Enlarged meristems and delayed growth in plp mutants result from lack of CaaX prenyltransferases

euAP2a, a key gene that regulates flowering time in peach (Prunus persica) by modulating thermo-responsive transcription programming

Frequent spring frost damage threatens temperate fruit production, and breeding of late-flowering cultivars is an effective strategy for preventing such damage. However, this effort is often hampered by the lack of specific genes and markers and a lack of understanding of the mechanisms. We examined a Late-Flowering Peach (LFP) germplasm and found that its floral buds require a longer chilling period to release from their dormancy and a longer warming period to bloom than the control cultivar, two key characteristics associated with flowering time. We discovered that a 983-bp deletion in euAP2a, an APETALA2 (AP2)-related gene with known roles in regulating floral organ identity and flowering time, was primarily responsible for late flowering in LFP. This deletion disrupts an miR172 binding site, resulting in a gain-of-function mutation in euAP2a. Transcriptomic analyses revealed that at different stages of floral development, two chilling-responsive modules and four warm-responsive modules, comprising approximately 600 genes, were sequentially activated, forming a unique transcription programming. Furthermore, we found that euAP2a was transiently downregulated during the activation of these thermal-responsive modules at various stages. However, the loss of such transient, stage-specific downregulation of euAP2a caused by the deletion of miR172 binding sites resulted in the deactivation or delay of these modules in the LFP flower buds, suggesting that euAP2a acts as a transcription repressor to control floral developmental pace in peaches by modulating the thermo-responsive transcription programming. The findings shed light on the mechanisms behind late flowering in deciduous fruit trees, which is instrumental for breeding frost-tolerant cultivars.

Methyl-Jasmonate Functions as a Molecular Switch Promoting Cross-Talk between Pathways for the Biosynthesis of Isoprenoid Backbones Used to Modify Proteins in Plants

In plants, the plastidial mevalonate (MVA)-independent pathway is required for the modification with geranylgeranyl groups of CaaL-motif proteins, which are substrates of protein geranylgeranyltransferase type-I (PGGT-I). As a consequence, fosmidomycin, a specific inhibitor of 1-deoxy-d-xylulose (DX)-5 phosphate reductoisomerase/DXR, the second enzyme in this so-called methylerythritol phosphate (MEP) pathway, also acts as an effective inhibitor of protein prenylation. This can be visualized in plant cells by confocal microscopy by expressing GFP-CaM-CVIL, a prenylation sensor protein. After treatment with fosmidomycin, the plasma membrane localization of this GFP-based sensor is altered, and a nuclear distribution of fluorescence is observed instead. In tobacco cells, a visual screen of conditions allowing membrane localization in the presence of fosmidomycin identified jasmonic acid methyl esther (MeJA) as a chemical capable of gradually overcoming inhibition. Using Arabidopsis protein prenyltransferase loss-of-function mutant lines expressing GFP-CaM-CVIL proteins, we demonstrated that in the presence of MeJA, protein farnesyltransferase (PFT) can modify the GFP-CaM-CVIL sensor, a substrate the enzyme does not recognize under standard conditions. Similar to MeJA, farnesol and MVA also alter the protein substrate specificity of PFT, whereas DX and geranylgeraniol have limited or no effect. Our data suggest that MeJA adjusts the protein substrate specificity of PFT by promoting a metabolic cross-talk directing the origin of the prenyl group used to modify the protein. MVA, or an MVA-derived metabolite, appears to be a key metabolic intermediate for this change in substrate specificity.

BnPLP1 Positively Regulates Flowering Time, Plant Height, and Main Inflorescence Length in Brassica napus

Protein prenylation mediated by the Arabidopsis thaliana PLURIPETALA (AtPLP) gene plays a crucial role in plant growth, development, and environmental response by adding a 15-carbon farnesyl group or one to two 20-carbon geranylgeranyl groups onto one to two cysteine residues at the C-terminus of the target protein. However, the homologous genes and their functions of AtPLP in rapeseed are unclear. In this study, bioinformatics analysis and gene cloning demonstrated the existence of two homologous genes of AtPLP in the Brassica napus L. genome, namely, BnPLP1 and BnPLP2. Evolutionary analysis revealed that BnPLP1 originated from the B. rapa L. genome, while BnPLP2 originated from the B. oleracea L. genome. Genetic transformation analysis revealed that the overexpression of BnPLP1 in Arabidopsis plants exhibited earlier flowering initiation, a prolonged flowering period, increased plant height, and longer main inflorescence length compared to the wild type. Contrarily, the downregulation of BnPLP1 expression in B. napus plants led to delayed flowering initiation, shortened flowering period, decreased plant height, and reduced main inflorescence length compared to the wild type. These findings indicate that the BnPLP1 gene positively regulates flowering time, plant height, and main inflorescence length. This provides a new gene for the genetic improvement of flowering time and plant architecture in rapeseed.

The cellular function of ROP GTPase prenylation is important for multicellularity in the moss Physcomitrium patens

A complete picture of how signaling pathways lead to multicellularity is largely unknown. Previously, we generated mutations in a protein prenylation enzyme, GGB, and showed that it is essential for maintaining multicellularity in the moss Physcomitrium patens. Here, we show that ROP GTPases act as downstream factors that are prenylated by GGB and themselves play an important role in the multicellularity of P. patens. We also show that the loss of multicellularity caused by the suppression of GGB or ROP GTPases is due to uncoordinated cell expansion, defects in cell wall integrity and the disturbance of the directional control of cell plate orientation. Expressing prenylatable ROP in the ggb mutant not only rescues multicellularity in protonemata but also results in development of gametophores. Although the prenylation of ROP is important for multicellularity, a higher threshold of active ROP is required for gametophore development. Thus, our results suggest that ROP activation via prenylation by GGB is a key process at both cell and tissue levels, facilitating the developmental transition from one dimension to two dimensions and to three dimensions in P. patens.

Protein Farnesylation Takes Part in Arabidopsis Seed Development

Protein farnesylation is a post-translational modification regulated by the ERA1 (Enhanced Response to ABA 1) gene encoding the β-subunit of the protein farnesyltransferase in Arabidopsis. The era1 mutants have been described for over two decades and exhibit severe pleiotropic phenotypes, affecting vegetative and flower development. We further investigated the development and quality of era1 seeds. While the era1 ovary contains numerous ovules, the plant produces fewer seeds but larger and heavier, with higher protein contents and a modified fatty acid distribution. Furthermore, era1 pollen grains show lower germination rates and, at flower opening, the pistils are immature and the ovules require one additional day to complete the embryo sac. Hand pollinated flowers confirmed that pollination is a major obstacle to era1 seed phenotypes, and a near wild-type seed morphology was thus restored. Still, era1 seeds conserved peculiar storage protein contents and altered fatty acid distributions. The multiplicity of era1 phenotypes reflects the diversity of proteins targeted by the farnesyltransferase. Our work highlights the involvement of protein farnesylation in seed development and in the control of traits of agronomic interest.

