Sucrose starvation induces the degradation of proteins in trans-Golgi network and secretory vesicle cluster in tobacco BY-2 cells

Vacuolar degradation of plant organelles

Plants continuously remodel and degrade their organelles due to damage from their metabolic activities and environmental stressors, as well as an integral part of their cell differentiation programs. Whereas certain organelles use local hydrolytic enzymes for limited remodeling, most of the pathways that control the partial or complete dismantling of organelles rely on vacuolar degradation. Specifically, selective autophagic pathways play a crucial role in recognizing and sorting plant organelle cargo for vacuolar clearance, especially under cellular stress conditions induced by factors like heat, drought, and damaging light. In these short reviews, we discuss the mechanisms that control the vacuolar degradation of chloroplasts, mitochondria, endoplasmic reticulum, Golgi, and peroxisomes, with an emphasis on autophagy, recently discovered selective autophagy receptors for plant organelles, and crosstalk with other catabolic pathways.

Disrupting cell wall integrity impacts endomembrane trafficking to promote secretion over endocytic trafficking

The plant cell wall provides a strong yet flexible barrier to protect cells from the external environment. Modifications of the cell wall, either during development or under stress conditions, can induce cell wall integrity responses and ultimately lead to alterations in gene expression, hormone production, and cell wall composition. These changes in cell wall composition presumably require remodelling of the secretory pathway to facilitate synthesis and secretion of cell wall components and cell wall synthesis/remodelling enzymes from the Golgi apparatus. Here, we used a combination of live-cell confocal imaging and transmission electron microscopy to examine the short-term and constitutive impact of isoxaben, which reduces cellulose biosynthesis, and Driselase, a cocktail of cell-wall-degrading fungal enzymes, on cellular processes during cell wall integrity responses in Arabidopsis. We show that both treatments altered organelle morphology and triggered rebalancing of the secretory pathway to promote secretion while reducing endocytic trafficking. The actin cytoskeleton was less dynamic following cell wall modification, and organelle movement was reduced. These results demonstrate active remodelling of the endomembrane system and actin cytoskeleton following changes to the cell wall.

Plant Heterotrophic Cultures: No Food, No Growth

Plant cells are capable of uptaking exogenous organic substances. This inherited trait allows the development of heterotrophic cell cultures in various plants. The most common of them are Nicotiana tabacum and Arabidopsis thaliana. Plant cells are widely used in academic studies and as factories for valuable substance production. The repertoire of compounds supporting the heterotrophic growth of plant cells is limited. The best growth of cultures is ensured by oligosaccharides and their cleavage products. Primarily, these are sucrose, raffinose, glucose and fructose. Other molecules such as glycerol, carbonic acids, starch, and mannitol have the ability to support growth occasionally, or in combination with another substrate. Culture growth is accompanied by processes of specialization, such as elongation growth. This determines the pattern of the carbon budget. Culture ageing is closely linked to substrate depletion, changes in medium composition, and cell physiological rearrangements. A lack of substrate leads to starvation, which results in a decrease in physiological activity and the mobilization of resources, and finally in the loss of viability. The cause of the instability of cultivated cells may be the non-optimal metabolism under cultural conditions or the insufficiency of internal regulation.

Secreted arabinogalactan protein from salt-adapted tobacco BY-2 cells appears to be glycosylphosphatidyl inositol-anchored and associated with lipophilic moieties

Arabinogalactan proteins (AGPs) are plant extracellular proteoglycans associated with the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. This moiety is thought to be cleaved by phospholipase for secretion. Salt-adapted tobacco BY-2 cells were reported to secrete large amounts of AGPs into the medium. To investigate this mechanism, we expressed a fusion protein of tobacco sweet potato sporamin and AGP (SPO-AGP) in BY-2 cells and analyzed its fate after salt-adapting the cells. A two-phase separation analysis using Triton X-114 indicated that a significant proportion of SPO-AGP in the medium was recovered in the detergent phase, suggesting that this protein is GPI-anchored. Differential ultracentrifugation and a gradient density fractionation implicated extracellular vesicles or particles with SPO-AGP in the medium. Endogenous AGP secreted from salt-adapted and nontransgenic BY-2 cells behaved similarly to SPO-AGP. These results suggest that a part of the secreted AGPs from salt-adapted tobacco BY-2 cells are GPI-anchored and associated with particles or vesicles.

Glycosylphosphatidylinositol-anchoring is required for the proper transport and extensive glycosylation of a classical arabinogalactan protein precursor in tobacco BY-2 cells

Many precursors of plant arabinogalactan proteins (AGPs) contain a C-terminal glycosylphosphatidylinositol (GPI)-anchoring signal. Using NtAGP1, a classical tobacco AGP, as a model, and green fluorescent protein (GFP) and sweet potato sporamin (SPO) as tags, we analyzed the localization and modification of AGP and its mutant without GPI-anchoring signal (AGPΔC) in tobacco BY-2 cells. The NtAGP1 fusion proteins migrated as large smear on SDS-polyacrylamide gel, and these proteins also localized preferentially to the plasma membrane. In contrast, fusions of AGPΔC with GFP and SPO yielded several forms: The largest were secreted, whereas others were recovered in the endomembrane organelles, including vacuoles. Comparison of the glycan structures of the microsomal SPO-AGP and the secreted SPO-AGPΔC using antibodies against the glycan epitopes of AGP indicated that the glycan structures of these proteins are different. These observations indicate that GPI-anchoring is required for the proper transport and glycosylation of the AGP precursor.

Subcellular coordination of plant cell wall synthesis

Organelles of the plant cell cooperate to synthesize and secrete a strong yet flexible polysaccharide-based extracellular matrix: the cell wall. Cell wall composition varies among plant species, across cell types within a plant, within different regions of a single cell wall, and in response to intrinsic or extrinsic signals. This diversity in cell wall makeup is underpinned by common cellular mechanisms for cell wall production. Cellulose synthase complexes function at the plasma membrane and deposit their product into the cell wall. Matrix polysaccharides are synthesized by a multitude of glycosyltransferases in hundreds of mobile Golgi stacks, and an extensive set of vesicle trafficking proteins govern secretion to the cell wall. In this review, we discuss the different subcellular locations at which cell wall synthesis occurs, review the molecular mechanisms that control cell wall biosynthesis, and examine how these are regulated in response to different perturbations to maintain cell wall homeostasis.