Antigravitropic PIN polarization maintains non-vertical growth in lateral roots.

Lateral roots are typically maintained at non-vertical angles with respect to gravity. These gravitropic setpoint angles are intriguing because their maintenance requires that roots are able to effect growth response both with and against the gravity vector, a phenomenon previously attributed to gravitropism acting against an antigravitropic offset mechanism. Here we show how the components mediating gravitropism in the vertical primary root-PINs and phosphatases acting upon them-are reconfigured in their regulation such that lateral root growth at a range of angles can be maintained. We show that the ability of Arabidopsis lateral roots to bend both downward and upward requires the generation of auxin asymmetries and is driven by angle-dependent variation in downward gravitropic auxin flux acting against angle-independent upward, antigravitropic flux. Further, we demonstrate a symmetry in auxin distribution in lateral roots at gravitropic setpoint angle that can be traced back to a net, balanced polarization of PIN3 and PIN7 auxin transporters in the columella. These auxin fluxes are shifted by altering PIN protein phosphoregulation in the columella, either by introducing PIN3 phosphovariant versions or via manipulation of levels of the phosphatase subunit PP2A/RCN1. Finally, we show that auxin, in addition to driving lateral root directional growth, acts within the lateral root columella to induce more vertical growth by increasing RCN1 levels, causing a downward shift in PIN3 localization, thereby diminishing the magnitude of the upward, antigravitropic auxin flux.

Light-induced displacement of PLASTID MOVEMENT IMPAIRED1 precedes light-dependent chloroplast movements.

Light-dependent chloroplast movements are an actin-dependent cellular response to changes in the light environment that help plants maximize photosynthetic potential and reduce photodamage. Over a dozen proteins are known to be required for normal chloroplast movements, but the molecular mechanisms regulating the transformation of light perception into chloroplast motility are not fully understood. Here, we show that in Arabidopsis (Arabidopsis thaliana) the actin-bundling plasma membrane-associated proteins THRUMIN1, PLASTID MOVEMENT IMPAIRED1 (PMI1), and KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT1 (KAC1) interact through the 14-3-3 proteins KAPPA and OMEGA. We also show that the interaction of PMI1 with 14-3-3 KAPPA and OMEGA is regulated by blue light activation of the Phototropin2 photoreceptor. Live-cell confocal microscopy revealed light-induced dynamic changes in the cellular localizations of PMI1 and KAC1. In particular, PMI1 was relocated away from irradiated areas of the plasma membrane in less than a minute after blue light exposure, consistent with PMI1 playing a critical role in initiating light-dependent chloroplast movements. We present a modified conceptual model for high light-dependent chloroplast movements in which PMI1 acts as the mobile signal that initiates a coordinated sequence of changes in protein-protein and protein-plasma membrane interactions that initiate the chloroplast movement response and determine where in the cell chloroplasts are able to anchor to the plasma membrane.

Light-dependent phosphorylation of THRUMIN1 regulates its association with actin filaments and 14-3-3 proteins.

Light-dependent chloroplast movements in leaf cells contribute to the optimization of photosynthesis. Low-light conditions induce chloroplast accumulation along periclinal cell surfaces, providing greater access to available light, whereas high light induces movement of chloroplasts to anticlinal cell surfaces, providing photodamage protection and allowing more light to reach underlying cell layers. The THRUMIN1 protein is required for normal chloroplast movements in Arabidopsis (Arabidopsis thaliana) and has been shown to localize at the plasma membrane and to undergo rapid light-dependent interactions with actin filaments through the N-terminal intrinsically disordered region (IDR). A predicted WASP-Homology 2 domain was found in the IDR but mutations in this domain did not disrupt localization of THRUMIN1:YFP to actin filaments. A series of other protein truncations and site-directed mutations of known and putative phosphorylation sites indicated that a phosphomimetic mutation (serine to aspartic acid) at position 170 disrupted localization of THRUMIN1 to actin filaments. However, the phosphomimetic mutant rescued the thrumin1-2 mutant phenotype for chloroplast movement and raises questions about the role of THRUMIN1's interaction with actin. Mutation of serine 146 to aspartic acid also resulted in cytoplasmic localization of THRUMIN1:YFP in Nicotiana benthamiana. Mutations to a group of putative zinc-binding cysteine clusters implicate the C-terminus of THRUMIN1 in chloroplast movement. Phosphorylation-dependent association of THRUMIN1 with 14-3-3 KAPPA and OMEGA were also identified. Together, these studies provide insights into the mechanistic role of THRUMIN1 in light-dependent chloroplast movements.

