Defective etioplasts observed in variegation mutants may reveal the light-independent regulation of white/yellow sectors of Arabidopsis leaves

Low-Temperature-Mediated Promoter Methylation Relates to the Expression of TaPOR2D, Affecting the Level of Chlorophyll Accumulation in Albino Wheat (Triticum aestivum L.)

Chlorophyll is an indispensable photoreceptor in plant photosynthesis. Its anabolic imbalance is detrimental to individual growth and development. As an essential epigenetic modification, DNA methylation can induce phenotypic variations, such as leaf color transformation, by regulating gene expression. Albino line XN1376B is a natural mutation of winter wheat cultivar XN1376; however, the regulatory mechanism of its albinism is still unclear. In this study, we found that low temperatures induced albinism in XN1376B. The number of chloroplasts decreased as the phenomenon of bleaching intensified and the fence tissue and sponge tissue slowly dissolved. We identified six distinct TaPOR (protochlorophyllide oxidoreductase) genes in the wheat genome, and TaPOR2D was deemed to be related to the phenomenon of albinism based on the expression in different color leaves (green leaves, white leaves and returned green leaves) and the analysis of promoters' cis-acting elements. TaPOR2D was localized to chloroplasts. TaPOR2D overexpression (TaPOR2D-OE) enhanced the chlorophyll significantly in Arabidopsis, especially at two weeks; the amount of chlorophyll was 6.46 mg/L higher than in WT. The methylation rate of the TaPOR2D promoter in low-temperature albino leaves is as high as 93%, whereas there was no methylation in green leaves. Correspondingly, three DNA methyltransferase genes (TaMET1, TaDRM and TaCMT) were up-regulated in white leaves. Our study clarified that the expression of TaPOR2D is associated with its promoter methylation at a low temperature; it affects the level of chlorophyll accumulation, which probably causes the abnormal development of plant chloroplasts in albino wheat XN1376B. The results provide a theoretical basis for in-depth analysis of the regulation of development of plant chloroplasts and color variation in wheat XN1376B leaves.

Arabidopsis Chloroplast protein for Growth and Fertility1 (CGF1) and CGF2 are essential for chloroplast development and female gametogenesis

BACKGROUND

Chloroplasts are essential organelles of plant cells for not only being the energy factory but also making plant cells adaptable to different environmental stimuli. The nuclear genome encodes most of the chloroplast proteins, among which a large percentage of membrane proteins have yet to be functionally characterized.

RESULTS

We report here functional characterization of two nuclear-encoded chloroplast proteins, Chloroplast protein for Growth and Fertility (CGF1) and CGF2. CGF1 and CGF2 are expressed in diverse tissues and developmental stages. Proteins they encode are associated with chloroplasts through a N-terminal chloroplast-targeting signal in green tissues but also located at plastids in roots and seeds. Mutants of CGF1 and CGF2 generated by CRISPR/Cas9 exhibited vegetative defects, including reduced leaf size, dwarfism, and abnormal cell death. CGF1 and CGF2 redundantly mediate female gametogenesis, likely by securing local energy supply. Indeed, mutations of both genes impaired chloroplast integrity whereas exogenous sucrose rescued the growth defects of the CGF double mutant.

CONCLUSION

This study reports that two nuclear-encoded chloroplast proteins, Chloroplast protein for Growth and Fertility (CGF1) and CGF2, play important roles in vegetative growth, in female gametogenesis, and in embryogenesis likely by mediating chloroplast integrity and development.

Down-regulation of specific plastid ribosomal proteins suppresses thf1 leaf variegation, implying a role of THF1 in plastid gene expression

Chloroplast development is regulated by many biological processes. However, these processes are not fully understood. Leaf variegation mutants have been used as powerful models to elucidate the genetic network of chloroplast development since the degree of leaf variegation is regulated by developmental and environmental cues. The thylakoid formation 1 (thf1) mutant is unique for its variegation in both leaves and cotyledons. Here, we reported a new suppressor gene of thf1 leaf variegation, designated sot8. Map-based cloning and DNA sequencing results showed that a single nucleotide mutation from G to A was detected in the second exon of the gene encoding the ribosomal protein small subunit 9 (PRPS9) in sot8-1, resulting in an amino acid change and a partial loss of PRPS9 function. However, sot8-1 was unable to rescue the thf1 phenotype in low photosystem II activity (Fv/Fm). In addition, we identified two T-DNA insertion mutants defective in plastid-specific ribosomal proteins (PSRPs), psrp2-1, and psrp5-1. Genetic analysis showed that knockdown of PSRP5 expression but not PSRP2 expression suppressed leaf variegation. Northern blotting results showed that precursors of plastid rRNAs over-accumulated in prps9-1 and psrp5-1, indicating that mutations in PRPS9 and PSRP5 cause a defect in rRNA processing. Consistently, inhibition of plastid protein synthesis by spectinomycin led to increased levels of plastid rRNA precursors in wild-type plants, suggesting that rRNA processing and plastid ribosomal assembly are coupled. Taken together, our data indicate that downregulating the expression of specific plastid ribosomal proteins suppresses thf1 leaf variegation, and provide new insights into a role of THF1 in plastid gene expression.

