Rapid peroxisomal responses to ROS suggest an alternative mechanistic model for post-biogenesis peroxisomal life cycle in plants

Organelle extensions in plant cells

The life strategy of plants includes their ability to respond quickly at the cellular level to changes in their environment. The use of targeted fluorescent protein probes and imaging of living cells has revealed several rapidly induced organelle responses that create the efficient sub-cellular machinery for maintaining homeostasis in the plant cell. Several organelles, including plastids, mitochondria, and peroxisomes, extend and retract thin tubules that have been named stromules, matrixules, and peroxules, respectively. Here, I combine all these thin tubular forms under the common head of organelle extensions. All extensions change shape continuously and in their elongated form considerably increase organelle outreach into the surrounding cytoplasm. Their pleomorphy reflects their interactions with the dynamic endoplasmic reticulum and cytoskeletal elements. Here, using foundational images and time-lapse movies, and providing salient information on some molecular and biochemically characterized mutants with increased organelle extensions, I draw attention to their common role in maintaining homeostasis in plant cells.

Communications Between the Endoplasmic Reticulum and Other Organelles During Abiotic Stress Response in Plants

To adapt to constantly changing environmental conditions, plants have evolved sophisticated tolerance mechanisms to integrate various stress signals and to coordinate plant growth and development. It is well known that inter-organellar communications play important roles in maintaining cellular homeostasis in response to environmental stresses. The endoplasmic reticulum (ER), extending throughout the cytoplasm of eukaryotic cells, is a central organelle involved in lipid metabolism, Ca²⁺ homeostasis, and synthesis and folding of secretory and transmembrane proteins crucial to perceive and transduce environmental signals. The ER communicates with the nucleus via the highly conserved unfolded protein response pathway to mitigate ER stress. Importantly, recent studies have revealed that the dynamic ER network physically interacts with other intracellular organelles and endomembrane compartments, such as the Golgi complex, mitochondria, chloroplast, peroxisome, vacuole, and the plasma membrane, through multiple membrane contact sites between closely apposed organelles. In this review, we will discuss the signaling and metabolite exchanges between the ER and other organelles during abiotic stress responses in plants as well as the ER-organelle membrane contact sites and their associated tethering complexes.

Simultaneous live-imaging of peroxisomes and the ER in plant cells suggests contiguity but no luminal continuity between the two organelles

Transmission electron micrographs of peroxisomes in diverse organisms, including plants, suggest their close association and even luminal connectivity with the endoplasmic reticulum (ER). After several decades of debate de novo peroxisome biogenesis from the ER is strongly favored in yeasts and mammals. Unfortunately many of the proteins whose transit through the ER constitutes a major evidence for peroxisome biogenesis from the ER do not exhibit a similar localization in plants. Consequently, at best the ER acts as a membrane source for peroxisome in plants. However, in addition to their de novo biogenesis from the ER an increase in peroxisome numbers also occurs through fission of existing peroxisomes. In recent years live-imaging has been used to visualize peroxisomes and the ER but the precise spatio-temporal relationship between the two organelles has not been well-explored. Here we present our assessment of the peroxisome-ER relationship through imaging of living Arabidopsis thaliana plants simultaneously expressing different color combinations of fluorescent proteins targeted to both organelles. Our observations on double transgenic wild type and a drp3a/apm1 mutant Arabidopsis plants suggest strong correlations between the dynamic behavior of peroxisomes and the neighboring ER. Although peroxisomes and ER are closely aligned there appears to be no luminal continuity between the two. Similarly, differentially colored elongated peroxisomes of a drp3a mutant expressing a photoconvertible peroxisomal matrix protein are unable to fuse and share luminal protein despite considerable intermingling. Substantiation of our observations is suggested through 3D iso-surface rendering of image stacks, which shows closed ended peroxisomes enmeshed among ER tubules possibly through membrane contact sites (MCS). Our observations support the idea that increase in peroxisome numbers in a plant cell occurs mainly through the fission of existing peroxisomes in an ER aided manner.

Integration of peroxisomes into an endomembrane system that governs cellular aging

The peroxisome is an organelle that has long been known for its essential roles in oxidation of fatty acids, maintenance of reactive oxygen species (ROS) homeostasis and anaplerotic replenishment of tricarboxylic acid (TCA) cycle intermediates destined for mitochondria. Growing evidence supports the view that these peroxisome-confined metabolic processes play an essential role in defining the replicative and chronological age of a eukaryotic cell. Much progress has recently been made in defining molecular mechanisms that link cellular aging to fatty acid oxidation, ROS turnover, and anaplerotic metabolism in peroxisomes. Emergent studies have revealed that these organelles not only house longevity-defining metabolic reactions but can also regulate cellular aging via their dynamic communication with other cellular compartments. Peroxisomes communicate with other organelles by establishing extensive physical contact with lipid bodies, maintaining an endoplasmic reticulum (ER) to peroxisome connectivity system, exchanging certain metabolites, and being involved in the bidirectional flow of some of their protein and lipid constituents. The scope of this review is to summarize the evidence that peroxisomes are dynamically integrated into an endomembrane system that governs cellular aging. We discuss recent progress in understanding how communications between peroxisomes and other cellular compartments within this system influence the development of a pro- or anti-aging cellular pattern. We also propose a model for the integration of peroxisomes into the endomembrane system governing cellular aging and critically evaluate several molecular mechanisms underlying such integration.

Plant peroxisomes: biogenesis and function

Peroxisomes are eukaryotic organelles that are highly dynamic both in morphology and metabolism. Plant peroxisomes are involved in numerous processes, including primary and secondary metabolism, development, and responses to abiotic and biotic stresses. Considerable progress has been made in the identification of factors involved in peroxisomal biogenesis, revealing mechanisms that are both shared with and diverged from non-plant systems. Furthermore, recent advances have begun to reveal an unexpectedly large plant peroxisomal proteome and have increased our understanding of metabolic pathways in peroxisomes. Coordination of the biosynthesis, import, biochemical activity, and degradation of peroxisomal proteins allows for highly dynamic responses of peroxisomal metabolism to meet the needs of a plant. Knowledge gained from plant peroxisomal research will be instrumental to fully understanding the organelle's dynamic behavior and defining peroxisomal metabolic networks, thus allowing the development of molecular strategies for rational engineering of plant metabolism, biomass production, stress tolerance, and pathogen defense.

Peroxisome metabolism and cellular aging

The essential role of peroxisomes in fatty acid oxidation, anaplerotic metabolism, and hydrogen peroxide turnover is well established. Recent findings suggest that these and other related biochemical processes governed by the organelle may also play a critical role in regulating cellular aging. The goal of this review is to summarize and integrate into a model the evidence that peroxisome metabolism actually helps define the replicative and chronological age of a eukaryotic cell. In this model, peroxisomal reactive oxygen species (ROS) are seen as altering organelle biogenesis and function, and eliciting changes in the dynamic communication networks that exist between peroxisomes and other cellular compartments. At low levels, peroxisomal ROS activate an anti-aging program in the cell; at concentrations beyond a specific threshold, a pro-aging course is triggered.