Broadening the impact of plant science through innovative, integrative, and inclusive outreach.

Population growth and climate change will impact food security and potentially exacerbate the environmental toll that agriculture has taken on our planet. These existential concerns demand that a passionate, interdisciplinary, and diverse community of plant science professionals is trained during the 21st century. Furthermore, societal trends that question the importance of science and expert knowledge highlight the need to better communicate the value of rigorous fundamental scientific exploration. Engaging students and the general public in the wonder of plants, and science in general, requires renewed efforts that take advantage of advances in technology and new models of funding and knowledge dissemination. In November 2018, funded by the National Science Foundation through the Arabidopsis Research and Training for the 21st century (ART 21) research coordination network, a symposium and workshop were held that included a diverse panel of students, scientists, educators, and administrators from across the US. The purpose of the workshop was to re-envision how outreach programs are funded, evaluated, acknowledged, and shared within the plant science community. One key objective was to generate a roadmap for future efforts. We hope that this document will serve as such, by providing a comprehensive resource for students and young faculty interested in developing effective outreach programs. We also anticipate that this document will guide the formation of community partnerships to scale up currently successful outreach programs, and lead to the design of future programs that effectively engage with a more diverse student body and citizenry.

Translation initiation factor 3 families: what are their roles in regulating cyanobacterial and chloroplast gene expression?

Initiation is a key control point for the regulation of translation in prokaryotes and prokaryotic-like translation systems such as those in plant chloroplasts. Genome sequencing and biochemical studies are increasingly demonstrating differences in many aspects of translation between well-studied microbes such as Escherichia coli and lesser studied groups such as cyanobacteria. Analyses of chloroplast translation have revealed its prokaryotic origin but also uncovered many unique aspects that do not exist in E. coli. Recently, a novel form of posttranscriptional regulation by light color was discovered in the filamentous cyanobacterium Fremyella diplosiphon that requires a putative stem-loop and involves the use of two different prokaryotic translation initiation factor 3s (IF3s). Multiple (up to five) putative IF3s have now been found to be encoded in 22 % of sequenced cyanobacterial genomes and 26 % of plant nuclear genomes. The lack of similar light-color regulation of gene expression in most of these species suggests that IF3s play roles in regulating gene expression in response to other environmental and developmental cues. In the plant Arabidopsis, two nuclear-encoded IF3s have been shown to localize to the chloroplasts, and the mRNA levels encoding these vary significantly in certain organ and tissue types and during several phases of development. Collectively, the accumulated data suggest that in about one quarter of photosynthetic prokaryotes and eukaryotes, IF3 gene families are used to regulate gene expression in addition to their traditional roles in translation initiation. Models for how this might be accomplished in prokaryotes versus eukaryotic plastids are presented.

PLASTID MOVEMENT IMPAIRED1 mediates ABA sensitivity during germination and implicates ABA in light-mediated Chloroplast movements.

The plant hormone abscisic acid (ABA) controls many aspects of plant growth and development, including seed development, germination and responses to water-deficit stress. A complex ABA signaling network integrates environmental signals including water availability and light intensity and quality to fine-tune the response to a changing environment. To further define the regulatory pathways that control water-deficit and ABA responses, we carried out a gene-trap tagging screen for water-deficit-regulated genes in Arabidopsis thaliana. This screen identified PLASTID MOVEMENT IMPAIRED1 (PMI1), a gene involved in blue-light-induced chloroplast movement, as functioning in ABA-response pathways. We provide evidence that PMI1 is involved in the regulation of seed germination by ABA, acting upstream of the intersection between ABA and low-glucose signaling pathways. Furthermore, PMI1 participates in the regulation of ABA accumulation during periods of water deficit at the seedling stage. The combined phenotypes of pmi1 mutants in chloroplast movement and ABA responses indicate that ABA signaling may modulate chloroplast motility. This result was further supported by the detection of altered chloroplast movements in the ABA mutants aba1-6, aba2-1 and abi1-1.

Morphological and anatomical determinants of mesophyll conductance in wild relatives of tomato (Solanum sect. Lycopersicon, sect. Lycopersicoides; Solanaceae).

