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Chloroplasts play an essential role in plant growth and development through manipulating photosynthesis and the production of hormones and metabolites. Although many genes or regulators involved in chloroplast biogenesis and development have been isolated and characterized, identification of novel components is still lacking. We isolated a rice (Oryza sativa) mutant, termed albino leaf 2 (al2), using genetic screening. Phenotypic analysis revealed that the al2 mutation caused obvious albino leaves at the early developmental stage, eventually leading to al2 seedling death. Electron microscopy investigations indicated that the chloroplast structure was disrupted in the al2 mutants at an early developmental stage and subsequently resulted in the breakdown of the entire chloroplast. Molecular cloning illustrated that AL2 encodes a chloroplast group IIA intron splicing facilitator (CRS1) in rice, which was confirmed by a genetic complementation experiment. Moreover, our results demonstrated that AL2 was constitutively expressed in various tissues, including green and non-green tissues. Interestingly, we found that the expression levels of a subset of chloroplast genes that contain group IIA and IIB introns were significantly reduced in the al2 mutant compared to that in the wild type, suggesting that AL2 is a functional CRS1 in rice. Differing from the orthologous CRS1 in maize and Arabidopsis that only regulates splicing of the chloroplast group II intron, our results demonstrated that the AL2 gene is also likely to be involved in the splicing of the chloroplast group I intron. They also showed that disruption of AL2 results in the altered expression of chloroplast-associated genes, including chlorophyll biosynthetic genes, plastid-encoded polymerases and nuclear-encoded chloroplast genes. Taken together, these findings shed new light on the function of nuclear-encoded chloroplast group I and II intron splicing factors in rice.

Chloroplast RNA metabolism is mediated by a multitude of nuclear encoded factors, many of which are highly specific for individual RNA processing events. In addition, a family of chloroplast ribonucleoproteins (cpRNPs) has been suspected to regulate larger sets of chloroplast transcripts. This together with their propensity for posttranslational modifications in response to external cues suggested a potential role of cpRNPs in the signal-dependent coregulation of chloroplast genes. We show here on a transcriptome-wide scale that the Arabidopsis thaliana cpRNPs CP31A and CP29A (for 31 kD and 29 kD chloroplast protein, respectively), associate with large, overlapping sets of chloroplast transcripts. We demonstrate that both proteins are essential for resistance of chloroplast development to cold stress. They are required to guarantee transcript stability of numerous mRNAs at low temperatures and under these conditions also support specific processing steps. Fine mapping of cpRNP—RNA interactions in vivo suggests multiple points of contact between these proteins and their RNA ligands. For CP31A, we demonstrate an essential function in stabilizing sense and antisense transcripts that span the border of the small single copy region and the inverted repeat of the chloroplast genome. CP31A associates with the common 3′-terminus of these RNAs and protects them against 3′-exonucleolytic activity.

Plastid-encoded RNA polymerase (PEP) plays an essential role in chloroplast development by governing the expression of genes involved in photosynthesis. At least 12 PEP-associated proteins (PAPs), including FSD3/PAP4, regulate PEP activity and chloroplast development by modulating formation of the PEP complex. In this study, we identified FSD3S, a splicing variant of FSD3; the FSD3 and FSD3S transcripts encode proteins with identical N-termini, but different C-termini. Characterization of FSD3 and FSD3S proteins showed that the C-terminal region of FSD3S contains a transmembrane domain, which promotes FSD3S localization to the chloroplast membrane but not to nucleoids, in contrast to FSD3, which localizes to the chloroplast nucleoid. We also found that overexpression of FSD3S negatively affects photosynthetic activity and chloroplast development by reducing expression of genes involved in photosynthesis. In addition, FSD3S failed to complement the chloroplast developmental defects in the fsd3 mutant. These results suggest FSD3 and FSD3S, with their distinct localization patterns, have different functions in chloroplast development, and FSD3S negatively regulates expression of PEP-dependent chloroplast genes, and development of chloroplasts.

