While textbook figures imply nuclei as resting spheres at the center of idealized cells, this picture fits few real situations. Plant nuclei come in many shapes and sizes, and can be actively transported within the cell. In several contexts, this nuclear movement is tightly coupled to a developmental program, the response to an abiotic signal, or a cellular reprogramming during either mutualistic or parasitic plant–microbe interactions. While many such phenomena have been observed and carefully described, the underlying molecular mechanism and the functional significance of the nuclear movement are typically unknown. Here, we survey recent as well as older literature to provide a concise starting point for applying contemporary molecular, genetic and biochemical approaches to this fascinating, yet poorly understood phenomenon.

Nuclear movement is involved in cellular and developmental processes across eukaryotic life, often driven by Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes, which bridge the nuclear envelope (NE) via the interaction of Klarsicht/ ANC-1/Syne-1 Homology (KASH) and Sad1/UNC-84 (SUN) proteins. Arabidopsis (Arabidopsis thaliana) LINC complexes are involved in nuclear movement and positioning in several cell types. Observations since the 1950s have described targeted nuclear movement and positioning during symbiosis initiation between legumes and rhizobia, but it has not been established whether these movements are functional or incidental. Here, we identify and characterize LINC complexes in the model legume Medicago truncatula. We show that LINC complex characteristics such as NE localization, dependence of KASH proteins on SUN protein binding for NE enrichment, and direct SUN-KASH binding are conserved between plant species. Using a SUN dominant-negative strategy, we demonstrate that LINC complexes are necessary for proper nuclear shaping and movement in Medicago root hairs, and are important for infection thread initiation and nodulation. 4

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

The Ran GTPase activating protein (RanGAP) is important to Ran signaling involved in nucleocytoplasmic transport, spindle organization, and postmitotic nuclear assembly. Unlike vertebrate and yeast RanGAP, plant RanGAP has an N-terminal WPP domain, required for nuclear envelope association and several mitotic locations of Arabidopsis thaliana RanGAP1. A double null mutant of the two Arabidopsis RanGAP homologs is gametophyte lethal. Here, we created a series of mutants with various reductions in RanGAP levels by combining a RanGAP1 null allele with different RanGAP2 alleles. As RanGAP level decreases, the severity of developmental phenotypes increases, but nuclear import is unaffected. To dissect whether the GAP activity and/or the subcellular localization of RanGAP are responsible for the observed phenotypes, this series of rangap mutants were transformed with RanGAP1 variants carrying point mutations abolishing the GAP activity and/or the WPPdependent subcellular localization. The data show that plant development is differentially affected by RanGAP mutant allele combinations of increasing severity and requires the GAP activity of RanGAP, while the subcellular positioning of RanGAP is dispensable. In addition, our results indicate that nucleocytoplasmic trafficking can tolerate both partial depletion of RanGAP and delocalization of RanGAP from the nuclear envelope.

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