Structure and function of endosomes in plant cells.

Endosomes are a heterogeneous collection of organelles that function in the sorting and delivery of internalized material from the cell surface and the transport of materials from the Golgi to the lysosome or vacuole. Plant endosomes have some unique features, with an organization distinct from that of yeast or animal cells. Two clearly defined endosomal compartments have been studied in plant cells, the trans-Golgi network (equivalent to the early endosome) and the multivesicular body (equivalent to the late endosome), with additional endosome types (recycling endosome, late prevacuolar compartment) also a possibility.

RNS2, a conserved member of the RNase T2 family, is necessary for ribosomal RNA decay in plants.

RNase T2 enzymes are conserved in most eukaryotic genomes, and expression patterns and phylogenetic analyses suggest that they may carry out an important housekeeping role. However, the nature of this role has been elusive. Here we show that RNS2, an intracellular RNase T2 from Arabidopsis thaliana, is essential for normal ribosomal RNA recycling.

Increase in catalase-3 activity as a response to use of alternative catabolic substrates during sucrose starvation.

Periods of carbohydrate deprivation are commonly encountered by plant cells. Plants respond to this nutrient stress by the mobilization of stored carbohydrates and the reallocation of other cellular macromolecules to degradative pathways. Previously we identified a number of metabolic genes that are upregulated in Arabidopsis thaliana cells during sucrose starvation.

Disruption of autophagy results in constitutive oxidative stress in Arabidopsis.

Plant cells frequently encounter oxidative stress, leading to oxidative damage and inactivation of proteins. We have recently demonstrated that oxidative stress induces autophagy in Arabidopsis seedlings in an AtATG18a-dependent manner and that RNAi-AtATG18a transgenic lines, which are defective in autophagosome formation, are hypersensitive to reactive oxygen species. Analysis of protein oxidation indicated that oxidized proteins are degraded in the vacuole after uptake by autophagy, and this degradation is impaired in RNAi-AtATG18a lines.

Degradation of oxidized proteins by autophagy during oxidative stress in Arabidopsis.

Upon encountering oxidative stress, proteins are oxidized extensively by highly reactive and toxic reactive oxidative species, and these damaged, oxidized proteins need to be degraded rapidly and effectively. There are two major proteolytic systems for bulk degradation in eukaryotes, the proteasome and vacuolar autophagy. In mammalian cells, the 20S proteasome and a specific type of vacuolar autophagy, chaperone-mediated autophagy, are involved in the degradation of oxidized proteins in mild oxidative stress.

Visualization of autophagy in Arabidopsis using the fluorescent dye monodansylcadaverine and a GFP-AtATG8e fusion protein.

Autophagy is a process that is thought to occur in all eukaryotes in which cells recycle cytoplasmic contents when subjected to environmental stress conditions or during certain stages of development. Upon induction of autophagy, double membrane-bound structures called autophagosomes engulf portions of the cytoplasm and transfer them to the vacuole or lysosome for degradation. In this study, we have characterized two potential markers for autophagy in plants, the fluorescent dye monodansylcadaverine (MDC) and a green fluorescent protein (GFP)-AtATG8e fusion protein, and propose that they both label autophagosomes in Arabidopsis.

AtATG18a is required for the formation of autophagosomes during nutrient stress and senescence in Arabidopsis thaliana.

Vacuolar autophagy is a major pathway by which eukaryotic cells degrade macromolecules, either to remove damaged or unnecessary proteins, or to produce respiratory substrates and raw materials to survive periods of nutrient deficiency. During autophagy, a double membrane forms around cytoplasmic components to generate an autophagosome, which is transported to the vacuole. The outer membrane fuses with the vacuole or lysosome, and the inner membrane and its contents are degraded by vacuolar or lysosomal hydrolases.

Transcriptome profiling of the response of Arabidopsis suspension culture cells to Suc starvation.

Upon encountering nutrient stress conditions, plant cells undergo extensive metabolic changes and induce nutrient recycling pathways for their continued survival. The role of nutrient mobilization in the response of Arabidopsis suspension cells to Suc starvation was examined. Vacuolar autophagy was induced within 24 h of starvation, with increased expression of vacuolar proteases that are likely to be required for degradation of cytoplasmic components delivered to the vacuole, and thus for nutrient recycling.