CRISPR/Cas9-targeted mutagenesis of OsERA1 confers enhanced responses to abscisic acid and drought stress and increased primary root growth under nonstressed conditions in rice

Abscisic acid (ABA) signaling components play an important role in the drought stress response in plants. Arabidopsis thaliana ENHANCED RESPONSE TO ABA1 (ERA1) encodes the β-subunit of farnesyltransferase and regulates ABA signaling and the dehydration response. Therefore, ERA1 is an important candidate gene for enhancing drought tolerance in numerous crops. However, a rice (Oryza sativa) ERA1 homolog has not been characterized previously. Here, we show that rice osera1 mutant lines, harboring CRISPR/Cas9-induced frameshift mutations, exhibit similar leaf growth as control plants but increased primary root growth. The osera1 mutant lines also display increased sensitivity to ABA and an enhanced response to drought stress through stomatal regulation. These results illustrate that OsERA1 is a negative regulator of primary root growth under nonstressed conditions and also of responses to ABA and drought stress in rice. These findings improve our understanding of the role of ABA signaling in the drought stress response in rice and suggest a strategy to genetically improve rice.

Targeting and signaling of Rho of plants guanosine triphosphatases require synergistic interaction between guanine nucleotide inhibitor and vesicular trafficking

Most eukaryotic cells are polarized. Common toolbox regulating cell polarization includes Rho guanosine triphosphatases (GTPases), in which spatiotemporal activation is regulated by a plethora of regulators. Rho of plants (ROPs) are the only Rho GTPases in plants. Although vesicular trafficking was hinted in the regulation of ROPs, it was unclear where vesicle-carried ROP starts, whether it is dynamically regulated, and which components participate in vesicle-mediated ROP targeting. In addition, although vesicle trafficking and guanine nucleotide inhibitor (GDI) pathways in Rho signaling have been extensively studied in yeast, it is unknown whether the two pathways interplay. Unclear are also cellular and developmental consequences of their interaction in multicellular organisms. Here, we show that the dynamic targeting of ROP through vesicles requires coat protein complex II and ADP-ribosylation factor 1-mediated post-Golgi trafficking. Trafficking of vesicle-carried ROPs between the plasma membrane and the trans-Golgi network is mediated through adaptor protein 1 and sterol-mediated endocytosis. Finally, we show that GDI and vesicle trafficking synergistically regulate cell polarization and ROP targeting, suggesting that the establishment and maintenance of cell polarity is regulated by an evolutionarily conserved mechanism.

Protein Prenylation in Plant Stress Responses

Protein prenylation is one of the most important posttranslational modifications of proteins. Prenylated proteins play important roles in different developmental processes as well as stress responses in plants as the addition of hydrophobic prenyl chains (mostly farnesyl or geranyl) allow otherwise hydrophilic proteins to operate as peripheral lipid membrane proteins. This review focuses on selected aspects connecting protein prenylation with plant responses to both abiotic and biotic stresses. It summarizes how changes in protein prenylation impact plant growth, deals with several families of proteins involved in stress response and highlights prominent regulatory importance of prenylated small GTPases and chaperons. Potential possibilities of these proteins to be applicable for biotechnologies are discussed.

How plants grow up

A plant's lateral structures, such as leaves, branches and flowers, literally hinge on the shoot axis, making its integrity and growth fundamental to plant form. In all plants, subapical proliferation within the shoot tip displaces cells downward to extrude the cylindrical stem. Following the transition to flowering, many plants show extensive axial elongation associated with increased subapical proliferation and expansion. However, the cereal grasses also elongate their stems, called culms, due to activity within detached intercalary meristems which displaces cells upward, elevating the grain-bearing inflorescence. Variation in culm length within species is especially relevant to cereal crops, as demonstrated by the high-yielding semi-dwarfed cereals of the Green Revolution. Although previously understudied, recent renewed interest the regulation of subapical and intercalary growth suggests that control of cell division planes, boundary formation and temporal dynamics of differentiation, are likely critical mechanisms coordinating axial growth and development in plants.

ROP GTPases Structure-Function and Signaling Pathways

Interactions between receptor like kinases and guanyl nucleotide exchange factors together with identification of effector proteins reveal putative ROP GTPases signaling cascades.

Quantitative Trait Transcripts Mapping Coupled with Expression Quantitative Trait Loci Mapping Reveal the Molecular Network Regulating the Apetalous Characteristic in Brassica napus L

The apetalous trait of rapeseed (Brassica napus, AACC, 2n = 38) is important for breeding an ideal high-yield rapeseed with superior klendusity to Sclerotinia sclerotiorum. Currently, the molecular mechanism underlying the apetalous trait of rapeseed is unclear. In this study, 14 petal regulators genes were chosen as target genes (TGs), and the expression patterns of the 14 TGs in the AH population, containing 189 recombinant inbred lines derived from a cross between apetalous "APL01" and normal "Holly," were analyzed in two environments using qRT-PCR. Phenotypic data of petalous degree (PDgr) in the AH population were obtained from the two environments. Both quantitative trait transcript (QTT)-association mapping and expression QTL (eQTL) analyses of TGs expression levels were performed to reveal regulatory relationships among TGs and PDgr. QTT mapping for PDgr determined that PLURIPETALA (PLP) was the major negative QTT associated with PDgr in both environments, suggesting that PLP negatively regulates the petal development of line "APL01." The QTT mapping of PLP expression levels showed that CHROMATIN-REMODELING PROTEIN 11 (CHR11) was positively associated with PLP expression, indicating that CHR11 acts as a positive regulator of PLP expression. Similarly, QTT mapping for the remaining TGs identified 38 QTTs, associated with 13 TGs, and 31 QTTs, associated with 10 TGs, respectively, in the first and second environments. Additionally, eQTL analyses of TG expression levels showed that 12 and 11 unconditional eQTLs were detected in the first and second environment, respectively. Based on the QTTs and unconditional eQTLs detected, we presented a hypothetical molecular regulatory network in which 14 petal regulators potentially regulated the apetalous trait in "APL01" through the CHR11-PLP pathway. PLP acts directly as the terminal signal integrator negatively regulating petal development in the CHR11-PLP pathway. These findings will aid in the understanding the molecular mechanism underlying the apetalous trait of rapeseed.

Heat-shock protein 40 is the key farnesylation target in meristem size control, abscisic acid signaling, and drought resistance

In this study, Barghetti et al. investigate the functions of protein farnesylation in plants. They show that defective farnesylation of a single factor—heat-shock protein 40 (HSP40), encoded by the J2 and J3 genes—is sufficient to confer ABA hypersensitivity, drought resistance, late flowering, and enlarged meristems, indicating that altered function of chaperone client proteins underlies most farnesyl transferase mutant phenotypes.

Protein farnesylation is central to molecular cell biology. In plants, protein farnesyl transferase mutants are pleiotropic and exhibit defective meristem organization, hypersensitivity to the hormone abscisic acid, and increased drought resistance. The precise functions of protein farnesylation in plants remain incompletely understood because few relevant farnesylated targets have been identified. Here, we show that defective farnesylation of a single factor—heat-shock protein 40 (HSP40), encoded by the J2 and J3 genes—is sufficient to confer ABA hypersensitivity, drought resistance, late flowering, and enlarged meristems, indicating that altered function of chaperone client proteins underlies most farnesyl transferase mutant phenotypes. We also show that expression of an abiotic stress-related microRNA (miRNA) regulon controlled by the transcription factor SPL7 requires HSP40 farnesylation. Expression of a truncated SPL7 form mimicking its activated proteolysis fragment of the membrane-bound SPL7 precursor partially restores accumulation of SPL7-dependent miRNAs in farnesyl transferase mutants. These results implicate the pathway directing SPL7 activation from its membrane-bound precursor as an important target of farnesylated HSP40, consistent with our demonstration that HSP40 farnesylation facilitates its membrane association. The results also suggest that altered gene regulation via select miRNAs contributes to abiotic stress-related phenotypes of farnesyl transferase mutants.

