The Arabidopsis dynamin-like proteins ADL1C and ADL1E play a critical role in mitochondrial morphogenesis

Antiviral Mx proteins have an ancient origin and widespread distribution among eukaryotes

First identified in mammals, Mx proteins are potent antivirals against a broad swathe of viruses. Mx proteins arose within the Dynamin superfamily of proteins (DSP), mediating critical cellular processes, such as endocytosis and mitochondrial, plastid, and peroxisomal dynamics. And yet, the evolutionary origins of Mx proteins are poorly understood. Using a series of phylogenomic analyses with stepwise increments in taxonomic coverage, we show that Mx proteins predate the interferon signaling system in vertebrates. Our analyses find an ancient monophyletic DSP lineage in eukaryotes that groups vertebrate and invertebrate Mx proteins with previously undescribed fungal MxF proteins, the relatively uncharacterized plant and algal Dynamin 4A/4C proteins, and representatives from several early-branching eukaryotic lineages. Thus, Mx-like proteins date back close to the origin of Eukarya. Our phylogenetic analyses also reveal that host-encoded and NCLDV (nucleocytoplasmic large DNA viruses)-encoded DSPs are interspersed in four distinct DSP lineages, indicating recurrent viral theft of host DSPs. Our analyses thus reveal an ancient history of viral and antiviral functions encoded by the Dynamin superfamily in eukaryotes.

The DEAD-box RNA helicase ZmRH48 is required for the splicing of multiple mitochondrial introns, mitochondrial complex biosynthesis, and seed development in maize

RNA helicases participate in nearly all aspects of RNA metabolism by rearranging RNAs or RNA-protein complexes in an adenosine triphosphate-dependent manner. Due to the large RNA helicase families in plants, the precise roles of many RNA helicases in plant physiology and development remain to be clarified. Here, we show that mutations in maize (Zea mays) DEAD-box RNA helicase 48 (ZmRH48) impair the splicing of mitochondrial introns, mitochondrial complex biosynthesis, and seed development. Loss of ZmRH48 function severely arrested embryogenesis and endosperm development, leading to defective kernel formation. ZmRH48 is targeted to mitochondria, where its deficiency dramatically reduced the splicing efficiency of five cis-introns (nad5 intron 1; nad7 introns 1, 2, and 3; and ccmFc intron 1) and one trans-intron (nad2 intron 2), leading to lower levels of mitochondrial complexes I and III. ZmRH48 interacts with two unique pentatricopeptide repeat (PPR) proteins, PPR-SMR1 and SPR2, which are required for the splicing of over half of all mitochondrial introns. PPR-SMR1 interacts with SPR2, and both proteins interact with P-type PPR proteins and Zm-mCSF1 to facilitate intron splicing. These results suggest that ZmRH48 is likely a component of a splicing complex and is critical for mitochondrial complex biosynthesis and seed development.

OsHXK3 encodes a hexokinase-like protein that positively regulates grain size in rice

We report the map-based cloning and functional characterization of SNG1, which encodes OsHXK3, a hexokinase-like protein that plays a pivotal role in controlling grain size in rice. Grain size is an important agronomic trait determining grain yield and appearance quality in rice. Here, we report the discovery of rice mutant short and narrow grain1 (sng1) with reduced grain length, width and weight. Map-based cloning revealed that the mutant phenotype was caused by loss of function of gene OsHXK3 that encodes a hexokinase-like (HKL) protein. OsHXK3 was associated with the mitochondria and was ubiquitously distributed in various organs, predominately in younger organs. Analysis of glucose (Glc) phosphorylation activities in young panicles and protoplasts showed that OsHXK3 was a non-catalytic hexokinase (HXK). Overexpression of OsHXK3 could not complement the Arabidopsis glucose insensitive2-1 (gin2-1) mutant, indicating that OsHXK3 lacked Glc signaling activity. Scanning electron microscopy analysis revealed that OsHXK3 affects grain size by promoting spikelet husk cell expansion. Knockout of other nine OsHXK genes except OsHXK3 individually did not change grain size, indicating that functions of OsHXKs have differentiated in rice. OsHXK3 influences gibberellin (GA) biosynthesis and homeostasis. Compared with wild type, OsGA3ox2 was significantly up-regulated and OsGA2ox1 was significantly down-regulated in young panicle of sng1, and concentrations of biologically active GAs were significantly decreased in young panicles of the mutants. The yield per plant of OsHXK3 overexpression lines (OE-4 and OE-35) was increased by 10.91% and 7.62%, respectively, compared to that of wild type. Our results provide evidence that an HXK lacking catalytic and sensory functions plays an important role in grain size and has the potential to increase yield in rice.

Contribution of Massive Mitochondrial Fusion and Subsequent Fission in the Plant Life Cycle to the Integrity of the Mitochondrion and Its Genome

Plant mitochondria have large genomes to house a small number of key genes. Most mitochondria do not contain a whole genome. Despite these latter characteristics, the mitochondrial genome is faithfully maternally inherited. To maintain the mitochondrial genes-so important for energy production-the fusion and fission of mitochondria are critical. Fission in plants is better understood than fusion, with the dynamin-related proteins (DRP 3A and 3B) driving the constriction of the mitochondrion. How the endoplasmic reticulum and the cytoskeleton are linked to the fission process is not yet fully understood. The fusion mechanism is less well understood, as obvious orthologues are not present. However, there is a recently described gene, MIRO2, that appears to have a significant role, as does the ER and cytoskeleton. Massive mitochondrial fusion (MMF or hyperfusion) plays a significant role in plants. MMF occurs at critical times of the life cycle, prior to flowering, in the enlarging zygote and at germination, mixing the cells' mitochondrial population-the so-called "discontinuous whole". MMF in particular aids genome repair, the conservation of critical genes and possibly gives an energy boost to important stages of the life cycle. MMF is also important in plant regeneration, an important component of plant biotechnology.

OsPPR939, a nad5 splicing factor, is essential for plant growth and pollen development in rice

P-subfamily PPR protein OsPPR939, which can be phosphorylated by OsS6K1, regulates plant growth and pollen development by involving in the splicing of mitochondrial nad5 introns 1, 2, and 3. In land plants, pentatricopeptide repeat (PPR) proteins play key roles in mitochondrial group II intron splicing, but how these nucleus-encoded proteins are imported into mitochondria is unknown. To date, a few PPR proteins have been characterized in rice (Oryza sativa). Here, we demonstrate that the mitochondrion-localized P-subfamily PPR protein OsPPR939 is required for the splicing of nad5 introns 1, 2, and 3 in rice. Complete knockout or partial disruption of OsPPR939 function resulted in different degrees of growth retardation and pollen sterility. The dramatically reduced splicing efficiency of these introns in osppr939-4 and osppr939-5 led to reduced mitochondrial complex I abundance and activity and enhanced expression of alternative respiratory pathway genes. Complementation with OsPPR939 rescued the defective plant morphology of osppr939-4 and restored its decreased splicing efficiency of nad5 introns 1, 2, and 3. Therefore, OsPPR939 plays crucial roles in plant growth and pollen development by splicing mitochondrial nad5 introns 1, 2, and 3. More importantly, the 12th amino acid Ser in the N-terminal targeting sequence of OsPPR939 is phosphorylated by OsS6K1, and truncated OsPPR939 with a non-phosphorylatable S12A mutation in its presequence could not be imported into mitochondria, suggesting that phosphorylation of this amino acid plays an important role in the mitochondrial import of OsPPR939. To our knowledge, the 12th residue Ser on OsPPR939 is the first experimentally proven phosphorylation site in PPR proteins. Our results provide a basis for investigating the regulatory mechanism of PPR proteins at the post-translational level.

