The Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries

Stomatal evolution and plant adaptation to future climate

Global climate change is affecting plant photosynthesis and transpiration processes, as well as increasing weather extremes impacting socio-political and environmental events and decisions for decades to come. One major research challenge in plant biology and ecology is the interaction of photosynthesis with the environment. Stomata control plant gas exchange and their evolution was a crucial innovation that facilitated the earliest land plants to colonize terrestrial environments. Stomata couple homoiohydry, together with cuticles, intercellular gas space, with the endohydric water-conducting system, enabling plants to adapt and diversify across the planet. Plants control stomatal movement in response to environmental change through regulating guard cell turgor mediated by membrane transporters and signaling transduction. However, the origin, evolution, and active control of stomata remain controversial topics. We first review stomatal evolution and diversity, providing fossil and phylogenetic evidence of their origins. We summarize functional evolution of guard cell membrane transporters in the context of climate changes and environmental stresses. Our analyses show that the core signaling elements of stomatal movement are more ancient than stomata, while genes involved in stomatal development co-evolved de novo with the earliest stomata. These results suggest that novel stomatal development-specific genes were acquired during plant evolution, whereas genes regulating stomatal movement, especially cell signaling pathways, were inherited ancestrally and co-opted by dynamic functional differentiation. These two processes reflect the different adaptation strategies during land plant evolution.

Spatially resolved proteomics of the Arabidopsis stomatal lineage identifies polarity complexes for cell divisions and stomatal pores

Cell polarity is used to guide asymmetric divisions and create morphologically diverse cells. We find that two oppositely oriented cortical polarity domains present during the asymmetric divisions in the Arabidopsis stomatal lineage are reconfigured into polar domains marking ventral (pore-forming) and outward-facing domains of maturing stomatal guard cells. Proteins that define these opposing polarity domains were used as baits in miniTurboID-based proximity labeling. Among differentially enriched proteins, we find kinases, putative microtubule-interacting proteins, and polar SOSEKIs with their effector ANGUSTIFOLIA. Using AI-facilitated protein structure prediction models, we identify potential protein-protein interaction interfaces among them. Functional and localization analyses of the polarity protein OPL2 and its putative interaction partners suggest a positive interaction with mitotic microtubules and a role in cytokinesis. This combination of proteomics and structural modeling with live-cell imaging provides insights into how polarity is rewired in different cell types and cell-cycle stages.

Membrane-associated NRPM proteins are novel suppressors of stomatal production in Arabidopsis

In Arabidopsis, stomatal development and patterning require tightly regulated cell division and cell-fate differentiation that are controlled by key transcription factors and signaling molecules. To identify new regulators of stomatal development, we assay the transcriptomes of plants bearing enriched stomatal lineage cells that undergo active division. A member of the novel regulators at the plasma membrane (NRPM) family annotated as hydroxyproline-rich glycoproteins was identified to highly express in stomatal lineage cells. Overexpressing each of the four NRPM genes suppressed stomata formation, while the loss-of-function nrpm triple mutants generated severely overproduced stomata and abnormal patterning, mirroring those of the erecta receptor family and MAPKKK yoda null mutants. Manipulation of the subcellular localization of NRPM1 surprisingly revealed its regulatory roles as a peripheral membrane protein instead of a predicted cell wall protein. Further functional characterization suggests that NRPMs function downstream of the EPF1/2 peptide ligands and upstream of the YODA MAPK pathway. Genetic and cell biological analyses reveal that NRPM may promote the localization and function of the ERECTA receptor proteins at the cell surface. Therefore, we identify NRPM as a new class of signaling molecules at the plasma membrane to regulate many aspects of plant growth and development.

The stomatal fates: Understanding initiation and enforcement of stomatal cell fate transitions

In the stomatal lineage, repeated arcs of initiation, stem-cell proliferation, and terminal cell fate commitment are displayed on the surface of aerial organs. Over the past two decades, the core transcription and signaling elements that guide cell divisions, patterning, and fate transitions were defined. Here we highlight recent work that extends the core using a variety of cutting-edge techniques in different plant species. New work has discovered transcriptional circuits that initiate and reinforce stomatal fate transitions, while also enabling the lineage to interpret and respond to environmental inputs. Recent developments show that some key stomatal factors are more flexible or potentially even interchangeable, opening up avenues to explore stomatal fates and regulatory networks.

Arabidopsis stomatal lineage cells establish bipolarity and segregate differential signaling capacity to regulate stem cell potential

Cell polarity combined with asymmetric cell divisions (ACDs) generates cellular diversity. In the Arabidopsis stomatal lineage, a single cortical polarity domain marked by BASL orients ACDs and is segregated to the larger daughter to enforce cell fate. We discovered a second, oppositely positioned polarity domain defined by OCTOPUS-LIKE (OPL) proteins, which forms prior to ACD and is segregated to the smaller (meristemoid) daughter. Genetic and misexpression analyses show that OPLs promote meristemoid-amplifying divisions and delay stomatal fate progression. Polarity mediates OPL segregation into meristemoids but is not required for OPL function. OPL localization and activity are largely independent of other stomatal polarity genes and of the brassinosteroid signaling components associated with OPLs in other contexts. While OPLs are unique to seed plants, ectopic expression in the liverwort Marchantia suppressed epidermal fate progression, suggesting that OPLs engage ancient and broadly conserved pathways to regulate cell division and cell fate.

