While metal nanoparticles are being increasingly used in many sectors of the economy, there is growing interest in the biological and environmental safety of their production. The main methods for nanoparticle production are chemical and physical approaches that are often costly and potentially harmful to the environment. The present review is devoted to the possibility of metal nanoparticle synthesis using plant extracts. This approach has been actively pursued in recent years as an alternative, efficient, inexpensive, and environmentally safe method for producing nanoparticles with specified properties. This review provides a detailed analysis of the various factors affecting the morphology, size, and yield of metal nanoparticles. The main focus is on the role of the natural plant biomolecules involved in the bioreduction of metal salts during the nanoparticle synthesis. Examples of effective use of exogenous biomatrices (peptides, proteins, and viral particles) to obtain nanoparticles in plant extracts are discussed.

Programmed cell death (PCD) is an integral part of plant development and of responses to abiotic stress or pathogens. Although the morphology of plant PCD is, in some cases, well characterised and molecular mechanisms controlling plant PCD are beginning to emerge, there is still confusion about the classification of PCD in plants. Here we suggest a classification based on morphological criteria. According to this classification, the use of the term 'apoptosis' is not justified in plants, but at least two classes of PCD can be distinguished: vacuolar cell death and necrosis. During vacuolar cell death, the cell contents are removed by a combination of autophagy-like process and release of hydrolases from collapsed lytic vacuoles. Necrosis is characterised by early rupture of the plasma membrane, shrinkage of the protoplast and absence of vacuolar cell death features. Vacuolar cell death is common during tissue and organ formation and elimination, whereas necrosis is typically found under abiotic stress. Some examples of plant PCD cannot be ascribed to either major class and are therefore classified as separate modalities. These are PCD associated with the hypersensitive response to biotrophic pathogens, which can express features of both necrosis and vacuolar cell death, PCD in starchy cereal endosperm and during self-incompatibility. The present classification is not static, but will be subject to further revision, especially when specific biochemical pathways are better defined. Research on plant cell death has grown considerably in the past few years, owing to the importance of cell death for plant development and defense. Just as animal cells engage several mechanisms leading to death, the road to cell demise in plants can also vary. The long evolutionary distance and distinct cellular architecture between the two kingdoms may account for the differences between the mechanisms of plant and animal cell death. It is therefore appropriate to assess the relevance of animal cell death nomenclature 1 to plants. At present, there is confusion in cell death terminology in plant biology, which drives our attempt to formulate a more logical classification. Although our molecular understanding of plant cell death regulation and execution is insufficient to create definitive classifications based on precise biochemical pathways, it is possible to begin classifying plant cell death scenarios based on morphological criteria, as was initially the case in animal cell death research 2,3 and is still used for the classification of cell death in animal science. 1 This document attempts to provide a classification of plant cell death. We urge authors, reviewers and editors to follow this classification to facilitate communication between scientists and accelerate research in this field.

Caspases are cysteine-dependent proteases and are important components of animal apoptosis. They introduce specific breaks after aspartate residues in a number of cellular proteins mediating programmed cell death (PCD). Plants encode only distant homologues of caspases, the metacaspases that are involved in PCD, but do not possess caspase-specific proteolytic activity. Nevertheless, plants do display caspase-like activities indicating that enzymes structurally distinct from classical caspases may operate as caspase-like proteases. Here, we report the identification and characterisation of a novel PCD-related subtilisinlike protease from tobacco and rice named phytaspase (plant aspartate-specific protease) that possesses caspase specificity distinct from that of other known caspase-like proteases. We provide evidence that phytaspase is synthesised as a proenzyme, which is autocatalytically processed to generate the mature enzyme. Overexpression and silencing of the phytaspase gene showed that phytaspase is essential for PCD-related responses to tobacco mosaic virus and abiotic stresses. Phytaspase is constitutively secreted into the apoplast before PCD, but unexpectedly is re-imported into the cell during PCD providing insights into how phytaspase operates.

The nucleolus and specific nucleolar proteins are involved in the life cycles of some plant and animal viruses, but the functions of these proteins and of nucleolar trafficking in virus infections are largely unknown. The ORF3 protein of the plant virus, groundnut rosette virus (an umbravirus), has been shown to cycle through the nucleus, passing through Cajal bodies to the nucleolus and then exiting back into the cytoplasm. This journey is absolutely required for the formation of viral ribonucleoprotein particles (RNPs) that, themselves, are essential for the spread of the virus to noninoculated leaves of the shoot tip. Here, we show that these processes rely on the interaction of the ORF3 protein with fibrillarin, a major nucleolar protein. Silencing of the fibrillarin gene prevents longdistance movement of groundnut rosette virus but does not affect viral replication or cell-to-cell movement. Repressing fibrillarin production also localizes the ORF3 protein to multiple Cajal bodylike aggregates that fail to fuse with the nucleolus. Umbraviral ORF3 protein and fibrillarin interact in vitro and, when mixed with umbravirus RNA, form an RNP complex. This complex has a filamentous structure with some regular helical features, resembling the RNP complex formed in vivo during umbravirus infection. The filaments formed in vitro are infectious when inoculated to plants, and their infectivity is resistant to RNase. These results demonstrate previously undescribed functions for fibrillarin as an essential component of translocatable viral RNPs and may have implications for other plant and animal viruses that interact with the nucleolus.Cajal bodies ͉ plant virus movement ͉ ribonucleoprotein particles T he nucleolus is a subnuclear domain and is the site of transcription and processing of rRNA and of ribosome biogenesis. In addition, the nucleolus also participates in other aspects of RNA metabolism and cell function (1, 2). The nucleolus is structurally and functionally associated with Cajal bodies (CBs), which are structures found in both animals and plants (3, 4). CBs contain different proteins including coilin, a protein essential for CB formation, and fibrillarin, a major nucleolar protein that is a core component of small nucleolar ribonucleoprotein particles (snoRNPs) and is required for rRNA processing (4-7). CBs are involved in the maturation of small nuclear RNPs (snRNPs) and snoRNPs, which traffic through CBs before accumulating in splicing speckles and the nucleolus, respectively (8, 9). Both the nucleolus and CBs have a role in RNA silencing in plants (10,11). Finally, a number of animal and plant viruses including the RNAcontaining tobacco etch virus and the DNA-containing tomato yellow leaf curl virus have a nucleolar phase in their life cycle (12, 13). Recently, we have shown that the ability of the umbravirus, groundnut rosette virus (GRV), to move long distances through the phloem, the specialized vascular system used by plants for the transport of assimilates and macromolecules, depends strictly on the...

The nucleolus and Cajal bodies (CBs) are prominent interacting subnuclear domains involved in a number of crucial aspects of cell function. Certain viruses interact with these compartments but the functions of such interactions are largely uncharacterized. Here, we show that the ability of the groundnut rosette virus open reading frame (ORF) 3 protein to move viral RNA long distances through the phloem strictly depends on its interaction with CBs and the nucleolus. The ORF3 protein targets and reorganizes CBs into multiple CB-like structures and then enters the nucleolus by causing fusion of these structures with the nucleolus. The nucleolar localization of the ORF3 protein is essential for subsequent formation of viral ribonucleoprotein (RNP) particles capable of virus long-distance movement and systemic infection. We provide a model whereby the ORF3 protein utilizes trafficking pathways involving CBs to enter the nucleolus and, along with fibrillarin, exit the nucleus to form viral 'transportcompetent' RNP particles in the cytoplasm.