development (Figure 3). When calyptrae are removed
1369-5266/© 2024 Elsevier Ltd. All rights are reserved, including those during early development under high humidity condi-
for text and data mining, AI training, and similar technologies. tions the apical region remains undifferentiated and the
sporophyte does not transition to capsule expansion and
Keywords
differentiation (Figure 1c,d; [10,11]). Typically during
Auxin, Bryophytes, Cuticle chemistry, Funaria hygrometrica, Gameto- this transition, the seta meristem ceases cell divisions.
phyte, Model systems, Mosses, Physcomitrium patens, Reproduc- However, calyptra removal results in this meristem
tive biology. continuing to divide [12]. Without the calyptra it no
longer builds a narrow cylindrical seta and instead pro-
duces an expanded, obconic-shaped stalk (Figure 1c).
Introduction To test whether this was a physiological and/or physical
Successful development and reproduction are central interaction, calyptrae were experimentally removed,
processes for biological organisms. In the vast majority of boiled using multiple solvents to extract any physio-
plants, the results of fertilization are highly branching, logically active compounds, and then replaced on the
free-living diploid sporophytes, whereas in bryophytes, sporophyte apex [13]. Even after this experimental
the diploid sporophytes are unbranched, physically manipulation, the sporophytes continued through their
attached to and dependent on the parental gametophyte regular developmental transitions, producing both a seta
throughout their life [1]. Bryophyte sporophytes are and capsule with normal morphology. These

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2 Growth and development 2024

Figure 1 auxin results in sporophytes with multiple sporangia,

demonstrating its role in branching suppression in the
moss sporophyte [15,16] The acropetal flow of auxin has
been proposed to play a role in capsule differentiation
[14], and thus calyptra removal during early develop-
ment, which results in a sporophyte that does not
transition to capsule differentiation (Figure 1c,d), may
be due to disruption of this auxin flow. Further studies of
auxin transport in moss sporophytes are needed to
determine which tissues auxin is moving through (i.e.,
epidermis, cortex, and/or central strand), the role
transmembrane proteins play (e.g., auxin efflux PIN-
FORMED [PIN] proteins; [17]), and how the phys-
ical presence of the calyptra may influence
auxin transport.

The moss calyptra also protects the undifferentiated

sporophyte apex from dehydration (Figure 3). Re-
searchers long observed that when calyptrae are
removed during early development under low humidity
conditions, the apex withers and the moss sporophyte
dies [18,19]. The ability of the calyptra to protect the
sporophyte from dehydration was attributed to a waxy
cuticle [7,20]. Despite the early articulation of this
hypothesis, it took nearly 100 years to confirm that a
waxy cuticle is present on the calyptra [21] and that it
develops precociously relative to the cuticle of the
sporophyte [22]. Under low humidity conditions,
experimental removal of calyptra cuticle waxes nega-
tively impacts survival, development, and fitness of the
sporophyte, thus demonstrating the importance of the
calyptra cuticle for dehydration protection [23].
Funaria hygrometrica sporophytes with and without calyptrae. (a,b)
Reproduced with permission from Ref. [21]. (a) Parental gametophyte
calyptra covering an immature, unexpanded sporophyte. Arrow indicates Calyptra morphological diversity
sporophyte apex. (b) Moss calyptra on the top of a mature sporophyte Comparative chemistry may enable us to better un-
capsule. (c,d) Calyptra removed from sporophyte apex in a high humidity
chamber. Sporophyte continues to grow via the activity of the seta meri-
derstand the structure and function of the calyptra
stem and the capsule does not expand or differentiate. (c) Instead of cuticle. In Funaria hygrometrica the calyptra was found
producing a narrow cylindrical seta the meristem produces an expanded to have thicker cuticle wax coverage in comparison to
obconic-shaped stalk. Reproduced with permission from Ref. [5]. (d) the leafy gametophyte (2.0 mg cm 2 versus
Median longitudinal section of the sporophyte apex. Reproduced with
0.94 mg cm 2, respectively; [24]). While the calyptra
permission from Ref. [12].
of this species has a smooth morphology (Figure 1a,b),
other taxa, such as members of the Polytrichaceae and
Orthotrichaceae, have calyptrae that are covered in
observations point toward the physical presence of the epidermal hairs (Figure 4aec). Ongoing research in
calyptra coordinating both seta development and our laboratory is testing the hypothesis that species
capsule differentiation, but further exploration of po- with smooth calyptrae have thicker layers of cuticle
tential physiological influences has not waxes in comparison to species with hairy calyptrae,
been undertaken. due to the added dehydration protection provided by
the hairs. In a separate study, a comparison of three
The plant hormone auxin also plays a role in moss Polytrichaceae species indicated potential differences
sporophyte development (Figure 3). During early in wax chemical composition between two species
development, auxin is transported basipetally through with hairy calyptrae (Pogonatum pensilvanicum and Poly-
the sporophyte and later, during capsule differentiation, trichum juniperinum) and one species with smooth
it is transported both acropetally and basipetally calyptrae (Polytrichadelphus pseudopolytricum; [25]).
[14,15]. Experimental disruption of the basipetal flow of Considering the results from these studies together

