Cortical microtubules act as a template to organize nano-scale patterning of exocytosis.

Targeting of exocytosis enables cellular morphogenesis, motility and polarized transport, yet relatively little is known about the targeting mechanisms in cellular systems. Here we show that the SEC/MUNC protein KEULE is a dynamic marker for individual secretory events and employ it as a live cell probe, that together with high-precision image analysis of thousands of events, reveal that cortical microtubule arrays act as two-dimensional templates that pattern exocytosis at the nano-scale in higher plant cells. This mechanism is distinct from previously described mechanisms involving motor-driven transport and defines ordered and adjacent linear domains where secretory events are higher and lower than expected, effectively redistributing exocytosis over most of the cell membrane.

SPR2 protects minus ends to promote severing and reorientation of plant cortical microtubule arrays.

The cortical microtubule arrays of higher plants are organized without centrosomes and feature treadmilling polymers that are dynamic at both ends. The control of polymer end stability is fundamental for the assembly and organization of cytoskeletal arrays, yet relatively little is understood about how microtubule minus ends are controlled in acentrosomal microtubule arrays, and no factors have been identified that act at the treadmilling minus ends in higher plants. Here, we identify SPIRAL2 (SPR2) as a protein that tracks minus ends and protects them against subunit loss.

How selective severing by katanin promotes order in the plant cortical microtubule array.

Plant morphogenesis requires differential and often asymmetric growth. A key role in controlling anisotropic expansion of individual cells is played by the cortical microtubule array. Although highly organized, the array can nevertheless rapidly change in response to internal and external cues.

The microtubule plus-end tracking proteins SPR1 and EB1b interact to maintain polar cell elongation and directional organ growth in Arabidopsis.

The microtubule plus-end tracking proteins (+TIPs) END BINDING1b (EB1b) and SPIRAL1 (SPR1) are required for normal cell expansion and organ growth. EB proteins are viewed as central regulators of +TIPs and cell polarity in animals; SPR1 homologs are specific to plants. To explore if EB1b and SPR1 fundamentally function together, we combined genetic, biochemical, and cell imaging approaches in Arabidopsis thaliana.

A mechanism for reorientation of cortical microtubule arrays driven by microtubule severing.

Environmental and hormonal signals cause reorganization of microtubule arrays in higher plants, but the mechanisms driving these transitions have remained elusive. The organization of these arrays is required to direct morphogenesis. We discovered that microtubule severing by the protein katanin plays a crucial and unexpected role in the reorientation of cortical arrays, as triggered by blue light.

Patterning and lifetime of plasma membrane-localized cellulose synthase is dependent on actin organization in Arabidopsis interphase cells.

The actin and microtubule cytoskeletons regulate cell shape across phyla, from bacteria to metazoans. In organisms with cell walls, the wall acts as a primary constraint of shape, and generation of specific cell shape depends on cytoskeletal organization for wall deposition and/or cell expansion. In higher plants, cortical microtubules help to organize cell wall construction by positioning the delivery of cellulose synthase (CesA) complexes and guiding their trajectories to orient newly synthesized cellulose microfibrils.

Cortical microtubule arrays are initiated from a nonrandom prepattern driven by atypical microtubule initiation.

The ordered arrangement of cortical microtubules in growing plant cells is essential for anisotropic cell expansion and, hence, for plant morphogenesis. These arrays are dismantled when the microtubule cytoskeleton is rearranged during mitosis and reassembled following completion of cytokinesis. The reassembly of the cortical array has often been considered as initiating from a state of randomness, from which order arises at least partly through self-organizing mechanisms.

Live cell imaging reveals structural associations between the actin and microtubule cytoskeleton in Arabidopsis.

In eukaryotic cells, the actin and microtubule (MT) cytoskeletal networks are dynamic structures that organize intracellular processes and facilitate their rapid reorganization. In plant cells, actin filaments (AFs) and MTs are essential for cell growth and morphogenesis. However, dynamic interactions between these two essential components in live cells have not been explored.

Arabidopsis cortical microtubules position cellulose synthase delivery to the plasma membrane and interact with cellulose synthase trafficking compartments.

Plant cell morphogenesis relies on the organization and function of two polymer arrays separated by the plasma membrane: the cortical microtubule cytoskeleton and cellulose microfibrils in the cell wall. Studies using in vivo markers confirmed that one function of the cortical microtubule array is to drive organization of cellulose microfibrils by guiding the trajectories of active cellulose synthase (CESA) complexes in the plasma membrane, thus orienting nascent microfibrils. Here we provide evidence that cortical microtubules also position the delivery of CESA complexes to the plasma membrane and interact with small CESA-containing compartments by a mechanism that permits motility driven by microtubule depolymerization.