The interface between plant organelles and non-biological nanostructures has the potential to impart organelles with new and enhanced functions. Here, we show that single-walled carbon nanotubes (SWNTs) passively transport and irreversibly localize within the lipid envelope of extracted plant chloroplasts, promote over three times higher photosynthetic activity than that of controls, and enhance maximum electron transport rates. The SWNT–chloroplast assemblies also enable higher rates of leaf electron transport in vivo through a mechanism consistent with augmented photoabsorption. Concentrations of reactive oxygen species inside extracted chloroplasts are significantly suppressed by delivering poly(acrylic acid)–nanoceria or SWNT–nanoceria complexes. Moreover, we show that SWNTs enable near-infrared fluorescence monitoring of nitric oxide both ex vivo and in vivo , thus demonstrating that a plant can be augmented to function as a photonic chemical sensor. Nanobionics engineering of plant function may contribute to the development of biomimetic materials for light-harvesting and biochemical detection with regenerative properties and enhanced efficiency. Imparting non-native functions to living plants using nanoparticles opens the possibility of creating synthetic materials that can grow and repair themselves using sunlight, water and carbon dioxide. It is now shown that, both in plant extracts and living leaves, carbon nanotubes traverse and localize within the lipid envelope of plant chloroplasts, enhance their photosynthetic activity, and enable near-infrared fluorescence monitoring of nitric oxide.

The chloroplast has a central position in oxygenic photosynthesis and primary metabolism. In addition to these functions, the chloroplast has recently emerged as a pivotal regulator of plant responses to abiotic and biotic stress conditions. Chloroplasts have their own independent genomes and gene-expression machinery and synthesize phytohormones and a diverse range of secondary metabolites, a significant portion of which contribute the plant response to adverse conditions. Furthermore, chloroplasts communicate with the nucleus through retrograde signaling, for instance, reactive oxygen signaling. All of the above facilitate the chloroplast’s exquisite flexibility in responding to environmental stresses. In this review, we summarize recent findings on the involvement of chloroplasts in plant regulatory responses to various abiotic and biotic stresses including heat, chilling, salinity, drought, high light environmental stress conditions, and pathogen invasions. This review will enrich the better understanding of interactions between chloroplast and environmental stresses, and will lay the foundation for genetically enhancing plant-stress acclimatization.

Chloroplasts and mitochondria are subcellular bioenergetic organelles with their own genomes and genetic systems. DNA replication and transmission to daughter organelles produces cytoplasmic inheritance of characters associated with primary events in photosynthesis and respiration. The prokaryotic ancestors of chloroplasts and mitochondria were endosymbionts whose genes became copied to the genomes of their cellular hosts. These copies gave rise to nuclear chromosomal genes that encode cytosolic proteins and precursor proteins that are synthesized in the cytosol for import into the organelle into which the endosymbiont evolved. What accounts for the retention of genes for the complete synthesis within chloroplasts and mitochondria of a tiny minority of their protein subunits? One hypothesis is that expression of genes for protein subunits of energy-transducing enzymes must respond to physical environmental change by means of a direct and unconditional regulatory control—control exerted by change in the redox state of the corresponding gene product. This hypothesis proposes that, to preserve function, an entire redox regulatory system has to be retained within its original membrane-bound compartment. Colocation of gene and gene product for redox regulation of gene expression (CoRR) is a hypothesis in agreement with the results of a variety of experiments designed to test it and which seem to have no other satisfactory explanation. Here, I review evidence relating to CoRR and discuss its development, conclusions, and implications. This overview also identifies predictions concerning the results of experiments that may yet prove the hypothesis to be incorrect.

