Green-revertible Chlorina 1 (grc1) is required for the biosynthesis of chlorophyll and the early development of chloroplasts in rice

The nuclear genes involved in chloroplast development and chlorophyll biosynthesis must be investigated to understand their functions in plant growth and development. In this study, we isolated and identified a unique leaf-color mutant of rice with a green-yellow phenotype before the four-leaf stage and named the mutation green-revertible chlorina 1 (grc1). The mutants had significantly lower plant height, number of tillers, and panicle length and headed significantly earlier than the wild type. The levels of chlorophylls, carotenoids, and chlorophyll precursors were also lower. The mutation in grc1 affected chloroplast ultrastructure, particularly thylakoid development. Genetic analysis indicated that the green-yellow phenotype was controlled by a single recessive gene. We mapped the grc1 gene to a 32.4-kb region on the long arm of chromosome 6. Through map-based cloning, we identified a 45-bp insertion in the genomic region of LOC_Os06g40080, which encoded a heme oxygenase. Expression of LOC_Os06g40080 was significantly down-regulated in the grc1 mutant. Subcellular localization showed that this heme oxygenase was localized in the chloroplast. In summary, we isolated and identified the gene for grc1, which plays an important role in chlorophyll biosynthesis and chloroplast development in rice.     

Young Seedling Stripe1 encodes a chloroplast nucleoid-associated protein required for chloroplast development in rice seedlings

Planta - Young Seedling Stripe1 (YSS1) was characterized as an important regulator of plastid-encoded plastid RNA polymerase (PEP) activity essential for chloroplast development at rice seedling...     

Chloroplast chlB gene is required for light-independent chlorophyll accumulation in Chlamydomonas reinhardtii

Light-independent chlorophyll synthesis occurs in some algae, lower plants, and gymnosperms, but not in angiosperms. We have identified a new chloroplast gene, chlB, that is required for the light-independent accumulation of chlorophyll in the green alga Chlamydomonas reinhardtii. The chlB gene was cloned, sequenced, and then disrupted by performing particle gun-mediated chloroplast transformation. The resulting homoplasmic mutant was unable to accumulate chlorophyll in the dark and thus exhibited a yellow-in-the-dark phenotype. The chlB gene encodes a polypeptide of 688 amino acid residues, and is distinct from two previously characterized chloroplast genes (chlN and chlL) also required for light-independent chlorophyll accumulation in C. reinhardtii. Three unidentified open reading frames in chloroplast genomes of liverwort, black pine, and Chlamydomonas moewusii were also identified as chlB genes, based on their striking sequence similarities to the C. reinhardtii chlB gene. A chlB-like gene is absent in chloroplast genomes of tobacco and rice, consistent with the lack of light-independent chlorophyll synthesis in these plants. Polypeptides encoded by the chloroplast chlB genes also show significant sequence similarities with the bchB gene product of Rhodobacter capsulatus. Comparisons among the chloroplast chlB and the bacterial bchB gene products revealed five highly conserved sequence areas that are interspersed by four stretches of highly variable and probably insertional sequences.     

Leaf permease1 gene of maize is required for chloroplast development.

Adjacent bundle sheath and mesophyll cells cooperate for carbon fixation in the leaves of C4 plants. Mutants with compromised plastid development should reveal the degree to which this cooperation is obligatory, because one can assay whether mesophyll cells with defective bundle sheath neighbors retain C4 characteristics or revert to C3 photosynthesis. The leaf permease1-mutable1 (lpe1-m1) mutant of maize exhibits disrupted chloroplast ultrastructure, preferentially affecting bundle sheath choroplasts under lower light. Despite the disrupted ultrastructure, the metabolic cooperation of bundle sheath and mesophyll cells for C4 photosynthesis remains intact. To investigate this novel mutation, the Activator transposon-tagged allele and cDNAs corresponding to the Lpe1 mRNA from wild-type plants were cloned. The Lpe1 gene encodes a polypeptide with significant similarity to microbial pyrimidine and purine transport proteins. An analysis of revertant sectors generated by Activator excision suggests that the Lpe1 gene product is cell autonomous and can be absent up to the last cell divisions in the leaf primordium without blocking bundle sheath chloroplast development.     

