Actomyosin and CSI1/POM2 cooperate to deliver cellulose synthase from Golgi to cortical microtubules in Arabidopsis

New insights into plant cell wall functions

The plant cell wall is an extremely complicated natural nanoscale structure composed of cellulose microfibrils embedded in a matrix of noncellulosic polysaccharides, further reinforced by the phenolic compound lignins in some cell types. Such network formed by the interactions of multiscale polymers actually reflects functional form of cell wall to meet the requirements of plant cell functionalization. Therefore, how plants assemble cell wall functional structure is fundamental in plant biology and critical for crop trait formation and domestication as well. Due to the lack of effective analytical techniques to characterize this fundamental but complex network, it remains difficult to establish direct links between cell-wall genes and phenotypes. The roles of plant cell walls are often underestimated as indirect. Over the past decades, many genes involved in cell wall biosynthesis, modification, and remodeling have been identified. The application of a variety of state-of-the-art techniques has made it possible to reveal the fine cell wall networks and polymer interactions. Hence, many exciting advances in cell wall biology have been achieved in recent years. This review provides an updated overview of the mechanistic and conceptual insights in cell wall functionality, and prospects the opportunities and challenges in this field.

Tethering of cellulose synthase complex to the plasma membrane relies on the isoform of EXO70A1 in Arabidopsis

In yeast and mammals, the EXO70 subunit of the exocyst complex plays a key role in mediating the tethering of exocytic vesicles to the plasma membrane (PM). In plants, however, the role of EXO70 in regulating vesicle tethering during exocytosis remains unclear. In land plants, EXO70 has undergone significant evolutionary expansion, resulting in multiple EXO70 paralogues that may allow the exocyst to form various isoforms with specific functions. Previous research in Arabidopsis has shown that generally disrupting exocyst function leads to various defects in cellulose synthase (CESA) complex (CSC) trafficking. In this study, we utilized real-time imaging combined with genetic approaches to explore the role of EXO70A1, a member of the EXO70 family in Arabidopsis, in CSC trafficking. The exo70a1 mutant exhibited a decrease in crystalline cellulose content and a reduced density of functional CSCs in the PM. Moreover, the delivery of tdTomato-CESA6 from the cortex to the PM was compromised in the mutant, leading to the accumulation of CSC vesicles at the cell cortex. However, the velocity of tdTomato-CESA6 in the PM was unaffected in exo70a1. These findings suggest that EXO70A1 has a specific role in tethering CSCs to the PM.

AtFH5 recruits and transports the arabinogalactan protein AGP23 to maintain the tip growth of pollen tube

Actin cytoskeleton drives the targeted transport of cell wall components to sustain the tip growth of pollen tubes for double fertilization; however, the underlying mechanism remains largely unknown. Arabidopsis formin 5 (AtFH5), an actin-nucleating protein, localizes at secretory vesicles and mediates actin polymerization-based vesicle trafficking in pollen. Here, we demonstrate that AtFH5 determines the recruitment and transport of cell wall components in AtFH5-labeled vesicles during the tip growth of pollen tubes. Through a screen of interacting proteins of AtFH5, we identify many cell wall-related proteins, with arabinogalactan protein 23 (AGP23) occupying the highest frequency. AtFH5 interacts with AGP23 via its N-terminal extracellular domain (ECD) and jointly regulate the pollen germination and tube growth process. Further observations reveal that AGP23 co-localizes with AtFH5 at moving vesicles, germination sites, and pollen tube tips, suggesting that AGP23 is delivered by AtFH5-labeled vesicles. Deletion of the ECD of AtFH5 interrupts the dynamic localization and cell-wall connection of AGP23 in pollen grains and tubes. Cytological and genetic evidence shows that AGP23 and AtFH5 work in the same pathway to modulate cell wall composition. Together, our data uncover a role of formin in directing the sorting and deposition of cell wall components via secretory vesicle trafficking during pollen germination and tube growth.

Regulation of cytoskeleton dynamics and its interplay with force in plant cells

The plant cytoskeleton is an intricate network composed of actin filaments and microtubules. The cytoskeleton undergoes continuous dynamic changes that provide the basis for rapidly responding to intrinsic and extrinsic stimuli, including mechanical stress. Microtubules can respond to alterations of mechanical stress and reorient along the direction of maximal tensile stress in plant cells. The cytoskeleton can also generate driving force for cytoplasmic streaming, organelle movement, and vesicle transportation. In this review, we discuss the progress of how the plant cytoskeleton responds to mechanical stress. We also summarize the roles of the cytoskeleton in generating force that drive organelles and nuclear transportation in plant cells. Finally, some hypotheses concerning the link between the roles of the cytoskeleton in force response and organelle movement, as well as several key questions that remain to be addressed in the field, are highlighted.

Novel molecular insights into the machinery driving secondary cell wall synthesis and patterning

The essential role of water-conducting xylem tissue in plant growth and crop yield is well-established. However, the molecular mechanisms underlying xylem formation and its unique functionality, which is acquired post-mortem, remain poorly understood. Recent advancements in genetic tools and model systems have significantly enhanced the ability to microscopically study xylem development, particularly its distinctive cell wall patterning. Early molecular mechanisms enabling pattern formation have been elucidated and validated through computational models. Despite these advancements, numerous questions remain unresolved but are approachable with current methodologies. This mini-review takes in the latest research findings in xylem cell wall synthesis and patterning and highlights prospective directions for future investigations.

Endomembrane trafficking driven by microtubule growth regulates stomatal movement in Arabidopsis

Microtubule-based vesicle trafficking usually relies upon kinesin and dynein motors and few reports describe microtubule polymerisation driving directional vesicle trafficking. Here we show that Arabidopsis END BINDING1b (EB1b), a microtubule plus-end binding protein, directly interacts with SYP121, a SNARE protein that mediates the trafficking of the K+ channel KAT1 and its distribution to the plasma membrane (PM) in Arabidopsis guard cells. Knockout of AtEB1b and its homologous proteins results in a modest but significant change in the distribution of KAT1 and SYP121 in guard cells and consequently delays light-induced stomatal opening. Live-cell imaging reveals that a portion of SYP121-associated endomembrane compartments co-localise with AtEB1b at the growing ends of microtubules, trafficking along with the growth of microtubules for targeting to the PM. Our study reveals a mechanism of vesicle trafficking driven by microtubule growth, which is involved in the redistribution of PM proteins to modulate guard cell movement.

Two roads diverge for cellulose synthase complex trafficking

Cellulose, an abundant and essential component of plant cell walls, is made by cellulose synthase complexes at the plasma membrane (PM). Recently, Liu et al. uncovered molecular mechanisms that suggest the existence of two distinct pathways for cellulose synthase trafficking from the Golgi apparatus to the PM.