Overexpression of the homoterpene synthase gene, OsCYP92C21, increases emissions of volatiles mediating tritrophic interactions in rice

Plant defence homoterpenes can be used to attract pest natural enemies. However, the biosynthetic pathway of homoterpenes is still unknown in rice, and the practical application of such indirect defence systems suffers from inherent limitations due to their low emissions from plants. Here, we demonstrated that the protein OsCYP92C21 is responsible for homoterpene biosynthesis in rice. We also revealed that the ability of rice to produce homoterpenes is dependent on the subcellular precursor pools. By increasing the precursor pools through specifically subcellular targeting expression, genetic transformation and genetic introgression, we significantly enhanced homoterpene biosynthesis in rice. The final introgressed GM rice plants exhibited higher homoterpene emissions than the wild type rice and the highest homoterpene emission reported so far for such GM plants even without the induction of herbivore attack. As a result, these GM rice plants demonstrated strong attractiveness to the parasitic wasp Cotesia chilonis. This study discovered the homoterpene biosynthesis pathway in rice, and lays the foundation for the utilisation of plant indirect defence mechanism in the "push-pull" strategy of integrated pest management through increasing precursor pools in the subcellular compartments and overexpressing homoterpene synthase by genetic transformation.

A fungal effector targets a heat shock-dynamin protein complex to modulate mitochondrial dynamics and reduce plant immunity

Mitochondria are essential for animal and plant immunity. Here, we report that the effector MoCDIP4 of the fungal pathogen Magnaporthe oryzae targets the mitochondria-associated OsDjA9-OsDRP1E protein complex to reduce rice immunity. The DnaJ protein OsDjA9 interacts with the dynamin-related protein OsDRP1E and promotes the degradation of OsDRP1E, which functions in mitochondrial fission. By contrast, MoCDIP4 binds OsDjA9 to compete with OsDRP1E, resulting in OsDRP1E accumulation. Knockout of OsDjA9 or overexpression of OsDRP1E or MoCDIP4 in transgenic rice results in shortened mitochondria and enhanced susceptibility to M. oryzae Overexpression of OsDjA9 or knockout of OsDRP1E in transgenic rice, in contrast, leads to elongated mitochondria and enhanced resistance to M. oryzae Our study therefore reveals a previously unidentified pathogen-infection strategy in which the pathogen delivers an effector into plant cells to target an HSP40-DRP complex; the targeting leads to the perturbation of mitochondrial dynamics, thereby inhibiting mitochondria-mediated plant immunity.

PPR14 Interacts With PPR-SMR1 and CRM Protein Zm-mCSF1 to Facilitate Mitochondrial Intron Splicing in Maize

In plants, splicing of organellar group II introns involves numerous nucleus-encoded trans-factors. But, how these trans-factors function and interact is not well understood. Here we report the function of a pentatricopeptide repeat (PPR) protein PPR14 and its physical relationship with other splicing factors in mitochondria. Null mutations of PPR14 severely arrest the embryo and endosperm development, causing an empty pericarp phenotype. PPR14 is required for the splicing of NADH dehydrogenase 2 (nad2) intron 3 and nad7 introns 1 and 2 in mitochondria. The absence of nad2 and nad7 transcripts leads to disruption of the mitochondrial complex I assembly and abolishes its NADH dehydrogenase activity. This is accompanied with increased levels of other mitochondrial complexes and elevated expression of the alternative oxidase proteins. As the function of PPR14 overlaps with PPR-SMR1 and the CRM-domain containing protein Zm-mCSF1, we tested their interactions. Protein-protein interaction analysis indicated that PPR14 interacts with PPR-SMR1 and Zm-mCSF1, suggesting that these three proteins may form a complex. As PPR proteins and CRM-domain containing proteins have many members in mitochondria and chloroplasts, we propose that organellar group II intron splicing is probably mediated by a dynamic complex that includes different PPR and CRM proteins in plants.

PPR-SMR1 is required for the splicing of multiple mitochondrial introns, interacts with Zm-mCSF1, and is essential for seed development in maize

Group II introns are ribozymes that can excise themselves from precursor-RNA transcripts, but plant organellar group II introns have structural deviations that inhibit ribozyme activity. Therefore, splicing of these introns requires the assistance of nuclear- and/or organellar-encoded splicing factors; however, how these splicing factors function remains unclear. In this study, we report the functions and interactions of two splicing factors, PPR-SMR1 and Zm-mCSF1, in intron splicing in maize mitochondria. PPR-SMR1 is a SMR domain-containing pentatricopeptide repeat (PPR) protein and Zm-mCSF1 is a CRM domain-containing protein, and both are targeted to mitochondria. Loss-of-function mutations in each of them severely arrests embryogenesis and endosperm development in maize. Functional analyses indicate that PPR-SMR1 and Zm-mCSF1 are required for the splicing of most mitochondrial group II introns. Among them, nad2-intron 2 and 3, and nad5-intron 1 are PPR-SMR1/Zm-mCSF1-dependent introns. Protein interaction assays suggest that PPR-SMR1 can interact with Zm-mCSF1 through its N-terminus, and that Zm-mCSF1 is self-interacting. Our findings suggest that PPR-SMR1, a novel splicing factor, acts in the splicing of multiple group II introns in maize mitochondria, and the protein-protein interaction between it and Zm-mCSF1 might allow the formation of large macromolecular splicing complexes.

The overexpression of cucumber (Cucumis sativus L.) genes that encode the branched-chain amino acid transferase modulate flowering time in Arabidopsis thaliana

The overexpression of CsBCATs promotes flowering in Arabidopsis by regulating the expression of flowering time genes. The branched-chain amino acid transferases (BCATs) play an important role in the metabolism of branched-chain amino acids (BCAAs), such as isoleucine, leucine, and valine. They function in both the synthesis and the degradation of this class of amino acids. We identified and characterized the three BCAT genes in cucumber (Cucumis sativus L.). The tissue-specific expression profiling in cucumber plants revealed that CsBCAT2 and CsBCAT7 were highly expressed in the reproductive tissues, whereas CsBCAT3 expression was highly detected in the vegetative tissues. The subcellular localization patterns of three CsBCATs were observed in the mitochondria. The functional analyses of CsBCATs showed that CsBCAT2 and CsBCAT3 restored the growth of bat1Δ/bat2Δ double knockout yeast (Saccharomyces cerevisiae), and CsBCAT3 and CsBCAT7 with different substrate preferences acted in a reverse reaction. The transgenic approach demonstrated that the overexpression of the three CsBCATs resulted in early flowering phenotypes, which were associated with the upregulation of FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) in a manner in which they were dependent on GIGANTEA (GI)/CONSTANS (CO) and SHORT VEGETATIVE PHASE (SVP)/FLOWERING LOCUS C (FLC) modules. Our results, which are observed in conjunction, suggest that there is an interconnection between BCAT genes that function in BCAA metabolism and the flowering time in plants.