Natural selection on photosynthetic performance is a primary factor determining leaf phenotypes. The complex CO2 diffusion path from substomatal cavities to the chloroplasts - the mesophyll conductance (g(m)) - limits photosynthetic rate in many species and hence shapes variation in leaf morphology and anatomy. Among sclerophyllous and succulent taxa, structural investment in leaves, measured as the leaf dry mass per area (LMA), has been implicated in decreased gm . However, in herbaceous taxa with high g(m), it is less certain how LMA impacts CO2 diffusion and whether it significantly affects photosynthetic performance. We addressed these questions in the context of understanding the ecophysiological significance of leaf trait variation in wild tomatoes, a closely related group of herbaceous perennials. Although g(m) was high in wild tomatoes, variation in g(m) significantly affected photosynthesis. Even in these tender-leaved herbaceous species, greater LMA led to reduced g(m). This relationship between g(m) and LMA is partially mediated by cell packing and leaf thickness, although amphistomy (equal distribution of stomata on both sides of the leaf) mitigates the effect of leaf thickness. Understanding the costs of increased LMA will inform future work on the adaptive significance of leaf trait variation across ecological gradients in wild tomatoes and other systems.

Mutations in GERANYLGERANYL DIPHOSPHATE SYNTHASE 1 affect chloroplast development in Arabidopsis thaliana (Brassicaceae).

PREMISE OF THE STUDY: Within plastids, geranylgeranyl diphosphate synthase is a key enzyme in the isoprenoid biosynthetic pathway that catalyzes the formation of geranylgeranyl diphosphate, a precursor molecule to several biochemical pathways including those that lead into the biosynthesis of carotenoids and abscisic acid, prenyllipids such as the chlorophylls, and diterpenes such as gibberellic acid. • METHODS: We have identified mutants in the GERANYLGERANYL DIPHOSPHATE SYNTHASE 1 (GGPS1) gene, which encodes the major plastid-localized enzyme geranylgeranyl diphosphate synthase in Arabidopsis thaliana. • KEY RESULTS: Two T-DNA insertion mutant alleles (ggps1-2 and ggps1-3) were found to result in seedling-lethal albino and embryo-lethal phenotypes, respectively, indicating that GGPS1 is an essential gene. We also identified a temperature-sensitive leaf variegation mutant (ggps1-1) in A. thaliana that is caused by a point mutation. Total chlorophyll and carotenoid levels were reduced in ggps1-1 white tissues as compared with green tissues. Phenotypes typically associated with a reduction in gibberellic acid were not seen, suggesting that gibberellic acid biosynthesis is not noticeably altered in the mutant. In contrast to other variegated mutants, the ggps1-1 green sector photosynthetic rate was not elevated relative to wild-type tissues. Chloroplast development in green sectors of variegated leaves appeared normal, whereas cells in white sectors contained abnormal plastids with numerous electron translucent bodies and poorly developed internal membranes. • CONCLUSIONS: Our results indicate that GGPS1 is a key gene in the chlorophyll biosynthetic pathway.

D6PK AGCVIII kinases are required for auxin transport and phototropic hypocotyl bending in Arabidopsis.

Phototropic hypocotyl bending in response to blue light excitation is an important adaptive process that helps plants to optimize their exposure to light. In Arabidopsis thaliana, phototropic hypocotyl bending is initiated by the blue light receptors and protein kinases phototropin1 (phot1) and phot2. Phototropic responses also require auxin transport and were shown to be partially compromised in mutants of the PIN-FORMED (PIN) auxin efflux facilitators. We previously described the D6 PROTEIN KINASE (D6PK) subfamily of AGCVIII kinases, which we proposed to directly regulate PIN-mediated auxin transport. Here, we show that phototropic hypocotyl bending is strongly dependent on the activity of D6PKs and the PIN proteins PIN3, PIN4, and PIN7. While early blue light and phot-dependent signaling events are not affected by the loss of D6PKs, we detect a gradual loss of PIN3 phosphorylation in d6pk mutants of increasing complexity that is most severe in the d6pk d6pkl1 d6pkl2 d6pkl3 quadruple mutant. This is accompanied by a reduction of basipetal auxin transport in the hypocotyls of d6pk as well as in pin mutants. Based on our data, we propose that D6PK-dependent PIN regulation promotes auxin transport and that auxin transport in the hypocotyl is a prerequisite for phot1-dependent hypocotyl bending.

Chloroplast movement provides photoprotection to plants by redistributing PSII damage within leaves.