Fats and function: protein lipid modifications in plant cell signalling

The post-translational lipid modifications N-myristoylation, prenylation and S-acylation are traditionally associated with increasing protein membrane affinity and localisation. However this is an over-simplification, with evidence now implicating these modifications in a variety of roles such as membrane microdomain partitioning, protein trafficking, protein complex assembly and polarity maintenance. Evidence for a regulatory role is also emerging, with changes or manipulation of lipid modifications offering a means of directly controlling various aspects of protein function. Proteomics advances have revealed an enrichment of signalling proteins in the lipid-modified proteome, potentially indicating an important role for these modifications in responding to stimuli. This review highlights some of the key themes and possible functions of lipid modification during signalling processes in plants.

Farnesylation-mediated subcellular localization is required for CYP85A2 function

Protein farnesylation refers to the addition of a 15-carbon farnesyl isoprenoid to the cysteine residue of the CaaX motif at the carboxy terminus of target proteins. In spite of its known roles in plant development and abiotic stress tolerance, how these processes are precisely regulated by farnesylation had remained elusive. We recently showed that CYP85A2, the cytochrome P450, which converts castasterone to brassinolide in the last step of brassinosteroid synthesis must be farnesylated in order to function in this pathway. Lack of either CYP85A2 or the farnesylation motif of CYP85A2 resulted in reduced brassinolide accumulation, hypersensitivity to ABA, and increased plant drought tolerance. In this study, we have assessed the influence of the N-terminal secretory signal and the C-terminal CaaX motif of CYP85A2 in mediating CYP85A2 function and targeting to endomembrane compartments. We show that CaaX motif could still target CYPA85A2 in the absence of an intact N-terminal secretory signal to the respective membrane compartments and partially rescue cyp85a2-2 phenotypes. However, in the absence of both the CaaX motif and the secretory signal, CYP85A2 is not targeted to the membranes and becomes unstable.

Virus-induced down-regulation of GmERA1A and GmERA1B genes enhances the stomatal response to abscisic acid and drought resistance in soybean

Drought is a major threat to global soybean production. The limited transformation potential and polyploid nature of soybean have hindered functional analysis of soybean genes. Previous research has implicated farnesylation in the plant's response to abscisic acid (ABA) and drought tolerance. We therefore used virus-induced gene silencing (VIGS) to evaluate farnesyltransferase genes, GmERA1A and GmERA1B (Glycine max Enhanced Response to ABA1-A and -B), as potential targets for increasing drought resistance in soybean. Apple latent spherical virus (ALSV)-mediated GmERA1-down-regulated soybean leaves displayed an enhanced stomatal response to ABA and reduced water loss and wilting under dehydration conditions, suggesting that GmERA1A and GmERA1B negatively regulate ABA signaling in soybean guard cells. The findings provide evidence that the ALSV-VIGS system, which bypasses the need to generate transgenic plants, is a useful tool for analyzing gene function using only a single down-regulated leaf. Thus, the ALSV-VIGS system could constitute part of a next-generation molecular breeding pipeline to accelerate drought resistance breeding in soybean.

Arabidopsis PROTEIN S-ACYL TRANSFERASE4 mediates root hair growth

Polar growth of root hairs is critical for plant survival and requires fine-tuned Rho of plants (ROP) signaling. Multiple ROP regulators participate in root hair growth. However, protein S-acyl transferases (PATs), mediating the S-acylation and membrane partitioning of ROPs, are yet to be found. Using a reverse genetic approach, combining fluorescence probes, pharmacological drugs, site-directed mutagenesis and genetic analysis with related root-hair mutants, we have identified and characterized an Arabidopsis PAT, which may be responsible for ROP2 S-acylation in root hairs. Specifically, functional loss of PAT4 resulted in reduced root hair elongation, which was rescued by a wild-type but not an enzyme-inactive PAT4. Membrane-associated ROP2 was significantly reduced in pat4, similar to S-acylation-deficient ROP2 in the wild type. We further showed that PAT4 and SCN1, a ROP regulator, additively mediate the stability and targeting of ROP2. The results presented here indicate that PAT4-mediated S-acylation mediates the membrane association of ROP2 at the root hair apex and provide novel insights into dynamic ROP signaling during plant tip growth.

The Interplay between Carbon Availability and Growth in Different Zones of the Growing Maize Leaf

Plants assimilate carbon in their photosynthetic tissues in the light. However, carbon is required during the night and in nonphotosynthetic organs. It is therefore essential that plants manage their carbon resources spatially and temporally and coordinate growth with carbon availability. In growing maize (Zea mays) leaf blades, a defined developmental gradient facilitates analyses in the cell division, elongation, and mature zones. We investigated the responses of the metabolome and transcriptome and polysome loading, as a qualitative proxy for protein synthesis, at dusk, dawn, and 6, 14, and 24 h into an extended night, and tracked whole-leaf elongation over this time course. Starch and sugars are depleted by dawn in the mature zone, but only after an extension of the night in the elongation and division zones. Sucrose (Suc) recovers partially between 14 and 24 h into the extended night in the growth zones, but not the mature zone. The global metabolome and transcriptome track these zone-specific changes in Suc. Leaf elongation and polysome loading in the growth zones also remain high at dawn, decrease between 6 and 14 h into the extended night, and then partially recover, indicating that growth processes are determined by local carbon status. The level of Suc-signaling metabolite trehalose-6-phosphate, and the trehalose-6-phosphate:Suc ratio are much higher in growth than mature zones at dusk and dawn but fall in the extended night. Candidate genes were identified by searching for transcripts that show characteristic temporal response patterns or contrasting responses to carbon starvation in growth and mature zones.

PLURIPETALA mediates ROP2 localization and stability in parallel to SCN1 but synergistically with TIP1 in root hairs

Prenylation, the post-translational attachment of prenyl groups to substrate proteins, can affect their distribution and interactomes. Arabidopsis PLURIPETALA (PLP) encodes the shared α subunit of two heterodimeric protein isoprenyltransferases, whose functional loss provides a unique opportunity to study developmental and cellular processes mediated by its prenylated substrates, such as ROP GTPases. As molecular switches, the distribution and activation of ROPs are mediated by various factors, including guanine nucleotide exchange factors, GTPase activating proteins, guanine nucleotide dissociation inhibitors (RhoGDIs), prenylation, and S-acylation. However, how these factors together ensure that dynamic ROP signalling is still obscure. We report here that a loss-of-function allele of PLP resulted in cytoplasmic accumulation of ROP2 in root hairs and reduced its stability. Consequently, two downstream events of ROP signalling, i.e. actin microfilament (MF) organization and the production of reactive oxygen species (ROS), were compromised. Genetic, cytological and biochemical evidence supports an additive interaction between prenylation and RhoGDI1/SCN1 in ROP2 distribution and stability whereas PLP acts synergistically with the protein S-acyl transferase TIP GROWTH DEFECTIVE1 during root hair growth. By using root hair growth as a model system, we uncovered complex interactions among prenylation, RhoGDIs, and S-acylation in dynamic ROP signalling.

