# Mechanical control of plant organ growth: Lessons from the seed

Jeanne Braat, Benoit Landrein

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Jeanne Braat, Benoit Landrein. Mechanical control of plant organ growth: Lessons from the seed. Current Opinion in Plant Biology, 2025, 85, pp.102737. 10.1016/j.pbi.2025.102737. hal-05201847

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# Mechanical control of plant organ growth: Lessons from the seed

Jeanne Braat and Benoit Landrein

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Plant organ growth is governed by the mechanical properties of individual cells but also by mechanical interactions between adjacent cells and tissues. These interactions generate forces that are sensed, triggering mechanical responses that influence essential cellular processes important for growth and differentiation. However, the extent to which cell mechanical properties and responses to forces shape organ size and form, as well as the molecular mechanisms underlying these processes, remain poorly understood due to the inherent complexity of plant organ morphogenesis. In this review, we highlight recent advancements in understanding the mechanics of plant organ development, focusing on insights gained from studying Arabidopsis seed development. We illustrate how mechanical interactions between tissues contribute to the regulation of seed growth and provide a framework for exploring the role of mechanics in shaping plant morphology.

## Addresses

Laboratoire Reproduction et Développement des Plantes, ENS de Lyon, UCB Lyon 1, CNRS, INRAE, Lyon, France

Corresponding author: Landrein, Benoit (benoit.landrein@ens-lyon.fr)

**Current Opinion in Plant Biology** 2025, 85:102737

This review comes from a themed issue on **Growth and development 2025**

Edited by Daniel Kierzkowski and Margot Smit

For complete overview of the section, please refer the article collection -
Growth and development 2025

Available online 16 May 2025

https://doi.org/10.1016/j.pbi.2025.102737

1369-5266/© 2025 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## Mechanics of plant cell growth

In plants, cell growth depends on the balance between the hydrostatic pressure of the cell and the mechanical properties of the surrounding cell wall [1]. Cell hydrostatic pressure arises from water entering the cell, driven by the osmotic concentration gradient between the solute-enriched cytoplasm and the surrounding environment. It can be regulated by controlling water movement through aquaporins and plasmodesmata, or by modulating osmolyte concentrations within the cell [2]. In rapidly expanding cells, the hydrostatic pressure

can reach values as high as 5–10 MPa [2]. This elevated pressure is counterbalanced by the mechanical resistance to deformation of the cell wall, which has two antagonistic functions associated with specific mechanical properties. The first function, referred to as *mechanical strength*, is to maintain cell integrity and prevent bursting. The second function, referred to as *dynamic extensibility*, is to loosen and yield above a certain turgor pressure, relieving stress and enabling water intake by the cell for growth to occur [3].

Wall loosening is an active process caused by changes in the connectivity of the polymer network. This induces a drop in turgor pressure, which drives uptake of water, increases cell volume, in turn causing a stretching of the loosened wall material. Wall loosening is regulated by expansins and by various enzymes whose activity can be modulated both transcriptionally and post-transcriptionally [4,5]. However, how these regulators influence the organization of the diverse wall components during growth remains a topic of debate. The prevailing hypothesis is that wall yielding relies on the ability of cellulose fibers to slide, which depends on their connections with hemicelluloses and on the properties of the gel-like matrix of pectins in which they are embedded [3,6]. Recently, Haas and colleagues proposed an alternative growth model which is not turgor-driven and emphasize a more central role for pectins [7], but this model remains contentious [8]. Recent studies have also identified RAPID ALKALINATION FACTOR (RALF) peptides as new regulators. They simultaneously function as structural components that organize cell wall architecture by interacting with extensins and influencing pectin compaction, and as feedback signaling molecules through their interactions with receptor-like kinases [9,10]. However, since studies on RALFs have primarily been conducted on tip-growing cells, whose walls are rich in pectin but low in cellulose, their relevance to cells undergoing diffuse growth, such as stems, hypocotyls or seeds, remains uncertain.

