The plant ESCRT component FREE1 regulates peroxisome-mediated turnover of lipid droplets in germinating Arabidopsis seedlings

The Atypical Pectin Methylesterase Family Member PME31 Promotes Seedling Lipid Droplet Utilization

In plants, the primary form of energy stored in seed lipid droplets, triacylglycerol (TAG), is catabolized during germination to support pre-photosynthetic growth. Although this process is essential for seedling development, it is incompletely understood. In a screen for Arabidopsis thaliana mutants displaying delayed degradation of the lipid droplet coat protein oleosin, five independent mutations in PECTIN METHYLESTERASE31 (PME31) were recovered. In addition to delayed oleosin degradation, pme31 mutant seedlings exhibited sustained lipid droplets and elevated levels of several TAG and diacylglycerol species. Although structural prediction classified PME31 as a pectinesterase, this structural family also includes a putative E. coli lipase, YbhC. Moreover, PME31 lacks an N-terminal signal peptide that would target it to the cell wall, where pectin resides. We found that a fluorescent PME31 reporter was cytosolic and partially associated with peroxisomes, the site of fatty acid catabolism, during lipid mobilization. Our findings suggest that, in contrast to canonical PMEs, which modify cell wall pectin, PME31 functions at peroxisomes to directly or indirectly promote lipid mobilization.

The green ESCRTs: Newly defined roles for ESCRT proteins in plants

Endocytosis and endosomal trafficking of plasma membrane proteins for degradation regulate cellular homeostasis and development. As part of these processes, ubiquitinated plasma membrane proteins (cargo) are recognized, clustered, and sorted into intraluminal vesicles of multivesicular endosomes by endosomal sorting complexes required for transport (ESCRT) proteins. At endosomes, ESCRT proteins recognize ubiquitinated cargo and mediate the deformation of the endosomal membrane in a negative geometry, away from the cytosol. ESCRTs are organized in five major complexes that are sequentially recruited to the endosomal membrane where they mediate its vesiculation and cargo sequestration. ESCRTs also participate in other membrane remodeling events and are widely conserved across organisms, both eukaryotes and prokaryotes. Plants contain both conserved and unique ESCRT components and show a general trend toward gene family expansion. Plant endosomes show a wide range of membrane budding patterns with potential implications in cargo sequestration efficiency, plant development, and hormone signaling. Understanding the diversification and specialization of plant ESCRT proteins can provide valuable insights in the mechanisms of ESCRT-mediated membrane bending. In this review, we discuss the endosomal function of ESCRT proteins, their unique features in plants, and the potential connections to the modes of plant endosomal vesiculation.

UBC18 E2 conjugating enzyme depends on SINAT1 E3 ligase to destabilize the ESCRT component FREE1 in plant iron deficiency responses

E2 ubiquitin-conjugating enzymes play a crucial role in the ubiquitination process by catalyzing ubiquitin transfer. Although the function of ubiquitin-protein ligases (E3s) in plants response to diverse abiotic stress by targeting specific substrates has been well studied, the involvement of E2s in environmental responses and their downstream targets are not well understood. In this study, we demonstrated that the E2 ubiquitin-conjugating enzyme 18 (UBC18) influences the stability of FREE1 to modulate iron deficiency stress. UBC18 affects the ubiquitination of FREE1 and promotes its degradation, and overexpression of UBC18 decreases plants' sensitivity to iron deficiency by reducing FREE1 level, whereas the ubc18 mutant exhibits sensitivity due to elevated FREE1 accumulation. This study also identified that lysine residues K227, K295, K315, and K540 are required for FREE1 ubiquitination and stability regulation. Mutating these lysine residues in FREE1 resulted in plants' sensitivity to iron starvation. Taken together, our findings shed light on the mechanism of UBC18 in responding to iron deficiency stress by modulating the abundance of FREE1, and further elucidate the role of ubiquitination sites in FREE1 stability regulation and the plant iron deficiency response.

Lipid droplets degradation mechanisms from microalgae to mammals, a comparative overview

Lipid droplets (LDs) are organelles composed of a hydrophobic core (mostly triacylglycerols and steryl esters) delineated by a lipid monolayer and found throughout the tree of life. LDs were seen for a long time as simple energy storage organelles but recent works highlighted their versatile roles in several fundamental cellular processes, particularly during stress response. LDs biogenesis occurs in the ER and their number and size can be dynamically regulated depending on their function, e.g. during development or stress. Understanding their biogenesis and degradation mechanisms is thus essential to better apprehend their roles. LDs degradation can occur in the cytosol by lipolysis or after their internalization into lytic compartments (e.g. vacuoles or lysosomes) using diverse mechanisms that depend on the considered organism, tissue, developmental stage or environmental condition. In this review, we summarize our current knowledge on the different LDs degradation pathways in several main phyla of model organisms, unicellular or pluricellular, photosynthetic or not (budding yeast, mammals, land plants and microalgae). We highlight the conservation of the main degradation pathways throughout evolution, but also the differences between organisms, or inside an organism between different organs. Finally, we discuss how this comparison can help to shed light on relationships between LDs degradation pathways and LDs functions.

