International Webinar Series

From Cellular Dynamics
to Morphology II

Nov 18 - Dec 16, 2021
Sponsered by the Grant-in-Aid for Scientiﬁc Research on Innovative Areas“Periodicty and its Modulation in Plants”

International Webinar Series ”From Cellular Dynamics to Morphology II”

Program[Session I] Thu 18. Nov. 17:00-19:30 JST; 19:00-21:30 AEDT
Chair: Hirokazu Tsukaya (University of Tokyo), Motomu Endo (NAIST)

Periodic plant organogenesis and its modulation at the shoot apex
Marcus Heisler (University of Sydney, Australia)
Mathematical model analysis on the generation of a steep spiral of spiromonostichy
Takaaki Yonekura (University of Tokyo, Japan)
Dimorphic leaf development in the aquatic plant Callitriche palustris
Hiroyuki Koga (University of Tokyo, Japan)[Session II] Wed 24. Nov. 17:00-19:30 JST; 9:00-11:30 CET
Chair: Masahiko Inami (University of Tokyo), Yohei Kondo (ExCELLS)

Towards new paradigms of movement for adaptive robots: lessons from plants
Barbara Mazzolai (Istituto Italiano di Tecnologia, Italy)
Human augmentation x Botany: Seeing touch, experiencing data, and plant inspired
physicality
Zendai Kashino (University of Tokyo, Japan)
P-MIRU (Polarized Multi-spectral Imaging of Reflection compatible with field Use)
Polarized light as a quick exploration tool for botanical and biological research
Alfonso Balandra (University of Tokyo, Japan)[Session III] Fri 26. Nov. 17:00-19:30 JST; 9:00-11:30 CET
Chair: Yoshihisa Oda (National Institute of Genetics), Minako Ueda (Tohoku University)

Are microtubules their own mechanosensors?
Olivier Hamant (INRAE/ENS de Lyon, France)
Secretory activity is required for normal morphogenesis of the oil body in Marchantia
polymorpha
Takehiko Kanazawa (National Institute for Basic Biology, Japan)
Dynamics of microtubule structure during cell division in land plants
Takema Sasaki (National Institute of Genetics, Japan)

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session IV] Tue 7. Dec. 9:00-11:30 JST; Mon 6. Dec. 18:00-20:30 CST
Chair: Takashi Ueda (National Institute for Basic Biology), Takayuki Kohchi (Kyoto University)

Flattening the curve: Mechanosensitive ion channels in plant cell and developmental biology
Elizabeth Haswell (Washington University in St. Louis, USA)How plants recover from injury and reconstruct their bodies
Momoko Ikeuchi (Niigata University, Japan)
New fluorescent probes for the analysis of plant cell behavior
Yoshikatsu Sato (Nagoya University, Japan)[Session V] Fri 10. Dec. 17:00-19:30 JST; 9:00-11:30 CET
Chair: Hidehiro Fukaki (Kobe University), Atsushi Mochizuki (Kyoto University)

Lateral root priming emerges from synergy between root tip growth and auxin reflux dynamics
Kirsten ten Tusscher (Utrecht University, The Netherlands)
A theoretical model for leaf shapes with or without branches
Akiko Nakamasu (Kumamoto University, Japan)
Polymorphism in the symmetries of gastric pouch arrangements in the sea anemone D. lineata
Safiye E. Sarper (Osaka University, Japan)[Session VI] Thu 16. Dec. 17:00-19:30 JST; 9:00-11:30 CET
Chair: Keiji Nakajima (NAIST), Hidehiro Fukaki (Kobe University)

The phloem nexus of root meristem organization
Christian Hardtke (University of Lausanne, Switzerland)
Robust control of the cambium maintenance during vascular development
Tomoyuki Furuya (Kobe University, Japan)
Root nodule formation controlled by host molecular networks in response to nitrogen-fixing
bacteria in Lotus japonicus
Takashi Soyano (National Institute for Basic Biology, Japan)

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session I] Thu 18. Nov. 17:00-19:30 JST; 19:00-21:30 AEDT

