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Epidermal cells of dark-grown plant seedlings reorient their cortical microtubule arrays in response to blue light from a net lateral orientation to a net longitudinal orientation with respect to the long axis of cells. The molecular mechanism underlying this microtubule array reorientation involves katanin, a microtubule severing enzyme, and a plant-specific microtubule associated protein called SPIRAL2. Katanin preferentially severs longitudinal microtubules, generating seeds that amplify the longitudinal array. Upon severing, SPIRAL2 binds nascent microtubule minus ends and limits their dynamics, thereby stabilizing the longitudinal array while the lateral array undergoes net depolymerization. To date, no experimental structural information is available for SPIRAL2 to help inform its mechanism. To gain insight into SPIRAL2 structure and function, we determined a 1.8 Å resolution crystal structure of the Arabidopsis thaliana SPIRAL2 C-terminal domain. The domain is composed of seven core α-helices, arranged in an α-solenoid. Amino-acid sequence conservation maps primarily to one face of the domain involving helices α1, α3, α5, and an extended loop, the α6-α7 loop. The domain fold is similar to, yet structurally distinct from the C-terminal domain of Ge-1 (an mRNA decapping complex factor involved in P-body localization) and, surprisingly, the C-terminal domain of the katanin p80 regulatory subunit. The katanin p80 C-terminal domain heterodimerizes with the MIT domain of the katanin p60 catalytic subunit, and in metazoans, binds the microtubule minus-end factors CAMSAP3 and ASPM. Structural analysis predicts that SPIRAL2 does not engage katanin p60 in a mode homologous to katanin p80. The SPIRAL2 structure highlights an interesting evolutionary convergence of domain architecture and microtubule minus-end localization between SPIRAL2 and katanin complexes, and establishes a foundation upon which structure-function analysis can be conducted to elucidate the role of this domain in the regulation of plant microtubule arrays.

Plants possess a large number of microtubule-based kinesin motor proteins. While the kinesin-2, 3, 9, and 11 families are absent from land plants, the kinesin-7 and 14 families are greatly expanded. In addition, some kinesins are specifically present only in land plants. The distinctive inventory of plant kinesins suggests that kinesins have evolved to perform specialized functions in plants. Plants assemble unique microtubule arrays during their cell cycle, including the interphase cortical microtubule array, preprophase band, anastral spindle and phragmoplast. In this review, we explore the functions of plant kinesins from a microtubule array viewpoint, focusing mainly on Arabidopsis kinesins. We emphasize the conserved and novel functions of plant kinesins in the organization and function of the different microtubule arrays.

Dynein and kinesin motor proteins transport cellular cargoes toward opposite ends of microtubule tracks. In neurons, microtubules are abundantly decorated with microtubule-associated proteins (MAPs) such as tau. Motor proteins thus encounter MAPs frequently along their path. To determine the effects of tau on dynein and kinesin motility, we conducted single-molecule studies of motor proteins moving along tau-decorated microtubules. Dynein tended to reverse direction, whereas kinesin tended to detach at patches of bound tau. Kinesin was inhibited at about a tenth of the tau concentration that inhibited dynein, and the microtubule-binding domain of tau was sufficient to inhibit motor activity. The differential modulation of dynein and kinesin motility suggests that MAPs can spatially regulate the balance of microtubule-dependent axonal transport.

Twisted growth serves myriad adaptive functions in plants. Unlike animal motions, plant motions require symmetry breaking during growth and typically involve microtubule-related genes. But how macroscopic twisting emerges from molecular-level perturbations remains unclear. Here, we show that microtubule-based symmetry breaking propagates across multiple organizational scales via the epidermis to produce handed root skewing. At the nanoscale, aberrant patterning of cellulose microfibrils is associated with microscale skewed cell expansion, both of which precede the millimeter scale emergence of helical epidermal cell files. The resulting chiral torsion of the epidermis mediates organ level symmetry breaking in the form of whole-root skewing through macroscale interactions between the root and its surrounding environment. We demonstrate the dominant role of the epidermis by complementation of microtubule activity in the epidermis alone, which is sufficient to restore transverse cortical microtubule orientation, wild-type-like morphology in cortical cells, and straight root growth. Nolan et al use Arabidopsis twisted mutants to show that frustrated distortion of the epidermis is the primary organ-level determinant of multilevel asymmetries that underlie skewed root growth.