A cell size threshold triggers commitment to stomatal fate in Arabidopsis

How flexible developmental programs integrate information from internal and external factors to modulate stem cell behavior is a fundamental question in developmental biology. Cells of the Arabidopsis stomatal lineage modify the balance of stem cell proliferation and differentiation to adjust the size and cell type composition of mature leaves. Here, we report that meristemoids, one type of stomatal lineage stem cell, trigger the transition from asymmetric self-renewing divisions to commitment and terminal differentiation by crossing a critical cell size threshold. Through computational simulation, we demonstrate that this cell size-mediated transition allows robust, yet flexible termination of stem cell proliferation, and we observe adjustments in the number of divisions before the differentiation threshold under several genetic manipulations. We experimentally evaluate several mechanisms for cell size sensing, and our data suggest that this stomatal lineage transition is dependent on a nuclear factor that is sensitive to DNA content.

Cortical polarity ensures its own asymmetric inheritance in the stomatal lineage to pattern the leaf surface

Asymmetric cell divisions specify differential cell fates across kingdoms. In metazoans, preferential inheritance of fate determinants into one daughter cell frequently depends on polarity-cytoskeleton interactions. Despite the prevalence of asymmetric divisions throughout plant development, evidence for analogous mechanisms that segregate fate determinants remains elusive. Here, we describe a mechanism in the Arabidopsis leaf epidermis that ensures unequal inheritance of a fate-enforcing polarity domain. By defining a cortical region depleted of stable microtubules, the polarity domain limits possible division orientations. Accordingly, uncoupling the polarity domain from microtubule organization during mitosis leads to aberrant division planes and accompanying cell identity defects. Our data highlight how a common biological module, coupling polarity to fate segregation through the cytoskeleton, can be reconfigured to accommodate unique features of plant development.

Maintaining asymmetry in cell division

Promoting asymmetric division through microtubule dynamics establishes cell fate.

Rewiring of hormones and light response pathways underlies the inhibition of stomatal development in an amphibious plant Rorippa aquatica underwater

Land plants have evolved the ability to cope with submergence. Amphibious plants are adapted to both aerial and aquatic environments through phenotypic plasticity in leaf form and function, known as heterophylly. In general, underwater leaves of amphibious plants are devoid of stomata, yet their molecular regulatory mechanisms remain elusive. Using the emerging model of the Brassicaceae amphibious species Rorippa aquatica, we lay the foundation for the molecular physiological basis of the submergence-triggered inhibition of stomatal development. A series of temperature shift experiments showed that submergence-induced inhibition of stomatal development is largely uncoupled from morphological heterophylly and likely regulated by independent pathways. Submergence-responsive transcriptome analysis revealed rapid reprogramming of gene expression, exemplified by the suppression of RaSPEECHLESS and RaMUTE within 1 h and the involvement of light and hormones in the developmental switch from terrestrial to submerged leaves. Further physiological studies place ethylene as a central regulator of the submergence-triggered inhibition of stomatal development. Surprisingly, red and blue light have opposing functions in this process: blue light promotes, whereas red light inhibits stomatal development, through influencing the ethylene pathway. Finally, jasmonic acid counteracts the inhibition of stomatal development, which can be attenuated by the red light. The actions and interactions of light and hormone pathways in regulating stomatal development in R. aquatica are different from those in the terrestrial species, Arabidopsis thaliana. Thus, our work suggests that extensive rewiring events of red light to ethylene signaling might underlie the evolutionary adaption to water environment in Brassicaceae.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

Protein polarization: Spatiotemporal precisions in cell division and differentiation

Specification of cell polarity is vital to normal cell growth, morphogenesis, and function. As other eukaryotes, plants generate cellular polarity that is coordinated with tissue polarity and organ axes. In development, new cell types are generated by stem-cell division and differentiation, a process often involving proteins that are polarized to cortical domains at the plasma membrane. In the past decade, pioneering work using the model plant Arabidopsis identified multiple proteins that are polarized in dividing cells to instruct divisional behaviors and/or specify cell fates. In this review, we use these polarized cell-division regulators as example to summarize key mechanisms underlying protein polarization in plant cells. Recent progress underscores that self-organizing amplification processes are commonly involved in establishing cell polarity, and cellular polarity is influenced by both tissue-level and local mechanochemical cues. In addition, protein polarization during asymmetric cell division shows a distinct feature of temporal control in the stomatal lineage. We further discuss possible coordination between protein polarization and the progression of cell cycle in this developmental context.

Evolution of polarity protein BASL and the capacity for stomatal lineage asymmetric divisions

Asymmetric and oriented stem cell divisions enable the continued production of patterned tissues. The molecules that guide these divisions include several "polarity proteins" that are localized to discrete plasma membrane domains, are differentially inherited during asymmetric divisions, and whose scaffolding activities can guide division plane orientation and subsequent cell fates. In the stomatal lineages on the surfaces of plant leaves, asymmetric and oriented divisions create distinct cell types in physiologically optimized patterns. The polarity protein BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) is a major regulator of stomatal lineage division and cell fate asymmetries in Arabidopsis, but its role in the stomatal lineages of other plants is unclear. Here, using phylogenetic and functional assays, we demonstrate that BASL is a eudicot-specific polarity protein. Dicot BASL orthologs can polarize in heterologous systems and rescue the Arabidopsis BASL mutant. The more widely distributed BASL-like proteins, although they share BASL's conserved C-terminal domain, are neither polarized nor do they function in asymmetric divisions of the stomatal lineage. Comparison of BASL protein localization and loss of function BASL phenotypes in Arabidopsis and tomato revealed previously unappreciated differences in how asymmetric cell divisions are employed for pattern formation in different species. This multi-species analysis therefore provides insight into the evolution of a unique polarity regulator and into the developmental choices available to cells as they build and pattern tissues.