Periodic plant organogenesis and its modulation
at the shoot apex

Marcus Heisler
University of Sydney, Australia

Over the last decade or so important progress has been made in identifying and understanding
a core set of patterning mechanisms that help shape plant aerial architecture. These include
a feedback loop between mechanical stress and interphase microtubules, the role of auxin andmechanical stress in regulating cell polarity and the role of adaxial and abaxial cell typeboundaries in modulating auxin response. What is perhaps most intriguing is how thesemechanisms integrate in a combinatorial manner to provide a means to generate a large
variety of commonly seen plant morphologies. Here I will discuss our current work onunderstanding these mechanisms and the links between them.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session I] Thu 18. Nov. 17:00-19:30 JST; 19:00-21:30 AEDTMathematical model analysis on the generation ofa steep spiral of spiromonostichy

Takaaki Yonekura
The University of Tokyo, Japan

Most plants are known to exhibit a regular and periodic pattern of leaf arrangement, and thispattern is called phyllotaxis. In particular, spiral phyllotaxis, in which leaves are arranged at aconstant angle to each other, have interested mathematicians and physicists because theirdivergence angle is related to the Fibonacci sequence, and the requirements for theirgeneration have been theoretically elucidated. As a result, it has come to be considered thatthe molecular entity of the process is that the position of a new primordium is determined insuch a way as to avoid the effects of inhibition emitted by existing leaf primordia. Recent
progress in molecular biology has also revealed that the essence of the inhibitory effect is the"up-the-gradient dynamics" of auxin transport toward higher auxin concentrations.
Despite the progress of research, the requirements for the generation of minor
phyllotactic patterns remain largely unknown. Using morphological analysis andmathematical modeling, we are trying to elucidate the requirements for the generation ofthese unaddressed patterns. Recently, we have focused on the requirements for the generationof spiromonostichy in Costaceae. Spiromonostichy is a kind of spiral phyllotaxis withextremely small divergence angle. It has been called a "genuine puzzle" because it seems todisagree with the inhibitory effect-based mechanism. We focused on the extremely large
plastochron ratio of the Costaceae and found that the inductive effect from existing leaf
primordia is necessary to explain this generation. Based on this hypothesis, we developed a
new mathematical model and found that this model can explain the generation of
spiromonostichy. Based on the result, I would like to give an overview of the extent to which
the previously reported phyllotactic patterns can be explained.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session I] Thu 18. Nov. 17:00-19:30 JST; 19:00-21:30 AEDT

Dimorphic leaf developmentin the aquatic plant Callitriche palustris

Hiroyuki Koga
The University of Tokyo, Japan

Aquatic plants, the plants that grow on wetlands or in water, often show significant phenotypic
plasticity in the leaf form. When the shoots of such plants are submerged in water, theysometimes form specific leaves that are drastically different from the leaves formed in the air.
This ability is called heterophylly. It is of interest in both developmental and evolutionary
biology because it provides an opportunity to elucidate the different modes of leaf
development in the same individual. We are addressing the molecular developmental basis of
the heterophylly and the evolutionary mechanism for acquiring such drastic phenotypic
plasticity, using Callitriche plants (water-starworts: Plantaginaceae, Lamiales) as a model. We
established laboratory culture lines of a heterophyllous species C. palustris and its closely
related species and explored the genetic factors involved in the differential leaf formation bycomparing gene expression patterns during the leaf development. In this talk, I will discuss
how C. palustris controls dimorphic leaf development depending on the surrounding
environment, and the differences and similarities in the mechanism of heterophylly to theother aquatic plants that acquired heterophylly independently.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session II] Wed 24. Nov. 17:00-19:30 JST; 9:00-11:30 CET

