Physiological Functions of the COPI Complex in Higher Plants

Abstract

COPI vesicles are essential to the retrograde transport of proteins in the early secretory pathway. The COPI coatomer complex consists of seven subunits, termed α-, β-, β′-, γ-, δ-, ε-, and ζ-COP, in yeast and mammals. Plant genomes have homologs of these subunits, but the essentiality of their cellular functions has hampered the functional characterization of the subunit genes in plants. Here we have employed virus-induced gene silencing (VIGS) and dexamethasone (DEX)-inducible RNAi of the COPI subunit genes to study the in vivo functions of the COPI coatomer complex in plants. The β′-, γ-, and δ-COP subunits localized to the Golgi as GFP-fusion proteins and interacted with each other in the Golgi. Silencing of β′-, γ-, and δ-COP by VIGS resulted in growth arrest and acute plant death in Nicotiana benthamiana, with the affected leaf cells exhibiting morphological markers of programmed cell death. Depletion of the COPI subunits resulted in disruption of the Golgi structure and accumulation of autolysosome-like structures in earlier stages of gene silencing. In tobacco BY-2 cells, DEX-inducible RNAi of β′-COP caused aberrant cell plate formation during cytokinesis. Collectively, these results suggest that COPI vesicles are essential to plant growth and survival by maintaining the Golgi apparatus and modulating cell plate formation.

Characterization of the role of several COPI complex isoforms during the early acceptance of compatible pollen grains inArabidopsis thaliana

“…A subunit that displayed a more subdued phenotype was that of the g-copi mutant; recall that when this line was homozygous, the mutant was lethal under our growing conditions and only heterozygous individuals were characterized. The heterozygous plants were characterized as previous studies showed that VIGS induced gene silencing of the N. benthamiana g-COPI homologue resulted in acute plant cell death, partially due to disassembly of the Golgi apparatus, so it was likely that the g-copi mutant would have a pollen-pistil interaction phenotype (Ahn et al 2015). As demonstrated by the RT-PCR results, expression of g-COPI was similar to Col-0 wild-type; however, the heterozygous plants displayed a pollination phenotype.…”

“…With increased anterograde traffic in the papilla cell to deliver vesicles to the pollen contact point to enable pollen grain hydration, there would be increased retrograde traffic to maintain membrane balance in the system. Coat Protein II (COPII) vesicles are involved in the anterograde transport of protein out from the ER while the Coat protein 1 (COPI) coated vesicles are the mediators of intra-Golgi transport and retrograde transport from the Golgi back towards the endoplasmic reticulum (Ahn et al 2015;Brandizzi and Barlowe 2013;Gimeno-Ferrer et al 2017). In regards to the intra-Golgi transport of COPI vesicles the directionality of their transport is still a matter of debate; however, their role in retrograde transport is clear (Jackson et al 2012).…”

“…In plants, the genes that encode the various components of the COPI subunits and machinery have been identified (a/b/b'/g/d/e/z) and in A. thaliana, several different isoforms of all of the coatomer subunits, except for g-COPI and d-COPI have been identified (Ahn et al 2015;Gao et al 2014;Robinson et al 2007;Woo et al 2015). At the ultrastructural level in A. thaliana, two structurally distinct type of COPI vesicles have been identified by electron tomography (Donohoe et al 2007;Gao et al 2014).…”

“…At the ultrastructural level in A. thaliana, two structurally distinct type of COPI vesicles have been identified by electron tomography (Donohoe et al 2007;Gao et al 2014). Characterization of the b'-gand d-COPI subunits examined their function in N. benthamiana and tobacco BY-2 cells, where it was demonstrated that the COPI complex was involved in cell plate formation (via the phragmoplast), Golgi maintenance and disruption lead to programmed cell death after COPI depletion via inducible gene silencing of b'-COPI, g-COPI and d-COPI (Ahn et al 2015). The a2-COPI isoform was characterized in the context of several a2-copi loss-of-function mutants.…”

Loss of Arabidopsis β-COP Function Affects Golgi Structure, Plant Growth and Tolerance to Salt Stress

“…Genes encoding the components of the COPI machinery have been identified in plants (Robinson et al, 2007;Gao et al, 2014;Ahn et al, 2015;Woo et al, 2015). In Arabidopsis, all coatomer subunits (except γand δ-COP) have more than one isoform, in contrast to yeast, that contains only one isoform for every subunit, and mammals, which contain 2 isoforms of the γand ζ-COP subunits but only one for the others.…”