Stomatal movement, which regulates gas exchange in plants, is controlled by a variety of environmental factors, including biotic and abiotic stresses. The stress hormone abscisic acid (ABA) initiates a signaling cascade, which leads to increased H 2 O 2 and Ca 21 levels and F-actin reorganization, but the mechanism of, and connection between, these events is unclear. SINE1, an outer nuclear envelope component of a plant Linker of Nucleoskeleton and Cytoskeleton complex, associates with F-actin and is, along with its putative paralog SINE2, expressed in guard cells. Here, we have determined that Arabidopsis (Arabidopsis thaliana) SINE1 and SINE2 play an important role in stomatal opening and closing. Loss of SINE1 or SINE2 results in ABA hyposensitivity and impaired stomatal dynamics but does not affect stomatal closure induced by the bacterial elicitor flg22. The ABA-induced stomatal closure phenotype is, in part, attributed to impairments in Ca 21 and F-actin regulation. Together, the data suggest that SINE1 and SINE2 act downstream of ABA but upstream of Ca 21 and F-actin. While there is a large degree of functional overlap between the two proteins, there are also critical differences. Our study makes an unanticipated connection between stomatal regulation and nuclear envelope-associated proteins, and adds two new players to the increasingly complex system of guard cell regulation.

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AUTS2 syndrome is a genetic disorder that causes intellectual disability, microcephaly, and other phenotypes. Syndrome severity is worse when mutations involve 3’ regions (exons 9-19) of the AUTS2 gene. Human AUTS2 protein has two major isoforms, full-length (1259 aa) and C-terminal (711 aa), the latter produced from an alternative transcription start site in exon 9. Structurally, AUTS2 contains the putative “AUTS2 domain” (∼200 aa) conserved among AUTS2 and its ohnologs, fibrosin, and fibrosin-like-1. Also, AUTS2 contains extensive low-complexity sequences and intrinsically disordered regions, features typical of RNA-binding proteins. During development, AUTS2 is expressed by specific progenitor cell and neuron types, including pyramidal neurons and Purkinje cells. AUTS2 localizes mainly in cell nuclei, where it regulates transcription and RNA metabolism. Some studies have detected AUTS2 in neurites, where it may regulate cytoskeletal dynamics. Neurodevelopmental functions of AUTS2 have been studied in diverse model systems. In zebrafish, auts2a morphants displayed microcephaly. In mice, excision of different Auts2 exons (7, 8, or 15) caused distinct phenotypes, variously including neonatal breathing abnormalities, cerebellar hypoplasia, dentate gyrus hypoplasia, EEG abnormalities, and behavioral changes. In mouse embryonic stem cells, AUTS2 could promote or delay neuronal differentiation. Cerebral organoids, derived from an AUTS2 syndrome patient containing a pathogenic missense variant in exon 9, exhibited neocortical growth defects. Emerging technologies for analysis of human cerebral organoids will be increasingly useful for understanding mechanisms underlying AUTS2 syndrome. Questions for future research include whether AUTS2 binds RNA directly, how AUTS2 regulates neurogenesis, and how AUTS2 modulates neural circuit formation.

ORCID ID: 0000-0002-4141-5400 (I.M.).In both plants and animals, the nucleus acts as an organizing center for the changes that occur during an organism's life cycle, whether as part of a developmental program or in response to environmental factors. Here, we cover recent research that explores roles of the nucleus other than gene expression in light response, fertilization, plant-pathogen interactions, bacterial symbiotic events, and hormone signaling. New strides have been made in understanding subnuclear organization, how nuclear organization responds to different environments and developmental stages, and how the cytoplasm-nucleoplasm connections are made that allow these responses. We further highlight new tools that have been developed to study dynamic changes in nuclear organization.The nucleus is compartmentalized by a double membrane called the nuclear envelope (NE), which consists of the outer nuclear membrane (ONM), the inner nuclear membrane (INM), and the embedded nuclear pore complexes (NPCs). The NPC is the primary pathway of macromolecular transport between the nucleus and the cytoplasm and is conserved throughout eukaryotes. The ONM and INM are populated by a variety of membrane-associated proteins, and in particular the inner membrane hosts a specific subset of proteins (Starr and Fridolfsson, 2010;Meier et al., 2017). In addition to functioning in nuclear morphology, chromatin attachment, and likely signal transduction, plant NE-associated proteins are involved in nuclear positioning and movement within the larger cellular context (Griffis et al., 2014). While plant nuclear ultrastructure reveals an inner nuclear membrane-associated meshwork similar to the animal lamina, plant genomes do not encode obvious lamin homologs. Instead, plant-specific long coiled-coil proteins with structural similarity to animal lamins might contribute to this meshwork. Plants also lack centrosomes, and the NE plays a role as the microtubuleorganizing center at the onset of mitosis. NE proteins functioning as calcium channels are involved in perinuclear calcium oscillations, which are an important step in the establishment of plant-symbiont interactions (recently reviewed in Meier et al., 2017).While the nuclear periphery is emerging as an essential organizing platform for the nucleus, intranuclear functional and structural organization might also originate from independent self-assembly processes, with the most conspicuous manifestation of these processes revealed as the nucleolus. New methods are now robustly adapted for plants to reveal greater details of functional nuclear and nucleolar organization. Due to the increasing realization that spatial organization is a crucial part of biological function at the subcellular level, many of these aspects have been recently reviewed

Abscisic acid (ABA) induces stomatal closure by utilizing complex signaling mechanisms, allowing for sessile plants to respond rapidly to ever-changing environmental conditions. ABA regulates the activity of plasma membrane ion channels and calcium-dependent protein kinases, Ca2+ oscillations, and reactive oxygen species (ROS) concentrations. Throughout ABA-induced stomatal closure, the cytoskeleton undergoes dramatic changes that appear important for efficient closure. However, the precise role of this cytoskeletal reorganization in stomatal closure and the nature of its regulation are unknown. We have recently shown that the plant KASH proteins SINE1 and SINE2 are connected to actin organization during ABA-induced stomatal closure but their role in microtubule (MT) organization remains to be investigated. We show here that depolymerizing MTs using oryzalin can restore ABA-induced stomatal closure deficits in sine1-1 and sine2-1 mutants. GFP-MAP4-visualized MT organization is compromised in sine1-1 and sine2-1 mutants during ABA-induced stomatal closure. Loss of SINE1 or SINE2 results in loss of radially organized MT patterning in open guard cells, aberrant MT organization during stomatal closure, and an overall decrease in the number of MT filaments or bundles. Thus, SINE1 and SINE2 are necessary for establishing MT patterning and mediating changes in MT rearrangement, which is required for ABA-induced stomatal closure.