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 The role of the calyptra Budke 3

Figure 2

Diagram of moss sporophyte development. Sporophytes begin development completely surrounded and protected by tissues of the parental gameto-
phyte. Initially the sporophyte grows by divisions of a single apical cell. Later a second meristematic region, the seta meristem, produces the seta/stalk,
which elevates the undifferentiated apical region. This region later differentiates into the sporangium/capsule. The sporangium includes cells that will
undergo meiosis to produce haploid spores, a operculum/lid that enables spore release, and an apophysis/neck that, if present, is where stomata are
located in some species. Throughout sporophyte development the apical region and seta meristem are covered by the gametophyte calyptra. Repro-
duced with permission from Ref. [5].

[24,25] a potential pattern emerges. The two species Evolutionary reductions in calyptra size appear to be
with smooth calyptrae (F. hygrometrica in Figure 1a,b correlated with smaller sporophytes and faster life
and P. pseudopolytricum) have wax mixtures that include cycles, but this hypothesis remains to be tested in a
alcohols and low levels of alkanes, but lack aldehydes. phylogenetic context. Calyptra shape ranges from spe-
In contrast, the two species with hairy calyptrae cies that have a narrow rostrum apically that abruptly
(P. pensilvanicum and P. juniperinum) lack alcohols, have transitions to a wider inflated base (Figure 4d,e) to those
high levels of alkanes, and aldehydes are present. that become gradually wider from the top to bottom
These differences in wax composition suggest that (Figure 4feh) to species that have a narrow tube-shape
cuticles on smooth and hairy calyptrae may function throughout (Figure 4iek). These morphologies have
differently, though our current level of understanding historically been divided into two broad categories.
does not enable us to determine the precise functional Cucullate calyptrae have a slit up one side (Figure 4iek)
differences for these compounds. Further exploration and mitrate calyptrae lack a prominent single slit, but
of calyptrae with different hairiness levels will enable can have one to multiple small slits at the bottom edge
us to determine if these patterns are consistent across (Figure 4deh). These morphological disruptions occur
a wider array of species. after calyptra development is complete. They are
caused by the expansion of the underlying sporophyte
In addition to having different levels of calyptra hairi- and are influenced by capsule morphology, which can
ness, calyptrae also vary in size and shape across the range from upright, resulting in mitrate calyptrae, to
approximately 13,000 species of mosses [26]. Calyptra inclined, resulting in cucullate calyptrae. Connecting
size ranges from very small (0.2 mm) in Physcomitrium this structural diversity in size, shape, and hairiness of
(Physcomitrella) patens (Hedw.) Mitt. [27] to relatively the calyptrae to their functional abilities, in terms of
large (up to 20 mm) in Dawsonia polytrichoides R. Br. [28]. protection and developmental coordination as well as

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4 Growth and development 2024