Towards new paradigms of movement
for adaptive robots: lessons from plants

Barbara Mazzolai
Istituto Italiano di Tecnologia, Italy

Nature is adaptive, constantly learning and evolving. In the 3.8 billion years, nature has
evolved and selected billions of species with characteristics most suited to their environment
to be more likely to survive and to sustain life for generations. Perfection is not nature’s goal.
Nature focuses on evolution. Therefore, nature experiments, re-combines the existing andadapts according to results. Thus, why are roboticists looking at natural organisms for the
robot revolution? By looking at natural organisms’ life and evolution strategies, nature canprovide engineers with the rules to design novel materials, devices able to adapt to
unstructured and even dangerous environments, and develop new manufacturing processes.
In this talk, I will present our approach to bioinspiration based on the investigationof plants and soft animals' features, with the double goal to identify and extract the key
principles underlying these biological functions and to translate them in a technologicalsolution, and to improve scientific knowledge on the biological system that we take as a model.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session II] Wed 24. Nov. 17:00-19:30 JST; 9:00-11:30 CET

Human augmentation x Botany: Seeing touch,experiencing data, and plant inspired physicality

Zendai Kashino
The University of Tokyo, Japan

Human Augmentation is a field that commonly aims to extend human ability using machines.
However, the essence of human augmentation is not to extend one’s ability through external
assistance, but to augmented one’s ability while maintaining a sense of physicality. By
achieving human augmentation while maintaining physicality, we can allow the augmented tofeel a sense of ownership their actions and perceptions. This, in turn, allows the conversion
of “information” into “experiences” which can facilitate an intuitive understanding of thenatural world.
As humans, botanists are both a subject of research and potential application area inthis field. Through human augmentation, it may be possible to allow an understanding ofplants which was previously unimaginable, leading to new avenues of botanical research. In
this talk, I introduce some of our work which aims to achieve this by imbuing botanical dataand its acquisition a sense of physicality and touch on how working with botanists has, in turn,inspired exploration into novel forms of human physicality as well.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session II] Wed 24. Nov. 17:00-19:30 JST; 9:00-11:30 CET

P-MIRU (Polarized Multi-spectral Imaging of Reflection
compatible with field Use) Polarized light as a quick
exploration tool for botanical and biological research

Alfonso Balandra
The University of Tokyo, Japan
Usually, the presence or absence microstructures in the leaves and flowers of plants could beobserved using circular polarization imagining, since these structures change the polarization
characteristics of the incident light, these could be responsible of hidden interactions with
pollinators capable to perceive such changes in light polarization in the surface of plants’flowers and leaves. P-MIRU is an imaging system that visualizes the surface and polar surface
structure of an object by irradiating the object with linearly polarized light of monochromatic
light and measuring and analyzing the degree of change in the reflected light. The system alsoenables the quick management and analysis of the captured data using cloud services. The
capture and quick analysis system will enable the rapid exploration of a great number ofbotanical and biological specimens.
In this talk we are going to overview the P-MIRU imagining system. First the project
origin and motivations are going to be explained. Then the talk will follow with an
introduction of the optical and electronic technologies used to develop the system and ageneral description of how the complete system works. Also, the current technique used toanalyze the degree of change in the reflected light will be introduced (Stokes Parameters). In
the talk P-MIRU’s last obtained results and the current status of the project will be presented.
Finally, future applications of the camera and future features for data management andanalysis using free cloud services will be discussed.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session III] Fri 26. Nov. 17:00-19:30 JST; 9:00-11:30 CET

Are microtubules their own mechanosensors?

Olivier Hamant
INRAE, CNRS, UCBL, ENS de Lyon, France

In plant cells, cortical microtubules (CMTs) generally control morphogenesis by guiding thesynthesis of stiff cellulose microfibrils in the wall. CMT orientation has been proposed todepend on geometrical cues, with microtubules aligning with the cell long axis in silico and invitro. Yet, CMTs are usually transverse in vivo, i.e., along predicted maximal cortical tension,
which is transverse for cylindrical pressurized vessels. Here, we confined protoplasts laterally
to impose a curvature ratio and modulated pressurization through osmotic changes. We findthat CMTs can be longitudinal or transverse in wall-less protoplasts and that the switch in
CMT orientation depends on pressurization. In particular, longitudinal CMTs become
transverse when cortical tension increases, consistent with observations in planta. To searchfor upstream regulators, we investigated the contribution of Receptor-Like Kinase (RLK) tothe CMT response to tensile stress. We found that both CMT and RLK pathways
independently control the mechanical integrity of the cell. Conversely, when both RLK
signaling and CMTs are impaired, plant cells behave like passive material. Altogether, theseresults reveal the key role of microtubule response to cortical tension in plant cellmorphogenesis, and further support the idea of an autonomous microtubule mechanosensing
pathway.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session III] Fri 26. Nov. 17:00-19:30 JST; 9:00-11:30 CETSecretory activity is required for normal morphogenesisof the oil body in Marchantia polymorpha