The Early-Acting Peroxin PEX19 Is Redundantly Encoded, Farnesylated, and Essential for Viability in Arabidopsis thaliana

Peroxisomes are single-membrane bound organelles that are essential for normal development in plants and animals. In mammals and yeast, the peroxin (PEX) proteins PEX3 and PEX19 facilitate the early steps of peroxisome membrane protein (PMP) insertion and pre-peroxisome budding from the endoplasmic reticulum. The PEX3 membrane protein acts as a docking site for PEX19, a cytosolic chaperone for PMPs that delivers PMPs to the endoplasmic reticulum or peroxisomal membrane. PEX19 is farnesylated in yeast and mammals, and we used immunoblotting with prenylation mutants to show that PEX19 also is fully farnesylated in wild-type Arabidopsis thaliana plants. We examined insertional alleles disrupting either of the two Arabidopsis PEX19 isoforms, PEX19A or PEX19B, and detected similar levels of PEX19 protein in the pex19a-1 mutant and wild type; however, PEX19 protein was nearly undetectable in the pex19b-1 mutant. Despite the reduction in PEX19 levels in pex19b-1, both pex19a-1 and pex19b-1 single mutants lacked notable peroxisomal β-oxidation defects and displayed normal levels and localization of peroxisomal matrix and membrane proteins. The pex19a-1 pex19b-1 double mutant was embryo lethal, indicating a redundantly encoded critical role for PEX19 during embryogenesis. Expressing YFP-tagged versions of either PEX19 isoform rescued this lethality, confirming that PEX19A and PEX19B act redundantly in Arabidopsis. We observed that pex19b-1 enhanced peroxisome-related defects of a subset of peroxin-defective mutants, supporting a role for PEX19 in peroxisome function. Together, our data indicate that Arabidopsis PEX19 promotes peroxisome function and is essential for viability.

ASG2 is a farnesylated DWD protein that acts as ABA negative regulator in Arabidopsis

The tagging-via-substrate approach designed for the capture of mammal prenylated proteins was adapted to Arabidopsis cell culture. In this way, proteins are in vivo tagged with an azide-modified farnesyl moiety and captured thanks to biotin alkyne Click-iT® chemistry with further streptavidin-affinity chromatography. Mass spectrometry analyses identified four small GTPases and ASG2 (ALTERED SEED GERMINATION 2), a protein previously associated to the seed germination gene network. ASG2 is a conserved protein in plants and displays a unique feature that associates WD40 domains and tetratricopeptide repeats. Additionally, we show that ASG2 has a C-terminal CaaX-box that is farnesylated in vitro. Protoplast transfections using CaaX prenyltransferase mutants show that farnesylation provokes ASG2 nucleus exclusion. Moreover, ASG2 interacts with DDB1 (DAMAGE DNA BINDING protein 1), and the subcellular localization of this complex depends on ASG2 farnesylation status. Finally, germination and root elongation experiments reveal that asg2 and the farnesyltransferase mutant era1 (ENHANCED RESPONSE TO ABSCISIC ACID (ABA) 1) behave in similar manners when exposed to ABA or salt stress. To our knowledge, ASG2 is the first farnesylated DWD (DDB1 binding WD40) protein related to ABA response in Arabidopsis that may be linked to era1 phenotypes.

Arabidopsis Rab Geranylgeranyltransferases Demonstrate Redundancy and Broad Substrate Specificity in Vitro

Posttranslational lipid modifications mediate the membrane attachment of Rab GTPases, facilitating their function in regulating intracellular vesicular trafficking. In Arabidopsis, most Rab GTPases have two C-terminal cysteines and potentially can be double-geranylgeranylated by heterodimeric Rab geranylgeranyltransferases (Rab-GGTs). Genes encoding two putative α subunits and two putative β subunits of Rab-GGTs have been annotated in the Arabidopsis thaliana genome, but little is known about Rab-GGT activity in Arabidopsis. In this study, we demonstrate that four different heterodimers can be formed between putative Arabidopsis Rab-GGT α subunits RGTA1/RGTA2 and β subunits RGTB1/RGTB2, but only RGTA1·RGTB1 and RGTA1·RGTB2 exhibit bona fide Rab-GGT activity, and they are biochemically redundant in vitro. We hypothesize that RGTA2 function might be disrupted by a 12-amino acid insertion in a conserved motif. We present evidence that Arabidopsis Rab-GGTs may have preference for prenylation of C-terminal cysteines in particular positions. We also demonstrate that Arabidopsis Rab-GGTs can not only prenylate a great variety of Rab GTPases in the presence of Rab escort protein but, unlike Rab-GGT in yeast and mammals, can also prenylate certain non-Rab GTPases independently of Rab escort protein. Our findings may help to explain some of the phenotypes of Arabidopsis protein prenyltransferase mutants.

The ERECTA, CLAVATA and class III HD-ZIP Pathways Display Synergistic Interactions in Regulating Floral Meristem Activities

In angiosperms, the production of flowers marks the beginning of the reproductive phase. At the emergence of flower primordia on the flanks of the inflorescence meristem, the WUSCHEL (WUS) gene, which encodes a homeodomain transcription factor starts to be expressed and establishes de novo stem cell population, founder of the floral meristem (FM). Similarly to the shoot apical meristem a precise spatial and temporal expression pattern of WUS is required and maintained through strict regulation by multiple regulatory inputs to maintain stem cell homeostasis. However, following the formation of a genetically determined fixed number of floral organs, this homeostasis is shifted towards organogenesis and the FM is terminated. In here we performed a genetic study to test how a reduction in ERECTA, CLAVATA and class III HD-ZIP pathways affects floral meristem activity and flower development. We revealed strong synergistic phenotypes of extra flower number, supernumerary whorls, total loss of determinacy and extreme enlargement of the meristem as compared to any double mutant combination indicating that the three pathways, CLV3, ER and HD-ZIPIII distinctively regulate meristem activity and that they act in parallel. Our findings yield several new insights into stem cell-driven development. We demonstrate the crucial requirement for coupling floral meristem termination with carpel formation to ensure successful reproduction in plants. We also show how regulation of meristem size and alternation in spatial structure of the meristem serve as a mechanism to determine flower organogenesis. We propose that the loss of FM determinacy due to the reduction in CLV3, ER and HD-ZIPIII activity is genetically separable from the AGAMOUS core mechanism of meristem termination.

Protein lipid modifications and the regulation of ROP GTPase function

In eukaryotes, the RHO superfamily of small G-proteins is implicated in the regulation of cell polarity and growth. Rho of Plants (ROPs)/RACs are plant-specific Rho family proteins that have been shown to regulate cell polarity, auxin transport and responses, ABA signalling, and response to pathogens. A hallmark of ROP/RAC function is their localization in specific plasma membrane domains. This short review focuses on the mechanisms responsible for membrane interactions of ROPs/RACs and how they affect ROP/RAC function.