Ordered cortical microtubule arrays are essential for normal plant morphogenesis, but how these arrays form is unclear. The dynamics of individual cortical microtubules are stochastic and cannot fully account for the observed order; however, using tobacco (Nicotiana tabacum) cells expressing either the MBD-DsRed (microtubule binding domain of the mammalian MAP4 fused to the Discosoma sp red fluorescent protein) or YFP-TUA6 (yellow fluorescent protein fused to the Arabidopsis α-tubulin 6 isoform) microtubule markers, we identified intermicrotubule interactions that modify their stochastic behaviors. The intermicrotubule interactions occur when the growing plus-ends of cortical microtubules encounter previously existing cortical microtubules. Importantly, the outcome of such encounters depends on the angle at which they occur: steep-angle collisions are characterized by approximately sevenfold shorter microtubule contact times compared with shallow-angle encounters, and steep-angle collisions are twice as likely to result in microtubule depolymerization. Hence, steep-angle collisions promote microtubule destabilization, whereas shallow-angle encounters promote both microtubule stabilization and coalignment. Monte Carlo modeling of the behavior of simulated microtubules, according to the observed behavior of transverse and longitudinally oriented cortical microtubules in cells, reveals that these simple rules for intermicrotubule interactions are necessary and sufficient to facilitate the self-organization of dynamic microtubules into a parallel configuration.

The cell wall consists of cellulose microfibrils embedded within a matrix of hemicellulose and pectin. Cellulose microfibrils are synthesized at the plasma membrane, whereas matrix polysaccharides are synthesized in the Golgi apparatus and secreted. The trafficking of vesicles containing cell wall components is thought to depend on actin-myosin. Here, we implicate microtubules in this process through studies of the kinesin-4 family member, Fragile Fiber1 (FRA1). In anfra1-5knockout mutant, the expansion rate of the inflorescence stem is halved compared with the wild type along with the thickness of both primary and secondary cell walls. Nevertheless, cell walls infra1-5have an essentially unaltered composition and ultrastructure. A functional triple green fluorescent protein-tagged FRA1 fusion protein moves processively along cortical microtubules, and its abundance and motile density correlate with growth rate. Motility of FRA1 and cellulose synthase complexes is independent, indicating that FRA1 is not directly involved in cellulose biosynthesis; however, the secretion rate of fucose-alkyne-labeled pectin is greatly decreased infra1-5, and the mutant has Golgi bodies with fewer cisternae and enlarged vesicles. Based on our results, we propose that FRA1 contributes to cell wall production by transporting Golgi-derived vesicles along cortical microtubules for secretion.

The model moss, Physcomitrium patens, is routinely cultured on cellophane placed over a solid nutrient medium. While this culture method is convenient for moss propagation, it is not suitable for studying how topographical features and mechanical cues from the environment influence the growth and development of moss. Here, we show that P. patens can be grown on fibrous scaffolds consisting of nanoscale, randomly oriented fibers composed of polyvinylidene tri‐fluoroethylene (NRP). The moss adheres tightly to NRP in contrast to the lack of adhesion to cellophane. Adhesion to the scaffold is associated with slower tip growth of moss protonema for some time, followed by an increase in tip growth rate that is equivalent to that on cellophane. In addition, the orientation of the first subapical cell division plane differs between NRP‐grown and cellophane‐grown protonema. Nonetheless, moss colonies grown on NRP did not show signs of nutrient or photosynthetic stress and developed normal gametophores. Together, these data establish NRP as a suitable substrate for the culture of P. patens and to probe the influence of mechanical forces on tip growth and cell division of moss.

The expanded availability of telehealth due to the COVID-19 pandemic presents a concern that telehealth may result in an unnecessary increase in utilization. We analyzed 4,114,651 primary care encounters (939,134 unique patients) from three healthcare systems between 2019 and 2021 and found little change in utilization as telehealth became widely available. Results suggest telehealth availability is not resulting in additional primary care visits and federal policies should support telehealth use.

Microtubules are polarized polymers that exhibit dynamic instability, with alternating phases of elongation and shortening, particularly at the more dynamic plus-end. Microtubule plus-end tracking proteins (+TIPs) localize to and track with growing microtubule plus-ends in the cell. +TIPs regulate microtubule dynamics and mediate interactions with other cellular components. The molecular mechanisms responsible for the +TIP tracking activity are not well understood, however. We reconstituted the +TIP tracking of mammalian proteins EB1 and CLIP-170 in vitro at single-molecule resolution using time-lapse total internal reflection fluorescence microscopy. We found that EB1 is capable of dynamically tracking growing microtubule plus-ends. Our single-molecule studies demonstrate that EB1 exchanges rapidly at microtubule plus-ends with a dwell time of <1 s, indicating that single EB1 molecules go through multiple rounds of binding and dissociation during microtubule polymerization. CLIP-170 exhibits lattice diffusion and fails to selectively track microtubule ends in the absence of EB1; the addition of EB1 is both necessary and sufficient to mediate plus-end tracking by CLIP-170. Single-molecule analysis of the CLIP-170-EB1 complex also indicates a short dwell time at growing plus-ends, an observation inconsistent with the copolymerization of this complex with tubulin for plus-end-specific localization. GTP hydrolysis is required for +TIP tracking, because end-specificity is lost when tubulin is polymerized in the presence of guanosine 5'-[α,β-methylene]triphosphate (GMPCPP). Together, our data provide insight into the mechanisms driving plus-end tracking by mammalian +TIPs and suggest that EB1 specifically recognizes the distinct lattice structure at the growing microtubule end.