A Plastid-Localized Pentatricopeptide Repeat Protein is Required for Both Pollen Development and Plant Growth in Rice

Several mitochondrial-targeted pentatricopeptide repeat (PPR) proteins involved in pollen development have been reported to be fertility restorer (Rf) proteins. However, the roles of plastid-localized PPR proteins in plant male reproduction are poorly defined. Here, we described a plastid-localized PPR-SMR protein, OsPPR676, which is required for plant growth and pollen development in rice. In this study, OsPPR676 was confirmed to be an interacted protein with Osj10gBTF3, β-subunit of nascent polypeptide-associated complex (β-NAC), by bimolecular fluorescence complementation assays, indicating that both proteins are probably involved in the same regulatory pathway of pollen development. Compared with other chloroplast-rich tissues, OsPPR676 was only weakly expressed in anther, but in the Mei and YM stages of pollen development, its expression was relatively strong in the tapetum. Disruption of OsPPR676 resulted in growth retardation of plants and partial sterility of pollens. Phenotypic analysis of different osppr676 mutant lines implied that the SMR domain was not essential for the function of OsPPR676. We further demonstrated that OsPPR676 is essential for production of plastid atpB subunit, and then plays crucial roles in biosynthesis of fatty acids, carbohydrates, and other organic matters via affecting activity of ATP synthase.

THI1, a Thiamine Thiazole Synthase, Interacts with Ca2+-Dependent Protein Kinase CPK33 and Modulates the S-Type Anion Channels and Stomatal Closure in Arabidopsis

Thiamine is required for both plant growth and development. Here, the involvement of a thiamine thiazole synthase, THI1, has been demonstrated in both guard cell abscisic acid (ABA) signaling and the drought response in Arabidopsis (Arabidopsis thaliana). THI1 overexpressors proved to be more sensitive to ABA than the wild type with respect to both the activation of guard cell slow type anion channels and stomatal closure; this effectively reduced the rate of water loss from the plant and thereby enhanced its level of drought tolerance. A yeast two-hybrid strategy was used to screen a cDNA library from epidermal strips of leaves for THI1 regulatory factors, and identified CPK33, a Ca(2+)-dependent protein kinase, as interactor with THI1 in a plasma membrane-delimited manner. Loss-of-function cpk33 mutants were hypersensitive to ABA activation of slow type anion channels and ABA-induced stomatal closure, while the CPK33 overexpression lines showed opposite phenotypes. CPK33 kinase activity was essential for ABA-induced stomatal closure. Consistent with their contrasting regulatory role over stomatal closure, THI1 suppressed CPK33 kinase activity in vitro. Together, our data reveal a novel regulatory role of thiamine thiazole synthase to kinase activity in guard cell signaling.

Arabidopsis dynamin-related protein 1E in sphingolipid-enriched plasma membrane domains is associated with the development of freezing tolerance

The freezing tolerance of Arabidopsis thaliana is enhanced by cold acclimation, resulting in changes in the compositions and function of the plasma membrane. Here, we show that a dynamin-related protein 1E (DRP1E), which is thought to function in the vesicle trafficking pathway in cells, is related to an increase in freezing tolerance during cold acclimation. DRP1E accumulated in sphingolipid and sterol-enriched plasma membrane domains after cold acclimation. Analysis of drp1e mutants clearly showed that DRP1E is required for full development of freezing tolerance after cold acclimation. DRP1E fused with green fluorescent protein was visible as small foci that overlapped with fluorescent dye-labelled plasma membrane, providing evidence that DRP1E localizes non-uniformly in specific areas of the plasma membrane. These results suggest that DRP1E accumulates in sphingolipid and sterol-enriched plasma membrane domains and plays a role in freezing tolerance development during cold acclimation.

NRGA1, a putative mitochondrial pyruvate carrier, mediates ABA regulation of guard cell ion channels and drought stress responses in Arabidopsis

Abscisic acid (ABA) regulates ion channel activity and stomatal movements in response to drought and other stresses. Here, we show that the Arabidopsis thaliana gene NRGA1 is a putative mitochondrial pyruvate carrier which negatively regulates ABA-induced guard cell signaling. NRGA1 transcript was abundant in the A. thaliana leaf and particularly in the guard cells, and its product was directed to the mitochondria. The heterologous co-expression of NRGA1 and AtMPC1 in yeast complemented a loss-of-function mitochondrial pyruvate carrier (MPC) mutant. The nrga1 loss-of-function mutant was very sensitive to the presence of ABA in the context of stomatal movements, and exhibited a heightened tolerance to drought stress. Disruption of NRGA1 gene resulted in increased ABA inhibition of inward K(+) currents and ABA activation of slow anion currents in guard cells. The nrga1/NRGA1 functional complementation lines restored the mutant's phenotypes. Furthermore, transgenic lines of constitutively overexpressing NRGA1 showed opposite stomatal responses, reduced drought tolerance, and ABA sensitivity of guard cell inward K(+) channel inhibition and anion channel activation. Our findings highlight a putative role for the mitochondrial pyruvate carrier in guard cell ABA signaling in response to drought.

OsAP65, a rice aspartic protease, is essential for male fertility and plays a role in pollen germination and pollen tube growth

Aspartic proteases (APs) comprise a large proteolytic enzyme family widely distributed in animals, microbes, viruses, and plants. The rice genome encodes 96 APs, of which only a few have been functionally characterized. Here, the identification and characterization of a novel AP gene, OsAP65, which plays an indispensable role in pollen tube growth in rice, is reported. The T-DNA insertion line of OsAP65 caused severe segregation distortion. In the progeny derived from an individual heterozygous for the T-DNA insertion, the wild type and T-DNA-carrying heterozygote segregated at a ratio close to 1:1, while homozygotes of disrupted OsAP65 (OsAP65-/-) were not recovered. Reciprocal crosses between heterozygotes and wild-type plants demonstrated that the mutant alleles could not be transmitted through the male gamete. Examination of the anthers from heterozygous plants revealed that the mutant pollen matured normally, but did not germinate or elongate. OsAP65 was expressed in various tissues and the transcript level in heterozygous plants was about half of the amount measured in the wild-type plants. The subcellular localization showed that OsAP65 is a pre-vacuolar compartment (PVC) protein. These results indicated that OsAP65 was essential for rice pollen germination and tube growth.

