Division of Labor Between Two Actin Nucleators—the Formin FH1 and the ARP2/3 Complex—in Arabidopsis Epidermal Cell Morphogenesis

Abstract

The ARP2/3 complex and formins are the only known plant actin nucleators. Besides their actin-related functions, both systems also modulate microtubule organization and dynamics. Loss of the main housekeeping Arabidopsis thaliana Class I membranetargeted formin FH1 (At3g25500) is known to increase cotyledon pavement cell lobing, while mutations affecting ARP2/3 subunits exhibit an opposite effect. Here we examine the role of FH1 and the ARP2/3 complex subunit ARPC5 (At4g01710) in epidermal cell morphogenesis with focus on pavement cells and trichomes using a model system of single fh1 and arpc5, as well as double fh1 arpc5 mutants. While cotyledon pavement cell shape in double mutants mostly resembled single arpc5 mutants, analysis of true leaf epidermal morphology, as well as actin and microtubule organization and dynamics, revealed a more complex relationship between the two systems and similar, rather than antagonistic, effects on some parameters. Both fh1 and arpc5 mutations increased actin network density and increased cell shape complexity in pavement cells and trichomes of first true leaves, in contrast to cotyledons. Thus, while the two actin nucleation systems have complementary roles in some aspects of cell morphogenesis in cotyledon pavement cells, they may act in parallel in other cell types and developmental stages.

Cooperative actin filament nucleation by the Arp2/3 complex and formins maintains the homeostatic cortical array in Arabidopsis epidermal cells

“…Simultaneous inhibition of the Arp2/3 complex and formins promotes de novo nucleation events When we combined genetic and chemical methods to down-regulate the Arp2/3 complex and formins simultaneously, surprisingly, the frequency of actin filament nucleation was significantly enhanced, with de novo nucleation events appearing to be the exclusive contributor to this enhancement. A similar experiment in Arabidopsis cotyledon epidermal pavement cells with the fh1 arpc5 double mutant also showed a minor increase in actin density (Cifrová et al, 2020).…”

“…Arabidopsis plants have defects in general growth and epidermal cell morphology when deficient for the Arp2/3 complex Previous studies of Arp2/3 complex mutants reveal defects in Arabidopsis epidermal cell growth and morphology (Cifrová et al, 2020;Facette et al, 2015;Harries et al, 2005;Le et al, 2003;Li et al, 2003;Mathur et al, 2003b;Szymanski et al, 1999;Yanagisawa et al, 2015). Here, we acquired a homozygous T-DNA insertion mutant for ARP2, arp2-1 (or wrm1-2; SALK_003448), and a point mutation of ARPC2, arpc2 (or dis2-1; (El-Assal et al, 2004), and confirmed the cell and organ growth phenotypes.…”

The Morphological Diversity of Plant Organs: Manipulating the Organization of Microtubules May Do the Trick

“…In several species, including Arabidopsis , pavement cells display an interlocking jigsaw puzzle-shape appearance, whose development relies on polarity establishment and is influenced by the cytoskeleton, hormonal signals, and mechanical stress (Bidhendi et al, 2019 ; Pan et al, 2020 ). The removal of actin filaments or actin regulating proteins (e.g., ARP2/3; SCAR/WAVE) leads to smaller lobes (Cifrová et al, 2020 ) suggesting that cortical actin filaments that localize to the growing lobe tips may regulate lobe outgrowth. In pavement cells, cortical microtubules are persistently enriched in periclinal walls at the convex side of lobes and mutants defective in MAPs often display defects in lobe formation and/or growth (Armour et al, 2015 ; Wong et al, 2019 ).…”

“…In addition, it is known that the structure of microtubules or actin can be influenced by each other. Cells with dysfunctional actin networks also exhibit alternations in microtubule organization (Cifrová et al, 2020 ). Our recent study has revealed a plant specific actin-microtubule bridging complex, consisting of Networked protein 3C (NET3C), Kinesin Light Chain-Related/Cellulose Microtubule Uncoupling1 (KLCR1/CMU1) and IQDs.…”

Bundling up the Role of the Actin Cytoskeleton in Primary Root Growth

“…Although some of its subunits have been shown to colocalize with actin-branching sites, its function is yet to be demonstrated in planta ( Fišerová et al, 2006 ). Other studies link the function of ARP2/3 to actin-microtubule interplay ( Zhang et al, 2013a ; Havelková et al, 2015 ; Cifrová et al, 2020 ). The lack of functional ARP2/3 complex subunits or its activators results in a reduction of primary root growth rate, accompanied by less cell divisions in the meristematic zone, more cells in the transition zone, and a shorter elongation zone ( Dyachok et al, 2008 , 2011 ; Takatsuka et al, 2018 ).…”