Abstract
The developmental program of seed formation, germination, and early seedling growth requires not only tight regulation of cell division and metabolism, but also concerted control of the structure and function of organelles, which relies on specific changes in their protein composition. Of particular interest is the switch from heterotrophic to photoautotrophic seedling growth, for which cytoplasmic lipid droplets (LDs) play a critical role as depots for energy-rich storage lipids. Here, we present the results of a bottom-up proteomics study analyzing the total protein fractions and LD-enriched fractions in eight different developmental phases during silique (seed) development, seed germination, and seedling establishment in Arabidopsis (Arabidopsis thaliana). The quantitative analysis of the LD proteome using LD-enrichment factors led to the identification of six previously unidentified and comparably low-abundance LD proteins, each of which was confirmed by intracellular localization studies with fluorescent protein fusions. In addition to these advances in LD protein discovery and the potential insights provided to as yet unexplored aspects in plant LD functions, our data set allowed for a comparative analysis of the LD protein composition throughout the various developmental phases examined. Among the most notable of the alterations in the LD proteome were those during seedling establishment, indicating a switch in the physiological function(s) of LDs after greening of the cotyledons. This work highlights LDs as dynamic organelles with functions beyond lipid storage.
Highlights
While the sporophyte of angiosperms is photoautotrophic during most of its life cycle, it is largely heterotrophic during its initial formation, including embryo development and early seedling establishment
In Arabidopsis (Arabidopsis thaliana) seeds, a combination of storage proteins and lipids accumulate in the embryo, with a minor proportion being deposited in the endosperm (Penfield et al, 2005)
Our proteomics survey allowed us to follow the dynamics of the lipid droplets (LDs) protein composition throughout silique development, germination, and seedling establishment, which will undoubtedly serve to help inform further research aimed at understanding the molecular mechanisms underlying LD biology in plants, as well as providing at better understanding of these developmental stages in Arabidopsis in general
Summary
While the sporophyte of angiosperms is photoautotrophic during most of its life cycle, it is largely heterotrophic during its initial formation, including embryo development and early seedling establishment. While the physiological role of LDIP remains to be determined, its ubiquitous expression, LD localization, and unique LD phenotype in the leaves of mutant plants (i.e., enlarged LDs) suggest that it is important in LD biogenesis and/or turnover Another recently-discovered plant LD protein is a member of the UBX domain-containing (PUX) protein family, PUX10, which was shown to localize to LDs in tobacco (Nicotiana tabacum) pollen tube and Arabidopsis seeds, and appears to play a role in the polyubiquitination pathway for degradation of other LD proteins (Deruyffelaere et al, 2018; Kretzschmar et al, 2018). Our proteomics survey allowed us to follow the dynamics of the LD protein composition throughout silique development, germination, and seedling establishment, which will undoubtedly serve to help inform further research aimed at understanding the molecular mechanisms underlying LD (protein) biology in plants, as well as providing at better understanding of these developmental stages in Arabidopsis in general
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