Overexpression of AtOGG1, a DNA glycosylase/AP lyase, enhances seed longevity and abiotic stress tolerance in Arabidopsis

Reactive oxygen species (ROS) are toxic by-products generated continuously during seed desiccation, storage, and germination, resulting in seed deterioration and therefore decreased seed longevity. The toxicity of ROS is due to their indiscriminate reactivity with almost any constituent of the cell, such as lipids, proteins, and DNA. The damage to the genome induced by ROS has been recognized as an important cause of seed deterioration. A prominent DNA lesion induced by ROS is 7,8-dihydro-8-oxoguanine (8-oxo-G), which can form base pairs with adenine instead of cytosine during DNA replication and leads to GC→TA transversions. In Arabidopsis, AtOGG1 is a DNA glycosylase/apurinic/apyrimidinic (AP) lyase that is involved in base excision repair for eliminating 8-oxo-G from DNA. In this study, the functions of AtOGG1 were elaborated. The transcript of AtOGG1 was detected in seeds, and it was strongly up-regulated during seed desiccation and imbibition. Analysis of transformed Arabidopsis protoplasts demonstrated that AtOGG1-yellow fluorescent protein fusion protein localized to the nucleus. Overexpression of AtOGG1 in Arabidopsis enhanced seed resistance to controlled deterioration treatment. In addition, the content of 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) in transgenic seeds was reduced compared to wild-type seeds, indicating a DNA damage-repair function of AtOGG1 in vivo. Furthermore, transgenic seeds exhibited increased germination ability under abiotic stresses such as methyl viologen, NaCl, mannitol, and high temperatures. Taken together, our results demonstrated that overexpression of AtOGG1 in Arabidopsis enhances seed longevity and abiotic stress tolerance.

The rice HGW gene encodes a ubiquitin-associated (UBA) domain protein that regulates heading date and grain weight

Heading date and grain weight are two determining agronomic traits of crop yield. To date, molecular factors controlling both heading date and grain weight have not been identified. Here we report the isolation of a hemizygous mutation, heading and grain weight (hgw), which delays heading and reduces grain weight in rice. Analysis of hgw mutant phenotypes indicate that the hemizygous hgw mutation decreases latitudinal cell number in the lemma and palea, both composing the spikelet hull that is known to determine the size and shape of brown grain. Molecular cloning and characterization of the HGW gene showed that it encodes a novel plant-specific ubiquitin-associated (UBA) domain protein localized in the cytoplasm and nucleus, and functions as a key upstream regulator to promote expressions of heading date- and grain weight-related genes. Moreover, co-expression analysis in rice and Arabidopsis indicated that HGW and its Arabidopsis homolog are co-expressed with genes encoding various components of ubiquitination machinery, implying a fundamental role for the ubiquitination pathway in heading date and grain weight control.

Protein-induced modulation of chloroplast membrane morphology

Organelles are surrounded by membranes with a distinct lipid and protein composition. While it is well established that lipids affect protein functioning and vice versa, it has been only recently suggested that elevated membrane protein concentrations may affect the shape and organization of membranes. We therefore analyzed the effects of high chloroplast envelope protein concentrations on membrane structures using an in vivo approach with protoplasts. Transient expression of outer envelope proteins or protein domains such as CHUP1-TM-GFP, outer envelope protein of 7 kDa-GFP, or outer envelope protein of 24 kDa-GFP at high levels led to the formation of punctate, circular, and tubular membrane protrusions. Expression of inner membrane proteins such as translocase of inner chloroplast membrane 20, isoform II (Tic20-II)-GFP led to membrane protrusions including invaginations. Using increasing amounts of DNA for transfection, we could show that the frequency, size, and intensity of these protrusions increased with protein concentration. The membrane deformations were absent after cycloheximide treatment. Co-expression of CHUP1-TM-Cherry and Tic20-II-GFP led to membrane protrusions of various shapes and sizes including some stromule-like structures, for which several functions have been proposed. Interestingly, some structures seemed to contain both proteins, while others seem to contain one protein exclusively, indicating that outer and inner envelope dynamics might be regulated independently. While it was more difficult to investigate the effects of high expression levels of membrane proteins on mitochondrial membrane shapes using confocal imaging, it was striking that the expression of the outer membrane protein Tom20 led to more elongate mitochondria. We discuss that the effect of protein concentrations on membrane structure is possibly caused by an imbalance in the lipid to protein ratio and may be involved in a signaling pathway regulating membrane biogenesis. Finally, the observed phenomenon provides a valuable experimental approach to investigate the relationship between lipid synthesis and membrane protein expression in future studies.

Co-regulation of transcellular and paracellular leak across microvascular endothelium by dynamin and Rac

Increased permeability of the microvascular endothelium to fluids and proteins is the hallmark of inflammatory conditions such as sepsis. Leakage can occur between (paracellular) or through (transcytosis) endothelial cells, yet little is known about whether these pathways are linked. Understanding the regulation of microvascular permeability is essential for the identification of novel therapies to combat inflammation. We investigated whether transcytosis and paracellular leakage are co-regulated. Using molecular and pharmacologic approaches, we inhibited transcytosis of albumin in primary human microvascular endothelium and measured paracellular permeability. Blockade of transcytosis induced a rapid increase in paracellular leakage that was not explained by decreases in caveolin-1 or increases in activity of nitric oxide synthase. The effect required caveolin-1 but was observed in cells depleted of clathrin, indicating that it was not due to the general inhibition of endocytosis. Inhibiting transcytosis by dynamin blockade increased paracellular leakage concomitantly with the loss of cortical actin from the plasma membrane and the displacement of active Rac from the plasmalemma. Importantly, inhibition of paracellular leakage by sphingosine-1-phosphate, which activates Rac and induces cortical actin, caused a significant increase in transcytosis of albumin in vitro and in an ex vivo whole-lung model. In addition, dominant-negative Rac significantly diminished albumin uptake by endothelia. Our findings indicate that transcytosis and paracellular permeability are co-regulated through a signaling pathway linking dynamin, Rac, and actin.

Plant dynamin-related protein families DRP1 and DRP2 in plant development

Two separate families of Arabidopsis dynamin-related proteins, DRP1 and DRP2, have been implicated in clathrin-mediated endocytosis and cell plate maturation during cytokinesis. The present review summarizes the current genetic, biochemical and cell biological knowledge about these two protein families, and suggests key directions for more fully understanding their roles and untangling their function in membrane trafficking. We focus particularly on comparing and contrasting these two protein families, which have very distinct domain structures and are independently essential for Arabidopsis development, yet which have been implicated in very similar cellular processes during cytokinesis and cell expansion.

The Arabidopsis chloroplast division protein DYNAMIN-RELATED PROTEIN5B also mediates peroxisome division

Peroxisomes are highly dynamic organelles involved in various metabolic pathways. The division of peroxisomes is regulated by factors such as the PEROXIN11 (PEX11) proteins that promote peroxisome elongation and the dynamin-related proteins (DRPs) and FISSION1 (FIS1) proteins that function together to mediate organelle fission. In Arabidopsis thaliana, DRP3A/DRP3B and FIS1A (BIGYIN)/FIS1B are two pairs of homologous proteins known to function in both peroxisomal and mitochondrial division. Here, we report that DRP5B, a DRP distantly related to the DRP3s and originally identified as a chloroplast division protein, also contributes to peroxisome division. DRP5B localizes to both peroxisomes and chloroplasts. Mutations in the DRP5B gene lead to peroxisome division defects and compromised peroxisome functions. Using coimmunoprecipitation and bimolecular fluorescence complementation assays, we further demonstrate that DRP5B can interact or form a complex with itself and with DRP3A, DRP3B, FIS1A, and most of the Arabidopsis PEX11 isoforms. Our data suggest that, in contrast with DRP3A and DRP3B, whose orthologs exist across plant, fungal, and animal kingdoms, DRP5B is a plant/algal invention to facilitate the division of their organelles (i.e., chloroplasts and peroxisomes). In addition, our results support the notion that proteins involved in the early (elongation) and late (fission) stages of peroxisome division may act cooperatively.