Takehiko Kanazawa
National Institute for Basic Biology, Japan

Eukaryotic cells possess a variety of endomembrane organelles with specific sets of proteins,
lipids, and polysaccharides to fulfill functions specific to each organelle. Delivery andlocalization of the various substances are accomplished by membrane traffic. In addition to
endomembrane organelles conserved among eukaryotes, eukaryotic cells have acquired
unique organelles in a lineage-specific manner. The emergence of a novel endomembraneorganelle is thought to be accompanied by the development of novel membrane trafficking
pathways to and from the organelle, whose mechanism, however, still remains elusive. We areinterested in the oil body, which was uniquely acquired by liverworts in bryophytes, as a modelof lineage-specific organelles. Recently, we discovered that secretory cargos are targeted tothe oil body in oil body cells in the liverwort, Marchantia polymorpha, which indicated thatthe oil body is formed by the redirection of the secretory pathway followed by fusion of
secretory vesicles inside the cell (Kanazawa et al., 2020). We also isolated mutants withaberrant morphology of the oil body, one of which was caused by a mutation in a key
machinery component of secretory trafficking. This finding suggests that unknown factors
required for normal oil body morphogenesis are transported to the oil body via the redirected
secretory pathway.
Kanazawa et al., (2020) The liverwort oil body is formed by redirection of the secretory
pathway. Nat Commun, 11: 6152

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session III] Fri 26. Nov. 17:00-19:30 JST; 9:00-11:30 CET

Dynamics of microtubule structure during cell divisionin land plants

Takema Sasaki
National Institute of Genetics, Japan

Eucaryotes have evolved diverse mechanisms for cell division. Land plants undergo cell
division by forming cell plates, which eventually partition their cytoplasm. Proper positioning
and construction of the cell plate require plant-unique microtubule structures, preprophaseband (PPB) and phragmoplast. We have identified a novel microtubule-associated protein
family, CORD (Sasaki et al. 2017 Plant Cell). CORD is conserved exclusively among land
plants, implying that CORD is involved in the evolution of microtubule structures in land
plants. In Arabidopsis, CORD genes were expressed in various tissues including xylem vessels
and mitotic meristem. We revealed that Arabidopsis CORD localizes to mitotic microtubules
and regulate the dynamics of microtubules in phragmoplast (Sasaki et al. 2019 Curr Biol). We
recently found that, in liverwort, CORD plays distinct roles in mitotic microtubules. Here, I
will present how CORD is involved in mitotic microtubules in land plants and will discusspossible roles of CORD in the evolution of plant cell division.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session IV] Tue 7. Dec. 9:00-11:30 JST; Mon 6. Dec. 18:00-20:30 CST

Flattening the curve: Mechanosensitive ion channels
in plant cell and developmental biology

Elizabeth Haswell
Washington University in St. Louis, USA

Mechanoperception, the fundamental process by which a physical stimulus is transduced intoa biochemical response, is ancient, universal, and critical for cellular function of organisms
across the evolutionary tree. Plants must sense gravity, water availability, pathogens, wind,and soil–all while regulating the internal forces that govern cell shape and tissuemorphogenesis. We view the membrane as an efficient platform for the perception of internaland external mechanical stimuli and are especially interested in the mechanosensory role of aclass of intrinsic membrane proteins called mechanosensitive (MS) ion channels. MS channels
are protein pores in the membrane that open in response to lateral tension, translating
physical force into a biochemical signal. An open MS ion channel can serve as an osmotic
release valve, releasing hydrostatic pressure by allowing ions to exit and/or as a receptor byallowing calcium to enter. Over the past 15 years, we and others have discovered that plant
MS ion channels from multiple families transport distinct ions with a range of efficiencies andtension sensitivities, localize to different cellular membranes and perform many different
physiological and developmental roles. In my talk, I will discuss our recent published andunpublished investigations into the role of MS ion channels found in the vacuole and theplasma membrane of pollen grains, pollen tubes, and other tip-growing cells.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session IV] Tue 7. Dec. 9:00-11:30 JST; Mon 6. Dec. 18:00-20:30 CST