The importance of lipid modified proteins in plants

Membranes have long been known to act as more than physical barriers within and between plant cells. Trafficking of membrane proteins, signalling from and across membranes, organisation of membranes and transport through membranes are all essential processes for plant cellular function. These processes rely on a myriad array of proteins regulated in a variety of manners and are frequently required to be directly associated with membranes. For integral membrane proteins, the mode of membrane association is readily apparent, but many peripherally associated membrane proteins are outwardly soluble proteins. In these cases the proteins are frequently modified by the addition of lipids allowing direct interaction with the hydrophobic core of membranes. These modifications include N-myristoylation, S-acylation (palmitoylation), prenylation and GPI anchors but until recently little was truly known about their function in plants. New data suggest that these modifications are able to act as more than just membrane anchors, and dynamic S-acylation in particular is emerging as a means of regulating protein function in a similar manner to phosphorylation. This review discusses how these modifications occur, their impact on protein function, how they are regulated, recent advances in the field and technical approaches for studying these modifications.

Prenylation is required for polar cell elongation, cell adhesion, and differentiation in Physcomitrella patens

Protein prenylation is required for a variety of growth and developmental processes in flowering plants. Here we report the consequences of loss of function of all known prenylation subunits in the moss Physcomitrella patens. As in Arabidopsis, protein farnesyltransferase and protein geranylgeranyltransferase type I are not required for viability. However, protein geranylgeranyltransferase type I activity is required for cell adhesion, polar cell elongation, and cell differentiation. Loss of protein geranylgeranyltransferase activity results in colonies of round, single-celled organisms that resemble unicellular algae. The loss of protein farnesylation is not as severe but also results in polar cell elongation and differentiation defects. The complete loss of Rab geranylgeranyltransferase activity appears to be lethal in P. patens. Labeling with antibodies to cell wall components support the lack of polarity establishment and the undifferentiated state of geranylgeranyltransferase type I mutant plants. Our results show that prenylated proteins play key roles in P. patens development and differentiation processes.

S-carvone suppresses cellulase-induced capsidiol production in Nicotiana tabacum by interfering with protein isoprenylation

S-Carvone has been described as a negative regulator of mevalonic acid (MVA) production by interfering with 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMGR) activity, a key player in isoprenoid biosynthesis. The impact of this monoterpene on the production of capsidiol in Nicotiana tabacum, an assumed MVA-derived sesquiterpenoid phytoalexin produced in response to elicitation by cellulase, was investigated. As expected, capsidiol production, as well as early stages of elicitation such as hydrogen peroxide production or stimulation of 5-epi-aristolochene synthase activity, were repressed. Despite the lack of capsidiol synthesis, apparent HMGR activity was boosted. Feeding experiments using (1-13C)Glc followed by analysis of labeling patterns by 13C-NMR, confirmed an MVA-dependent biosynthesis; however, treatments with fosmidomycin, an inhibitor of the MVA-independent 2-C-methyl-D-erythritol 4-phosphate (MEP) isoprenoid pathway, unexpectedly down-regulated the biosynthesis of this sesquiterpene as well. We postulated that S-carvone does not directly inhibit the production of MVA by inactivating HMGR, but possibly targets an MEP-derived isoprenoid involved in the early steps of the elicitation process. A new model is proposed in which the monoterpene blocks an MEP pathway-dependent protein geranylgeranylation necessary for the signaling cascade. The production of capsidiol was inhibited when plants were treated with some inhibitors of protein prenylation or by further monoterpenes. Moreover, S-carvone hindered isoprenylation of a prenylable GFP indicator protein expressed in N. tabacum cell lines, which can be chemically complemented with geranylgeraniol. The model was further validated using N. tabacum cell extracts or recombinant N. tabacum protein prenyltransferases expressed in Escherichia coli. Our study endorsed a reevaluation of the effect of S-carvone on plant isoprenoid metabolism.

The role of lipid post-translational modification in plant developmental processes

Most eukaryotic proteins are post-translationally modified, and modification has profound effects on protein function. One key modification is the attachment of a lipid group to certain amino acids; this typically facilitates subcellular targeting (association with a membrane) and protein-protein interactions (by virtue of the large hydrophobic moiety). Most widely recognized are lipid modifications of proteins involved in developmental signaling, but proteins with structural roles are also lipid-modified. The three known types of intracellular protein lipid modifications are S-acylation, N-myristoylation, and prenylation. In plants, genetic analysis of the enzymes involved, along with molecular analysis of select target proteins, has recently shed light on the roles of lipid modification in key developmental processes, such as meristem function, flower development, polar cell elongation, cell differentiation, and hormone responses. In addition, while lipid post-translational mechanisms are generally conserved among eukaryotes, plants differ in the nature and function of target proteins, the effects of lipid modification on target proteins, and the roles of lipid modification in developmental processes.

Turning moss into algae: prenylation targets in Physcomitrella patens

Prenylation is a series of lipid posttranslational modifications that are involved in several key aspects of plant development. We recently knocked out every prenylation subunit in Physcomitrella patens. Like in Arabidopsis, knockout of protein farnesyltransferase and protein geranylgeranyltransferase in P. patens does not result in lethality; however, effects on development are extensive. In particular, the knockout of protein geranylgeranyltransferase results in small unicellular plants that resemble algae. Here we perform an analysis of predicted geranylgeranyltransferase target proteins in P. patens, and draw attention to those most likely to play a role in the knockout phenotype.

In vitro prenylation assay of Arabidopsis proteins

Protein prenylation, like other lipid posttranslational modifications of eukaryotic proteins, plays important roles in protein-membrane association and protein-protein interactions. In Arabidopsis, hundreds of proteins involved in a great variety of biological processes are potential prenylation substrates that need to be verified, including heterotrimeric G proteins and most Rop and Rab small GTPases. Also, genetic evidence suggests substrate cross-specificity among different prenyltransferases and/or the existence of unidentified prenylation players. In this chapter we describe a direct and flexible in vitro enzymatic assay designed for testing prenylation activity and substrate specificity in vitro. This protocol takes Arabidopsis Rab-GGT as example and starts with preparation of purified protein components of the reaction, followed by reconstitution of the prenylation reaction in vitro, and autoradiographic detection for qualitative and semiquantitative analysis.

Assaying protein S-acylation in plants

S-acylation is increasingly being recognized as an important posttranslational modification of proteins controlling activity, subcellular localization, microdomain residence, and stability. Heterotrimeric G-proteins and GPCRs are particularly well studied S-acylated proteins, and fast, cheap, reliable methods are required for the analysis of S-acylation states of these proteins. Various approaches have been developed to study S-acylation, but they are time consuming, expensive, frequently require radiolabels and generally only suitable for cell culture, making them impractical for work in plant systems. Here a rapid and inexpensive method is described for the analysis of the S-acylation state of AGG2 that can be performed on any cell or tissue sample using standard laboratory equipment and methods. This method is also applicable to any protein that can be detected by western blotting.