Plants use light to fix carbon through the process of photosynthesis but light also causes photoinhibition, by damaging photosystem II (PSII). Plants can usually adjust their rate of PSII repair to equal the rate of damage, but under stress conditions or supersaturating light-intensities damage may exceed the rate of repair. Light-induced chloroplast movements are one of the many mechanisms plants have evolved to minimize photoinhibition. We found that chloroplast movements achieve a measure of photoprotection to PSII by altering the distribution of photoinhibition through depth in leaves. When chloroplasts are in the low-light accumulation arrangement a greater proportion of PSII damage occurs near the illuminated surface than for leaves where the chloroplasts are in the high-light avoidance arrangement. According to our findings chloroplast movements can increase the overall efficiency of leaf photosynthesis in at least two ways. The movements alter light profiles within leaves to maximize photosynthetic output and at the same time redistribute PSII damage throughout the leaf to reduce the amount of inhibition received by individual chloroplasts and prevent a decrease in photosynthetic potential.

The role of a 14-3-3 protein in stomatal opening mediated by PHOT2 in Arabidopsis.

The 14-3-3 λ isoform is required for normal stomatal opening mediated by PHOT2 in Arabidopsis thaliana. Arabidopsis phototropin2 (PHOT2) interacts with the λ-isoform 14-3-3 protein both in yeast two-hybrid screening and in an in vitro pull-down assay. Further yeast two-hybrid analysis also showed that the PHOT2 C-terminal kinase domain was required for the interaction. Site-directed mutagenesis indicated that PHOT2 Ser-747 is essential for the yeast interaction. Phenotypic characterization of a loss-of-function 14-3-3 λ mutant in a phot1 mutant background showed that the 14-3-3 λ protein was necessary for normal PHOT2-mediated blue light-induced stomatal opening. PHOT2 Ser-747 was necessary for complementation of the blue light-activated stomatal response in a phot1 phot2 double mutant. The 14-3-3 λ mutant in the phot1 mutant background allowed normal phototropism and normal chloroplast accumulation and avoidance responses. It also showed normal stomatal opening mediated by PHOT1 in a phot2 mutant background. The 14-3-3 κ mutant had no effect on stomatal opening in response to blue light. Although the 14-3-3 λ mutant had no chloroplast movement phenotype, the 14-3-3 κ mutation caused a weaker avoidance response at an intermediate blue light intensity by altering the balance between the avoidance and accumulation responses. The results highlight the strict specificity of phototropin-mediated signal transduction pathways.

Changes in leaf optical properties associated with light-dependent chloroplast movements.

We surveyed 24 plant species to examine how leaf anatomy influenced chloroplast movement and how the optical properties of leaves change with chloroplast position. All species examined exhibited light-dependent chloroplast movements but the associated changes in leaf absorptance varied considerably in magnitude. Chloroplast movement-dependent changes in leaf absorptance were greatest in shade species, in which absorptance changes of >10% were observed between high- and low-light treatments. Using the Kubelka-Munk theory, we found that changes in the absorption (k) and chlorophyll a absorption efficiency (k*) associated with chloroplast movement correlated with cell diameter, such that the narrower, more columnar cells found in sun leaves restricted the ability of chloroplasts to move. The broader, more spherical cells of shade leaves allowed greater chloroplast rearrangements and in low-light conditions allowed efficient light capture. Across the species tested, light-dependent chloroplast movements modulated leaf optical properties and light absorption efficiency by manipulating the package (sieve or flattening) effect but not the detour (path lengthening) effect.

Intracellular invasion of green algae in a salamander host.

The association between embryos of the spotted salamander (Ambystoma maculatum) and green algae ("Oophila amblystomatis" Lamber ex Printz) has been considered an ectosymbiotic mutualism. We show here, however, that this symbiosis is more intimate than previously reported. A combination of imaging and algal 18S rDNA amplification reveals algal invasion of embryonic salamander tissues and cells during development. Algal cells are detectable from embryonic and larval Stages 26-44 through chlorophyll autofluorescence and algal 18S rDNA amplification. Algal cell ultrastructure indicates both degradation and putative encystment during the process of tissue and cellular invasion. Fewer algal cells were detected in later-stage larvae through FISH, suggesting that the decline in autofluorescent cells is primarily due to algal cell death within the host. However, early embryonic egg capsules also contained encysted algal cells on the inner capsule wall, and algal 18S rDNA was amplified from adult reproductive tracts, consistent with oviductal transmission of algae from one salamander generation to the next. The invasion of algae into salamander host tissues and cells represents a unique association between a vertebrate and a eukaryotic alga, with implications for research into cell-cell recognition, possible exchange of metabolites or DNA, and potential congruence between host and symbiont population structures.

