A mutable albino allele in rice reveals that formation of thylakoid membranes requires the SNOW-WHITE LEAF1 gene

Albino lethal 13, a chloroplast-imported protein required for chloroplast development in rice

Chloroplasts play a vital role in plant growth and development, which are the main sites of photosynthesis and the production of hormones and metabolites. Despite their significance, the regulatory mechanisms governing chloroplast development remain unclear. In our investigation, we identified a rice mutant with defective chloroplasts in rice (Oryza sativa L.), named albino lethal 13 (osal13), which displayed a distinct albino phenotype in leaves, ultimately resulting in seedling lethality. Molecular cloning revealed that OsAL13 encodes a novel rice protein with no homologous gene or known conserved domain. This gene was located in the chloroplast and exhibited constitutive expression in various tissues, particularly in green tissues and regions of active cell growth. Our study's findings reveal that RNAi-mediated knockdown of OsAL13 led to a pronounced albino phenotype, reduced chlorophyll and carotenoid contents, a vesicle chloroplast structure, and a decrease in the expression of chloroplast-associated genes. Consequently, the pollen fertility and seed setting rate were lower compared with the wild type. In contrast, the overexpression of OsAL13 resulted in an increased photosynthetic rate, a higher total grain number per panicle, and enhanced levels of indole-3-acetic acid (IAA) in the roots and gibberellin A3 (GA3) in the shoot. These outcomes provide new insights on the role of OsAL13 in regulating chloroplast development in rice.

Crucial role of SWL1 in chloroplast biogenesis and development in Arabidopsis thaliana

KEY MESSAGE

The disruption of the SWL1 gene leads to a significant down regulation of chloroplast and secondary metabolites gene expression in Arabidopsis thaliana. And finally results in a dysfunction of chloroplast and plant growth. Although the development of the chloroplast has been a consistent focus of research, the corresponding regulatory mechanisms remain unidentified. In this study, the CRISPR/Cas9 system was used to mutate the SWL1 gene, resulting in albino cotyledons and variegated true leaf phenotype. Confocal microscopy and western blot of chloroplast protein fractions revealed that SWL1 localized in the chloroplast stroma. Electron microscopy indicated chloroplasts in the cotyledons of swl1 lack well-defined grana and internal membrane structures, and similar structures have been detected in the albino region of variegated true leaves. Transcriptome analysis revealed that down regulation of chloroplast and nuclear gene expression related to chloroplast, including light harvesting complexes, porphyrin, chlorophyll metabolism and carbon metabolism in the swl1 compared to wild-type plant. In addition, proteomic analysis combined with western blot analysis, showed that a significant decrease in chloroplast proteins of swl1. Furthermore, the expression of genes associated with secondary metabolites and growth hormones was also reduced, which may be attributed to SWL1 associated with absorption and fixation of inorganic carbon during chloroplast development. Together, the above findings provide valuable information to elucidate the exact function of SWL1 in chloroplast biogenesis and development.

Molecular and biochemical characterization of rice developed through conventional integration of nDart1-0 transposon gene

Mutations, the genetic variations in genomic sequences, play an important role in molecular biology and biotechnology. During DNA replication or meiosis, one of the mutations is transposons or jumping genes. An indigenous transposon nDart1-0 was successfully introduced into local indica cultivar Basmati-370 from transposon-tagged line viz., GR-7895 (japonica genotype) through conventional breeding technique, successive backcrossing. Plants from segregating populationsshowed variegated phenotypes were tagged as BM-37 mutants. Blast analysis of the sequence data revealed that the GTP-binding protein, located on the BAC clone OJ1781_H11 of chromosome 5, contained an insertion of DNA transposon nDart1-0. The nDart1-0 has "A" at position 254 bp, whereas nDart1 homologs have "G", which efficiently distinguishes nDart1-0 from its homologs. The histological analysis revealed that the chloroplast of mesophyll cells in BM-37 was disrupted with reduction in size of starch granules and higher number of osmophillic plastoglobuli, which resulted in decreased chlorophyll contents and carotenoids, gas exchange parameters (Pn, g, E, Ci), and reduced expression level of genes associated with chlorophyll biosynthesis, photosynthesis and chloroplast development. Along with the rise of GTP protein, the salicylic acid (SA) and gibberellic acid (GA) and antioxidant contents(SOD) and MDA levels significantly enhanced, while, the cytokinins (CK), ascorbate peroxidase (APX), catalase (CAT), total flavanoid contents (TFC) and total phenolic contents (TPC) significantly reduced in BM-37 mutant plants as compared with WT plants. These results support the notion that GTP-binding proteins influence the process underlying chloroplast formation. Therefore, it is anticipated that to combat biotic or abiotic stress conditions, the nDart1-0 tagged mutant (BM-37) of Basmati-370 would be beneficial.