How plants recover from injury and reconstruct their bodies

Momoko Ikeuchi
Niigata University

Life is full of damaging stresses. Plants generate callus upon injury to heal wound sites, yet
regulatory mechanisms of tissue repair remain elusive. We recently identified WUSCHELRELATED HOMEOBOX 13 (WOX13) as a key regulator of callus formation and organ
adhesion in Arabidopsis thaliana (Ikeuchi et al., 2021). Strikingly, wox13 mutant is totally
deficient in organ reconnection in petiole grafting system, suggesting that WOX13 is pivotal
for the establishment of organ reconnection. We uncovered that WOX13 directly regulates
various cell wall-modifying enzyme genes. Furthermore, the chemical composition of cell wallmonosaccharides was markedly different in the wox13 mutant. These data together suggestthat WOX13 modifies cell wall properties, which may facilitate efficient callus formation andorgan reconnection. Noteworthy, WOX13 belongs to the ancient subclade of WOX family,
whose sequence is widely conserved across land plants. Our data suggest that molecularfunction of WOX13 is partially conserved between the moss Physcomitrium patens andArabidopsis. I will discuss an evolutionary perspective on regulatory mechanisms of cellular
reprogramming upon injury.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session IV] Tue 7. Dec. 9:00-11:30 JST; Mon 6. Dec. 18:00-20:30 CST

New fluorescent probes for the analysis of plant cell behavior

Yoshikatsu Sato
Nagoya University, Japan

Plants have a sessile lifestyle so that must withstand any stress where they germinated, even
if they are placed in an unfavorable condition. In order to adapt to fluctuating environmental
conditions, plants have evolved their own developmental processes and physiologicalresponses during the course of evolution. The control of turgor pressure is one of the notable
examples, which is the intracellular force that pushes the cell membrane against the cell wall.
Remarkably, the force is comparable to that of a car tire and plants use the force for their
growth as well as their posture regulation. It remained unknown how plant cells would behave
when they encountered physical disturbance that were not easily altered by turgor pressure.
We used microfluidic devices to analyze the ability of plant cells to penetrate narrow gaps in
tip-growing cells, such as pollen tubes and root hairs in angiosperms, and the moss
protonemata and showed that all tip-growing cells we tested have a capacity to deform theirshape and penetrated the 1 µm gap (Yanagisawa et al. 2017 Sci Rep 7 1403). These abilities
are thought to be useful in growing narrow spaces of female tissue for pollen tubes, and inelongating the narrow spaces of soil particles for root hairs. We realized that new tools areneeded to further understand the mechanical properties in cell wall and the dynamics of cell
nuclei that maintain a certain distance from the elongation end. In this talk, I will present thelatest status of our imaging analysis platform aimed at understanding the mechanicaloptimization of tip-growing cells with flexible mechanical properties.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session V] Fri 10. Dec. 17:00-19:30 JST; 9:00-11:30 CET

Lateral root priming emerges from synergy
between root tip growth and auxin reflux dynamics