Heat shock protein cognate 70-4 and an E3 ubiquitin ligase, CHIP, mediate plastid-destined precursor degradation through the ubiquitin-26S proteasome system in Arabidopsis

Plastid-targeted proteins pass through the cytosol as unfolded precursors. If proteins accumulate in the cytosol, they can form nonspecific aggregates that cause severe cellular damage. Here, we demonstrate that high levels of plastid precursors are degraded through the ubiquitin-proteasome system (UPS) in Arabidopsis thaliana cells. The cytosolic heat shock protein cognate 70-4 (Hsc70-4) and E3 ligase carboxy terminus of Hsc70-interacting protein (CHIP) were highly induced in plastid protein import2 plants, which had a T-DNA insertion at Toc159 and showed an albino phenotype and a severe defect in protein import into chloroplasts. Hsc70-4 and CHIP together mediated plastid precursor degradation when import-defective chloroplast-targeted reporter proteins were transiently expressed in protoplasts. Hsc70-4 recognized specific sequence motifs in transit peptides and thereby led to precursor degradation through the UPS. CHIP, which interacted with Hsc70-4, functioned as an E3 ligase in the Hsc70-4-mediated protein degradation. The physiological role of Hsc70-4 was confirmed by analyzing Hsc70-4 RNA interference plants in an hsc70-1 mutant background. Plants with lower Hsc70 levels exhibited abnormal embryogenesis, resulting in defective seedlings that displayed high levels of reactive oxygen species and monoubiquitinated Lhcb4 precursors. We propose that Hsc70-4 and CHIP mediate plastid-destined precursor degradation to prevent cytosolic precursor accumulation and thereby play a critical role in embryogenesis.

Rice OGR1 encodes a pentatricopeptide repeat-DYW protein and is essential for RNA editing in mitochondria

RNA editing is the alteration of RNA sequences via insertion, deletion and conversion of nucleotides. In flowering plants, specific cytidine residues of RNA transcribed from organellar genomes are converted into uridines. Approximately 35 editing sites are present in the chloroplasts of higher plants; six pentatricopeptide repeat genes involved in RNA editing have been identified in Arabidopsis. However, although approximately 500 editing sites are found in mitochondrial RNAs of flowering plants, only one gene in Arabidopsis has been reported to be involved in such editing. Here, we identified rice mutants that are defective in seven specific RNA editing sites on five mitochondrial transcripts. Their various phenotypes include delayed seed germination, retarded growth, dwarfism and sterility. Mutant seeds from heterozygous plants are opaque. This mutation, named opaque and growth retardation 1 (ogr1), was generated by T-DNA insertion into a gene that encodes a pentatricopeptide repeat protein containing the DYW motif. The OGR1-sGFP fusion protein is localized to mitochondria. Ectopic expression of OGR1 in the mutant complements the altered phenotypes. We conclude that OGR1 is essential for RNA editing in rice mitochondria and is required for normal growth and development.

The Arabidopsis peroxisome division mutant pdd2 is defective in the DYNAMIN-RELATED PROTEIN3A (DRP3A) gene

In plants, the division of peroxisomes is mediated by several classes of proteins, including PEROXIN11 (PEX11), FISSION1 (FIS1) and DYNAMIN-RELATED PROTEIN3 (DRP3). DRP3A and DRP3B are two homologous dynamin-related proteins playing overlapping roles in the division of both peroxisomes and mitochondria, with DRP3A performing a stronger function than DRP3B in peroxisomal fission. Here, we report the identification and characterization of the peroxisome division defective 2 (pdd2) mutant, which was later proven to be another drp3A allele. The pdd2 mutant generates a truncated DRP3A protein and exhibits pale green and retarded growth phenotypes. Intriguingly, this mutant displays much stronger peroxisome division deficiency in root cells than in leaf mesophyll cells. Our data suggest that the partial GTPase effector domain retained in pdd2 may have contributed to the distinct mutant phenotype of this mutant.

Pathogen inducible voltage-dependent anion channel (AtVDAC) isoforms are localized to mitochondria membrane in Arabidopsis

Voltage-dependent anion channels (VDACs) are reported to be porin-type, beta-barrel diffusion pores. They are prominently localized in the outer mitochondrial membrane and are involved in metabolite exchange between the organelle and the cytosol. In this study, we have investigated a family of VDAC isoforms in Arabidopsis thaliana (AtVDAC). We have shown that the heterologous expression of AtVDAC proteins can functionally complement a yeast mutant lacking the endogenous mitochondrial VDAC gene. AtVDACs tagged with GFP were localized to mitochondria in both yeast and plant cells. We also looked at the response of AtVDACs to biotic and abiotic stresses and found that four AtVDAC transcripts were rapidly up-regulated in response to a bacterial pathogen.

Rice XB15, a protein phosphatase 2C, negatively regulates cell death and XA21-mediated innate immunity

Perception of extracellular signals by cell surface receptors is of central importance to eukaryotic development and immunity. Kinases that are associated with the receptors or are part of the receptors themselves modulate signaling through phosphorylation events. The rice (Oryza sativa L.) XA21 receptor kinase is a key recognition and signaling determinant in the innate immune response. A yeast two-hybrid screen using the intracellular portion of XA21, including the juxtamembrane (JM) and kinase domain as bait, identified a protein phosphatase 2C (PP2C), called XA21 binding protein 15 (XB15). The interaction of XA21 and XB15 was confirmed in vitro and in vivo by glutathione-S-transferase (GST) pull-down and co-immunoprecipitation assays, respectively. XB15 fusion proteins purified from Escherichia coli and from transgenic rice carry PP2C activity. Autophosphorylated XA21 can be dephosphorylated by XB15 in a temporal- and dosage-dependent manner. A serine residue in the XA21 JM domain is required for XB15 binding. Xb15 mutants display a severe cell death phenotype, induction of pathogenesis-related genes, and enhanced XA21-mediated resistance. Overexpression of Xb15 in an XA21 rice line compromises resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae. These results demonstrate that Xb15 encodes a PP2C that negatively regulates the XA21-mediated innate immune response.

Rice peptide deformylase PDF1B is crucial for development of chloroplasts

Because protein synthesis begins with N-formylmethionine in plant endosymbiotic organelles, removal of the formyl group by peptide deformylase (PDF) is essential to allowing the excision of the first methionine. Rice contains three copies (OsPDF1A, OsPDF1B and OsPDF1B2) of the PDF genes. Unlike OsPDF1A and OsPDF1B, OsPDF1B2 is apparently non-functional, with several deleterious substitutions and deletions. OsPDF1A is more strongly expressed in the roots, while OsPDF1B is expressed at higher levels in mature leaves. Transient expression of PDF-green fluorescent protein (GFP) fusion proteins in the protoplasts demonstrates that, unlike OsPDF1A, OsPDF1B is localized in both the chloroplasts and the mitochondria. We used T-DNA insertional alleles to elucidate functional roles associated with OsPDF1B. Homozygous plants of pdf1b/pdf1b exhibited the phenotypes of chlorina and growth retardation. Histochemical analysis showed that the length of their mesophyll cells was increased 4- to 5-fold, resulting in a reduction in the total number of cells. Transmission electron microscopy analyses revealed that chloroplasts were severely damaged and mitochondria appeared to be mildly altered in the pdf1b mutants. Expression of genes encoded in the chloroplasts and mitochondria was altered in the mutants. Based on these results, we conclude that OsPDF1B is essential for the development of chloroplast and perhaps mitochondria.

