Arabidopsis thaliana EARLY RESPONSIVE TO DEHYDRATION 7 Localizes to Lipid Droplets via Its Senescence Domain.

Nathan M Doner,Robert T Mullen,Siqi Sun,Till Ischebeck,Kent D Chapman,Satinder K Gidda,Kerstin Schmitt,John M Dyer,Damien Seay,Marina Mehling,Gerhard H Braus

doi:10.3389/fpls.2021.658961

Abstract

Lipid droplets (LDs) are neutral-lipid-containing organelles found in all kingdoms of life and are coated with proteins that carry out a vast array of functions. Compared to mammals and yeast, relatively few LD proteins have been identified in plants, particularly those associated with LDs in vegetative (non-seed) cell types. Thus, to better understand the cellular roles of LDs in plants, a more comprehensive inventory and characterization of LD proteins is required. Here, we performed a proteomics analysis of LDs isolated from drought-stressed Arabidopsis leaves and identified EARLY RESPONSIVE TO DEHYDRATION 7 (ERD7) as a putative LD protein. mCherry-tagged ERD7 localized to both LDs and the cytosol when ectopically expressed in plant cells, and the protein’s C-terminal senescence domain (SD) was both necessary and sufficient for LD targeting. Phylogenetic analysis revealed that ERD7 belongs to a six-member family in Arabidopsis that, along with homologs in other plant species, is separated into two distinct subfamilies. Notably, the SDs of proteins from each subfamily conferred targeting to either LDs or mitochondria. Further, the SD from the ERD7 homolog in humans, spartin, localized to LDs in plant cells, similar to its localization in mammals; although, in mammalian cells, spartin also conditionally localizes to other subcellular compartments, including mitochondria. Disruption of ERD7 gene expression in Arabidopsis revealed no obvious changes in LD numbers or morphology under normal growth conditions, although this does not preclude a role for ERD7 in stress-induced LD dynamics. Consistent with this possibility, a yeast two-hybrid screen using ERD7 as bait identified numerous proteins involved in stress responses, including some that have been identified in other LD proteomes. Collectively, these observations provide new insight to ERD7 and the SD-containing family of proteins in plants and suggest that ERD7 may be involved in functional aspects of plant stress response that also include localization to the LD surface.

Highlights

Cells store neutral lipids, such as triacylglycerols (TAG) and sterol esters, in cytoplasmic lipid droplets (LDs), an organelle found in all kingdoms of life (Chapman et al, 2013; Lundquist et al, 2020)
Consistent with this, we observed a significant increase in the number of Lipid droplets (LDs) stained with the neutral lipid-selective dye BODIPY 493/503 in leaves from drought stress-treated vs. control Arabidopsis plants (Supplementary Figure 2)
Shown in Supplementary Dataset 3 are the summed label-free quantification (LFQ) and enrichment values for all detected proteins at different subcellular localizations based on PPDB (Sun et al, 2009; Ischebeck et al, 2020), including proteins annotated to be localized at LDs, which were the most enriched in the LD fractions, as expected

Summary

INTRODUCTION

Cells store neutral lipids, such as triacylglycerols (TAG) and sterol esters, in cytoplasmic lipid droplets (LDs), an organelle found in all kingdoms of life (Chapman et al, 2013; Lundquist et al, 2020). Among the six SD-containing proteins in Arabidopsis, the SDs of ERD7 and both of its two closest homologs convey LD targeting in plant cells, as does the SD in human spartin, while the SD of other, more distantly-related Arabidopsis SD-containing proteins target to mitochondria We discuss these and other findings, including a yeast two-hybrid (Y2H) screen that identified numerous stress-related proteins, and how they will provide new avenues for investigating the underlying mechanisms linking LDs and stress responses in plants

MATERIALS AND METHODS

RESULTS

DISCUSSION

DATA AVAILABILITY STATEMENT