Robust banded protoxylem pattern formation through microtubule-based directional ROP diffusion restriction

Abstract

In plant vascular tissue development, different cell wall patterns are formed, offering different mechanical properties optimised for different growth stages. Critical in these patterning processes are Rho of Plants (ROP) proteins, a class of evolutionarily conserved small GTPase proteins responsible for local membrane domain formation in many organisms. While the spotted metaxylem pattern can easily be understood as a result of a Turing-style reaction-diffusion mechanism, it remains an open question how the consistent orientation of evenly spaced bands and spirals as found in protoxylem is achieved. We hypothesise that this orientation results from an interaction between ROPs and an array of transversely oriented cortical microtubules that acts as a directional diffusion barrier. Here, we explore this hypothesis using partial differential equation models with anisotropic ROP diffusion and show that a horizontal microtubule array acting as a vertical diffusion barrier to active ROP can yield a horizontally banded ROP pattern. We then study the underlying mechanism in more detail, finding that it can only orient curved pattern features but not straight lines. This implies that, once formed, banded and spiral patterns cannot be reoriented by this mechanism. Finally, we observe that ROPs and microtubules together only form ultimately static patterns if the interaction is implemented with sufficient biological realism.

Long-term single-cell imaging and simulations of microtubules reveal driving forces for wall pattering during proto-xylem development

“…A Turing-like reaction-diffusion mechanism (Turing 1952) is suggested to initiate patterning of ROP11 in meta-xylem formation, in which ROPGEF4 and ROPGAP3 may act as activator and inhibitor, respectively (Oda and Fukuda 2012b). Alternatively, the difference in diffusion constants between active and inactive ROP11 may drive the Turing mechanism (Jacobs et al 2019). Both mechanisms would explain how ROP11-induced microtubule-free regions are established to spatially control cell wall deposition (Oda and Fukuda 2012a; Oda and Fukuda 2013).…”

“…A modelling study on ROP patterning suggested that microtubule arrays, which form a barrier to ROP diffusion, cannot change the orientation of an existing pattern. This indicates that microtubules need to be oriented transversely prior to the onset of band separation (Jacobs et al 2019).…”

Nucleation complex behaviour is critical for cortical microtubule array homogeneity and patterning

“…Various ROPs and the aforementioned effectors are also expressed during protoxylem formation [57, 58] and striated AtROP7 patterns are observed in protoxylem [59]. Results so far indicate that the corresponding microtubule patterns are not simply a readout of a ROP pattern, as changes in microtubule dynamics affect both the dynamics and outcome of the patterning process [55, 20, 60].…”

Secondary cell wall patterning—connecting the dots, pits and helices

“…Theoretical studies regarding how Turing-like RD processes may operate in protoxylem revealed that diffusion anisotropy, as produced by microtubule-based diffusion restriction, is critical to this system. Under such conditions, a Turing-like RD mechanism favours banded patterns whose orientation is defined by the overall orientation of the diffusion anisotropy [160]. Interestingly, this outcome is independent of whether these systems would form spot, stripe or gap patterns in absence of the diffusion anisotropy.…”

“…For instance, Jacobs et al . [160] used partial differential equation models to study active ROP diffusion restriction as a mechanism of ROP pattern orientation in protoxylem development.…”