## Mechanical control of plant organ growth

Since plant cells are connected through their shared cell walls, plant organ growth depends not only on the mechanical properties of individual cells but also on mechanical interactions between neighboring cells and
tissues. These interactions often imply complex load applications that can lead to instances of localized elevated stress [11,12]. These stresses can be sensed by cells, influencing patterning, growth, and differentiation [13]. A clear example of this occurs during lateral root emergence. This process primarily depends on changes in cell mechanical properties induced by auxin, which affects both the turgor pressure of pericycle cells—destined to form the lateral root primordium—and the mechanical properties of the walls in the overlying endodermis, cortex, and epidermis layers [14,15]. However, mechanical signals also play a role, as recent findings suggest that the radial expansion of pericycle cells reduces stress in the shared pericycledendodermis wall, thereby redirecting auxin flow to endodermal cells and promoting further modifications in their wall properties [16]. Remarkably, lateral root emergence can not only be triggered hormonally, but also by mechanical cues, as observed in root bending and ablation experiments [17,18].

The growth of many aerial plant organs is thought to follow the *epidermal growth control theory*, stating that growth is driven by the pressure of inner tissues under compression but constrained by the mechanical properties of the outer layer, often the epidermis, which is under tension [19]. In this model, the mechanical properties of epidermal walls, often thicker than those of inner tissues to withstand the tensile stress generated by internal pressure in underlying layers [19,20], are critical for organ morphogenesis and are tightly regulated, notably by brassinosteroid hormones [21,22]. Interestingly, this model also suggests that epidermal cells experience specific force patterns influenced not only by cell shape but also by the overall morphology of the organ [23]. These stress patterns have been shown to regulate various cellular processes through mechanoperation, including gene expression, protein polarity, and cytoskeleton organization [13]. They could thus provide positional information, enabling epidermal cells to assess their position within the organ and perceive changes in organ size and shape during morphogenesis. However, testing this hypothesis is challenging because the visualization of mechanical forces is only at an exploratory stage [24], and the molecular mechanisms underlying mechanoperation and mechanotransduction remain largely unknown in plants [13].

## The seed as a model system to study plant organ growth

The developing seed of Arabidopsis serves as a highly attractive model system for studying mechanisms of plant organ growth due to its simple ovoid shape and organization. Seeds in this species consist of three genetically and physically distinct compartments: the embryo, the endosperm and the seed coat, that are nested within one another like Russian dolls (Figure 1a).

Seed growth mostly occurs during the first week following fertilization when the embryo is still very small. It thus largely depends on biochemical and mechanical interactions between the endosperm and the seed coat [25]. The endosperm is a zygotic tissue that forms a single polynucleate cell with a large vacuole, which occupies most of the seed's inner space during early growth. It acts as a hydrostatic pressure source that promotes seed growth. The seed coat is a maternal tissue composed of five protective layers (the two outer layers form the outer integument, the three inner layers, the inner integument) derived from the differentiation of the ovule integuments [26] (Figure 1a). This structure encloses both endosperm and embryo and regulates growth by restricting expansion through its wall mechanical properties.

Cell-to-cell communication *via* biochemical signals plays a crucial role in coordinating the behavior of each compartment during seed development. This is particularly evident after fertilization when auxin produced by the paternal genome in the central cell not only triggers endosperm development but is also transported to the ovule integuments to initiate seed coat differentiation [27,28] by promoting brassinosteroid signaling [29,30]. Recent findings also show that the expression of numerous endosperm-specific genes depends on the presence of the embryo, suggesting the existence of a previously uncharacterized communication pathway between both compartments that acts early during seed development [31]. As seed development progresses, a peptide-based signaling mechanism between the endosperm and embryo has also been identified as essential for proper cuticle deposition at the embryo's surface [32]. These discoveries underscore the critical role of inter-compartmental communication to seed development.

## Mechanical control of seed morphogenesis

Recent research has also shed light on the pivotal role of mechanical signals in synchronizing endosperm and seed coat growth. Similar to other organs governed by epidermal growth control, endosperm pressure puts seed coat cells under tension, triggering responses that affect their mechanical properties [33,34]. Two distinct types of mechanical responses have been identified, each occurring in a specific outer integument layer at a given developmental stage [34,35]. During the early stages of seed morphogenesis, endosperm pressure promotes the organization of cortical microtubules according to shape-driven stresses in the abaxial epidermis of the outer integument, the outermost layer of the seed coat [34] (Figure 1b). As cortical microtubules act as tracks for the polarized deposition of cellulose fibers in the wall [36], this mechanically driven organization promotes the early elongation of the seed. However, this response is transient, with growth becoming progressively less anisotropic until it
<figure><img src="image_4.png" /><figcaption>Figure 1</figcaption></figure>