Seed management using NGS technology to rapidly eliminate a deleterious allele from rice breeder seeds

Spontaneous mutations are stochastic phenomena that occur in every population. However, deleterious mutated allele present in seeds distributed to farmers must be detected and removed. Here, we eliminated undesirable mutations from the parent population in one generation through a strategy based on next-generation sequencing (NGS). This study dealt with a spontaneous albino mutant in the 'Hinohikari' rice variety grown at the Miyazaki Comprehensive Agricultural Experiment Station, Japan. The incidence of albinism in the population was 1.36%. NGS analysis revealed the genomic basis for differences between green and albino phenotypes. Every albino plant had a C insertion in the Snow-White Leaf1 (SWL1) gene on chromosome 4 causing a frameshift mutation. Selfing plants heterozygous for the mutant allele, swl1-R332P, resulted in a 3:1 green/albino ratio, confirming that a single recessive gene controls albinism. Ultrastructural leaf features in the swl1-R332P mutants displayed deformed chlorophyll-associated organelles in albino plants that were similar to those of previously described swl1 mutants. Detection of the causative gene and its confirmation using heterozygous progenies were completed within a year. The NGS technique outlined here facilitates rapid identification of spontaneous mutations that can occur in breeder seeds.

Bulk segregant analysis-sequencing and RNA-Seq analyses reveal candidate genes associated with albino phenotype in Brassica napus

Inheritable albino mutants are excellent models for exploring the mechanism of chloroplast biogenesis and development. However, only a few non-lethal albino mutations have been reported to date in Brassica species. Here, we describe a resynthesized Brassica napus mutant, whose leaf, stem, and silique tissues showed an inheritable albino phenotype under field conditions after the bud stage but green phenotype in the greenhouse during the whole growing season, indicating that the albino phenotype depends on environmental conditions. Compared with the green leaves of the field-grown wild-type (GL) and greenhouse-grown mutant (WGL) plants, white leaves of the field-grown mutant (WL) showed significantly lower chlorophyll contents and structural defects in chloroplasts. Genetic analysis revealed that the albino phenotype of WL is recessive and is controlled by multiple genes. Bulk segregant analysis-sequencing (BSA-Seq) indicated that the candidate regions responsible for the albino phenotype spanned a total physical distance of approximately 49.68 Mb on chromosomes A03, A07, A08, C03, C04, C06, and C07. To gain insights into the molecular mechanisms that control chloroplast development in B. napus, we performed transcriptome (RNA-Seq) analysis of GL, WGL, and WL samples. GO and KEGG enrichment analyses suggested that differentially expressed genes (DEGs) associated with leaf color were significantly enriched in photosynthesis, ribosome biogenesis and chlorophyll metabolism. Further analysis indicated that DEGs involved in chloroplast development and chlorophyll metabolism were likely the main factors responsible for the albino phenotype in B. napus. A total of 59 DEGs were screened in the candidate regions, and four DEGs (BnaC03G0522600NO, BnaC07G0481600NO, BnaC07G0497800NO, and BnaA08G0016300NO) were identified as the most likely candidates responsible for the albino phenotype. Altogether, this study provides clues for elucidating the molecular mechanisms underlying chloroplast development in B. napus.

