Intercellular communication in plants, as in other multicellular organisms, allows cells in tissues to coordinate their responses for development and in response to environmental stimuli. Much of this communication is facilitated by plasmodesmata (PD), consisting of membranes and cytoplasm, that connect adjacent cells to each other. PD have long been viewed as passive conduits for the movement of a variety of metabolites and molecular cargoes, but this perception has been changing over the last two decades or so. Research from the last few years has revealed the importance of PD as signaling hubs and as crucial players in hormone signaling. The adoption of advanced biochemical approaches, molecular tools, and high-resolution imaging modalities has led to several recent breakthroughs in our understanding of the roles of PD, revealing the structural and regulatory complexity of these ‘protoplasmic connecting threads’. We highlight several of these findings that we think well illustrate the current understanding of PD as functioning at the nexus of plant physiology, development, and acclimation to the environment.
]]>Plasmodesmata (PD) are intercellular nanopores that traverse the cell wall of plant cells, connecting adjacent cells and providing a route for movement of molecules between cells. Although PD were first described >100 years ago (see discussion in Gunning, 1976), they are still viewed as 'protoplasmic connecting threads', and we still have a limited understanding of their biogenesis, composition/ultrastructure, how cargo move through these structures, and even their functions. However, with the increasing use of tools of modern molecular biology, proteomics, and especially advanced imaging modalities, we are finally probing critical aspects of the form, formation, and function of PD. Over the last few decades, the community studying PD has provided experimental evidence for PD as conduits for the trafficking of various molecules, including ions, hormones, and small RNA molecules, enabling direct communication and coordination between plant cells. Thus, they play a crucial role in various physiological processes, such as nutrient transport, developmental signaling, and defense responses. Recent findings have reinforced the concept that they are essential for the integrity and function of plant tissues and contribute significantly to the adaptability and resilience of plants in their environment.
PD comprise a continuous plasma membrane (PM) that extends through the cell wall, connecting neighboring cells. A common constituent of PD in land plants is a strand of the endoplasmic reticulum (ER), called the desmotubule, that spans the center of the plasmodesmatal opening (Box 1). The space between the PM and the desmotubule is known as the cytoplasmic sleeve, and to date it has been identified as the main route for intercellular trafficking via PD. The last decade or so has yielded valuable insights into the PM lipid and protein constituents of PD, such that PD are now viewed as highly specialized membrane contact sites (MCS; Brault et al., 2019;Petit et al., 2019;Ishikawa et al., 2020) (Box 1). In this short article, we will highlight some of the progress in elucidating important aspects of the composition and function of PD made in the last few years that extend their roles into signaling and hormone distribution (Box 2). These findings highlight the complexity of PD and uncover even more questions about how these structures are regulated and contribute to integrated tissue-and organism-level responses.
The protein and lipid composition of PD is still being elucidated, although much progress has been made in the last several years (Box 1). Consistent with a single evolutionary origin of PD in land plants (Brunkard and Zambryski. 2017), recent analyses suggest that the plasmodesmatal protein composition is largely conserved across evolution. Analysis of plasmodesmatal proteomes from the moss Physcomitrium patens revealed commonalities with known proteomes from existing angiosperm plasmodesmatal proteomes. Johnston et al. (2023) generated an Arabidopsis proteome from mature leaves and another from mixed tissues of the moss P. patens. The localization of candidate proteins in PD was confirmed by confocal microscopy. A comparative analysis using previous proteomes from Arabidopsis (Fernandez-Calvino et al., 2011;Brault et al., 2019) and Populus trichocarpa (poplar) (Leijon et al., 2018) analysis generated 16 orthogroups of plasmodesmal proteins conserved among these organisms. Several of these orthogroups contained proteins with verified plasmodesmatal localization, for example a glycosylphosphatidylinositol (GPI)-anchored β-1,3-glucanase and a multiple C2 domains and transmembrane region protein (MCTP) (Box 1).