THRUMIN1 is a light-regulated actin-bundling protein involved in chloroplast motility.

Chloroplast movement in response to changing light conditions optimizes photosynthetic light absorption. This repositioning is stimulated by blue light perceived via the phototropin photoreceptors and is transduced to the actin cytoskeleton. Some actin-based motility systems use filament reorganizations rather than myosin-based translocations. Recent research favors the hypothesis that chloroplast movement is driven by actin reorganization at the plasma membrane, but no proteins affecting chloroplast movements have been shown to associate with both the plasma membrane and actin filaments in vivo. Here we identified THRUMIN1 as a critical link between phototropin photoreceptor activity at the plasma membrane and actin-dependent chloroplast movements. THRUMIN1 bundles filamentous actin in vitro, and it localizes to the plasma membrane and displays light- and phototropin-dependent localization to microfilaments in vivo. These results suggest that phototropin-induced actin bundling via THRUMIN1 is important for chloroplast movement. A mammalian homolog of THRUMIN1, GRXCR1, has been implicated in auditory responses and hair cell stereocilla development as a regulator of actin architecture. Studies of THRUMIN1 will help elucidate the function of this family of eukaryotic proteins.

A mutation in Arabidopsis seedling plastid development1 affects plastid differentiation in embryo-derived tissues during seedling growth.

Oilseed plants like Arabidopsis (Arabidopsis thaliana) develop green photosynthetically active embryos. Upon seed maturation, the embryonic chloroplasts degenerate into a highly reduced plastid type called the eoplast. Upon germination, eoplasts redifferentiate into chloroplasts and other plastid types. Here, we describe seedling plastid development1 (spd1), an Arabidopsis seedling albino mutant capable of producing normal green vegetative tissues. Mutant seedlings also display defects in etioplast and amyloplast development. Precocious germination of spd1 embryos showed that the albino seedling phenotype of spd1 was dependent on the passage of developing embryos through the degreening and dehydration stages of seed maturation, suggesting that SPD1 is critical during eoplast development or early stages of eoplast redifferentiation. The SPD1 gene was found to encode a protein containing a putative chloroplast-targeting sequence in its amino terminus and also domains common to P-loop ATPases. Chloroplast localization of the SPD1 protein was confirmed by targeting assays in vivo and in vitro. Although the exact function of SPD1 remains to be defined, our findings reveal aspects of plastid development unique to embryo-derived cells.

Integration of Phot1, Phot2, and PhyB signalling in light-induced chloroplast movements.

In Arabidopsis thaliana, chloroplasts move towards the periclinal cell walls upon exposure to low blue light intensities and to anticlinal walls under high light. The regulation of these chloroplast movements involves members of both the phototropin and phytochrome families of photoreceptors. Examination of fluence-rate response dependencies in phot1 and phot2 mutants revealed that although both photoreceptors are capable of inducing chloroplast accumulation under low-light conditions, the signals from these photoreceptors appear to be antagonistic. Chloroplast movements in wild-type plants were intermediate between those of the single phot mutants, consistent with each operating through separate signalling cascades. Mutants in phot2 showed transient chloroplast avoidance responses upon exposure to intense blue light, and slow but sustained chloroplast avoidance under intense white light, indicating that in the absence of phot2, phot1 is capable of generating both a low and a high-light response signal. Mutations in phytochrome B (phyB) caused an enhanced avoidance response at intermediate and high light intensities. Examination of phyB, phot1phyB, and phot2phyB mutants indicated that this enhancement is caused by PhyB inhibition of the high-light avoidance response in wild-type plants. In addition, our results suggest that the inhibition by PhyB is not exclusive to either of the phot1 or phot2 signalling pathways.

A novel phototropic response to red light is revealed in microgravity.

The aim of this study was to investigate phototropism in plants grown in microgravity conditions without the complications of a 1-g environment. Experiments performed on the International Space Station (ISS) were used to explore the mechanisms of both blue-light- and red-light-induced phototropism in plants. This project utilized the European Modular Cultivation System (EMCS), which has environmental controls for plant growth as well as centrifuges for gravity treatments used as a 1-g control. Images captured from video tapes were used to analyze the growth, development, and curvature of Arabidopsis thaliana plants that developed from seed in space. A novel positive phototropic response to red light was observed in hypocotyls of seedlings that developed in microgravity. This response was not apparent in seedlings grown on Earth or in the 1-g control during the space flight. In addition, blue-light-based phototropism had a greater response in microgravity compared with the 1-g control. Although flowering plants are generally thought to lack red light phototropism, our data suggest that at least some flowering plants may have retained a red light sensory system for phototropism. Thus, this discovery may have important implications for understanding the evolution of light sensory systems in plants.

