Bellis perennis: a useful tool for protein localization studies

Insertion of plastidic β-barrel proteins into the outer envelopes of plastids involves an intermembrane space intermediate formed with Toc75-V/OEP80

The insertion of organellar membrane proteins with the correct topology requires the following: First, the proteins must contain topogenic signals for translocation across and insertion into the membrane. Second, proteinaceous complexes in the cytoplasm, membrane, and lumen of organelles are required to drive this process. Many complexes required for the intracellular distribution of membrane proteins have been described, but the signals and components required for the insertion of plastidic β-barrel-type proteins into the outer membrane are largely unknown. The discovery of common principles is difficult, as only a few plastidic β-barrel proteins exist. Here, we provide evidence that the plastidic outer envelope β-barrel proteins OEP21, OEP24, and OEP37 from pea (Pisum sativum) and Arabidopsis thaliana contain information defining the topology of the protein. The information required for the translocation of pea proteins across the outer envelope membrane is present within the six N-terminal β-strands. This process requires the action of translocon of the outer chloroplast (TOC) membrane. After translocation into the intermembrane space, β-barrel proteins interact with TOC75-V, as exemplified by OEP37 and P39, and are integrated into the membrane. The membrane insertion of plastidic β-barrel proteins is affected by mutation of the last β-strand, suggesting that this strand contributes to the insertion signal. These findings shed light on the elements and complexes involved in plastidic β-barrel protein import.

White stripe leaf 12 (WSL12), encoding a nucleoside diphosphate kinase 2 (OsNDPK2), regulates chloroplast development and abiotic stress response in rice (Oryza sativa L.)

Chloroplast is a crucial organelle for plant photosynthesis and maintaining normal life activities in higher plants. Although some genes related to chloroplast development and pigment synthesis have been identified or cloned in rice, little is known about the relationship between these genes and abiotic stress response. In this study, we identified a novel mutant white stripe leaf 12 (wsl12) affecting pigment synthesis, chloroplast development and abiotic stress response in rice. The mutant phenotype was obvious at seeding and tillering stages and in response to the temperature change. Genetic analysis of reciprocal crosses between wsl12 and wild-type plants showed that wsl12 was a recessive mutant in a single nuclear locus. Map-based cloning revealed that the WSL12 locus encoded OsNDPK2, one of the three nucleoside diphosphate kinases (OsNDPKs). WSL12 expressed in all tested tissues, while it highly expressed in leaves and young tissues. The WSL12 protein localized to the chloroplast. The wsl12 mutant showed higher superoxide anion level and enhanced sensitivity to abscisic acid (ABA) and salinity. The transcription pattern of many genes involved in chlorophyll biosynthesis, ABA synthesis, light signaling pathway, reactive oxygen species-scavenging pathway and the other two OsNDPKs was altered in the wsl12 mutant. These results indicate that the OsNDPK2 encoded by WSL12 plays an important role in chloroplast development and chlorophyll biosynthesis by regulating the expression levels of related genes. In addition, WSL12 also affects the response to abiotic stress, such as ABA and salinity in rice, and is beneficial to molecular breeding of stress tolerance.

Clues to the functions of plant NDPK isoforms

This review describes the five nucleoside diphosphate kinase (NDPK) genes found in both model plants Arabidopsis thaliana (thale cress) and Oryza sativa L. (rice). Phylogenetic and sequence analyses of these genes allow the definition of four types of NDPK isoforms with different predicted subcellular localization. These predictions are supported by experimental evidence for most NDPK types. Data mining also provides evidence for the existence of a novel NDPK type putatively localized in the endoplasmic reticulum. Phylogenic analyses indicate that plant types I, II, and III belong to the previously identified Nme group I whereas type IV belongs to Nme group II. Additional analysis of the literature offers clues supporting the idea that the various plant NDPK types have different functions. Hence, cytosolic type I NDPKs are involved in metabolism, growth, and stress responses. Type II NDPKs are localized in the chloroplast and mainly involved in photosynthetic development and oxidative stress management. Type III NDPKs have dual targeting to the mitochondria and the chloroplast and are principally involved in energy metabolism. The subcellular localization and precise function of the novel type IV NDPKs, however, will require further investigations.

The folding capacity of the mature domain of the dual-targeted plant tRNA nucleotidyltransferase influences organelle selection

tRNA-NTs (tRNA nucleotidyltransferases) are required for the maturation or repair of tRNAs by ensuring that they have an intact cytidine-cytidine-adenosine sequence at their 3′-termini. Therefore this enzymatic activity is found in all cellular compartments, namely the nucleus, cytoplasm, plastids and mitochondria, in which tRNA synthesis or translation occurs. A single gene codes for tRNA-NT in plants, suggesting a complex targeting mechanism. Consistent with this, distinct signals have been proposed for plastidic, mitochondrial and nuclear targeting. Our previous research has shown that in addition to N-terminal targeting information, the mature domain of the protein itself modifies targeting to mitochondria and plastids. This suggests the existence of an as yet unknown determinate for the distribution of dual-targeted proteins between these two organelles. In the present study, we explore the enzymatic and physicochemical properties of tRNA-NT variants to correlate the properties of the enzyme with the intracellular distribution of the protein. We show that alteration of tRNA-NT stability influences its intracellular distribution due to variations in organelle import capacities. Hence the fate of the protein is determined not only by the transit peptide sequence, but also by the physicochemical properties of the mature protein.

Phytochrome cytoplasmic signaling

Extensive studies in both lower and higher plants indicate that plant phytochrome photoreceptors signal not only by regulating transcription in the nucleus but also by acting within the cytoplasm, the latter signaling routes acting within minutes or even seconds and also providing directional information. Directional signals seem to arise from phytochromes attached anisotropically to the plasma membrane. Neochromes-phytochrome-phototropin hybrid photoreceptors probably attached to the plasma membrane-provide this signal in various ferns and perhaps certain algae but are absent from other groups. In mosses and probably higher plants too, a subpopulation of canonical phytochromes interact with phototropins at the plasma membrane and thereby steer directional responses. Phytochromes also seem able to regulate translation in the cytoplasm. This review discusses putative phytochrome functions in these contexts.

A phytochrome-phototropin light signaling complex at the plasma membrane

Phytochromes are red/far-red photochromic photoreceptors central to regulating plant development. Although they are known to enter the nucleus upon light activation and, once there, regulate transcription, this is not the complete picture. Various phytochrome effects are manifested much too rapidly to derive from changes in gene expression, whereas others seem to occur without phytochrome entering the nucleus. Phytochromes also guide directional responses to light, excluding a genetic signaling route and implying instead plasma membrane association and a direct cytoplasmic signal. However, to date, no such association has been demonstrated. Here we report that a phytochrome subpopulation indeed associates physically with another photoreceptor, phototropin, at the plasma membrane. Yeast two-hybrid methods using functional photoreceptor molecules showed that the phytochrome steering growth direction in Physcomitrella protonemata binds several phototropins specifically in the photoactivated Pfr state. Split-YFP studies in planta showed that the interaction occurs exclusively at the plasma membrane. Coimmunoprecipitation experiments provided independent confirmation of in vivo phy-phot binding. Consistent with this interaction being associated with a cellular signal, we found that phytochrome-mediated tropic responses are impaired in Physcomitrella phot(-) mutants. Split-YFP revealed a similar interaction between Arabidopsis phytochrome A and phototropin 1 at the plasma membrane. These associations additionally provide a functional explanation for the evolution of neochrome photoreceptors. Our results imply that the elusive phytochrome cytoplasmic signal arises through binding and coaction with phototropin at the plasma membrane.