Taking their attempt to identify bona fide plasmodesmatal proteins further, Gombos et al. (2023) leveraged an iterative bioinformatic and experimental approach based on previously published plasmodesmatal proteomes and included experimental verification of localization of identified proteins to PD. A resource that describes the analytical pipeline used for this work is publicly available (https://pddb.uni- hohenheim.de/). Using this approach, they identified members of the GH17 family as moss plasmodesmatal proteins, a Box 1. Structure and composition of plasmodesmata PD are nanopores with outer diameters ranging from 25 nm to 50 nm depending on the tissue and species, and they extend for the thickness of the cell wall, typically ~100 nm. The center of PD of land plants is usually occupied by an appressed strand of cortical ER called the desmotubule (DT) whose lumen is largely occluded by proteins (Tilney et al., 1991) (Fig. 1). The cytosol-filled space between the DT and PM is called the cytoplasmic sleeve or annulus, and it is further divided into nanochannels by the spoke proteins. A nanochannel is typically only 2-3 nm wide, approximately the soluble radius of GFP, but the cytoplasmic sleeve is likely to be the main route for plasmodesmatal trafficking.
Over 1300 proteins were identified in the putative plasmodesmatal proteome of Arabidopsis thaliana suspension cell cultures, and localization of several to PD has been confirmed (Fernandez-Calvino et al., 2011). The plasmodesmatal association of the confirmed PD-localized proteins is supported by recent studies from other plants including moss (Leijon et al., 2018;Gombos et al., 2023;Johnston et al., 2023). Protein functions most often associated with plasmodesmatal proteins include callose metabolism [callose synthases (CALS), β-1,3-glucanases, callose binding], stress responses (peroxidases, calreticulin, LRR kinases), and membrane trafficking (syntaxins, VAMPs, and SNAP proteins). Multiple C2 domains and transmembrane region proteins (Brault et al., 2019) are constituents of PD and they have been designated as the likely spokes of PD (Ding et al., 1992;Brault et al., 2019). The spokes of PD probably control the spacing between the DT and PM, and this distance is correlated with developmental states of the PD (Nicolas et al., 2017). Callose synthases associate with signaling nodes formed by PDLPs and NHL3 (Tee et al., 2023). CML41 signals through PDLPs during defense responses, leading to callose deposition at PD (Xu et al., 2017). Numerous cell wall-associated proteins also localize to PD. EXO and EXO-like proteins and members of the cell wall-modifying XTH family are also found in moss PD (Gombos et al., 2023). The kinase Medtr1g073320 is part of the M. truncatula plasmodesmatal proteome (Kirk et al., 2022).
The lipid composition of plasmodesmatal PMs is distinct from that of the bulk PM. The PMs of PD from Arabidopsis suspension cells are enriched in sterols and sphingolipids with saturated very long chain fatty acids (Grison et al., 2015). This is supported by the presence of the lipid microdomain marker protein remorin (Raffaele et al., 2009;Simon-Plas et al., 2011), and numerous GPI-anchored proteins in the plasmodesmatal proteome (Fernandez-Calvino et al., 2011). Indeed, several ER-PM contact site proteins were identified as part of plasmodesmatal proteomes including synaptotagmins and vesicle-associated membrane protein-associated proteins (VAPs) (Schapire et al., 2008;Wang et al., 2014). Box 1. Continued
finding supported by the identification of callose at moss PD (Renzaglia et al., 2024). Other cell wall-modifying enzymes including XYLOGLUCAN ENDOTRANSGLYCOSYLASE/ HYDROLASE (XTH) proteins were also found as part of the moss proteome. An interesting group of moss plasmodesmalocalized proteins (PDLPs) is the EXORDIUM (EXO) and EXORDIUM-LIKE proteins. These proteins have been implicated in brassinosteroid (BR) signaling and cell expansion (Coll-Garcia et al., 2004;Schroder et al., 2012), an important consideration we revisit later in this discussion.
Another bioinformatics tool, PIP1, makes publicly available an in silico pipeline to identify plasmodesmatal proteins based on homology (https://github.com/PhilPlantMan/PIP1/) (Kirk et al., 2022). The potential utility of this pipeline was demonstrated by the verification of localization of Medtr1g073320 in PD identified from the Medicago truncatula genome. PIP1 also leverages interaction networks to identify experimental conditions that could potentially regulate PD. The advent of these tools and approaches represents an important development in the field of PD and should increase the frequency with which potential plasmodesmatal proteins from diverse plant species are identified.
