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Cortical microtubules influence plant cell shape by guiding cellulose deposition. Epidermal hypocotyl cells in Arabidopsis thaliana create distinct cortical microtubule array patterns to enable axial cell growth. How these array patterns are created and maintained during cell wall formation is a critical and unsolved problem in cell biology. Previous work showed that arrays aligned longitudinally with the cell's growth axis have a “split bipolar” organization, with microtubules treadmilling toward the apical or basal ends of the cell from a region of antiparallel overlap at the cell's midzone. The underlying order or architecture of these coaligned arrays prompted us to ask whether microtubules oriented transversely to the cell's axis are organized to a similar degree. Creating new fluorescently tagged End-Binding Protein 1b (EB1b) probes to circumvent gain-of-function effects observed for GFP-EB1b, we found that transverse arrays form persistent, nearly unipolar domains of microtubules treadmilling around the short axis of the cell, independent of the EB1b probe used. Our findings reveal an organizational strategy for transverse arrays distinct from that of longitudinal arrays, with implications for the mechanisms of array pattern creation and maintenance. 

ABSTRACT Plant cells create a plasma membrane‐associated network of microtubules that are nucleated by γ‐tubulin ring complexes primarily through microtubule‐dependent microtubule nucleation (MDMN). This dynamic array organizes into specific patterns in response to developmental and environmental cues to influence primary cell wall construction. The molecular mechanisms directing the creation of cortical microtubule array patterns are largely unknown. The hetero‐octameric AUGMIN complex facilitates mitotic spindle formation by associating γ‐tubulin ring complexes with existing spindle microtubules and creating parallel branched microtubules through MDMN. AUGMIN8, the key linker protein connecting the AUGMIN complex to the parent microtubule, is encoded by a paralogous family of QWRF genes in flowering plants. Members of the QWRF family are distinguished by an unstructured N‐terminal half encoded in a single 5′ exon. We hypothesize that the QWRF paralogs form interchangeable AUGMIN microtubule binding subunits that confer specific roles to the AUGMIN complex in mitotic and non‐mitotic microtubule arrays. We identify four QWRF family members expressed inArabidopsishypocotyl cells and investigate the sites of QWRF interaction with cortical microtubules using transient transformation of fluorescently tagged constructs in the heterologousNicotiana benthamianasystem. We show that full‐length QWRF8 and QWRF4 associate with non‐mitotic, cortical microtubules as distributed puncta where QWRF8 shows evidence for two independent sites of microtubule association. Sequence comparisons and in vivo assay with homologous fragments from QWRF1, 2, 4, and 5 define a shared N‐terminal conserved microtubule association domain. We additionally identify protein regions leading to the formation of microtubule‐associated “QWRF bodies” potentially linked to discontinuous localization on microtubules. We identify the “QWRF” protein motif as a conserved domain associating the AUGMIN8 paralogs with AUGMIN6, part of the larger AUGMIN complex. 

Abstract Light, temperature, water, and nutrient availability influence how plants grow to maximize access to resources. Axial growth, the linear extension of tissues by coordinated axial cell expansion, plays a central role in these adaptive morphological responses. Using Arabidopsis (Arabidopsis thaliana) hypocotyl cells to explore axial growth control mechanisms, we investigated WAVE-DAMPENED2-LIKE4 (WDL4), an auxin-induced, microtubule-associated protein and member of the larger WDL gene family shown to modulate hypocotyl growth under changing environmental conditions. Loss-of-function wdl4 seedlings exhibited a hyper-elongation phenotype under light conditions, continuing to elongate when wild-type Col-0 hypocotyls arrested and reaching 150% to 200% of wild-type length before shoot emergence. wdl4 seedling hypocotyls showed dramatic hyper-elongation (500%) in response to temperature elevation, indicating an important role in morphological adaptation to environmental cues. WDL4 was associated with microtubules under both light and dark growth conditions, and no evidence was found for altered microtubule array patterning in loss-of-function wdl4 mutants under various conditions. Examination of hormone responses showed altered sensitivity to ethylene and evidence for changes in the spatial distribution of an auxin-dependent transcriptional reporter. Our data provide evidence that WDL4 regulates hypocotyl cell elongation without substantial changes to microtubule array patterning, suggesting an unconventional role in axial growth control.