Kirsten ten Tusscher
Utrecht University, The Netherlands

Root system architecture critically determines a plants access to water and nutrients. As aconsequence, root architecture has evolved to be a highly plastic property, with the number,
length and orientation of root system branches adapting to water and nutrient levels as wellas their spatial distribution. Still, the earliest stage of lateral root formation, called priming, inwhich groups of cells gain the competence for future lateral root development, appears to behighly regular in nature.
In Arabidopsis, priming has been characterized to involve regular temporaloscillations in auxin level and/or signaling in the root tip, accompanied by fluctuations in alarge number of genes. Through growth, these temporal oscillations subsequently become
translated into spatially periodic patches of primed cells along the primary root axis. Previous
studies demonstrated that auxin production, transport and sensing are all critical for priming,
while also a major correlation with root growth was observed. This led us to speculate thatpriming may arise from an interplay between root tip auxin and growth dynamics. Given theemergent nature of such a hypothesized mechanism, we used a computational modelingdriven approach for our investigations.
From our modeling approach we deciphered that the so-called reflux loop dictatingroot tip auxin transport dynamics results in a zone of auxin loading at the boundary of themeristem and elongation zone. Additionally, stem cell driven growth dynamics result inperiodic variations in the size at which cells arrive at this, driving oscillatory fluctuations intheir passive auxin uptake capacity. Combined this explains the experimentally observed auxin
oscillations. We experimentally validated the predictions resulting from our uncovered
priming mechanism and contrasted these with predictions emanating from alternative,
previously suggested mechanisms. Importantly, our priming mechanisms at least partly
explains adaptation of main and lateral root properties to environmental factors such as
nutrient levels as two sides of the same coin, rather than needing to invoke two differential
adaptation mechanisms.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session V] Fri 10. Dec. 17:00-19:30 JST; 9:00-11:30 CET

A theoretical model for leaf shapes with or without branches

Akiko Nakamasu
Kumamoto University, Japan

Diversity in leaf shapes can be observed among plant species or even in a single plant. We areusing a simple model for the understanding of how leaf shapes are generated autonomously.
In this model, we just treat deformations of a leaf boundary. This assumption may extremely
simple, though, it can show characteristics observed in leaf shapes.
For example, growths coupled with a periodicity on the boundary give a steadysequence of a development of branch pattern. It can explain asymmetries and developmental
changes in leaf divarications. Then, other leaves than branches can also be obtained bymodulations of the periodical growth.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session V] Fri 10. Dec. 17:00-19:30 JST; 9:00-11:30 CETPolymorphism in the symmetries of gastricpouch arrangements in the sea anemone D. lineata

Safiye E. Sarper
Osaka University, Japan

Symmetry in the arrangement of organs and body parts is the distinctive morphologicalfeature of plants and animals. Compared to the recurrent evolution of plant symmetry(especially between bilateral and radial symmetries of flowers in many angiosperm orders),
such symmetry evolution limitedly appeared in animals where most animal phylum always
demonstrate bilateral symmetry. In animals, cnidaria (jellyfish, coral, sea anemone) is a rare
phylum including both bilateral and radial symmetrical species. In cnidarians, symmetries
appear in their internal organs, such as gastric pouches and muscles. However, how different
symmetries appear during the developmental process remains unknown. In this talk, we'll
report intraspecific variations in the symmetric arrangement of gastric pouches, muscles, andsiphonoglyphs, the Anthozoan-specific organ that drives water into the organism, in D. lineata(Diadumenidae, Actiniaria-sea anemone). We found that the positional arrangement of theinternal organs was constrained to either biradial or bilateral symmetries depending on thenumber of siphonoglyph organs. Based on the morphological observations, a mathematicalmodel of internal organ positioning was employed to predict the developmental backgrounds
responsible for the biradial and bilateral symmetries. In the model, we assumed that the
specification of gastric pouches is orchestrated by lateral inhibition and activation, which
results in different symmetries depending on the number of siphonoglyph organs. Thus, we
propose that a common developmental program can generate either bilateral or biradialsymmetries depending on the number of siphonoglyph organs formed in the early
developmental stages.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session VI] Thu 16. Dec. 17:00-19:30 JST; 9:00-11:30 CETThe phloem nexus of root meristem organization