Alongside questions about the composition of PD are those regarding how cellular cargo is transported to and through PD. Plant viruses have long been used as probes to answer these questions (Heinlein, 2015;Reagan and Burch-Smith, 2020). Identification of plasmodesmal targeting signals on the PLASMODESMATA LOCATED PROTEIN5 (PDLP5) protein and several plasmodesmatal-localized receptor kinases was accomplished by novel computational approaches. This non-conventional signal consists of amino acid sequences in the extracellular region that is located close to the membrane, and these signals could target PM-localized receptor kinases to PD (Luna et al., 2023). This marks a breakthrough in understanding how the plasmodesmatal proteome is assembled, extending work identifying the GPI anchor as a likely plasmodesma-targeting sequence on plant proteins (Zavaliev et al., 2016) and the identification of plasmodesma-targeting sequences of viral movement proteins (Yuan et al., 2016(Yuan et al., , 2018)). Interestingly the DUF26 domains of the PDLPs are structurally similar to carbohydrate-binding lectins, perhaps explaining their involvement in callose homeostasis (Vaattovaara et al., 2019). How mobile cellular RNA cargo is transported in cells to arrive at PD for intercellular and then systemic trafficking has also been examined. Using FLOWERING LOCUS T (FT) mRNA as a probe, cyclophilins of the ROC family were identified as potential adaptors that connect RNA molecules to vesicles that traffic along the cytoskeleton for delivery to PD (Luo et al., 2024). The use of these molecular tools in combination with new computational approaches promises further breakthroughs in unraveling the mechanisms of plasmodesmatal trafficking.
Possibly because of their location within the cell wall, PD are often viewed as static organelles, rather like plumbing in a concrete structure. Instead, PD, both as a collective group and as individuals, are dynamic and respond to developmental and environmental signals. This has been well documented by studies of PD in the cambium of Populus nigra over seasons (Fuchs et al., 2010). These studies and others have shown that the numbers of PD at a given cell-cell interface can change in response to environmental and developmental signals, presumably to provide the optimal degree of connectivity for tissue function. This concept was reinforced by the exciting demonstration that in C 4 plants the number of PD connecting the mesophyll (M) and bundle sheath (BS) changes depending on the light conditions (Schreier et al., 2024). The increased plasmodesmal connectivity is a conserved trait among C 4 plants, resulting in 8-to 13-fold higher plasmodesmatal frequency in C 4 M-BS interfaces compared with closely related C 3 species. Notably, inhibiting photochemical reactions and chloroplast development significantly reduced formation of PD during deetiolation, yet this effect is mitigated by the exogenous supply of sucrose. The necessity of functional chloroplasts becomes evident as sucrose alone proved insufficient to support formation of PD in plants lacking functional chloroplasts (Schreier et al., 2024). These observations extend previous knowledge on the relationships between light and plasmodesmatal development, and between chloroplasts and PD in leaves of C 4 plants (Wang et al., 2017;Danila et al., 2019). It is important to note that the observations about increases in plasmodesmatal density made by Schreier et al. (2024) required the application of high-resolution, three-dimensional imaging (volume EM). This is exemplary of the utility of such techniques and the necessity for the wider, skilled adoption of such approaches by those who study PD. However, caution must be exercised that these tools, which can be expensive in terms of instrument costs and user time, do not make other valid approaches obsolete.
Temperature is another environmental factor that influences PD. This has been well documented to be the case for responses to chilling, where dynamic regulation of callose levels at PD have been shown to modulate intercellular flux in meristems to control dormancy responses (Rinne and van der Schoot, 1998;Bilska and Sowinski, 2010;Rinne et al., 2018;Tylewicz et al., 2018;Gao et al., 2021). While induction of callose at PD and dormancy involve abscisic acid (ABA), release from dormancy involves gibberellic acid (Gao et al., 2021). Recent studies in Arabidopsis reveal that elevated temperatures (30 °C) induce callose accumulation at PD at the sieve element-phloem-pole pericycle (PPP) and PPP-endodermal interfaces where sugars are exiting the phloem (unloading; Liu et al., 2022). This likely defect in sugar export due to reduced plasmodesmamediated intercellular trafficking in the growing parts of the root was proposed to be the cause of slower root growth in plants treated with higher temperatures. Plasmodesmamediated trafficking in angiosperm wood, in the xylem parenchyma, is dependent on the season, indicating responses to environmental signaling (Slupianek et al., 2023).
The dynamics of regulation of PD in growth transition from slow to fast growth after dormancy release were recently studied, revealing a novel mechanism for regulating the callose level at PD. Lillium spp. (lily) VERNALIZATION INSENSITIVE 3-LIKE 1 (LoVIL1), part of the Polycomb Repressive Complex that methylates target loci, promotes plasmodesmatal permeability via epigenetic repression of CALLOSE SYNTHASE 3 (LoCALS3) expression (Pan et al., 2023). This enables cellular communication through PD and sugar symplastic transport in shoot apical meristems (SAMs), activating cell activity and facilitating bulb growth transition. In addition, a novel transcription factor, LoNFYA7, may also play a role in H3K9ac regulation through an unknown mechanism, with H3K9ac potentially fine-tuning LoNFYA7 transcription. It will be interesting to determine whether such a strategy to regulate callose levels at PD is more widespread, potentially opening up new avenues for manipulating PD for biotechnology.