Biogenesis of thylakoid membranes

Thylakoids mediate photosynthetic electron transfer and represent one of the most elaborate energy-transducing membrane systems. Despite our detailed knowledge of its structure and function, much remains to be learned about how the machinery is put together. The concerted synthesis and assembly of lipids, proteins and low-molecular-weight cofactors like pigments and transition metal ions require a high level of spatiotemporal coordination. While increasing numbers of assembly factors are being functionally characterized, the principles that govern how thylakoid membrane maturation is organized in space are just starting to emerge. In both cyanobacteria and chloroplasts, distinct production lines for the fabrication of photosynthetic complexes, in particular photosystem II, have been identified. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Differential expression of a novel gene EaF82a in green and yellow sectors of variegated Epipremnum aureum leaves is related to uneven distribution of auxin

EaF82, a gene identified in previous studies of the variegated plant Epipremnum aureum, exhibited a unique expression pattern with greater transcript abundance in yellow sectors than green sectors of variegated leaves, but lower abundance in regenerated pale yellow plants than in green plants derived from leaf tissue culture. Studies of its full-length cDNA and promoter region revealed two members with only the EaF82a expressed. Immunoblotting confirmed that EaF82a encodes a 12 kDa protein and its accumulation consistent with its gene expression patterns in different color tissues. Transient expression of EaF82a-sGFP fusion proteins in protoplasts showed that EaF82a seems to be present in the cytosol as unidentified spots. Sequence motif search reveals a potential auxin responsive element in promoter region. Using transgenic Arabidopsis seedlings carrying EaF82a promoter driving the bacterial uidA (GUS) gene, an increased GUS activity was observed when IAA (indole-3-acetic acid) concentration was elevated. In E. aureum, EaF82a is more abundant at the site where axillary buds emerge and at the lower side of bending nodes where more IAA accumulates relative to the upper side. The measurement of endogenous IAA levels in different color tissues revealed the same pattern of IAA distribution as that of EaF82a expression, further supporting that EaF82a is an IAA responsive gene. EaF82a expression in etiolated transgenic Arabidopsis seedlings responded to IAA under the influence of light suggesting a microenvironment of uneven light condition affects the EaF82a transcript levels and protein accumulation in variegated leaves.

The puzzle of chloroplast vesicle transport - involvement of GTPases

In the cytosol of plant cells vesicle transport occurs via secretory pathways among the endoplasmic reticulum network, Golgi bodies, secretory granules, endosome, and plasma membrane. Three systems transfer lipids, proteins and other important molecules through aqueous spaces to membrane-enclosed compartments, via vesicles that bud from donor membranes, being coated and uncoated before tethered and fused with acceptor membranes. In addition, molecular, biochemical and ultrastructural evidence indicates presence of a vesicle transport system in chloroplasts. Little is known about the protein components of this system. However, as chloroplasts harbor the photosynthetic apparatus that ultimately supports most organisms on the planet, close attention to their pathways is warranted. This may also reveal novel diversification and/or distinct solutions to the problems posed by the targeted intra-cellular trafficking of important molecules. To date two homologs to well-known yeast cytosolic vesicle transport proteins, CPSAR1 and CPRabA5e (CP, chloroplast localized), have been shown to have roles in chloroplast vesicle transport, both being GTPases. Bioinformatic data indicate that several homologs of cytosolic vesicle transport system components are putatively chloroplast-localized and in addition other proteins have been implicated to participate in chloroplast vesicle transport, including vesicle-inducing protein in plastids 1, thylakoid formation 1, snowy cotyledon 2/cotyledon chloroplast biogenesis factor, curvature thylakoid 1 proteins, and a dynamin like GTPase FZO-like protein. Several putative potential cargo proteins have also been identified, including building blocks of the photosynthetic apparatus. Here we discuss details of the largely unknown putative chloroplast vesicle transport system, focusing on GTPase-related components.