Protein interaction network of Arabidopsis thaliana female gametophyte development identifies novel proteins and relations

Although the female gametophyte in angiosperms consists of just seven cells, it has a complex biological network. In this study, female gametophyte microarray data from Arabidopsis thaliana were integrated into the Arabidopsis interactome database to generate a putative interaction map of the female gametophyte development including proteome map based on biological processes and molecular functions of proteins. Biological and functional groups as well as topological characteristics of the network were investigated by analyzing phytohormones, plant defense, cell death, transporters, regulatory factors, and hydrolases. This approach led to the prediction of critical members and bottlenecks of the network. Seventy-four and 24 upregulated genes as well as 171 and 3 downregulated genes were identified in subtracted networks based on biological processes and molecular function respectively, including novel genes such as the pathogenesis-related protein 4, ER type Ca(2+) ATPase 3, dihydroflavonol reductase, and ATP disulfate isomerase. Biologically important relationships between genes, critical nodes, and new essential proteins such as AT1G26830, AT5G20850, CYP74A, AT1G42396, PR4 and MEA were found in the interactome's network. The positions of novel genes, both upregulated and downregulated, and their relationships with biological pathways, in particular phytohormones, were highlighted in this study.

RNAi-mediated disruption of squalene synthase improves drought tolerance and yield in rice

About one-third of the world's rice area is in rain-fed lowlands and most are prone to water shortage. The identification of genes imparting tolerance to drought in the model cereal plant, rice, is an attractive strategy to engineer improved drought tolerance not only rice but other cereals as well. It is demonstrated that RNAi-mediated disruption of a rice farnesyltransferase/squalene synthase (SQS) by maize squalene synthase improves drought tolerance at both the vegetative and reproductive stages. Twenty-day-old seedlings of wild type (Nipponbare) and seven independent events of transgenic RNAi lines showed no difference in morphology. When subjected to water stress for a period of 32 d under growth chamber conditions, transgenic positives showed delayed wilting, conserved more soil water, and improved recovery. When five independent events along with wild-type plants were subjected to drought at the reproductive stage under greenhouse conditions, the transgenic plants lost water more slowly compared with the wild type, through reduced stomatal conductance and the retention of high leaf relative water content (RWC). After 28 d of slow progressive soil drying, transgenic plants recovered better and flowered earlier than wild-type plants. The yield of water-stressed transgenic positive plants ranged from 14-39% higher than wild-type plants. When grown in plates with Yoshida's nutrient solution with 1.2% agar, transgenic positives from three independent events showed increased root length and an enhanced number of lateral roots. The RNAi-mediated inactivation produced reduced stomatal conductance and subsequent drought tolerance.

Transcriptome analysis reveals coordinated spatiotemporal regulation of hemoglobin and nitrate reductase in response to nitrate in maize roots

Given the importance of nitrogen for plant growth and the environmental costs of intense fertilization, an understanding of the molecular mechanisms underlying the root adaptation to nitrogen fluctuations is a primary goal for the development of biotechnological tools for sustainable agriculture. This research aimed to identify the molecular factors involved in the response of maize roots to nitrate. cDNA-amplified fragment length polymorphism was exploited for comprehensive transcript profiling of maize (Zea mays) seedling roots grown with varied nitrate availabilities; 336 primer combinations were tested and 661 differentially regulated transcripts were identified. The expression of selected genes was studied in depth through quantitative real-time polymerase chain reaction and in situ hybridization. Over 50% of the genes identified responded to prolonged nitrate starvation and a few were identified as putatively involved in the early nitrate signaling mechanisms. Real-time results and in situ localization analyses demonstrated co-regulated transcriptional patterns in root epidermal cells for genes putatively involved in nitric oxide synthesis/scavenging. Our findings, in addition to strengthening already known mechanisms, revealed the existence of a new complex signaling framework in which brassinosteroids (BRI1), the module MKK2-MAPK6 and the fine regulation of nitric oxide homeostasis via the co-expression of synthetic (nitrate reductase) and scavenging (hemoglobin) components may play key functions in maize responses to nitrate.

Roles for farnesol and ABA in Arabidopsis flower development

The Arabidopsis FOLK (At5g58560) gene encodes farnesol kinase, which phosphorylates farnesol to farnesyl phosphate. Loss-of-function mutations in the FOLK gene are associated with enhanced sensitivity to abscisic acid (ABA), suggesting that FOLK negatively regulates ABA signaling. Moreover, folk flowers develop supernumerary carpels under water stress, providing evidence for a molecular link between farnesol metabolism, abiotic stress signaling and flower development. Here, we show that farnesol increases ABA sensitivity and that ABA affects flower development in Arabidopsis.

How prenylation and S-acylation regulate subcellular targeting and function of ROP GTPases

Rho of Plants (ROP) small G proteins function at discrete domains of the plasma and possibly endo membranes. ROPs are synthesized as soluble proteins and their attachment to membranes and partitioning in membrane microdomains are facilitated by the posttranslational lipid modifications prenylation and/or S-acylation. Based on their amino acid sequences, ROPs can be classified into two major subgroups: type-I ROPs terminate with a canonical CaaX box motif and are prenylated primarily by geranylgeranyltransferase-I (GGT-I) and to a lesser extent by farnesyltransferase (FT). Type-II ROPs terminate with a plant specific GC-CG box domain and are attached to the plasma membrane by stable S-acylation. In addition, type-I and possibly also type-II ROPs undergo activation dependent transient S-acylation in the G-domain and consequent partitioning into lipid rafts. Surprisingly, although geranylgeranylation is required for the membrane attachment of type-I ROPs and the γ subunits of heterotrimeric G proteins, Arabidopsis mutants lacking GGT-I function have a mild phenotype compared to wild type plants. The mild phenotype of the ggt-I mutants suggested that farnesylation by FT may compensate for the loss of GGT-I function and that possibly the prenylated type-I and S-acylated type-II ROPS have some overlapping functions. In a paper recently published in Plant Physiology we examined the role of the prenyl group type in type-I ROP function and membrane interaction dynamics and the functional redundancy between type-I and type-II ROPs. This study complements a second paper in which we examined the role of G-domain transient S-acylation in the membrane interaction dynamics and signaling by type-I ROPs. Together these two studies provide a framework for realizing the role of prenylation and S-acylation in subcellular targeting, membrane interaction dynamics and signaling by ROP GTPases.

Farnesol kinase is involved in farnesol metabolism, ABA signaling and flower development in Arabidopsis

Farnesol, which is toxic to plant cells at high concentrations, is sequentially phosphorylated to farnesyl phosphate and farnesyl diphosphate. However, the genes responsible for the sequential phosphorylation of farnesol have not been identified and the physiological role of farnesol phosphorylation has not been fully elucidated. To address these questions, we confirmed the presence of farnesol kinase activity in Arabidopsis (Arabidopsis thaliana) membranes and identified the corresponding gene (At5g58560, FOLK). Heterologous expression in recombinant yeast cells established farnesol as the preferred substrate of the FOLK-encoded kinase. Moreover, loss-of-function mutations in the FOLK gene abolished farnesol kinase activity, caused an abscisic acid-hypersensitive phenotype and promoted the development of supernumerary carpels under water-stress conditions. In wild-type plants, exogenous abscisic acid repressed FOLK gene expression. These observations demonstrate a role for farnesol kinase in negative regulation of abscisic acid signaling, and provide molecular evidence for a link between farnesol metabolism, abiotic stress signaling and flower development.