Auxin is well known for its critical roles in plant development, and the PIN, AUX1/LAX, and ABCB proteins are crucial for the active transmembrane transport of auxin (Zhang et al., 2023). However, cell-to-cell trafficking of auxin via PD is also essential for establishing auxin gradients (Gao et al., 2020;Mellor et al., 2020;Band, 2021). These findings have explained previous observations about the involvement of auxin and PD in root development (Han et al., 2014). They have also been supported by numerous subsequent publications that agree with the findings that the PD-auxin axis is crucial for plant development. Lateral root development (Sager et al., 2020), cell division of the root apical meristem (Olatunji et al., 2023), and root branching in response to transient water stress (xerobranching) (Mehra et al., 2022) have all recently been shown to require PD-mediated auxin transport. It should be noted that plasmodesmatal transport of ABA is critical for xerobranching. Importantly, ABA could also negatively regulate PD-mediated intercellular trafficking by inducing expression of PDLP2 and 3, leading to callose accumulation at PD (Mehra et al., 2022). This, in turn, limited auxin cell-to-cell transport via PD and altered radial auxin gradients and root growth.
The role of PD and intercellular trafficking of auxin in leaf development has recently been investigated. Using the gnome (gn) mutants as a starting point, the roles of auxin signaling and transport in leaf vein patterning were investigated (Linh and Scarpella, 2022). The inclusion of mutants with defects in plasmodesmatal callose metabolism (gsl8, cals3) in the analysis demonstrated that the movement of auxin or auxin-generated signals via PD in addition to polar auxin transport was critical for leaf vein patterning. Leaf hyponastic responses have long been known to be auxin dependent. Auxin trafficking via PD was found to act in parallel to and non-redundantly with polar auxin transport by the PIN proteins (Li et al., 2023).
Another exciting recent development concerns the relationship between PD and BR hormones (Wang et al., 2023). Studies in Arabidopsis revealed that BRs can move between via PD. More specifically, the movement of BR precursors over short distances between root cells via PD is necessary for the completion of BR biosynthesis. Further, plasmodesma permeability was regulated by BR levels, with higher BR levels leading to decreased permeability. As was observed for other hormones, callose levels at PD were the determining factor in establishing plasmodesmatal permeability to BR factors and, although the endogenous players in BR-mediated regulation of PD are yet to be determined, BR was found to transcriptionally regulate the expression of many callose metabolism genes. Given the well-established interplay between auxin and BR in plant cell division, it will be very interesting to examine their cooperative effects on PD, if any.
PD have long been understood to have roles in systemic transport of signals (see discussions in Gunning, 1976). It has been proposed that reactive oxygen species (ROS) can travel cellto-cell via PD and act as systemic signals in response to stresses including high light (Fichman et al., 2021). Apoplastic ROS generated by respiratory burst oxidase homolog D (RBOHD) was found to increase plasmodesmatal permeability through its effects on PDLPs to allow wave propagation. The finding that pdlp1 and pdlp5 mutants have lost or have severely reduced propagation of systemic signaling is somewhat surprising since these mutants had no change in intercellular trafficking (Thomas et al., 2008) or had increased intercellular trafficking compared with wild-type plants (Lee et al., 2011). It therefore is unclear how the systemic signals are propagated since these findings would suggest that 'closed PD' with greatly reduced cytoplasmic space between the desmotubule and PM (Box 1) are the conduits for the systemic signal. Are the plasmodesmatal membranes the route for signal transmission? Further complicating the issue, is that ROS have been identified as a potent inducer of callose accumulation at PD (Tee et al., 2023;Vu et al., 2023, and references therein), again leading to 'closed PD' and reduced intercellular trafficking. This raises a question for careful examination and consideration: do the ROS produced in response to different stresses have distinct effects on PD, opening or closing PD to regulate systemic signaling?