The Arabidopsis PHYTOCHROME KINASE SUBSTRATE2 protein is a phototropin signaling element that regulates leaf flattening and leaf positioning.

In Arabidopsis (Arabidopsis thaliana), the blue light photoreceptor phototropins (phot1 and phot2) fine-tune the photosynthetic status of the plant by controlling several important adaptive processes in response to environmental light variations. These processes include stem and petiole phototropism (leaf positioning), leaf flattening, stomatal opening, and chloroplast movements. The PHYTOCHROME KINASE SUBSTRATE (PKS) protein family comprises four members in Arabidopsis (PKS1-PKS4). PKS1 is a novel phot1 signaling element during phototropism, as it interacts with phot1 and the important signaling element NONPHOTOTROPIC HYPOCOTYL3 (NPH3) and is required for normal phot1-mediated phototropism. In this study, we have analyzed more globally the role of three PKS members (PKS1, PKS2, and PKS4). Systematic analysis of mutants reveals that PKS2 (and to a lesser extent PKS1) act in the same subset of phototropin-controlled responses as NPH3, namely leaf flattening and positioning. PKS1, PKS2, and NPH3 coimmunoprecipitate with both phot1-green fluorescent protein and phot2-green fluorescent protein in leaf extracts. Genetic experiments position PKS2 within phot1 and phot2 pathways controlling leaf positioning and leaf flattening, respectively. NPH3 can act in both phot1 and phot2 pathways, and synergistic interactions observed between pks2 and nph3 mutants suggest complementary roles of PKS2 and NPH3 during phototropin signaling. Finally, several observations further suggest that PKS2 may regulate leaf flattening and positioning by controlling auxin homeostasis. Together with previous findings, our results indicate that the PKS proteins represent an important family of phototropin signaling proteins.

The "sensational" power of movement in plants: A Darwinian system for studying the evolution of behavior.

Darwin's research on botany and plant physiology was a landmark attempt to integrate plant movements into a biological perspective of behavior. Since antiquity, people have sought to explain plant movements via mechanical or physiological forces, and yet they also constructed analogies between plant and animal behavior. During the Renaissance and Enlightenment, thinkers began to see that physiochemical explanations of plant movements could equally apply to animal behavior and even human thought. Darwin saw his research on plant movements as a strategic front against those who argued that his theory of evolution could not account for the acquisition of new behavioral traits. He believed that his research explained how the different forms of plant movement evolved as modified habits of circumnutation, and he presented evidence that plants might have a brain-like organ, which could have acquired various types of plant sensitivity during evolution. Upon publication of The Power of Movement in Plants, his ideas were overwhelmingly rejected by plant physiologists. Subsequently, plant biologists came to view the work as an important contribution to plant physiology and biology, but its intended contribution to the field of evolution and behavior has been largely overlooked.

Mutations in a plastid-localized elongation factor G alter early stages of plastid development in Arabidopsis thaliana.

BACKGROUND: Proper development of plastids in embryo and seedling tissues is critical for plant development. During germination, plastids develop to perform many critical functions that are necessary to establish the seedling for further growth. A growing body of work has demonstrated that components of the plastid transcription and translation machinery must be present and functional to establish the organelle upon germination. RESULTS: We have identified Arabidopsis thaliana mutants in a gene that encodes a plastid-targeted elongation factor G (SCO1) that is essential for plastid development during embryogenesis since two T-DNA insertion mutations in the coding sequence (sco1-2 and sco1-3) result in an embryo-lethal phenotype. In addition, a point mutation allele (sco1-1) and an allele with a T-DNA insertion in the promoter (sco1-4) of SCO1 display conditional seedling-lethal phenotypes. Seedlings of these alleles exhibit cotyledon and hypocotyl albinism due to improper chloroplast development, and normally die shortly after germination. However, when germinated on media supplemented with sucrose, the mutant plants can produce photosynthetically-active green leaves from the apical meristem. CONCLUSION: The developmental stage-specific phenotype of the conditional-lethal sco1 alleles reveals differences in chloroplast formation during seedling germination compared to chloroplast differentiation in cells derived from the shoot apical meristem. Our identification of embryo-lethal mutant alleles in the Arabidopsis elongation factor G indicates that SCO1 is essential for plant growth, consistent with its predicted role in chloroplast protein translation.