Mechanical control of seed morphogenesis

a. Organization of the seed compartments during seed morphogenesis (0–8 Days post-anthesis (DPA)).

b. Seed morphogenesis is driven by two distinct mechanical responses triggered in the seed coat's outer integument at different developmental stages. Early on (1–3 DPA), endosperm pressure induces tension in the abaxial epidermis, organizing cortical microtubules (CMTs) according to shape-driven stresses to facilitate initial seed elongation. Later (from 3 DPA onward), this tension strengthens the adaxial epidermis (wall 3) through pectin accumulation, making it load-bearing and restricting further growth, ultimately setting the seed's final size.

becomes fully isotropic approximately three days after pollination (Figure 1b). Despite its temporary nature, this response is still crucial for proper seed shaping, as evidenced by the altered seed morphology observed in mutants with disrupted microtubule organization or impaired cellulose deposition guidance [34].

The response of cortical microtubules to shape-driven stresses in the abaxial epidermis is unsurprising, as such responses have already been well-characterized in other

organs under epidermal growth control [13]. However, the seed coat's outer integument exhibits a distinct response to endosperm-driven stresses that is triggered later during morphogenesis [35]. This response occurs in the adaxial epidermis of the outer integument, the second outermost layer of the seed coat (Figure 1b). Cells in this layer are particularly intriguing because they begin stiffening their inner longitudinal wall (i.e., the wall facing the seed's interior) a few days after pollination, notably by loading it with demethylesterified pectin
[35]. This process is thought to enable this layer to progressively take on a load-bearing role, replacing the abaxial epidermis which differentiates to produce mucilage [25]. Interestingly, this layer also exhibits a specific transcriptional response to mechanical signals, characterized by the induction of the mechanosensitive gene *ELA1* (EUI-LIKE P450 A1), which encodes a gibberellin-degrading enzyme involved in organ growth regulation [35].

Building on these observations, a recent study proposed a mechanical model of seed growth centered on the interactions between the endosperm and the seed coat [37]. This model, described as an *incoherent mechanosensitive feedforward loop*, posits that endosperm pressure-induced tension in the seed coat directly promotes growth by triggering wall relaxation. However, it also inhibits growth indirectly by activating mechanosensitive stiffening of the walls in the adaxial epidermal cells of the outer integument. Through a combination of experimental evidence and computer simulations, the authors demonstrated that this model not only explains how seed size is regulated in wild-type (WT) plants but also accounts for the counterintuitive phenotype observed in the endosperm-defective *iku2* mutant [38]. In this mutant, increased endosperm pressure prematurely stiffens the seed coat walls, resulting in an early cessation of growth and, ultimately, the formation of smaller seeds [37]. The early stiffening of *iku2* seed coat walls is marked by a precocious accumulation of demethylesterified pectins, which could contribute to stiffening if these pectins cross-link with calcium to form rigid egg-box pectic domains [36]. However, mechanical signals may also induce other wall modifications to promote seed coat wall stiffening.

Echoing studies in mammals that identified hydraulics and mechanics as key determinants of embryo size [39], this study offers a possible scaling mechanism for seeds that relies on mechanical signaling. This mechanism may also operate in other plant organs following the epidermal growth control theory, such as leaves, stems, and floral organs, provided that the mechanical properties of epidermal walls are also influenced by the tension exerted by the expansion of inner tissues in these organs. This could notably be the case in developing anthers where recent studies revealed mechanical interactions between inner and outer tissues resembling those described in seeds. In this organ, growth is promoted by the relaxation of the walls in inner tissues but is restricted by the mechanical properties of outer tissues, the epidermis at early stages and the endothecium later on [40,41]. Strikingly, these tissue interactions may not only regulate another morphogenesis but may also be necessary for the correct specification of the germ cells though mechanical signaling [40]. Additionally, echoing findings in the shoot apical meristem where differences in pressure between neighboring cells generate growth

heterogeneities [42], the mechanical model of seed growth underscores the often-overlooked role of active turgor pressure regulation in organ development, as recently highlighted in discussions by Ali and colleagues [43]. However, like other studies aiming at determining the contribution of mechanical signals to organ morphogenesis, this model is limited by our lack of knowledge of the molecular mechanisms of mechanotransception and of mechanotransduction in plants [13].