Several Ca 2+ -permeable channels were also found to be critical for amplification or maintenance of the ROS signal. The effect of the ROS wave is to initiate acclimation responses that allow plants to cope with the stress that instigated the ROS wave. A mutant screen has identified H 2 O 2 -INDUCED CA 2+ INCREASES 1 (HPCA1) as the ROS receptor in Arabidopsis plants (Fichman et al., 2022). HPCA1 is part of a signaling circuit induced by high light stress or bacterial infection that includes Ca 2+ channels and sensors that regulate ROS production by RBOHD, coordinating systemic ROS and Ca 2+ signaling (Box 2). These and other studies (e.g. Toyota et al., 2018) suggest that symplastic pathways via PD are critical for systemic Ca 2+ signaling. However, recent evidence suggests that wound-induced Ca 2+ waves are instead propagated systemically via the apoplast driven by bulk flow and dependent on amino acid diffusion (Bellandi et al., 2022). Importantly, Bellandi and co-workers found that PD were not involved in Ca 2+ wave propagation since accumulation of callose at PD, rendering them 'closed', had no effect on systemic wound signaling. Further, the wound-induced Ca 2+ wave did not involve RBOHD/F, suggesting a distinct means for signal propagation from that described for high light-or biotic stress-induced waves. Together, these contrasting findings reveal the complexity of systemic signaling, and highlight that both the symplastic and apoplastic routes may have specific contributions to signaling.
Another recent study brings the interplay between symplastic and apoplastic signaling into focus. Using Arabidopsis radical induced cell death1 (rcd1) as a system to study root-toshoot signaling, Jin and co-workers built on their previous work (Liang et al., 2012(Liang et al., , 2014;;Jin et al., 2022) to examine how extracellular ROS, specifically superoxide, act to control the symplastic trafficking of small RNAs. Their findings suggest that extracellular superoxide impacts phytosterol levels in the plasma membrane. These changes then lead to altered numbers of PD and a concomitant change in symplastic trafficking, coupled with defects in the development of pits that regulate sap transport.
Biotic and abiotic stresses trigger a cascade of defense reactions and modulation of the intercellular transport in plant tissues via changes in plasmodesmatal permeability. During plant-virus interaction, viral movement proteins (MPs) ensure the transport of virions or viral nucleic acid-protein complexes using independently evolved strategies. These strategies and the host proteins co-opted for viral intercellular spread have been intensely investigated (Heinlein, 2015;Reagan and Burch-Smith, 2020). More recently, ER-and desmotubule-associated reticulons (Box 1), RTN3 and 6, were found to directly interact with viral MPs from cucumber mosaic virus (CMV), and several other viruses (Tilsner and Kriechbaumer, 2022). However, interaction between CMV MP and RTN3 reduced the ability of the MP to induce increased intercellular movement of the green fluorescent protein (GFP) probe, suggesting that the interaction modulates MP function. Several other ER proteins also interact with the CMV MP (Ham et al., 2023). The interaction between the MP and the ER luminal binding proteins (BiPs) was bridged by the integral membrane AtERdj2a. In contrast to the role of reticulons in limiting MP-mediated increase in intercellular trafficking, BiPs were necessary for viral systemic spread, presumably via their role in cell-to-cell viral spread. The ER lumen localization of BiPs reported in this study challenges the long-held notion that the ER lumen does not occupy the desmotubule, which is instead occluded by proteins that make possible the extreme membrane curvature of the structure. Nonetheless, it is consistent with a few demonstrations of ER lumen continuity between cells (Tilsner et al., 2011). This highlights another aspect of plasmodesma structure that requires further investigation.
Retrograde signaling from chloroplast to nucleus has previously been proposed to control plasmodesma formation and regulation (Burch-Smith et al., 2011;Burch-Smith and Zambryski, 2012). Supporting this idea, potato virus X (PVX) infection triggered Kunitz peptidase inhibitor-like protein (KPILP) expression, which repressed expression of nuclear-encoded chloroplast genes associated with retrograde signaling (Ershova et al., 2022). This was also associated with reduced photoassimilate accumulation and increased intercellular trafficking. Over the last decade or so, numerous studies have highlighted the chloroplast as a critical player in plant-virus interactions (Delprato et al., 2015;Bhattacharyya and Chakraborty, 2018;Bwalya and Kim, 2023). However, it remains to be established if chloroplast-to-nucleus retrograde signaling is commonly involved.
