Plasma Membrane Domain Patterning and Self-Reinforcing Polarity in Arabidopsis

Abstract

Cell polarity is a key feature in the development of multicellular organisms. For instance, asymmetrically localized plasma-membrane-integral PIN-FORMED (PIN) proteins direct transcellular fluxes of the phytohormone auxin that govern plant development. Finetuned auxin flux is important for root protophloem sieve element differentiation and requires the interacting plasma-membrane-associated BREVIS RADIX (BRX) and PROTEIN KINASE ASSOCIATED WITH BRX (PAX) proteins. We observed ''donut-like'' polar PIN localization in developing sieve elements that depends on complementary, ''muffin-like'' polar localization of BRX and PAX. Plasma membrane association and polarity of PAX, and indirectly BRX, largely depends on phosphatidylinositol-4,5-bisphosphate. Consistently, mutants in phosphatidylinositol-4phosphate 5-kinases (PIP5Ks) display protophloem differentiation defects similar to brx mutants. The same PIP5Ks are in complex with BRX and display ''muffin-like'' polar localization. Our data suggest that the BRX-PAX module recruits PIP5Ks to reinforce PAX polarity and thereby the polarity of all three proteins, which is required to maintain a local PIN minimum.

Local auxin competition explains fragmented differentiation patterns

“…Protein immunolocalization. Immunostaining was performed on 5-day-old seedlings as previously described 11 . Samples were imaged by confocal laserscanning microscopy.…”

“…This auxin pattern emerges from polar auxin transport dynamics, with a key role for plasma-membrane-integral PIN-FORMED (PIN) auxin efflux carriers. PINs are rootward localized (i.e., at the plasma membrane that faces the root tip) in developing protophloem cells 11,12 , similar to most inner cell files 13,14 , and transport shootderived auxin delivered by bulk transport through mature phloem to the periphery of the meristem, as well as locally synthesized auxin, and auxin redirected by the root tip reflux loop 15,16 .…”

“…Whereas PAX stimulates PIN-mediated auxin efflux, BRX inhibits this activation. Because threshold auxin levels negatively regulate BRX plasma-membrane association and also stimulate PAX activity through phosphorylation 11,12,17 , a dynamic steadystate equilibrium ensues that fine-tunes PIN activity and thereby auxin flux through PPSE cell files. Yet, counterintuitively, both brx and pax loss-of-function mutants display discontinuous protophloem, which manifests in reduced root growth and other systemic effects 6,12,18 .…”

Plasma membrane nano-organization specifies phosphoinositide effects on Rho-GTPases and actin dynamics in tobacco pollen tubes

“…In previous studies, a contribution of phosphoinositides to the regulation of important physiological processes has also been reported for the vegetative portion of plants. These studies describe roles for phosphoinositides in clathrin recruitment (König et al, 2008;Ischebeck et al, 2013); the polarization of auxin efflux carriers of the PIN-FORMED (PIN)-family (Mei et al, 2012;Ischebeck et al, 2013; Tejos et al, 2014), of D6P-kinases (Barbosa et al, 2016) and of BRX and PAX proteins (Marhava et al, 2020); the control of cytokinesis (Lin et al, 2019); cell wall deposition (Rodriguez-Villalon et al, 2015), and roles in membrane trafficking during plant pathogen interactions (Shimada et al, 2019;Qin et al, 2020). While these reports together may suggest the presence of PtdIns(4,5)P 2 in membrane nanodomains occupied by phosphoinositides, in Arabidopsis hypocotyl cells we detected such nanodomains by SD only upon application of salt (Supplemental Fig.…”

OBERON3 and SUPPRESSOR OF MAX2 1-LIKE proteins form a regulatory module specifying phloem identity

“…A similar trend was observed for brx;smxl5 double mutants. Like OPS, the BRX gene ensures continues SE formation in this case, however, by downregulation of BAM3 transcription and steepening the auxin gradient in developing phloem cells (Scacchi et al 2010;Depuydt et al 2013;Marhava et al 2018;Marhava et al 2019). Similar to ops;smxl5 double mutants, brx;smxl5 double mutants developed shorter roots than brx and smxl5 single mutants and largely failed to differentiate SE ( Figure 3A, Supplementary Figure 3).…”