dsg, a gene required for cell-cell interaction early in Myxococcus development.

dsg mutants of Myxococcus xanthus are conditionally defective in fruiting body development, including sporulation. Unable to develop on their own, these mutants can assemble fruiting bodies with spores if they are mixed with wild-type cells. To elucidate the developmental defect in dsg mutants by close comparison with wild type, such mutants have been backcrossed by transduction, using a closely linked insertion of transposon Tn5 for selection. Backcrossed dsg mutants form aggregates that are larger, less compact, and less symmetrical than dsg+ fruiting bodies. Also, the starvation-induced sporulation in dsg aggregates is delayed and reduced. However, dsg mutants can be induced by glycerol or dimethyl sulfoxide to sporulate at levels approaching those of wild type. dsg mutants may thus have a primary defect early in development which diminishes their capacity to aggregate and which indirectly decreases the number of fruiting body spores. The linked insertion of Tn5 also facilitated cloning the dsg gene. The cloned dsg+ allele was shown to be dominant to both the dsg-429 and dsg-439 alleles, and both mutant alleles were shown to belong to the same genetic complementation group. Subcloning of restriction fragments, deletions, and insertions of transposon Tn5 agree in locating the dsg gene to an 850-base-pair segment of the cloned region.     

Micro-scale chlorophyll analysis and developmental expression of a cytokinin oxidase/dehydrogenase gene during leaf development and senescence

Chlorophyll content is a baseline measurement in many plant studies. With the miniaturization of many molecular and biochemical assays the sample size needed for these assays has been greatly reduced. Chlorophyll is commonly determined by spectrophotometry. Traditionally this analysis has been carried out in 10 mm cuvettes with volumes of 0.2–2.0 ml. The NanoDrop™ spectrophotometer requires a sample volume of 1–2 μl which provides the opportunity to analyze far smaller samples. Chlorophyll analyses are a critical part of any study of senescence and the cytokinins are considered to play a key role in regulating senescence. The tissue specific levels of cytokinins are thought to be regulated, at least in part, by expression of the cytokinin degradation enzyme cytokinin oxidase/dehydrogenase (CKX). Here an assay is described for the spectral determination of chlorophyll using the NanoDrop™ spectrophotometer that allows the determination of chlorophyll from part samples of clover leaves while leaving sufficient tissue for quantitative real-time PCR study of CKX expression. The expression of TrCKX2 increased during leaf development along the stolon of Trifolium repens but was stable during the onset and progression of senescence suggesting that TrCKX2 was not an initiator of leaf senescence but may have facilitated the progression.     

A MAP kinase gene, BMK1, is required for conidiation and pathogenicity in the rice leaf spot pathogen Bipolaris oryzae

We isolated and characterized BMK1, a gene encoding a mitogen-activated protein kinase (MAPK), from the rice leaf spot pathogen Bipolaris oryzae. The deduced amino acid sequence showed significant homology with Fus3/Kss1 MAPK homologues from other phytopathogenic fungi. The BMK1 disruptants showed impaired hyphal growth, no conidial production, and loss of virulence against rice leaves, indicating that the BMK1 is essential for conidiation and pathogenicity in B. oryzae.     

A chloroplast gene is required for the light-independent accumulation of chlorophyll in Chlamydomonas reinhardtii.

The light-independent pathway of chlorophyll synthesis which occurs in some lower plants and algae is still largely unknown. We have characterized a chloroplast mutant, H13, of Chlamydomonas reinhardtii which is unable to synthesize chlorophyll in the dark and is also photosystem I deficient. The mutant has a 2.8 kb deletion as well as other rearrangements of its chloroplast genome. By performing particle gun mediated chloroplast transformation of H13 with defined wild-type chloroplast DNA fragments, we have identified a new chloroplast gene, chlN, coding for a 545 amino acid protein which is involved in the light-independent accumulation of chlorophyll, probably at the step of reduction of protochlorophyllide to chlorophyllide. The chlN gene is also found in the chloroplast genomes of liverwort and pine, but is absent from the chloroplast genomes of tobacco and rice.     