Figure 3

Roles of the calyptra in sporophyte development. These roles include protection from dehydration [18,19] and the physical presence of the calyptra
coordinating development [10–13]. Cuticular waxes on the surface of the parental calyptrae [21,22,24] have been demonstrated to provide dehydration
protection for the sporophyte [23]. Epidermal hairs on the calyptrae may also play a role in dehydration protection, but this has yet to be studied
experimentally. Auxin transport in moss sporophytes occurs basipetally during the early development and later, during capsule differentiation, auxin is
transported both acropetally and basipetally [14,15]. Disruption of the basipetal flow of auxin results in sporophytes with multiple sporangia, demonstrating
its role in branching suppression [15,16]. The physical presence of the calyptra may play a role in coordinating sporophyte development by influencing
auxin transport, but this hypothesis has yet to be test. This illustration was created with BioRender.com.

the diverse habitats these species occupy remains an Species with larger calyptrae and taller sporophytes
underexplored area of study. devote more time to sporophyte development, enabling
this relationship to be studied at multiple develop-
Model systems for studying calyptra mental stages [22] and their larger size facilitates easier
function manipulation experiments [11,23]. These larger species
The majority of research studies that integrate func- are also often morphologically complex, enabling ex-
tional genomics and development in mosses focus on plorations of the developmental influence of the calyp-
the model species Physcomitrium patens (Funariaceae; trae on structures such as the peristome, which has been
[29e33]. This species was developed as a model system identified as a key innovation in the evolution and
due to its rapid life cycle [34], sequenced genome [35], diversification of mosses [37]. Genomic resources have
and its ability to be genetically transformed using ho- been developed for several species that have both rela-
mologous recombination [36]. Research employing tively large calyptrae and sporophytes, as well as com-
P. patens has expanded our understanding of plant growth plex sporophyte morphologies, including Ceratodon
and development, but unfortunately this species is not purpureus (Hedw.) Brid. [38], Syntrichia caninervis Mitt.
an optimal system for studying the functional relation- [39], Takakia lepidozioides S.Hatt. & Inoue [40], Entodon
ships between the parental calyptra and sporophyte. In seductrix (Hedw.) Müll. Hal. [41], Hypnum curvifolium
this species, both the calyptra and sporophyte are Hedw. [41], F. hygrometrica Hedw. [42], and Physcomi-
morphologically reduced (0.2 and 0.4 mm in height, trium pyriforme (Hedw.) Brid. [42]. The latter two spe-
respectively) and the capsule lacks structures that aid in cies, along with P. patens, are in the Funariaceae and thus
spore dispersal, including peristome teeth and an are well positioned for comparative developmental
operculum [27]. The rapid life cycle also results in a studies [43]. Combining these genetic resources with
short time span for studying sporophyte development in advances in CRISPR-Cas9 genome editing [44] will
relation to the calyptra. Due to these challenges, there enable us to develop these species as model systems to
do not appear to be any published studies focusing on study the influence of the calyptra on gene expression
calyptra cuticle waxes or the calyptraesporophyte rela- and the production of RNAs and proteins that coordi-
tionship in P. patens. nate sporophyte development.

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 The role of the calyptra Budke 5

Figure 4 combination with phylogenetic comparative methods

will enable us to expand both our ecological and evolu-
tionary understanding of moss calyptrae.

Declaration of competing interest

The authors declare the following financial interests/
personal relationships which may be considered as po-
tential competing interests: Jessica M. Budke reports
financial support was provided by US National Science
Foundation. If there are other authors, they declare that
they have no known competing financial interests or
personal relationships that could have appeared to in-
fluence the work reported in this paper.

Data availability
No data was used for the research described in
the article.

Acknowledgements
The author’s research on moss calyptrae is supported by a grant from the
United States National Science Foundation [DEB-2046467]. Thanks to
helpful feedback from two anonymous reviewers and to Luke Busta for
thoughtful discussions on calyptra cuticle chemistry that enhanced
the manuscript.

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