Transformation of Long-Lived Albino Epipremnum aureum 'Golden Pothos' and Restoring Chloroplast Development

Chloroplasts are organelles responsible for chlorophyll biosynthesis, photosynthesis, and biosynthesis of many metabolites, which are one of key targets for crop improvement. Elucidating and engineering genes involved in chloroplast development are important approaches for studying chloroplast functions as well as developing new crops. In this study, we report a long-lived albino mutant derived from a popular ornamental plant Epipremnum aureum 'Golden Pothos' which could be used as a model for analyzing the function of genes involved in chloroplast development and generating colorful plants. Albino mutant plants were isolated from regenerated populations of variegated 'Golden Pothos' whose albino phenotype was previously found to be due to impaired expression of EaZIP, encoding Mg-protoporphyrin IX monomethyl ester cyclase. Using petioles of the mutant plants as explants with a traceable sGFP gene, an efficient transformation system was developed. Expressing Arabidopsis CHL27 (a homolog of EaZIP) but not EaZIP in albino plants restored green color and chloroplast development. Interestingly, in addition to the occurrence of plants with solid green color, plants with variegated leaves and pale-yellow leaves were also obtained in the regenerated populations. Nevertheless, our study shows that these long-lived albino plants along with the established efficient transformation system could be used for creating colorful ornamental plants. This system could also potentially be used for investigating physiological processes associated with chlorophyll levels and chloroplast development as well as certain biological activities, which are difficult to achieve using green plants.

Establishment of nDart1-tagged lines of Koshihikari, an elite variety of rice in Japan

To utilize a transposon-tagged mutant as a breeding material in rice, an endogenous DNA transposon, nDart1-0, was introduced into Koshihikari by successive backcrossing together with aDart1-27, an active autonomous element. The founder line for nDart1-tagged lines of Koshihikari carried nDart1-0 on chromosome 9 and transposed nDart1-12s on chromosomes 1 and 8 and nDart1-3 on chromosome 11. In nDart1-tagged lines, there were the most abnormal phenotypic mutants and many aberrant chlorophyll mutants at seedling stage. At mature stage, many semi-sterile mutants were observed. Dwarf, reduced culm number and lesion mimic mutants were also found. In total, 43.2% of the lines segregated some phenotypic mutants. Thus, the nDart1-tagged lines of Koshihikari are expected to be potentially useful for screening stress-tolerant mutants under abiotic or biotic stress conditions.

Transgenerational activation of an autonomous DNA transposon, Dart1-24, by 5-azaC treatment in rice

Dart1-24, one of the 37 autonomous DNA transposon Dart1s, was heritably activated by the demethylation of the 5' region following 5-azaC treatment of rice seeds. Transposons are controlled by epigenetic regulations. To obtain newly activated autonomous elements of Dart1, a DNA transposon, in rice, seeds of a stable pale yellow leaf (pyl-stb) mutant caused by the insertion of nDart1-0, a nonautonomous element in OsClpP5, were treated with 5-azaC, a demethylating agent. In the 5-azaC-treated M1 plants, 60-70% of the plants displayed variegated pale yellow leaf (pyl-v) phenotype, depending on the concentration of 5-azaC used, suggesting that inactivated Dart1 might become highly activated by 5-azaC treatment and nDart1-0 was excised from OsClpP5 by the activated Dart1s. Although the M2 plants derived from most of these pyl-v plants showed stable pyl phenotypes, some variegated M1 plants generated pyl-v M2 progeny. These results indicated that most M1 pyl-v phenotypes at M1 were not heritable. Dart1-24, 1-27 and 1-28 were expressed in the M2 pyl-v plants, and mapping analysis confirmed that Dart1-24 was newly activated. Further, the transgenerational activation of Dart1-24 was demonstrated to be caused by the demethylation of nucleotides in its 5' region.

Genetic analysis and fine mapping of a qualitative trait locus wpb1 for albino panicle branches in rice

Chloroplast plays an important role in the plant life cycle. However, the details of its development remain elusive in rice. In this study, we report the fine-mapping of a novel rice gene wpb1 (white panicle branch 1), which affects chloroplast biogenesis, from a tropical japonica variety that results in an albino panicle branches at and after the heading stage. The wpb1 variety was crossed with Nipponbare to generate the F₂ and BC₁F₂ populations. Green and white panicle branch phenotypes with a 3:1 segregation ratio was observed in the F₂ population. Bulked segregant analysis (BSA) based on whole genome resequencing was conducted to determine the wpb1 locus. A candidate interval spanning from 11.35 to 23.79M (physical position) on chromosome 1 was identified. The results of BSA analysis were verified by a 40K rice SNP-array using the BC₁F₂ population. A large-scale F₂ population was used to pinpoint wpb1, and the locus was further narrowed down to a 95-kb interval. Furthermore, our results showed that the expression levels of the majority of the genes involved in Chl biosynthesis, photosynthesis and chloroplast development were remarkably affected in wpb1 variety and in F₂ plants with a white panicle branch phenotype. In line with the results mentioned above, anatomical structural examination and chlorophyll (Chl) content measurement suggested that wpb1 might play an important role in the regulation of chloroplast development. Further cloning and functional characterization of the wpb1 gene will shed light on the molecular mechanism underlying chloroplast development in rice.

