Tomato bushy stunt virus (TBSV), a positive-strand RNA virus, causes extensive inward vesiculations of the peroxisomal boundary membrane and formation of peroxisomal multivesicular bodies (pMVBs). Although pMVBs are known to contain protein components of the viral membrane-bound RNA replication complex, the mechanisms of protein targeting to peroxisomal membranes and participation in pMVB biogenesis are not well understood. We show that the TBSV 33-kD replication protein (p33), expressed on its own, targets initially from the cytosol to peroxisomes, causing their progressive aggregation and eventually the formation of peroxisomal ghosts. These altered peroxisomes are distinct from pMVBs; they lack internal vesicles and are surrounded by novel cytosolic vesicles that contain p33 and appear to be derived from evaginations of the peroxisomal boundary membrane. Concomitant with these changes in peroxisomes, p33 and resident peroxisomal membrane proteins are relocalized to the peroxisomal endoplasmic reticulum (pER) subdomain. This sorting of p33 is disrupted by the coexpression of a dominant-negative mutant of ADP-ribosylation factor1, implicating coatomer in vesicle formation at peroxisomes. Mutational analysis of p33 revealed that its intracellular sorting is also mediated by several targeting signals, including three peroxisomal targeting elements that function cooperatively, plus a pER targeting signal resembling an Arg-based motif responsible for vesicle-mediated retrieval of escaped ER membrane proteins from the Golgi. These results provide insight into virus-induced intracellular rearrangements and reveal a peroxisome-to-pER sorting pathway, raising new mechanistic questions regarding the biogenesis of peroxisomes in plants.

Programmed cell death (PCD) functions in the developmental remodeling of leaf shape in higher plants, a process analogous to digit formation in the vertebrate limb. In this study, we provide a cytological characterization of the time course of events as PCD remodels young expanding leaves of the lace plant. Tonoplast rupture is the first PCD event in this system, indicated by alterations in cytoplasmic streaming, loss of anthocyanin color, and ultrastructural appearance. Nuclei become terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling positive soon afterward but do not become morphologically altered until late stages of PCD. Genomic DNA is fragmented, but not into internucleosomal units. Other cytoplasmic changes, such as shrinkage and degradation of organelles, occur later. This form of PCD resembles tracheary element differentiation in cytological execution but requires unique developmental regulation so that discrete panels of tissue located equidistantly between veins undergo PCD while surrounding cells do not.

SUMMARYPlants survive against myriad environmental odds while remaining rooted to a single spot. The time scale over which plant cells can respond to environmental cues is seldom appreciated. Fluorescent protein-assisted live imaging of peroxisomes reveals that they respond within seconds of exposure to hydrogen peroxide and hydroxyl radicals by producing dynamic extensions called peroxules. Observations of the Arabidopsis flu mutant and treatments with xenobiotics eliciting singlet oxygen and superoxide reactive oxygen species suggest that the observed responses are specific for hydroxyl radicals. Prolonged exposure to hydroxyl radicals inhibits peroxule extension, and instead causes motile and spherical peroxisomes in a cell to become immotile and elongate several-fold. Expression of photo-convertible EosFP-PTS1 demonstrates that vermiform peroxisomes result from rapid stretching of individual peroxisomes, while the subsequent 'beads-on-a-string' morphology results from differential protein distribution within an elongated tubule. Over time, the beads in elongated peroxisomes also extend peroxules randomly before undergoing asynchronous, asymmetrical fission. Peroxule extension does not appear to involve cytoskeletal elements directly, but is closely aligned with and reflects the dynamics of ER tubules. Peroxisomal responses reveal a rapidly invoked subcellular machinery that is involved in recognition of hydroxyl stress thresholds, and its possible remediation locally through extension of peroxules or globally by increasing peroxisome numbers. A matrix protein retro-flow mechanism that supports peroxisome-ER connectivity in plant cells is suggested.