RAC/ROP GTPases and auxin signaling

Auxin functions as a key morphogen in regulating plant growth and development. Studies on auxin-regulated gene expression and on the mechanism of polar auxin transport and its asymmetric distribution within tissues have provided the basis for realizing the molecular mechanisms underlying auxin function. In eukaryotes, members of the Ras and Rho subfamilies of the Ras superfamily of small GTPases function as molecular switches in many signaling cascades that regulate growth and development. Plants do not have Ras proteins, but they contain Rho-like small G proteins called RACs or ROPs that, like fungal and metazoan Rhos, are regulators of cell polarity and may also undertake some Ras functions. Here, we discuss the advances made over the last decade that implicate RAC/ROPs as mediators for auxin-regulated gene expression, rapid cell surface-located auxin signaling, and directional auxin transport. We also describe experimental data indicating that auxin-RAC/ROP crosstalk may form regulatory feedback loops and theoretical modeling that attempts to connect local auxin gradients with RAC/ROP regulation of cell polarity. We hope that by discussing these experimental and modeling studies, this perspective will stimulate efforts to further refine our understanding of auxin signaling via the RAC/ROP molecular switch.

Differential effects of prenylation and s-acylation on type I and II ROPS membrane interaction and function

Prenylation primarily by geranylgeranylation is required for membrane attachment and function of type I Rho of Plants (ROPs) and Gγ proteins, while type II ROPs are attached to the plasma membrane by S-acylation. Yet, it is not known how prenylation affects ROP membrane interaction dynamics and what are the functional redundancy and specificity of type I and type II ROPs. Here, we have used the expression of ROPs in mammalian cells together with geranylgeranylation and CaaX prenylation-deficient mutants to answer these questions. Our results show that the mechanism of type II ROP S-acylation and membrane attachment is unique to plants and likely responsible for the viability of plants in the absence of CaaX prenylation activity. The prenylation of ROPs determines their steady-state distribution between the plasma membrane and the cytosol but has little effect on membrane interaction dynamics. In addition, the prenyl group type has only minor effects on ROP function. Phenotypic analysis of the CaaX prenylation-deficient pluripetala mutant epidermal cells revealed that type I ROPs affect cell structure primarily on the adaxial side, while type II ROPs are functional and induce a novel cell division phenotype in this genetic background. Taken together, our studies show how prenyl and S-acyl lipid modifications affect ROP subcellular distribution, membrane interaction dynamics, and function.

Identification of a novel abscisic acid-regulated farnesol dehydrogenase from Arabidopsis

In Arabidopsis (Arabidopsis thaliana), farnesylcysteine is oxidized to farnesal and cysteine by a membrane-associated thioether oxidase called farnesylcysteine lyase. Farnesol and farnesyl phosphate kinases have also been reported in plant membranes. Together, these observations suggest the existence of enzymes that catalyze the interconversion of farnesal and farnesol. In this report, Arabidopsis membranes are shown to possess farnesol dehydrogenase activity. In addition, a gene on chromosome 4 of the Arabidopsis genome (At4g33360), called FLDH, is shown to encode an NAD(+)-dependent dehydrogenase that oxidizes farnesol more efficiently than other prenyl alcohol substrates. FLDH expression is repressed by abscisic acid (ABA) but is increased in mutants with T-DNA insertions in the FLDH 5' flanking region. These T-DNA insertion mutants, called fldh-1 and fldh-2, are associated with an ABA-insensitive phenotype, suggesting that FLDH is a negative regulator of ABA signaling.

Characterization, sub-cellular localization and expression profiling of the isoprenylcysteine methylesterase gene family in Arabidopsis thaliana

BACKGROUND

Isoprenylcysteine methylesterases (ICME) demethylate prenylated protein in eukaryotic cell. Until now, knowledge about their molecular information, localization and expression pattern is largely unavailable in plant species. One ICME in Arabidopsis, encoded by At5g15860, has been identified recently. Over-expression of At5g15860 caused an ABA hypersensitive phenotype in transgenic Arabidopsis plants, indicating that it functions as a positive regulator of ABA signaling. Moreover, ABA induced the expression of this gene in Arabidopsis seedlings. The current study extends these findings by examining the sub-cellular localization, expression profiling, and physiological functions of ICME and two other ICME-like proteins, ICME-LIKE1 and ICME-LIKE2, which were encoded by two related genes At1g26120 and At3g02410, respectively.

RESULTS

Bioinformatics investigations showed that the ICME and other two ICME-like homologs comprise a small subfamily of carboxylesterase (EC 3.1.1.1) in Arabidopsis. Sub-cellular localization of GFP tagged ICME and its homologs showed that the ICME and ICME-like proteins are intramembrane proteins predominantly localizing in the endoplasmic reticulum (ER) and Golgi apparatus. Semi-quantitative and real-time quantitative PCR revealed that the ICME and ICME-like genes are expressed in all examined tissues, including roots, rosette leaves, cauline leaves, stems, flowers, and siliques, with differential expression levels. Within the gene family, the base transcript abundance of ICME-LIKE2 gene is very low with higher expression in reproductive organs (flowers and siliques). Time-course analysis uncovered that both ICME and ICME-like genes are up-regulated by mannitol, NaCl and ABA treatment, with ICME showing the highest level of up-regulation by these treatments. Heat stress resulted in up-regulation of the ICME gene significantly but down-regulation of the ICME-LIKE1 and ICME-LIKE2 genes. Cold and dehydration stimuli led to no significant change of both ICME and ICME-like gene expression. Mutant icme-like2-1 showed increased sensitivity to ABA but slightly decreased sensitivity to salt and osmotic stresses during seed germination.

CONCLUSIONS

It is concluded that the ICME family is involved in stress and ABA signaling in Arabidopsis, probably through mediating the process of demethylating prenylated proteins. Identification of these prenylated proteins will help to better understand the significance of protein prenylation in Planta.

The CaaX specificities of Arabidopsis protein prenyltransferases explain era1 and ggb phenotypes

BACKGROUND

Protein prenylation is a common post-translational modification in metazoans, protozoans, fungi, and plants. This modification, which mediates protein-membrane and protein-protein interactions, is characterized by the covalent attachment of a fifteen-carbon farnesyl or twenty-carbon geranylgeranyl group to the cysteine residue of a carboxyl terminal CaaX motif. In Arabidopsis, era1 mutants lacking protein farnesyltransferase exhibit enlarged meristems, supernumerary floral organs, an enhanced response to abscisic acid (ABA), and drought tolerance. In contrast, ggb mutants lacking protein geranylgeranyltransferase type 1 exhibit subtle changes in ABA and auxin responsiveness, but develop normally.

RESULTS

We have expressed recombinant Arabidopsis protein farnesyltransferase (PFT) and protein geranylgeranyltransferase type 1 (PGGT1) in E. coli and characterized purified enzymes with respect to kinetic constants and substrate specificities. Our results indicate that, whereas PFT exhibits little specificity for the terminal amino acid of the CaaX motif, PGGT1 exclusively prenylates CaaX proteins with a leucine in the terminal position. Moreover, we found that different substrates exhibit similar K(m) but different k(cat) values in the presence of PFT and PGGT1, indicating that substrate specificities are determined primarily by reactivity rather than binding affinity.