Proteomic evidence for genetic epistasis: ClpR4 mutations switch leaf variegation to virescence in Arabidopsis

Chloroplast development in plants is regulated by a series of coordinated biological processes. In this work, a genetic suppressor screen for the leaf variegation phenotype of the thylakoid formation 1 (thf1) mutant combined with a proteomic assay was employed to elucidate this complicated network. We identified a mutation in ClpR4, named clpR4-3, which leads to leaf virescence and also rescues the var2 variegation. Proteomic analysis showed that the chloroplast proteome of clpR4-3 thf1 is dominantly controlled by clpR4-3, providing molecular mechanisms that cause genetic epistasis of clpR4-3 to thf1. Classification of the proteins significantly mis-regulated in the mutants revealed that those functioning in the expression of plastid genes are oppositely regulated while proteins functioning in antioxidative stress, protein folding, and starch metabolism are changed in the same direction between thf1 and clpR4-3. The levels of FtsHs including FtsH2/VAR2, FtsH8, and FtsH5/VAR1 are greatly reduced in thf1 compared with those in the wild type, but are higher in clpR4-3 thf1 than in thf1. Quantitative PCR analysis revealed that FtsH expression in clpR4-3 thf1 is regulated post-transcriptionally. In addition, a number of ribosomal proteins are less expressed in the clpR4-3 proteome, which is in line with the reduced levels of rRNAs in clpR4-3. Furthermore, knocking out PRPL11, one of the most downregulated proteins in the clpR4-3 thf1 proteome, rescues the leaf variegation phenotype of the thf1 and var2 mutants. These results provide insights into molecular mechanisms by which the virescent clpR4-3 mutation suppresses leaf variegation of thf1 and var2.

Genetic interactions reveal that specific defects of chloroplast translation are associated with the suppression of var2-mediated leaf variegation

Arabidopsis thaliana L. yellow variegated (var2) mutant is defective in a chloroplast FtsH family metalloprotease, AtFtsH2/VAR2, and displays an intriguing green and white leaf variegation. This unique var2-mediated leaf variegation offers a simple yet powerful tool for dissecting the genetic regulation of chloroplast development. Here, we report the isolation and characterization of a new var2 suppressor gene, SUPPRESSOR OF VARIEGATION8 (SVR8), which encodes a putative chloroplast ribosomal large subunit protein, L24. Mutations in SVR8 suppress var2 leaf variegation at ambient temperature and partially suppress the cold-induced chlorosis phenotype of var2. Loss of SVR8 causes unique chloroplast rRNA processing defects, particularly the 23S-4.5S dicistronic precursor. The recovery of the major abnormal processing site in svr8 23S-4.5S precursor indicate that it does not lie in the same position where SVR8/L24 binds on the ribosome. Surprisingly, we found that the loss of a chloroplast ribosomal small subunit protein, S21, results in aberrant chloroplast rRNA processing but not suppression of var2 variegation. These findings suggest that the disruption of specific aspects of chloroplast translation, rather than a general impairment in chloroplast translation, suppress var2 variegation and the existence of complex genetic interactions in chloroplast development.

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.

Arabidopsis thylakoid formation 1 is a critical regulator for dynamics of PSII-LHCII complexes in leaf senescence and excess light

In higher plants, photosystem II (PSII) is a large pigment-protein supramolecular complex composed of the PSII core complex and the plant-specific peripheral light-harvesting complexes (LHCII). PSII-LHCII complexes are highly dynamic in their quantity and macro-organization to various environmental conditions. In this study, we reported a critical factor, the Arabidopsis Thylakoid Formation 1 (THF1) protein, which controls PSII-LHCII dynamics during dark-induced senescence and light acclimation. Loss-of-function mutations in THF1 lead to a stay-green phenotype in pathogen-infected and senescent leaves. Both LHCII and PSII core subunits are retained in dark-induced senescent leaves of thf1, indicative of the presence of PSII-LHCII complexes. Blue native (BN)-polyacrylamide gel electrophoresis (PAGE) and immunoblot analysis showed that, in dark- and high-light-treated thf1 leaves, a type of PSII-LHCII megacomplex is selectively retained while the stability of PSII-LHCII supercomplexes significantly decreased, suggesting a dual role of THF1 in dynamics of PSII-LHCII complexes. We showed further that THF1 interacts with Lhcb proteins in a pH-dependent manner and that the stay-green phenotype of thf1 relies on the presence of LHCII complexes. Taken together, the data suggest that THF1 is required for dynamics of PSII-LHCII supramolecular organization in higher plants.