Lipid droplets (LDs) are ubiquitous organelles in plant cells, but their physiological roles are largely unknown. To gain insight into the function of LDs in plants, we have characterized the Arabidopsis homologs of SEIPIN proteins, which are crucial factors for LD biogenesis in yeast and animals. is expressed almost exclusively in embryos, while and have broader expression profiles with maximal levels in embryos and pollen, where LDs accumulate most abundantly. Genetic analysis demonstrates that all three contribute to proper LD biogenesis in embryos, whereas in pollen, only and play a significant role. The double and triple mutants accumulate extremely enlarged LDs in seeds and pollen, which hinders their subsequent mobilization during germination. Interestingly, electron microscopy analysis reveals the presence of nuclear LDs attached to type I nucleoplasmic reticulum in triple mutant embryos, supporting that SEIPINs are essential for maintaining the correct polarity of LD budding at the nuclear envelope, restricting it to the outer membrane. In pollen, the perturbations in LD biogenesis and turnover are coupled to reduced germination in vitro and with lower fertilization efficiency in vivo. In seeds, germination per se is not affected in and triple mutants, but there is a striking increase in seed dormancy levels. Our findings reveal the relevance of SEIPIN-dependent LD biogenesis in pollen transmission and in adjusting the timing of seed germination, two key adaptive traits of great importance in agriculture.
Several oligosaccharide fragments derived from plant cell walls activate plant immunity and behave as typical damage-associated molecular patterns (DAMPs). Some of them also behave as negative regulators of growth and development, and due to their antithetic effect on immunity and growth, their concentrations, activity, time of formation, and localization is critical for the so-called “growth-defense trade-off.” Moreover, like in animals, over accumulation of DAMPs in plants provokes deleterious physiological effects and may cause hyper-immunity if the cellular mechanisms controlling their homeostasis fail. Recently, a mechanism has been discovered that controls the activity of two well-known plant DAMPs, oligogalacturonides (OGs), released upon hydrolysis of homogalacturonan (HG), and cellodextrins (CDs), products of cellulose breakdown. The potential homeostatic mechanism involves specific oxidases belonging to the family of berberine bridge enzyme-like (BBE-like) proteins. Oxidation of OGs and CDs not only inactivates their DAMP activity, but also makes them a significantly less desirable food source for microbial pathogens. The evidence that oxidation and inactivation of OGs and CDs may be a general strategy of plants for controlling the homeostasis of DAMPs is discussed. The possibility exists of discovering additional oxidative and/or inactivating enzymes targeting other DAMP molecules both in the plant and in animal kingdoms.
A member of the Arabidopsis Berberine Bridge Enzyme-like (BBE-l) protein family named CELLODEXTRIN OXIDASE 2 (CELLOX2) has been characterized in this paper and shown to display structural and enzymatic features similar to the previously characterized CELLOX1. These include the capability to oxidize the mixed-linked β-1->3/β-1->4-glucans (MLGs), recently described as cell wall-derived damage-associated molecular patterns (DAMPs) that activate plant immunity. The two paralogous genes show a different expression profile. Unlike CELLOX1, CELLOX2 is not expressed in seedlings or in adult plants and is not involved in immunity against Botrytis cinerea. Both genes are expressed in a concerted manner in the seed coat during development: whereas CELLOX2 transcripts are detected mainly during the heart stage, CELLOX1 transcripts are detected later, when the expression of CELLOX2 decreases. Analysis of seeds of cellox1 and cellox2 knock-out mutants show alterations in the structure of the coat and mucilage, but not in their monosaccharide composition. We propose that the cell wall structure of specific organs is not only the result of a coordinated synthesis/degradation of polysaccharides but also of their exposure to enzymatic oxidation. Our results also reinforce the view that the family of BBE-l proteins is at least in part devoted to the control of the activity of cell wall-derived oligosaccharides acting as DAMPs.
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