Rice XB15, a protein phosphatase 2C, negatively regulates cell death and XA21-mediated innate immunity

Perception of extracellular signals by cell surface receptors is of central importance to eukaryotic development and immunity. Kinases that are associated with the receptors or are part of the receptors themselves modulate signaling through phosphorylation events. The rice (Oryza sativa L.) XA21 receptor kinase is a key recognition and signaling determinant in the innate immune response. A yeast two-hybrid screen using the intracellular portion of XA21, including the juxtamembrane (JM) and kinase domain as bait, identified a protein phosphatase 2C (PP2C), called XA21 binding protein 15 (XB15). The interaction of XA21 and XB15 was confirmed in vitro and in vivo by glutathione-S-transferase (GST) pull-down and co-immunoprecipitation assays, respectively. XB15 fusion proteins purified from Escherichia coli and from transgenic rice carry PP2C activity. Autophosphorylated XA21 can be dephosphorylated by XB15 in a temporal- and dosage-dependent manner. A serine residue in the XA21 JM domain is required for XB15 binding. Xb15 mutants display a severe cell death phenotype, induction of pathogenesis-related genes, and enhanced XA21-mediated resistance. Overexpression of Xb15 in an XA21 rice line compromises resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae. These results demonstrate that Xb15 encodes a PP2C that negatively regulates the XA21-mediated innate immune response.

Effects of arc3, arc5 and arc6 mutations on plastid morphology and stromule formation in green and nongreen tissues of Arabidopsis thaliana

Mutations in the ARC3, ARC5 and ARC6 genes of Arabidopsis thaliana affect chloroplast division. We investigated whether ARC3, ARC5 and ARC6 are also involved in determining plastid morphology in nongreen tissues, where stromules, stroma-filled tubular extensions of the plastid envelope membrane, are more abundant than in mesophyll cells. Using plastid-targeted green fluorescent protein to observe plastids throughout the organs of these mutants, we have discovered a number of new mutant phenotypes. The size of arc3 plastids was heterogeneous in various tissues. arc5 plastids appeared wild-type in the majority of nongreen tissues examined. However, in cells of stamen filaments, the arc5 mutant showed an increase in the frequency of stromules. Increased stromule frequency was observed for a number of organs in the arc6 mutant. Some arc6 cells contained heterogeneous mixtures of plastids; epidermal cells of hypocotyls, stamen filaments and the bases of petals possessed both very large chloroplasts as well as much smaller nongreen plastids. Quantitative analysis in hypocotyl cells revealed that the alteration in stromule length in arc3 and arc6 mutants occurred despite wild-type plastid densities. Thus, in hypocotyls, the effects of the arc3 and arc6 mutations on stromule length and frequency are independent of changes in plastid division. Although electron micrographs of stromules emanating from chloroplasts have rarely been reported, within the arc3 mutant, narrow, 40-50 nm diameter, recoiled stromules could be followed for about 10 microm in electron micrographs of leaf tissue.

Mitochondrial biogenesis and function in Arabidopsis

Mitochondria represent the powerhouse of cells through their synthesis of ATP. However, understanding the role of mitochondria in the growth and development of plants will rely on a much deeper appreciation of the complexity of this organelle. Arabidopsis research has provided clear identification of mitochondrial components, allowed wide-scale analysis of gene expression, and has aided reverse genetic manipulation to test the impact of mitochondrial component loss on plant function. Forward genetics in Arabidopsis has identified mitochondrial involvement in mutations with notable impacts on plant metabolism, growth and development. Here we consider the evidence for components involved in mitochondria biogenesis, metabolism and signalling to the nucleus.

Mitochondrial biogenesis and function in Arabidopsis

Mitochondria represent the powerhouse of cells through their synthesis of ATP. However, understanding the role of mitochondria in the growth and development of plants will rely on a much deeper appreciation of the complexity of this organelle. Arabidopsis research has provided clear identification of mitochondrial components, allowed wide-scale analysis of gene expression, and has aided reverse genetic manipulation to test the impact of mitochondrial component loss on plant function. Forward genetics in Arabidopsis has identified mitochondrial involvement in mutations with notable impacts on plant metabolism, growth and development. Here we consider the evidence for components involved in mitochondria biogenesis, metabolism and signalling to the nucleus.

Dynamics of Arabidopsis dynamin-related protein 1C and a clathrin light chain at the plasma membrane

Plant morphogenesis depends on polarized exocytic and endocytic membrane trafficking. Members of the Arabidopsis thaliana dynamin-related protein 1 (DRP1) subfamily are required for polarized cell expansion and cytokinesis. Using a combination of live-cell imaging techniques, we show that a functional DRP1C green fluorescent fusion protein (DRP1C-GFP) was localized at the division plane in dividing cells and to the plasma membrane in expanding interphase cells. In both tip growing root hairs and diffuse-polar expanding epidermal cells, DRP1C-GFP organized into dynamic foci at the cell cortex, which colocalized with a clathrin light chain fluorescent fusion protein (CLC-FFP), suggesting that DRP1C may participate in clathrin-mediated membrane dynamics. DRP1C-GFP and CLC-GFP foci dynamics are dependent on cytoskeleton organization, cytoplasmic streaming, and functional clathrin-mediated endocytic traffic. Our studies provide insight into DRP1 and clathrin dynamics in the plant cell cortex and indicate that the clathrin endocytic machinery in plants has both similarities and striking differences to that in mammalian cells and yeast.

Variation in molybdenum content across broadly distributed populations of Arabidopsis thaliana is controlled by a mitochondrial molybdenum transporter (MOT1)

Molybdenum (Mo) is an essential micronutrient for plants, serving as a cofactor for enzymes involved in nitrate assimilation, sulfite detoxification, abscisic acid biosynthesis, and purine degradation. Here we show that natural variation in shoot Mo content across 92 Arabidopsis thaliana accessions is controlled by variation in a mitochondrially localized transporter (Molybdenum Transporter 1 - MOT1) that belongs to the sulfate transporter superfamily. A deletion in the MOT1 promoter is strongly associated with low shoot Mo, occurring in seven of the accessions with the lowest shoot content of Mo. Consistent with the low Mo phenotype, MOT1 expression in low Mo accessions is reduced. Reciprocal grafting experiments demonstrate that the roots of Ler-0 are responsible for the low Mo accumulation in shoot, and GUS localization demonstrates that MOT1 is expressed strongly in the roots. MOT1 contains an N-terminal mitochondrial targeting sequence and expression of MOT1 tagged with GFP in protoplasts and transgenic plants, establishing the mitochondrial localization of this protein. Furthermore, expression of MOT1 specifically enhances Mo accumulation in yeast by 5-fold, consistent with MOT1 functioning as a molybdate transporter. This work provides the first molecular insight into the processes that regulate Mo accumulation in plants and shows that novel loci can be detected by association mapping.