Nucleostemin-like 1 is required for embryogenesis and leaf development in Arabidopsis

Arabidopsis NSN1 encodes a nucleolar GTP-binding protein and is required for flower development. Defective flowers were formed in heterozygous nsn1/+?plants. Homozygous nsn1 plants were dwarf and exhibited severe defects in reproduction. Arrests in embryo development in nsn1 could occur at any stage of embryogenesis. Cotyledon initiation and development during embryogenesis were distorted in nsn1 plants. At the seedling stage, cotyledons and leaves of nsn1 formed upward curls. The curled leaves developed meristem-like outgrowths or hyperplasia tissues in the adaxial epidermis. Long and enlarged pavement cells, characteristic of the abaxial epidermis of wild type plants, were found in the adaxial epidermis in nsn1 leaves, suggesting a disoriented leaf polarity in the mutant. The important role of NSN1 in embryo development and leaf differentiation was consistent with the high level expression of the NSN1 gene in the developing embryos and the primordia of cotyledons and leaves. The CLAVATA 3 (CLV3) gene, a stem cell marker in the Arabidopsis shoot apical meristem (SAM), was expressed in expanded regions surrounding the SAM of nsn1 plants, and induced ectopically in the meristem-like outgrowths in cotyledons and leaves. The nsn1 mutation up-regulated the expression levels of several genes implicated in the meristem identity and the abaxial cell fate, and repressed the expression of other genes related to the specification of cotyledon boundary and abaxial identity. These results demonstrate that NSN1 represents a novel GTPase required for embryogenesis, leaf development and leaf polarity establishment in Arabidopsis.     

OsCSLD1, a cellulose synthase-like D1 gene, is required for root hair morphogenesis in rice

Root hairs are long tubular outgrowths that form on the surface of specialized epidermal cells. They are required for nutrient and water uptake and interact with the soil microflora. Here we show that the Oryza sativa cellulose synthase-like D1 (OsCSLD1) gene is required for root hair development, as rice (Oryza sativa) mutants that lack OsCSLD1 function develop abnormal root hairs. In these mutants, while hair development is initiated normally, the hairs elongate less than the wild-type hairs and they have kinks and swellings along their length. Because the csld1 mutants develop the same density and number of root hairs along their seminal root as the wild-type plants, we propose that OsCSLD1 function is required for hair elongation but not initiation. Both gene trap expression pattern and in situ hybridization analyses indicate that OsCSLD1 is expressed in only root hair cells. Furthermore, OsCSLD1 is the only member of the four rice CSLD genes that shows root-specific expression. Given that the Arabidopsis (Arabidopsis thaliana) gene KOJAK/AtCSLD3 is required for root hair elongation and is expressed in the root hair, it appears that OsCSLD1 may be the functional ortholog of KOJAK/AtCSLD3 and that these two genes represent the root hair-specific members of this family of proteins. Thus, at least part of the mechanism of root hair morphogenesis in Arabidopsis is conserved in rice.     

VARICOSE, a WD-domain protein, is required for leaf blade development

To gain insight into the processes controlling leaf development, we characterized an Arabidopsis mutant, varicose (vcs), with leaf and shoot apical meristem defects. The vcs phenotype is temperature dependent; low temperature growth largely suppressed defects, whereas high growth temperatures resulted in severe leaf and meristem defects. VCS encodes a putative WD-domain containing protein, suggesting a function involving protein-protein interactions. Temperature shift experiments indicated that VCS is required throughout leaf development, but normal secondary vein patterning required low temperature early in leaf development. The low-temperature vcs phenotype is enhanced in axr1-3 vcs double mutants and in vcs mutants grown in the presence of polar auxin transport inhibitors, however, vcs has apparently normal auxin responses. Taken together, these observations suggest a role for VCS in leaf blade formation.