CONCLUSIONS

The data presented here potentially explain the relatively strong phenotype of era1 mutants and weak phenotype of ggb mutants. Specifically, the substrate specificities of PFT and PGGT1 suggest that PFT can compensate for loss of PGGT1 in ggb mutants more effectively than PGGT1 can compensate for loss of PFT in era1 mutants. Moreover, our results indicate that PFT and PGGT1 substrate specificities are primarily due to differences in catalysis, rather than differences in substrate binding.

Arabidopsis RAB geranylgeranyl transferase beta-subunit mutant is constitutively photomorphogenic, and has shoot growth and gravitropic defects

RAB GTPases are important directional regulators of intracellular vesicle transport. Membrane localization of RAB GTPases is mediated by C-terminal double geranylgeranylation. This post-translational modification is catalyzed by the alpha-beta-heterodimer catalytic core of RAB geranylgeranyl transferase (RAB-GGT), which cooperates with the RAB escort protein (REP) that presents a nascent RAB. Here, we show that RAB-geranylgeranylation activity is significantly reduced in two homozygous mutants of the major Arabidopsis beta-subunit of RAB-GGT (AtRGTB1), resulting in unprenylated RAB GTPases accumulation in the cytoplasm. Both endocytosis and exocytosis are downregulated in rgtb1 homozygotes defective in shoot growth and morphogenesis. Root gravitropism is normal in rgtb1 roots, but is significantly compromised in shoots. Mutants are defective in etiolation and show constitutive photomorphogenic phenotypes that cannot be rescued by brassinosteroid treatment, similarly to the det3 mutant that is also defective in the secretory pathway. Transcriptomic analysis revealed an upregulation of specific RAB GTPases in etiolated wild-type plants. Taken together, these data suggest that the downregulation of the secretory pathway is interpreted as a photomorphogenic signal in Arabidopsis.

Farnesylcysteine lyase is involved in negative regulation of abscisic acid signaling in Arabidopsis

The Arabidopsis FCLY gene encodes a specific farnesylcysteine (FC) lyase, which is responsible for the oxidative metabolism of FC to farnesal and cysteine. In addition, fcly mutants with quantitative decreases in FC lyase activity exhibit an enhanced response to ABA. However, the enzymological properties of the FCLY-encoded enzyme and its precise role in ABA signaling remain unclear. Here, we show that recombinant Arabidopsis FC lyase expressed in insect cells exhibits high selectivity for FC as a substrate and requires FAD and molecular oxygen for activity. Arabidopsis FC lyase is also shown to undergo post-translational N-glycosylation. FC, which is a competitive inhibitor of isoprenylcysteine methyltransferase (ICMT), accumulates in fcly mutants. Moreover, the enhanced response of fcly mutants to ABA is reversed by ICMT overexpression. These observations support the hypothesis that the ABA hypersensitive phenotype of fcly plants is the result of FC accumulation and inhibition of ICMT.

Protein isoprenylation: the fat of the matter

Protein isoprenylation refers to the covalent attachment of a 15-carbon farnesyl or 20-carbon geranylgeranyl moiety to a cysteine residue at or near the carboxyl terminus. This post-translational lipid modification, which mediates protein-membrane and protein-protein interactions, is necessary for normal control of abscisic acid and auxin signaling, meristem development, and other fundamental processes. Recent studies have also revealed roles for protein isoprenylation in cytokinin biosynthesis and innate immunity. Most isoprenylated proteins are further modified by carboxyl terminal proteolysis and methylation and, collectively, these modifications are necessary for the targeting and function of isoprenylated proteins.

Proteins prenylated by type I protein geranylgeranyltransferase act positively on the jasmonate signalling pathway triggering the biosynthesis of monoterpene indole alkaloids in Catharanthus roseus

In Catharanthus roseus, the first step of monoterpenoid indole alkaloids (MIA) biosynthesis results from the condensation of the indole precursor tryptamine with the terpenoid precursor secologanin. Secologanin biosynthesis requires two successive biosynthetic pathways, the plastidial methyl-D: -erythritol 4-phosphate (MEP) pathway and the monoterpene secoiridoid pathway. In C. roseus cell culture, the expression of several genes encoding enzymes of these two pathways is dramatically down-regulated by auxin, while strongly enhanced by cytokinin and methyl-jasmonate. Furthermore, our previous studies have shown that protein prenylation events are also involved in the transcriptional activation of some of these genes. In the present work, we investigate the involvement of protein prenylation in the jasmonate signalling pathway leading to MIA biosynthesis. Inhibition of protein prenyltransferase down-regulates the methyl-jasmonate-induced expression of MEP and monoterpene secoiridoid pathway genes and thus abolishes MIA biosynthesis. Jointly, it also inhibits the methyl-jasmonate-induced expression of the AP2/ERF transcription factor ORCA3 that acts as a central regulator of MIA biosynthesis. Finally, a specific silencing of protein prenyltransferases mediated by RNA interference in C. roseus cells shows that inhibition of type I protein geranylgeranyltransferase (PGGT-I) down-regulates the methyl-jasmonate-induced expression of ORCA3, suggesting that PGGT-I prenylated proteins are part of the early steps of jasmonate signalling leading to MIA biosynthesis.

The plastidial 2-C-methyl-D-erythritol 4-phosphate pathway provides the isoprenyl moiety for protein geranylgeranylation in tobacco BY-2 cells

Protein farnesylation and geranylgeranylation are important posttranslational modifications in eukaryotic cells. We visualized in transformed Nicotiana tabacum Bright Yellow-2 (BY-2) cells the geranylgeranylation and plasma membrane localization of GFP-BD-CVIL, which consists of green fluorescent protein (GFP) fused to the C-terminal polybasic domain (BD) and CVIL isoprenylation motif from the Oryza sativa calmodulin, CaM61. Treatment with fosmidomycin (Fos) or oxoclomazone (OC), inhibitors of the plastidial 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, caused mislocalization of the protein to the nucleus, whereas treatment with mevinolin, an inhibitor of the cytosolic mevalonate pathway, did not. The nuclear localization of GFP-BD-CVIL in the presence of MEP pathway inhibitors was completely reversed by all-trans-geranylgeraniol (GGol). Furthermore, 1-deoxy-d-xylulose (DX) reversed the effects of OC, but not Fos, consistent with the hypothesis that OC blocks 1-deoxy-d-xylulose 5-phosphate synthesis, whereas Fos inhibits its conversion to 2-C-methyl-d-erythritol 4-phosphate. By contrast, GGol and DX did not rescue the nuclear mislocalization of GFP-BD-CVIL in the presence of a protein geranylgeranyltransferase type 1 inhibitor. Thus, the MEP pathway has an essential role in geranylgeranyl diphosphate (GGPP) biosynthesis and protein geranylgeranylation in BY-2 cells. GFP-BD-CVIL is a versatile tool for identifying pharmaceuticals and herbicides that interfere either with GGPP biosynthesis or with protein geranylgeranylation.