Computational Modeling of Cytoskeletal and Cell Wall Systems during Cell Morphogenesis

In plants, the actin and microtubule cytoskeletons control cell morphogenesis by modulating local cell wall properties. We previously used a combination of finite element (FE) computational modeling and multivariate live cell imaging to demonstrate that the actin filament nucleator ARP2/3 generates actin meshworks in a microtubule depletion zone at the branch apex of developing trichomes (Yanagisawa et al., 2015). The actin meshwork appears to have two functions: 1) it organizes actin bundle networks for bidirectional transport and for uniform distribution of secretory organelles; 2) it dynamically links the geometry of the cell wall with the size of the microtubule depletions zone to enable branch tapering during elongation. At present the cellular mechanisms by which ARP2/3 is clustered and activated is not known. In this study, we focus on the upstream regulators of ARP2/3 in the trichome system. Rho of plant (ROP) small GTPases function as critical regulators to re-organize cytoskeletons. SPIKE1 is a guanine nucleotide exchange factor, which converts the inactive ROP-GDP form into the active ROP-GTP form. ROP-GTP activates the WAVE/SCAR regulatory complex (W/SRC) which then unleashes the actin filament nucleation activity of ARP2/3. Here we report on the localization and dynamics of a fully functional YFP-tagged version of SPIKE1. We show that SPIKE1 and activated ROP-GTP localize at the trichome branch apex to mediate cell tapering. We also discover that SPIKE1, W/SRC and ARP2/3 cluster beneath the plasma membrane at putative ER-subdomains that can persist for tens of minutes. W/SRC localization is altered in spike1 trichomes and pavement cells, indicating that SPIKE1 is required for normal clustering and activation of ARP2/3. Interestingly, polarized accumulation of SPIKE1 is lost in developing trichomes when microtubules are depolymerized. These results indicate that SPIKE1 works in close cooperation with microtubules to determine an ARP2/3 activation domain that dynamically regulates cell wall heterogeneity and cell morphogenesis. These results will be discussed in the context of discovering growth control modules that may be adapted to different cell types to engineer desired cell shapes.