Stroma-filled tubules named stromules are sporadic extensions of plastids. Earlier, photobleaching was used to demonstrate fluorescent protein diffusion between already interconnected plastids and formed the basis for suggesting that all plastids are able to form networks for exchanging macromolecules. However, a critical appraisal of literature shows that this conjecture is not supported by unequivocal experimental evidence. Here, using photoconvertible mEosFP, we created color differences between similar organelles that enabled us to distinguish clearly between organelle fusion and nonfusion events. Individual plastids, despite conveying a strong impression of interactivity and fusion, maintained well-defined boundaries and did not exchange fluorescent proteins. Moreover, the high pleomorphy of etioplasts from dark-grown seedlings, leucoplasts from roots, and assorted plastids in the accumulation and replication of chloroplasts5 (arc5), arc6, and phosphoglucomutase1 mutants of Arabidopsis thaliana suggested that a single plastid unit might be easily mistaken for interconnected plastids. Our observations provide succinct evidence to refute the long-standing dogma of interplastid connectivity. The ability to create and maintain a large number of unique biochemical factories in the form of singular plastids might be a key feature underlying the versatility of green plants as it provides increased internal diversity for them to combat a wide range of environmental fluctuations and stresses.

The inner bark tissues of three temperate hardwoods contain specific proteins which undergo seasonal fluctuations. Increases in particular proteins, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, occur within the bark of several Acer, Populus and Salix spp. during late summer and early autumn. These proteins are abundant in the bark throughout the winter and their levels decline the following spring. Light and electron microscopy showed that the parenchyma cells of the inner bark are packed with spherical organelles throughout the overwintering period. These organelles are rich in protein and analogous to protein bodies found in cells of mature seeds. The protein bodies of the parenchyma cells are replaced by large central vacuoles during spring and summer, presumably as a result of the mobilization of the storage protein and fusion of the protein bodies. The high levels of specific proteins in inner bark tissues and the presence of protein bodies within the parenchyma cells indicate that the living cells of the bark act as a nitrogen reserve in overwintering temperate hardwoods.

The ricinosome (precursor protease vesicle) is an organelle found exclusively in plant cells. Ricinosomes contain a 45-kDa pro-cysteine endopeptidase (CysEP) with a C-terminal KDEL endoplasmic reticulum retention signal. CysEP is a member of a unique group of papain-type cysteine peptidases found specifically in senescing and ricinosome-containing tissues. During seed development in the castor oil plant (Ricinus communis L.), the cells of the nucellus are killed as the major seed storage organ, the cellular endosperm, expands and begins to accumulate reserves. The destruction of the maternal seed tissues is a developmentally programmed cell death. castor bean ͉ papain-type KDEL peptidase ͉ seed maturation

The morphology of developing Ricinus communis L. cv. Hale seed was studied from fertilization to quiescence, a period of approximately 60 days under the growth conditions used. Fresh and dry weights and dimensions of the whole seed, endosperm, and embryo were also determined. The development of castor bean seed has been divided into 10 stages of approximately equal duration. We have constructed a morphological timetable which allows for the consistent and repeatable selection of seeds at each of the aforementioned stages.

Successful development and dehiscence of the anther and release of pollen are dependent upon the programmed cell death (PCD) of the tapetum and other sporophytic tissues. Ultrastructural examination of the developing and dehiscing anther of tomato (Solanum lycopersicum) revealed that cells of the interlocular septum, the connective tissue, the middle layer/ endothecium, and the epidermal cells surrounding the stomium all exhibit features consistent with progression through PCD. Ricinosomes, a subset of precursor protease vesicles that are unique to some incidents of plant PCD, were also present in all of these cell types. These novel organelles are known to harbor KDEL-tailed cysteine proteinases that act in the final stages of corpse processing following cell death. Indeed, a tomato KDEL-tailed cysteine proteinase, SlCysEP, was identified and its gene was cloned, sequenced, and characterized. SlCysEP transcript and protein were restricted to the anthers of the senescing tomato flower. Present in the interlocular septum and in the epidermal cells surrounding the stomium relatively early in development, SlCysEP accumulates later in the sporophytic tissues surrounding the locules as dehiscence ensues. At the ultrastuctural level, immunogold labeling localized SlCysEP to the ricinosomes within the cells of these tissues, but not in the tapetum. It is suggested that the accumulation of SlCysEP and the appearance of ricinosomes act as very early predictors of cell death in the tomato anther.