Arabidopsis dynamin-like protein DRP1A: a null mutant with widespread defects in endocytosis, cellulose synthesis, cytokinesis, and cell expansion

Dynamin-related proteins are large GTPases that deform and cause fission of membranes. The DRP1 family of Arabidopsis thaliana has five members of which DRP1A, DRP1C, and DRP1E are widely expressed. Likely functions of DRP1A were identified by studying rsw9, a null mutant of the Columbia ecotype that grows continuously but with altered morphology. Mutant roots and hypocotyls are short and swollen, features plausibly originating in their cellulose-deficient walls. The reduction in cellulose is specific since non-cellulosic polysaccharides in rsw9 have more arabinose, xylose, and galactose than those in wild type. Cell plates in rsw9 roots lack DRP1A but still retain DRP1E. Abnormally placed and often incomplete cell walls are preceded by abnormally curved cell plates. Notwithstanding these division abnormalities, roots and stems add new cells at wild-type rates and organ elongation slows because rsw9 cells do not grow as long as wild-type cells. Absence of DRP1A reduces endocytotic uptake of FM4-64 into the cytoplasm of root cells and the hypersensitivity of elongation and radial swelling in rsw9 to the trafficking inhibitor monensin suggests that impaired endocytosis may contribute to the development of shorter fatter roots, probably by reducing cellulose synthesis.

yucca6, a dominant mutation in Arabidopsis, affects auxin accumulation and auxin-related phenotypes

Auxin plays critical roles in many aspects of plant growth and development. Although a number of auxin biosynthetic pathways have been identified, their overlapping nature has prevented a clear elucidation of auxin biosynthesis. Recently, Arabidopsis (Arabidopsis thaliana) mutants with supernormal auxin phenotypes have been reported. These mutants exhibit hyperactivation of genes belonging to the YUCCA family, encoding putative flavin monooxygenase enzymes that result in increased endogenous auxin levels. Here, we report the discovery of fertile dominant Arabidopsis hypertall1-1D and hypertall1-2D (yucca6-1D, -2D) mutants that exhibit typical auxin overproduction phenotypic alterations, such as epinastic cotyledons, increased apical dominance, and curled leaves. However, unlike other auxin overproduction mutants, yucca6 plants do not display short or hairy root phenotypes and lack morphological changes under dark conditions. In addition, yucca6-1D and yucca6-2D have extremely tall (>1 m) inflorescences with extreme apical dominance and twisted cauline leaves. Microarray analyses revealed that expression of several indole-3-acetic acid-inducible genes, including Aux/IAA, SMALL AUXIN-UP RNA, and GH3, is severalfold higher in yucca6 mutants than in the wild type. Tryptophan (Trp) analog feeding experiments and catalytic activity assays with recombinant YUCCA6 indicate that YUCCA6 is involved in a Trp-dependent auxin biosynthesis pathway. YUCCA6:GREEN FLUORESCENT PROTEIN fusion protein indicates YUCCA6 protein exhibits a nonplastidial subcellular localization in an unidentified intracellular compartment. Taken together, our results identify YUCCA6 as a functional member of the YUCCA family with unique roles in growth and development.

A mutation in the GTP hydrolysis site of Arabidopsis dynamin-related protein 1E confers enhanced cell death in response to powdery mildew infection

We screened for mutants of Arabidopsis thaliana that displayed enhanced disease resistance to the powdery mildew pathogen Erysiphe cichoracearum and identified the edr3 mutant, which formed large gray lesions upon infection with E. cichoracearum and supported very little sporulation. The edr3-mediated disease resistance and cell death phenotypes were dependent on salicylic acid signaling, but independent of ethylene and jasmonic acid signaling. In addition, edr3 plants displayed enhanced susceptibility to the necrotrophic fungal pathogen Botrytis cinerea, but showed normal responses to virulent and avirulent strains of Pseudomonas syringae pv. tomato. The EDR3 gene was isolated by positional cloning and found to encode Arabidopsis dynamin-related protein 1E (DRP1E). The edr3 mutation caused an amino acid substitution in the GTPase domain of DRP1E (proline 77 to leucine) that is predicted to block GTP hydrolysis, but not GTP binding. A T-DNA insertion allele in DRP1E did not cause powdery mildew-induced lesions, suggesting that this phenotype is caused by DRP1E being locked in the GTP-bound state, rather than by a loss of DRP1E activity. Analysis of DRP1E-green fluorescent protein fusion proteins revealed that DRP1E is at least partially localized to mitochondria. These observations suggest a mechanistic link between salicylic acid signaling, mitochondria and programmed cell death in plants.

Blue light-induced association of phototropin 2 with the Golgi apparatus

Phototropins 1 and 2 (phot1 and phot2) function as blue light (BL) photoreceptors for phototropism, chloroplast relocation, stomatal opening and leaf flattening in Arabidopsis thaliana. Phototropin consists of two functional domains, the N-terminal photosensory domain and the C-terminal Ser/Thr kinase domain. However, little is known about the signal transduction pathway that links the photoreceptors and the physiological responses downstream of BL perception. To understand the mechanisms by which phot2 initiates these responses, we transformed the phot1phot2 double mutant of Arabidopsis with constructs encoding translationally fused phot2:green fluorescent protein (P2G). P2G was fully functional for the phot2-specific physiological responses in these transgenic plants. It localized strongly to the plasma membrane and weakly to the cytoplasm in the dark. Upon illumination with BL, punctate P2G staining was formed within a few minutes in addition to the constitutive plasma membrane staining. This punctate distribution pattern matched well with that of the Golgi-localized KAM1DeltaC:mRFP. Brefeldin A (BFA), an inhibitor of vesicle trafficking, induced accumulation of P2G around the perinuclear region even in darkness, but the punctate pattern was not observed. After treatment of these cells with BL, P2G exhibited the punctate distribution pattern that matched with that of the Golgi marker. Hence, the light-dependent association of P2G with the Golgi apparatus was BFA-insensitive. A structure/function analysis indicated that the kinase domain was essential for the Golgi localization of phot2. The BL-induced Golgi localization of phot2 may be one of important signaling steps in the phot2 signal transduction pathway.

Mitochondrial morphology and dynamics in yeast and multicellular eukaryotes

Mitochondria form dynamic tubular networks that continually change their shape and move throughout the cell. In eukaryotes, these organellar gymnastics are controlled by numerous pathways that preserve proper mitochondrial morphology and function. The best understood of these are the fusion and fission pathways, which rely on conserved GTPases and their binding partners to regulate organelle connectivity and copy number in healthy cells and during apoptosis. In budding yeast, mitochondrial shape is also maintained by proteins acting in the tubulation pathway. Novel proteins and pathways that control mitochondrial dynamics continue to be discovered, indicating that the mechanisms governing this organelle's behavior are more sophisticated than previously appreciated. Here we review recent advances in the field of mitochondrial dynamics and highlight the importance of these pathways to human health.

Plant mitochondrial dynamics

Higher plant mitochondria are dynamic, pleomorphic organelles. The higher plant chondriome (all mitochondria in a cell collectively) is typically composed of numerous, physically discrete, mitochondria. However, frequent inter-mitochondrial fusion, enabling the mixing and recombination of mtDNA, ensures that the higher plant chondriome functions, at least genetically, as a discontinuous whole. Nothing is known about the genes controlling mitochondrial fusion in plants; there are no plant homologues of most of the genes known to be involved in fusion in other organisms. In contrast, the mitochondrial fission apparatus is generally conserved. Higher plant mitochondria use dynamin-like and Fis-type proteins for division; like yeast and animals, higher plants have lost the mitochondrial-specific form of the prokaryote-derived protein, FtsZ. In addition to being providers of energy for life, mitochondria provide a trigger for death. The role of mitochondrial dynamics in the initiation and promulgation of cell death is conserved in higher plants although there are specific differences in the genes and mechanisms involved relative to other higher eukaryotes.