Developmental pleiotropy of SDP1 from seedling to mature stages in B. napus

Rapeseed, an important oil crop, relies on robust seedling emergence for optimal yields. Seedling emergence in the field is vulnerable to various factors, among which inadequate self-supply of energy is crucial to limiting seedling growth in early stage. SUGAR-DEPENDENT1 (SDP1) initiates triacylglycerol (TAG) degradation, yet its detailed function has not been determined in B. napus. Here, we focused on the effects of plant growth during whole growth stages and energy mobilization during seedling establishment by mutation in BnSDP1. Protein sequence alignment and haplotypic analysis revealed the conservation of SDP1 among species, with a favorable haplotype enhancing oil content. Investigation of agronomic traits indicated bnsdp1 had a minor impact on vegetative growth and no obvious developmental defects when compared with wild type (WT) across growth stages. The seed oil content was improved by 2.0-2.37% in bnsdp1 lines, with slight reductions in silique length and seed number per silique. Furthermore, bnsdp1 resulted in lower seedling emergence, characterized by a shrunken hypocotyl and poor photosynthetic capacity in the early stages. Additionally, impaired seedling growth, especially in yellow seedlings, was not fully rescued in medium supplemented with exogenous sucrose. The limited lipid turnover in bnsdp1 was accompanied by induced amino acid degradation and PPDK-dependent gluconeogenesis pathway. Analysis of the metabolites in cotyledons revealed active amino acid metabolism and suppressed lipid degradation, consistent with the RNA-seq results. Finally, we proposed strategies for applying BnSDP1 in molecular breeding. Our study provides theoretical guidance for understanding trade-off between oil accumulation and seedling energy mobilization in B. napus.

ABLs and TMKs are co-receptors for extracellular auxin

Extracellular perception of auxin, an essential phytohormone in plants, has been debated for decades. Auxin-binding protein 1 (ABP1) physically interacts with quintessential transmembrane kinases (TMKs) and was proposed to act as an extracellular auxin receptor, but its role was disputed because abp1 knockout mutants lack obvious morphological phenotypes. Here, we identified two new auxin-binding proteins, ABL1 and ABL2, that are localized to the apoplast and directly interact with the extracellular domain of TMKs in an auxin-dependent manner. Furthermore, functionally redundant ABL1 and ABL2 genetically interact with TMKs and exhibit functions that overlap with those of ABP1 as well as being independent of ABP1. Importantly, the extracellular domain of TMK1 itself binds auxin and synergizes with either ABP1 or ABL1 in auxin binding. Thus, our findings discovered auxin receptors ABL1 and ABL2 having functions overlapping with but distinct from ABP1 and acting together with TMKs as co-receptors for extracellular auxin.

Molecular Machinery of Lipid Droplet Degradation and Turnover in Plants

Lipid droplets (LDs) are important organelles conserved across eukaryotes with a fascinating biogenesis and consumption cycle. Recent intensive research has focused on uncovering the cellular biology of LDs, with emphasis on their degradation. Briefly, two major pathways for LD degradation have been recognized: (1) lipolysis, in which lipid degradation is catalyzed by lipases on the LD surface, and (2) lipophagy, in which LDs are degraded by autophagy. Both of these pathways require the collective actions of several lipolytic and proteolytic enzymes, some of which have been purified and analyzed for their in vitro activities. Furthermore, several genes encoding these proteins have been cloned and characterized. In seed plants, seed germination is initiated by the hydrolysis of stored lipids in LDs to provide energy and carbon equivalents for the germinating seedling. However, little is known about the mechanism regulating the LD mobilization. In this review, we focus on recent progress toward understanding how lipids are degraded and the specific pathways that coordinate LD mobilization in plants, aiming to provide an accurate and detailed outline of the process. This will set the stage for future studies of LD dynamics and help to utilize LDs to their full potential.

Plant condensates: no longer membrane-less?

Cellular condensation is a reinvigorated area of study in biology, with scientific discussions focusing mainly on the forces that drive condensate formation, properties, and functions. Usually, condensates are called 'membrane-less' to highlight the absence of a surrounding membrane and the lack of associated contacts. In this opinion article we take a different direction, focusing on condensates that may be interfacing with membranes and their possible functions. We also highlight changes in condensate material properties brought about by condensate-membrane interactions, proposing how condensates-membrane interfaces could potentially affect interorganellar communication, development, and growth, but also adaptation in an evolutionary context. We would thus like to stimulate research in this area, which is much less understood in plants compared with the animal field.

Recent advances in plant endomembrane research and new microscopical techniques

The endomembrane system consists of various membrane-bound organelles including the endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN), endosomes, and the lysosome/vacuole. Membrane trafficking between distinct compartments is mainly achieved by vesicular transport. As the endomembrane compartments and the machineries regulating the membrane trafficking are largely conserved across all eukaryotes, our current knowledge on organelle biogenesis and endomembrane trafficking in plants has mainly been shaped by corresponding studies in mammals and yeast. However, unique perspectives have emerged from plant cell biology research through the characterization of plant-specific regulators as well as the development and application of the state-of-the-art microscopical techniques. In this review, we summarize our current knowledge on the plant endomembrane system, with a focus on several distinct pathways: ER-to-Golgi transport, protein sorting at the TGN, endosomal sorting on multivesicular bodies, vacuolar trafficking/vacuole biogenesis, and the autophagy pathway. We also give an update on advanced imaging techniques for the plant cell biology research.