The recent studies on the mechanical control of seed size and shape highlight the crucial role of the two outer integument layers in seed morphogenesis [34,37]. These layers can modulate their wall properties and transduce mechanical forces into specific cellular responses. The changes in wall properties observed in outer integument cells at specific developmental stages are closely tied to the differentiation programs of each layer, as illustrated by the phenotype of the outer integument differentiation mutant *apetala2* [44,45]. The *ap2* mutant produces large, round seeds and partially rescues the small-seed phenotype of the endosperm mutant *iku2* by preventing the mechanosensitive stiffening of the seed coat [37]. Additional evidence supporting this link comes from several studies showing that the transcription factor SEEDSTICK modulates seed coat wall composition by regulating the expression of genes involved in pectin and xyloglucan metabolism [46–48]. Interestingly, these transcriptional regulators may not only function as differentiation switches but also as finely tuned growth modulators. Indeed, a recent study revealed that the photoperiod regulator CONSTANS adjusts seed size in response to day length by tuning *APETALA2* expression during seed development [49]. Although these findings further support that some regulators of outer integument differentiation control seed growth by modulating wall composition, much remains to be understood about how changes in wall composition translate into changes in mechanical properties. Moreover, whether these transcriptional regulators also enable outer integument cells to exhibit tissue-specific responses to mechanical signals remains an open question.

## Seed development is governed by mechanical interactions

Strikingly, seed morphogenesis may involve additional mechanical interactions beyond those triggered in the seed coat's outer integument by endosperm expansion. Notably, Lima and colleagues recently investigated how brassinosteroids (BR) regulate seed development post-fertilization [30]. Their findings revealed that these hormones are active in the seed coat, where they modulate growth by tuning the expression of wall regulators. They also demonstrated that BR-dependent promotion of seed growth impacts the early development of the coenocytic endosperm, a structure characterized by successive rounds of nuclear division without cytokinesis [30]. Intriguingly, this effect could be replicated by
physically constraining the seeds [30]. This study aligns
with recent research on stems, which shows that BR can
influence the development of inner tissues in a non-cell-
autonomous manner, by relieving the mechanical con-
straints imposed by the epidermis [22]. However, un-
derstanding how the seed coat's physical restriction of
the endosperm influences its development remains
challenging. One plausible mechanism is that the endo-
sperm senses its own size *via* its cytoskeleton, which
guides its proliferation. Supporting this idea, Foteh Ali
and colleagues showed that actin and microtubule cyto-
skeletons form asters around each endosperm nucleus
during coenocytic development, ensuring their even
distribution [50]. Disruption of these asters was found to
impair endosperm proliferation and ultimately affect
seed growth [50]. Finally, embryo development, like that
of the endosperm, may also be influenced by mechanical
restriction from the seed coat. Rolletscheck and col-
leagues indeed demonstrated that imposing mechanical
constraints on developing Brassica embryos, mimicking
those exerted by the seed coat, significantly affects their
development, notably triggering an early metabolic
switch to maturation [51,52].

Conclusion

In recent years, the Arabidopsis seed has emerged as a
powerful model system for studying how mechanical
interactions between tissues and cellular responses to
mechanical signals influence plant organ growth. Based
on this research, the idea that different seed compart-
ments, and even distinct layers within a given
compartment, possess a unique “mechanical identity”
has been promoted. This identity, that may be encoded
in their differentiation program, would not only deter-
mine their mechanical properties but also regulate how
they transduce mechanical signals into tissue-specific
cellular responses. However, as with other plant
organs, much remains to be understood about the mo-
lecular basis of cellular mechanical identities, including
how mechanical properties are controlled and how me-
chanical signals drive tissue-specific cellular responses.

Declaration of competing interest

The authors declare the following financial interests/
personal relationships which may be considered as po-
tential competing interests:

Benoit Landrein reports financial support was provided
by French National Research Agency. If there are other
authors, they declare that they have no known
competing financial interests or personal relationships
that could have appeared to influence the work reported
in this paper.

Acknowledgments

This work was supported by a young researcher
grant from the French Agency of Research (ANR)
(ANR-23-CE13-0009 MECHASEED). We apologize to

colleagues whose work we were unable to cite in this
review due to space limitations.

Data availability

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

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