As with revealing the effects of light on development of PD, 3D electron microscopy has also proven vital in revealing more about how PD determine the outcome of plant-virus interactions. Using the phloem-limited reovirus, Southern rice black-streaked dwarf virus (SRBSDV), Lv et al. (2024) employed serial block face-SEM to illuminate the role of PD and irregular PD in preventing viral spread out of the vascularized tumors that are the sites of viral replication. PD with drastic modifications to their typical structures were also observed in those samples (although the authors chose to designate those as 'flexible gateways'). This study exemplifies a promising way to better understand PD in defense, as plasmodesmatal density and viral infection in large numbers of cells can be simultaneously measured, and demonstrates the high resolution at which plasmodesma structure can be determined. Studies of PD-associated bodies (PAMBs) formed by the Hibiscus green spot virus (HGSV) MPs are another example of the power of volume (3D) EM to elucidate the relationship between PD and viruses (Atabekova et al., 2023).
dsRNA is well known for its role in inducing the antiviral RNA silencing machinery (Niehl and Heinlein 2019;Ding, 2023). Building on the finding that dsRNA can also induce other types of defense, namely pattern-triggered immunity (PTI) (Korner et al., 2013;Niehl et al., 2016), Huang et al. (2023) characterized the cellular machinery involved in dsRNAtriggered PTI. Callose accumulation at PD and calcium signaling via CALMODULIN-LIKE protein 41 (CML41) (Xu et al., 2017) were identified as critical components of the PTI response, while ROS, specifically H 2 O 2 , and mitogen-activated protein kinases (MAPKs) 3 and 6 were not implicated, in contrast to most other PTI responses (Lang and Colcombet, 2020). Further, PDLP1, 2, and 3 were identified as involved in callose accumulation but not PDLP5 (Box 2).
PD also have pivotal roles in responding to pathogens besides viruses. Closure of PD in response to microbial elicitors such as chitin and flg22 is orchestrated by specific receptor complexes and modulators; however, notably both chitin and flg22 responses converge on callose synthesis with an apparent central node of response. Recent investigations revealed that one such node involves the interaction of PDLPs and NON-RACE SPECIFIC DISEASE RESISTANCE/HIN1 HAIRPIN-INDUCED-LIKE protein 3 (NHL3), upstream of CALLOSE SYNTHASE 1 (CALS1) activation (Tee et al., 2023) (Box 2). Salicylic aid (SA) elicits a signaling cascade that converges on microbial-triggered responses downstream of ROS production by RBOHD, an NADPH oxidase. PDLP1 and PDLP5, SA-triggered responses, interact with NHL3, establishing a central integrator for plasmodesmal immune signaling (Tee et al., 2023). This suggests that NHL3 interacts with PDLPs to transmit information and activate callose synthesis, ultimately leading to closure of PD in response to diverse immune elicitors.
Effector proteins from fungal pathogens have recently been found to traffic between cells via PD (Cao et al., 2018;Blekemolen et al., 2022;Talbi et al., 2023;Ohtsu et al., 2024), as have those from bacterial pathogens (Aung et al., 2020;Li et al., 2021). Other effectors have been shown to localize to PD and interfere with callose deposition concomitant with increased intercellular trafficking (Tomczynska et al., 2020;Rahman et al., 2021). Together with the myriad studies on virus-PD interactions, these findings reinforce the idea that PD have important roles in the establishment of pathogen infections of plant hosts. In the future we expect that the roles of PD in beneficial hostmicrobe interactions will be investigated with similar intensity to that which has been given to understanding pathogenic interactions. While several studies have revealed the important roles of PD and symplastic trafficking in nodule formation and function, molecular details are lacking. Happily, progress is being made in this area. A member of the TETRASPANIN family of proteins known to associate with PD, TETRASPANIN8-1, was necessary for nodule formation by Rhizobium tropici and arbuscule formation by Rhizophagus irregularis on common bean (Phaseolus vulgaris) (Parra-Aguilar et al., 2023). While TETRASPANIN8-1 does not itself localize to PD, this finding raises the possibility that PD-associated TETRASPANINs may have similar roles in plant-microbe interactions.
With the advent and adoption of new analytical techniques, we expect the next few years to continue to reveal much more about the form, formation, and function of PD. As the finding that PD are critical for correct establishment of auxin gradients that are necessary for development has revealed much about the role of PD, we expect that further investigation of the interplay between PD and other phytohormones will also illuminate other critical aspects of the contributions of PD to plant survival. The rise of advanced microscopy techniques will provide unprecedented insights into the 3D architecture and dynamics of PD. By unraveling the intricate composition of PD with precision, new clues will emerge to elucidate a wide range of mechanisms underlying plant growth, adaptation, and interaction with pathogens, thus advancing agricultural practices and biotechnological applications.
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