Plastid intramembrane proteolysis

Modulation of photosynthesis and other proteins during water-stress

Protein changes under drought or water stress conditions have been widely investigated. These investigations have given us enormous understanding of how drought is manifested in plants and how plants respond and adopt to such conditions. Chlorophyll fluoroescence, gas exchange, OMICS, biochemical and molecular analyses have shed light on regulation of physiology and photosynthesis of plants under drought. Use of proteomics has greatly increased the repertoire of drought-associated proteins which nevertheless, need to be investigated for their mechanistic and functional roles. Roles of such proteins have been succinctly discussed in various review articles, however more information on their functional role in countering drought is needed. In this review, recent developments in the field, alterations in the abundance of plant proteins in response to drought, monitored through numerous proteomic and immuno-blot analyses, and how these could affect plants growth and development, are discussed.

The role of chloroplast protein remodeling in stress responses and shaping of the plant peptidome

In addition to photosynthesis, chloroplasts perform a variety of important cellular functions in the plant cell, which can, for example, regulate plant responses to abiotic and biotic stress conditions. Under stress, intensive chloroplast protein remodeling and degradation can occur, releasing large numbers of endogenous peptides. These protein-derived peptides can be found intracellularly, but also in the plant secretome. Although the pathways of chloroplast protein degradation and the types of chloroplast proteases implicated in this process have received much attention, the role of the resulting peptides is less well understood. In this review we summarize the data on peptide generation processes during the remodeling of the chloroplast proteome under stress conditions and discuss the mechanisms leading to these changes. We also review the experimental evidence which supports the concept that peptides derived from chloroplast proteins can function as regulators of plant responses to (a)biotic stresses.

Genetic and Molecular Genetic Basis of Nuclear-Plastid Incompatibilities

Genetic analysis of nuclear-cytoplasm incompatibilities is not straightforward and requires an elaborated experimental design. A number of species have been genetically studied, but notable advances in genetic mapping of nuclear loci involved in nuclear-plastid incompatibility have been achieved only in wheat and pea. This review focuses on the study of the genetic background underlying nuclear-plastid incompatibilities, including cases where the molecular genetic basis of such incompatibility has been unveiled, such as in tobacco, Oenothera, pea, and wheat.

Targeted Deletion of a Plasmodium Site-2 Protease Impairs Life Cycle Progression in the Mammalian Host

Site-2 proteases (S2P) belong to the M50 family of metalloproteases, which typically perform essential roles by mediating activation of membrane–bound transcription factors through regulated intramembrane proteolysis (RIP). Protease-dependent liberation of dormant transcription factors triggers diverse cellular responses, such as sterol regulation, Notch signalling and the unfolded protein response. Plasmodium parasites rely on regulated proteolysis for controlling essential pathways throughout the life cycle. In this study we examine the Plasmodium-encoded S2P in a murine malaria model and show that it is expressed in all stages of Plasmodium development. Localisation studies by endogenous gene tagging revealed that in all invasive stages the protein is in close proximity to the nucleus. Ablation of PbS2P by reverse genetics leads to reduced growth rates during liver and blood infection and, hence, virulence attenuation. Strikingly, absence of PbS2P was compatible with parasite life cycle progression in the mosquito and mammalian hosts under physiological conditions, suggesting redundant or dispensable roles in vivo.

Essentials of Proteolytic Machineries in Chloroplasts

Plastids are unique organelles that can alter their structure and function in response to environmental and developmental stimuli. Chloroplasts are one type of plastid and are the sites for various metabolic processes, including photosynthesis. For optimal photosynthetic activity, the chloroplast proteome must be properly shaped and maintained through regulated proteolysis and protein quality control mechanisms. Enzymatic functions and activities are conferred by protein maturation processes involving consecutive proteolytic reactions. Protein abundances are optimized by the balanced protein synthesis and degradation, which is depending on the metabolic status. Malfunctioning proteins are promptly degraded. Twenty chloroplast proteolytic machineries have been characterized to date. Specifically, processing peptidases and energy-driven processive proteases are the major players in chloroplast proteome biogenesis, remodeling, and maintenance. Recently identified putative proteases are potential regulators of photosynthetic functions. Here we provide an updated, comprehensive overview of chloroplast protein degradation machineries and discuss their importance for photosynthesis. Wherever possible, we also provide structural insights into chloroplast proteases that implement regulated proteolysis of substrate proteins/peptides.

