Microtubule reorientation in the blue spotlight: Cutting and CLASPing at dynamic hot spots

Microtubule reorientation into a longitudinal network during the phototropic response in Arabidopsis thaliana depends on their severing by katanin at crossovers. Lindeboom et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201805047) show that at newly generated plus ends, the anti-catastrophe activity of CLASP is essential for further growth.

Dissecting the mechanism of abscisic acid-induced dynamic microtubule reorientation using live cell imaging

Abscisic acid (ABA) is involved in plant development and responses to environmental stress including the formation of longitudinal microtubule arrays in elongating cells, although the underlying mechanism for this is unknown. We explored ABA-induced microtubule reorientation in leek (Allium porrum L.) leaf epidermal cells transiently expressing a GFP–MBD microtubule reporter. After 14–18 h incubation with ABA, the frequency of cells with longitudinal arrays of cortical microtubules along the outer epidermal wall increased with dose-dependency until saturation at 20 μM. Time-course imaging of individual cells revealed a gradual increase in the occurrence of discordant, dynamic microtubules deviating from the normal transverse microtubule array within 2–4 h of exposure to ABA, followed by reorientation into a completely longitudinal array within 5–8 h. Approximately one-half of the ABA-induced reorientation occurred independently of cytoplasmic streaming following the application of cytochalasin D. Reorientation occurred also in the elongation zone of Arabidopsis root tips. Transient expression of AtEB1b–GFP reporter and analysis of ‘comet’ velocities in Allium revealed that the microtubule growth rate increased by 55% within 3 h of exposure to ABA. ABA also increased the sensitivity of microtubules to depolymerisation by oryzalin and exacerbated oryzalin-induced radial swelling of Arabidopsis root tips. The swelling was further aggravated in AtPLDδ-null mutant, suggesting PLDδ plays a role in microtubule stability. We propose that ABA-induced reorientation of transverse microtubule array initially involves destabilisation of the array combined with the formation of dynamic, discordant microtubules.

P115 RhoGEF and microtubules decide the direction apoptotic cells extrude from an epithelium

To preserve epithelial barrier function, dying cells are squeezed out of an epithelium by “apoptotic cell extrusion.” Specifically, a cell destined for apoptosis signals its live neighboring epithelial cells to form and contract a ring of actin and myosin II that squeezes the dying cell out of the epithelial sheet. Although most apoptotic cells extrude apically, we find that some exit basally. Localization of actin and myosin IIA contraction dictates the extrusion direction: basal extrusion requires circumferential contraction of neighboring cells at their apices, whereas apical extrusion also requires downward contraction along the basolateral surfaces. To activate actin/myosin basolaterally, microtubules in neighboring cells reorient and target p115 RhoGEF to this site. Preventing microtubule reorientation restricts contraction to the apex, driving extrusion basally. Extrusion polarity has important implications for tumors where apoptosis is blocked but extrusion is not, as basal extrusion could enable these cells to initiate metastasis.