Straying off the Highway: Trafficking of Secreted Plant Proteins and Complexity in the Plant Cell Wall Proteome

Abstract

From simply skimming through the abstract lists of more or less any collection of both basic and applied plant-related journals, it is immediately apparent that the plant cell wall represents a nexus of many fields of research: growth and development, plant-pathogen interactions, abiotic stress, self-and interorganismal recognition, signaling systems, numerous primary and specialized metabolic processes, biomaterials and bioproducts, and many others. That said, and to deal with an issue of semantics, while the term cell wall can refer specifically to the structural matrix that surrounds all plant cells, for the purposes of this Update it is used more broadly, also to include the apoplast, or extracellular environment. Given its multifunctional nature then, it is not surprising that the apoplast houses a dynamic and complex proteome, and the compendium of cell wall proteins continues to grow as researchers from disparate disciplines discover new roles for extracellular proteins. In addition, however, as cell wall proteomics projects develop and the subcellular localizations of an ever-growing list of plant proteins are determined, a number of surprises have been thrown up, both in terms of the identity of secreted proteins and the trafficking pathways that they follow. The purpose of this Update is to give some examples of previously unsuspected aspects of plant cell wall protein trafficking that are challenging longheld assumptions, rather than to provide an exhaustive review, and to highlight some questions that can be categorized into the "who, how, where, and when" of the cell wall proteome.

Cytoskeletal Components Define Protein Location to Membrane Microdomains*

“…The secretory route from ER and trans-Golgi network to the destination membranes has not been revealed in detail. It is also unknown if trafficking to the plasma membrane is a direct process or if it includes some intermediate compartments endosomal compartments (68,69). Although there are indications that the trans-Golgi network to plasma membrane trafficking involves exocyst complex (70), exosomes, and probably multivesicular bodies (71), it still needs to be revealed if the cytoskeleton is necessary for this process.…”

Profiling the Secretome and Extracellular Proteome of the Potato Late Blight Pathogen Phytophthora infestans

“…Although hyphal lysis may occur, there are evidences that the presence of intracellular proteins in the extracellular medium-such as enolases, ribonucleases, and related proteins-might act as virulence factors and be involved in a variety of extracellular functions (22,60,61). Therefore, proteins identified by de novo sequencing were annotated by sequence homology with other oomycetes to search for additional evidence supporting their identification.…”

Differentially Secreted Proteins of Antarctic and Mesophilic Strains ofSynechocystis SalinaandChlorella Vulgarisafter UV-B and Temperature Stress Treatment

“…other widely used model organisms to study the mechanisms of stress response and adaptation to environmental changes are eukaryotic microalgae. the ecM or cell wall in algae and plants is a dynamic structure, which not only provides structural support to the cell, but is also involved in plant development and morphogenesis, and protects cells from biotic and abiotic stresses (44). Profound changes in cell wall composition in response to environmental stresses have been found in these organisms.…”