Increasing leaf transpiration efficiency (TE) may provide leads for growing rice like dryland cereals such as wheat (Triticum aestivum). To explore avenues for improving TE in rice, variations in stomatal conductance (gs ) and mesophyll conductance (gm ) and their anatomical determinants were evaluated in two cultivars from each of lowland, aerobic, and upland groups of Oryza sativa, one cultivar of O. glaberrima, and two cultivars of T. aestivum, under three water regimes. The TE of upland rice, O. glaberrima, and wheat was more responsive to the gm/gs ratio than that of lowland and aerobic rice. Overall, the explanatory power of the particular anatomical trait varied among species. Low stomatal density mostly explained the low gs in drought-tolerant rice, whereas rice genotypes with smaller stomata generally responded more strongly to drought. Compared with rice, wheat had a higher gm , which was associated with thicker mesophyll tissue, mesophyll and chloroplasts more exposed to intercellular spaces, and thinner cell walls. Upland rice, O. glaberrima, and wheat cultivars minimized the decrease in gm under drought by maintaining high ratios of chloroplasts to exposed mesophyll cell walls. Rice TE could be improved by increasing the gm/gs ratio via modifying anatomical traits.

Plastids, the defining organelles of plant cells, undergo physiological and morphological changes to fulfill distinct biological functions. In particular, the differentiation of chloroplasts into chromoplasts results in an enhanced storage capacity for carotenoids with industrial and nutritional value such as beta-carotene (provitamin A). Here, we show that synthetically inducing a burst in the production of phytoene, the first committed intermediate of the carotenoid pathway, elicits an artificial chloroplast-to-chromoplast differentiation in leaves. Phytoene overproduction initially interferes with photosynthesis, acting as a metabolic threshold switch mechanism that weakens chloroplast identity. In a second stage, phytoene conversion into downstream carotenoids is required for the differentiation of chromoplasts, a process that involves a concurrent reprogramming of nuclear gene expression and plastid morphology for improved carotenoid storage. We hence demonstrate that loss of photosynthetic competence and enhanced production of carotenoids are not just consequences but requirements for chloroplasts to differentiate into chromoplasts.

ClpP4 is a nuclear-encoded plastid protein that functions as a proteolytic subunit of the ATP-dependent Clp protease of higher plants. Given the lack of viable clpP4 knockout mutants, antisense clpP4 repression lines were prepared to study the functional importance of ClpP4 in Arabidopsis thaliana. Screening of transformants revealed viable lines with up to 90% loss of wild type levels of ClpP4 protein, while those with > 90% were severely bleached and strongly retarded in vegetative growth, failing to reach reproductive maturity. Of the viable antisense plants, repression of clpP4 expression produced a pleiotropic phenotype, of which slow growth and leaf variegation were most prominent. Chlorosis was most severe in younger leaves, with the affected regions localized around the mid-vein and exhibiting impaired chloroplast development and mesophyll cell differentiation. Chlorosis lessened during leaf expansion until all had regained the wild type appearance upon maturity. This change in phenotype correlated with the developmental expression of ClpP4 in the wild type, in which ClpP4 was less abundant in mature leaves due to post-transcriptional/translational regulation. Repression of ClpP4 caused a concomitant down-regulation of other nuclear-encoded ClpP paralogs in the antisense lines, but no change in other chloroplast-localized Clp proteins. Greening of the young chlorotic antisense plants upon maturation was accelerated by increased light, either by longer photoperiod or by higher growth irradiance; conditions that both raised levels of ClpP4 in wild type leaves. In contrast, shift to low growth irradiance decreased the relative amount of ClpP4 in wild type leaves, and caused newly developed leaves of fully greened antisense lines to regain the chlorotic phenotype.

Leaves are beautifully specialized organs designed to maximize the use of light and CO₂ for photosynthesis. Engineering leaf anatomy therefore holds great potential to enhance photosynthetic capacity. Here we review the effect of the dominant leaf anatomical traits on leaf photosynthesis and confirm that a high chloroplast surface area exposed to intercellular airspace per unit leaf area (S c) is critical for efficient photosynthesis. The possibility of improving S c through appropriately increasing mesophyll cell density is further analyzed. The potential influences of modifying mesophyll cell morphology on CO₂ diffusion, light distribution within the leaf, and other physiological processes are also discussed. Some potential target genes regulating leaf mesophyll cell proliferation and expansion are explored. Indeed, more comprehensive research is needed to understand how manipulating mesophyll cell morphology through editing the potential target genes impacts leaf photosynthetic capacity and related physiological processes. This will pinpoint the targets for engineering leaf anatomy to maximize photosynthetic capacity.