Christian Hardtke
University of Lausanne, Switzerland

Higher animals and plants are two evolutionary outcomes of complex multicellularity. An
important driver of cellular specialization and body plan expansion in both phyla was theevolution of vascular distribution networks. Most prominently, the cardiovascular network of
mammals distributes oxygen and nutrients throughout the body, driven by the pumping
action of the heart. By comparison, the vascular transport systems of the dominant group of
higher plants, the angiosperms, are fundamentally different. For instance, transport ofphotosynthates in the phloem network is governed by dynamic source-sink relations anddriven by a pressure differential that builds up through locally controlled cellular osmolarity.
The conducting phloem channels are the so-called sieve tubes, which not only transport
nutrients but also developmental signals such as the phytohormone auxin. Sieve tubes areformed from interconnected individual sieve elements that are meticulously aligned in cell
files as they develop. Sieve elements represent a cellular between-life-and-death state,because they lack numerous organelles, including the nucleus, and are nurtured by theirneighboring, so-called companion cells. The peculiar sieve element differentiation process
can be observed in growth apices. For example, in the root meristem of Arabidopsis thaliana
seedlings their development from stem cell into mature sieve element is laid out in a
spatiotemporal gradient. My lab has characterized a molecular network that guides thisdifferentiation process through the interplay between controlled transcellular auxin transport
across developing sieve element cell files and autocrine receptor kinase signaling pathways
that respond to small peptide ligands. Key players in this network constitute a molecularrheostat that finetunes transcellular auxin flux and contributes to auxin canalization indeveloping sieve elements. Such intersection with phytohormone pathways also revealed a keyrole of developing protophloem in root meristem organization, in a sometimes non-cellautonomous, compensatory fashion.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session VI] Thu 16. Dec. 17:00-19:30 JST; 9:00-11:30 CET

Robust control of the cambium maintenanceduring vascular development

Tomoyuki Furuya
Kobe University, Japan

Like trees, plants undergo continuous radial growth owing to the activity of vascular stem cells
in the cambium. Thus, vascular stem cells should be maintained throughout the radial growth.
However, it was not fully understood that the molecular mechanism for balancing between
the proliferation and the differentiation in vascular stem cells. Here, to discover novel
regulators for vascular stem cells, we newly constructed a gene co-expression network
equipped with temporal and cell-type information during vascular development by combining
multiple transcriptome datasets of an ectopic vascular cell differentiation system, VISUAL.
We extracted a total of 394 cambium-related genes based on a gene co-expression network.
Of them, a BES/BZR transcription factor BEH3 was found as a potential stem cell regulator.
The beh3 mutants exhibited a larger variation in vascular size than the WT, suggesting therole of BEH3 as a stabilizer of vascular development. BEH3 had a weaker transcriptional
repressor activity than other BES/BZR homologs known as negative regulators of vascular
stem cell maintenance and inhibits the activity of these other factors via competition to bindto the same DNA motif. Indeed, mathematical modelling supported that the competitive
relationship among BES/BZR members leads to decreasing “phenotypic variation” in vascularsize, demonstrating the importance of the opposite-behaving member in the robust regulationof stem cell activity.

International Webinar Series ”From Cellular Dynamics to Morphology II”[Session VI] Thu 16. Dec. 17:00-19:30 JST; 9:00-11:30 CETRoot nodule formation controlled by host molecular networks
in response to nitrogen-fixing bacteria in Lotus japonicus

Takashi Soyano
National Institute for Basic Biology, Japan

Legumes form root nodules to host nitrogen-fixing bacteria, collectively called rhizobia.
Invasion and release of rhizobia into host cells are restricted in nodule primordia originatingroot cortical cells. Root nodules provide endosymbiotic bacteria with an environment suitable
for nitrogen fixation while avoiding excessive symbiosis by confining nitrogen-fixing bacteria
in the symbiotic organ. Nodule formation is indispensable for establishing root nodule
symbiosis. We have shown that nodule primordium development is regulated by a lateral rootrelated transcription factor, ASL18/LBD16, of which expression is positively regulated by ahost nodulation-specific transcription factor evolutionarily derived from a member of NINlike proteins responsible for nitrate responses. Therefore, nodule symbiosis has evolved
through cooption and rearrangement of molecular pathways conserved in most plants.
Recently, we found that expression of host symbiotic genes oscillates in the susceptible region
near the root tip after inoculation with rhizobia, similar to the root clock in Arabidopsis.
Stimuli from rhizobia trigger a series of nodulation processes, but it is likely that the hostmoderately regulates infection and nodule distribution. The periodicity of host symbiotic gene
expression may be concerned with the pattering of nodulation in roots growing linearly.