The ubiquitin-protein ligase MIEL1 localizes to peroxisomes to promote seedling oleosin degradation and lipid droplet mobilization

Lipid droplets are organelles conserved across eukaryotes that store and release neutral lipids to regulate energy homeostasis. In oilseed plants, fats stored in seed lipid droplets provide fixed carbon for seedling growth before photosynthesis begins. As fatty acids released from lipid droplet triacylglycerol are catabolized in peroxisomes, lipid droplet coat proteins are ubiquitinated, extracted, and degraded. In Arabidopsis seeds, the predominant lipid droplet coat protein is OLEOSIN1 (OLE1). To identify genes modulating lipid droplet dynamics, we mutagenized a line expressing mNeonGreen-tagged OLE1 expressed from the OLE1 promoter and isolated mutants with delayed oleosin degradation. From this screen, we identified four miel1 mutant alleles. MIEL1 (MYB30-interacting E3 ligase 1) targets specific MYB transcription factors for degradation during hormone and pathogen responses [D. Marino et al., Nat. Commun. 4, 1476 (2013); H. G. Lee and P. J. Seo, Nat. Commun. 7, 12525 (2016)] but had not been implicated in lipid droplet dynamics. OLE1 transcript levels were unchanged in miel1 mutants, indicating that MIEL1 modulates oleosin levels posttranscriptionally. When overexpressed, fluorescently tagged MIEL1 reduced oleosin levels, causing very large lipid droplets. Unexpectedly, fluorescently tagged MIEL1 localized to peroxisomes. Our data suggest that MIEL1 ubiquitinates peroxisome-proximal seed oleosins, targeting them for degradation during seedling lipid mobilization. The human MIEL1 homolog (PIRH2; p53-induced protein with a RING-H2 domain) targets p53 and other proteins for degradation and promotes tumorigenesis [A. Daks et al., Cells 11, 1515 (2022)]. When expressed in Arabidopsis, human PIRH2 also localized to peroxisomes, hinting at a previously unexplored role for PIRH2 in lipid catabolism and peroxisome biology in mammals.

Lipid droplets are versatile organelles involved in plant development and plant response to environmental changes

Since decades plant lipid droplets (LDs) are described as storage organelles accumulated in seeds to provide energy for seedling growth after germination. Indeed, LDs are the site of accumulation for neutral lipids, predominantly triacylglycerols (TAGs), one of the most energy-dense molecules, and sterol esters. Such organelles are present in the whole plant kingdom, from microalgae to perennial trees, and can probably be found in all plant tissues. Several studies over the past decade have revealed that LDs are not merely simple energy storage compartments, but also dynamic structures involved in diverse cellular processes like membrane remodeling, regulation of energy homeostasis and stress responses. In this review, we aim to highlight the functions of LDs in plant development and response to environmental changes. In particular, we tackle the fate and roles of LDs during the plant post-stress recovery phase.

Identification and characterization of the LDAP family revealed GhLDAP2_Dt enhances drought tolerance in cotton

Lipid droplet-associated proteins (LDAPs) play essential roles in tissue growth and development and in drought stress responses in plants. Cotton is an important fiber and cash crop; however, the LDAP family has not been characterized in cotton. In this study, a total of 14, six, seven, and seven genes were confirmed as LDAP family members in Gossypium hirsutum, Gossypium raimondii, Gossypium arboreum, and Gossypium stocksii, respectively. Additionally, expansion in the LDAP family occurred with the formation of Gossypium, which is mirrored in the number of LDAPs found in five Malvaceae species (Gossypioides kirkii, Bombax ceiba, Durio zibethinus, Theobroma cacao, and Corchorus capsularis), Arabidopsis thaliana, and Carica papaya. The phylogenetic tree showed that the LDAP genes in cotton can be divided into three groups (I, II, and III). The analysis of gene structure and conserved domains showed that LDAPs derived from group I (LDAP1/2/3) are highly conserved during evolution, while members from groups II and III had large variations in both domains and gene structures. The gene expression pattern analysis of LDAP genes showed that they are expressed not only in the reproductive organs (ovule) but also in vegetative organs (root, stem, and leaves). The expression level of two genes in group III, GhLDAP6_At/Dt, were significantly higher in fiber development than in other tissues, indicating that it may be an important regulator of cotton fiber development. In group III, GhLDAP2_At/Dt, especially GhLDAP2_Dt was strongly induced by various abiotic stresses. Decreasing the expression of GhLDAP2_Dt in cotton via virus-induced gene silencing increased the drought sensitivity, and the over-expression of GhLDAP2_Dt led to increased tolerance to mannitol-simulated osmotic stress at the germination stage. Thus, we conclude that GhLDAP2_Dt plays a positive role in drought tolerance.