The Chloroplast Protease AMOS1/EGY1 Affects Phosphate Homeostasis under Phosphate Stress

Plastid intramembrane proteases in Arabidopsis (Arabidopsis thaliana) are involved in jasmonic acid biosynthesis, chloroplast development, and flower morphology. Here, we show that Ammonium-Overly-Sensitive1 (AMOS1), a member of the family of plastid intramembrane proteases, plays an important role in the maintenance of phosphate (P) homeostasis under P stress. Loss of function of AMOS1 revealed a striking resistance to P starvation. amos1 plants displayed retarded root growth and reduced P accumulation in the root compared to wild type (Col-0) under P-replete control conditions, but remained largely unaffected by P starvation, displaying comparable P accumulation and root and shoot growth under P-deficient conditions. Further analysis revealed that, under P-deficient conditions, the cell wall, especially the pectin fraction of amos1, released more P than that of wild type, accompanied by a reduction of the abscisic acid (ABA) level and an increase in ethylene production. By using an ABA-insensitive mutant, abi4, and applying ABA and ACC exogenously, we found that ABA inhibits cell wall P remobilization while ethylene facilitates P remobilization from the cell wall by increasing the pectin concentration, suggesting ABA can counteract the effect of ethylene. Furthermore, the elevated ABA level and the lower ethylene production also correlated well with the mimicked P deficiency in amos1 Thus, our study uncovers the role of AMOS1 in the maintenance of P homeostasis through ABA-antagonized ethylene signaling.

Substrates and physiological functions of secretase rhomboid proteases

Rhomboids are conserved intramembrane serine proteases with widespread functions. They were the earliest discovered members of the wider rhomboid-like superfamily of proteases and pseudoproteases. The secretase class of rhomboid proteases, distributed through the secretory pathway, are the most numerous in eukaryotes, but our knowledge of them is limited. Here we aim to summarise all that has been published on secretase rhomboids in a concise encyclopaedia of the enzymes, their substrates, and their biological roles. We also discuss emerging themes of how these important enzymes are regulated.

Chloroplast signaling within, between and beyond cells

The most conspicuous function of plastids is the oxygenic photosynthesis of chloroplasts, yet plastids are super-factories that produce a plethora of compounds that are indispensable for proper plant physiology and development. Given their origins as free-living prokaryotes, it is not surprising that plastids possess their own genomes whose expression is essential to plastid function. This semi-autonomous character of plastids requires the existence of sophisticated regulatory mechanisms that provide reliable communication between them and other cellular compartments. Such intracellular signaling is necessary for coordinating whole-cell responses to constantly varying environmental cues and cellular metabolic needs. This is achieved by plastids acting as receivers and transmitters of specific signals that coordinate expression of the nuclear and plastid genomes according to particular needs. In this review we will consider the so-called retrograde signaling occurring between plastids and nuclei, and between plastids and other organelles. Another important role of the plastid we will discuss is the involvement of plastid signaling in biotic and abiotic stress that, in addition to influencing retrograde signaling, has direct effects on several cellular compartments including the cell wall. We will also review recent evidence pointing to an intriguing function of chloroplasts in regulating intercellular symplasmic transport. Finally, we consider an intriguing yet less widely known aspect of plant biology, chloroplast signaling from the perspective of the entire plant. Thus, accumulating evidence highlights that chloroplasts, with their complex signaling pathways, provide a mechanism for exquisite regulation of plant development, metabolism and responses to the environment. As chloroplast processes are targeted for engineering for improved productivity the effect of such modifications on chloroplast signaling will have to be carefully considered in order to avoid unintended consequences on plant growth and development.