Mitochondria as a connected population: ensuring continuity of the mitochondrial genome during plant cell dedifferentiation through massive mitochondrial fusion

Mitochondrial fusion in plants and its role in development are poorly understood. Cultured tobacco mesophyll protoplasts provide an excellent experimental system for visualizing mitochondrial dynamics. Before protoplasts first divide, mitochondria undergo a phase of extensive elongation before fission causes an increase in number, followed by actin filament (AF)-dependent dispersion that distributes mitochondria uniformly throughout the cytoplasm. Here, by fusing protoplasts containing either green fluorescent protein- or MitoTracker-labelled mitochondria, we show that elongation results from fusion during early (4-8 h) protoplast culture. This massive mitochondrial fusion (MMF) leads to near-complete mixing of the mitochondrial population within 24 h. Staining isolated mitochondria with 4',6-diamidino-2-phenylindole (DAPI) revealed that in freshly prepared protoplasts mitochondrial nucleoids were unequally distributed, with many mitochondria failing to stain with DAPI, suggesting the presence of an incomplete mitochondrial genome. Following MMF, nucleoids were distributed evenly throughout the population, thereby ensuring continuity of the mitochondrial genome in daughter cells. Massive mitochondrial fusion appears to be specific to dedifferentiation, since it also occurs in mesophyll protoplasts of Arabidopsis and Medicago but not in protoplasts from already dedifferentiated cells such as BY-2 or callus cultures. Efficient MMF requires an inner membrane electrical gradient, cytoplasmic protein synthesis, microtubules and functional kinesin but not ATP or AFs, indicating fundamental differences from mitochondrial fusion in non-plant systems. Our studies reveal that individual mitochondria are connected over time by fusion events, a finding that allows a clearer interpretation of how novel mitochondrial genotypes develop following cell fusion, and indicates that developmentally regulated fusion ensures continuity of the mitochondrial genome.

Heterodimerization and endocytosis of Arabidopsis brassinosteroid receptors BRI1 and AtSERK3 (BAK1)

In Arabidopsis thaliana brassinosteroid (BR), perception is mediated by two Leu-rich repeat receptor-like kinases, BRASSINOSTEROID INSENSITIVE1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) (Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR-like KINASE3 [AtSERK3]). Genetic, biochemical, and yeast (Saccharomyces cerevisiae) interaction studies suggested that the BRI1-BAK1 receptor complex initiates BR signaling, but the role of the BAK1 receptor is still not clear. Using transient expression in protoplasts of BRI1 and AtSERK3 fused to cyan and yellow fluorescent green fluorescent protein variants allowed us to localize each receptor independently in vivo. We show that BRI1, but not AtSERK3, homodimerizes in the plasma membrane, whereas BRI1 and AtSERK3 preferentially heterodimerize in the endosomes. Coexpression of BRI1 and AtSERK3 results in a change of the steady state distribution of both receptors because of accelerated endocytosis. Endocytic vesicles contain either BRI1 or AtSERK3 alone or both. We propose that the AtSERK3 protein is involved in changing the equilibrium between plasma membrane-located BRI1 homodimers and endocytosed BRI1-AtSERK3 heterodimers.

Cytokinesis: lines of division taking shape

Plant cytokinesis requires an orchestrated interplay of membrane and cytoskeleton dynamics, which results in the formation of the membrane that partitions the cytoplasm of the dividing cell. Until recently, phragmoplast-assisted cytokinesis of somatic cells was regarded as mechanistically different from 'non-conventional' modes of cytokinesis, such as endosperm cellularisation or male meiotic cytokinesis. However, features that are similar among these diverse modes of cytokinesis have now been revealed by electron tomography, suggesting common underlying mechanisms that are also supported by genetic and molecular studies. Further insight into the complex process of cytokinesis has been gained from the identification of new components and from the analysis of known components.

The Arabidopsis rab5 homologs rha1 and ara7 localize to the prevacuolar compartment

Rha1, an Arabidopsis Rab5 homolog, plays a critical role in vacuolar trafficking in plant cells. In this study, we investigated the localization of Rha1 and Ara7, two Arabidopsis proteins that have highly similar amino acid sequence homology to Rab5 in animal cells. Both Ara7 and Rha1 gave a punctate staining pattern and colocalized when transiently expressed as GFP- (green fluorescent protein) or small epitope-tagged forms in Arabidopsis protoplasts. In protoplasts, transiently expressed Rha1 and Ara7 colocalized with AtPEP12p and VSR(At-1), two proteins that are known to be present at the prevacuolar compartment (PVC). Furthermore, endogenous Rha1 also gave a punctate staining pattern and colocalized with AtPEP12p to the PVC. Mutations in the first and second GTP-binding motifs alter the localizations of GFP: Rha1[S24N] in the cytosol and Rha1[Q69L] in the tonoplast of the central vacuole. Also, mutations in the effector domain and the prenylation site inhibit membrane association of Rha1. Based on these results, we propose that Rha1 and Ara7 localize to the PVC and that GTP-binding motifs as well as the effector domain are important for localization of Rha1 to the PVC.

Bax-induced cell death of Arabidopsis is meditated through reactive oxygen-dependent and -independent processes

An Arabidopsis protoplast system was developed for dissecting plant cell death in individual cells. Bax, a mammalian pro-apoptotic member of the Bcl-2 family, induces apoptotic-like cell death in Arabidopsis. Bax accumulation in Arabidopsis mesophyll protoplasts expressing murine Bax cDNA from a glucocorticoid-inducible promoter results in cytological characteristics of apoptosis, namely DNA fragmentation, increased vacuolation, and loss of plasma membrane integrity. In vivo targeting analysis monitored using jellyfish green fluorescent protein (GFP) reporter indicated full-length Bax was localized to the mitochondria, as it does in animal cells. Deletion of the carboxyl-terminal transmembrane domain of Bax completely abolished targeting to mitochondria. Bax expression was followed by reactive oxygen species (ROS) accumulation. Treatment of protoplasts with the antioxidant N -acetyl- -cysteine (NAC) during induction of Bax expression strongly suppressed Bax-mediated ROS production and the cell death phenotype. However, some population of the ROS depleted cells still induced cell death, indicating that there is a process that Bax-mediated plant cell death is independent of ROS accumulation. Accordingly, suppression of Bax-mediated plant cell death also takes place in two different processes. Over-expression of a key redox-regulator, Arabidopsis nucleoside diphosphate kinase 2 (AtNDPK2) down-regulated ROS accumulation and suppressed Bax-mediated cell death and transient expression of Arabidopsis Bax inhibitor-1 (AtBI-1) substantially suppressed Bax-induced cell death without altering cellular ROS level. Taken together, our results collectively suggest that the Bax-mediated cell death and its suppression in plants is mediated by ROS-dependent and -independent processes.