What happens before the wall? Interaction between XTH33 and ARP2/3 complex

The study of the plant cell wall (CW) plays a central and crucial role in all aspects of plant biology. CW is a dynamic compartment composed of cellulose microfibrils embedded in a complex network of hemicelluloses, pectins and, in a minor part, proteins. Interaction models exist for crosstalk between cellulose and xyloglucans (XGs), the major component of hemicelluloses, but these models are continually updated. Furthermore, pectins can be covalently crosslinked with XGs. The synthesis and remodeling of CW polymers are essential for plant cell growth and differentiation and for responding to different stresses. In particular, the Xyloglucan endotransglucosylases/hydrolases (XTHs) [E.C. 2.4.1.207] mediate both the integration of newly secreted XGs into the wall and the remodeling of pre-existing wall XGs 1. Thus, CW biogenesis, growth, differentiation and remodeling, as well as wall-related signaling and defense responses depend on the presence of complex and specific mechanisms. It is well known that the secretion route has a pivotal role in this scenario. Vesicle trafficking and organelle movement are primarily dependent on a cytoskeletal action. It was demonstrated that actin cytoskeleton is involved in the vesicles’ distribution2. The cytoskeleton regulates CW assembly, perceives changes in the CW, and many other aspects. In spite of this the interaction between CW proteins and cytoskeleton has never been investigated. The ARP2/3 complex is one of the most conserved actin nucleators. It is composed of two actin-like subunits (ARP2 and ARP3) and five unique subunits (ARP2/3 complex component - ARPC1 to ARPC5). Plant ARP2/3 complex has been reported to participate in actin reorganization at the plasma membrane during polarized growth of trichomes and at the plasma membrane-endoplasmic reticulum contact sites3. It is reported how the mutation in the ARP2/3 complex subunits can affect the shape of the cotyledon pavement cells suggesting a possible role in the re-distribution of cell wall component 3. To undercover this possible intriguing role, we have focused our attention on studying the interconnection between an Xyloglucan endotransglucosylase/hydrolase isoform, XTH33, remodeling CW enzyme, and the ARP2/3 complex. It has been previously demonstrated that XTH33 reaches plasma membrane and the cell wall through a Conventional Protein Secretion (CPS) pathway 1. In stable Arabidopsis transformed plants mutated in different subunits of ARP2/3 complex, we have observed how the localization of XTH33 chimera changed. In particular, in the mutant arpc5 background seedlings the XTH33-RFP has lost its CW localization to fill the Vacuole. Intriguingly, the XTH33-RFP/arpc5 plants also show a strong growth in the first phase of development and a completely remodeling of cells shape that are not visible studying the interaction with the other subunits. These preliminary results demonstrate a re-localization of XTH33 mediated by the action of the actin filaments, in particular, the involvement of the ARPC5 subunit of the ARP2/3 complex. This opens the way for further analyses in order to understand the underlying cytoskeletal mechanisms under these processes.