Phenotypic and proteomic characteristics of sorghum (Sorghum bicolor) albino lethal mutant sbe6-a1

Leaf color mutants are ideal materials for chloroplast development and photosynthetic mechanism research. Here, we characterized an EMS (ethyl methane sulfonate)-mutagenized sorghum (Sorghum bicolor) mutant, sbe6-a1, in which the severe disruption in chloroplast structure and a chlorophyll deficiency promote an albino leaf phenotype and lead to premature death. The proteomic analyses of mutant and its progenitor wild-type (WT) were performed using a Q Exactive plus Orbitrap mass spectrometer and 4,233 proteins were accurately quantitated. The function analysis showed that most of up-regulated proteins in mutant sbe6-a1 had not been well characterized. GO-enrichment analysis of the differentially abundant proteins (DAPs) showed that up-regulated DAPs were significantly enriched in catabolic process and located in mitochondria, while down regulated DAPs were located in chloroplasts and participated in photosynthesis and some other processes. KEGG pathway-enrichment analyses indicated that the degradation and metabolic pathways of fatty acids, as well as some amino acids and secondary metabolites, were significantly enhanced in the mutant sbe6-a1, while photosynthesis-related pathways, some secondary metabolites' biosynthesis and ribosomal pathways were significantly inhibited. Analysis also shows that some DAPs, such as FBAs, MDHs, PEPC, ATP synthase, CABs, CHLM, PRPs, pathogenesis-related protein, sHSP, ACP2 and AOX may be closely associated with the albino phenotype. Our analysis will promote the understanding of the molecular phenomena that result in plant albino phenotypes.

LARGE GRAIN Encodes a Putative RNA-Binding Protein that Regulates Spikelet Hull Length in Rice

Grain size is a key determiner of grain weight, one of the yield components in rice (Oryza sativa). Therefore, to increase grain yield, it is important to elucidate the detailed mechanisms regulating grain size. The Large grain (Lgg) mutant, found in the nonautonomous DNA-based active rice transposon1 (nDart1)-tagged lines of Koshihikari, is caused by a truncated nDart1-3 and 355 bp deletion in the 5' untranslated region of LGG, which encodes a putative RNA-binding protein, through transposon display and cosegregation analysis between grain length and LGG genotype in F2 and F3. Clustered regularly interspaced short palindromic repeats/CRISPR-associated 9-mediated knockout and overexpression of LGG led to longer and shorter grains than wild type, respectively, showing that LGG regulates spikelet hull length. Expression of LGG was highest in the 0.6-mm-long young panicle and gradually decreased as the panicle elongated. LGG was also expressed in roots and leaves. These results show that LGG functions at the very early stage of panicle development. Longitudinal cell numbers of spikelet hulls of Lgg, knockout and overexpressed plants were significantly different from those of the wild type, suggesting that LGG might regulate longitudinal cell proliferation in the spikelet hull. RNA-Seq analysis of 1-mm-long young panicles from LGG knockout and overexpressing plants revealed that the expressions of many cell cycle-related genes were reduced in knockout plants relative to LGG-overexpressing plants and wild type, whereas some genes for cell proliferation were highly expressed in knockout plants. Taken together, these results suggest that LGG might be a regulator of cell cycle and cell division in the rice spikelet hull.

A naturally occurring conditional albino mutant in rice caused by defects in the plastid-localized OsABCI8 transporter

A wide range of molecules are transported across membranes by the ATP binding cassette (ABC) transporters. Plants possess a collection of ABC proteins bearing similarities to the components of prokaryotic multi subunit ABC transporters, designed as ABC group I. However the functions of most of them are not well understood. Here, we characterized a naturally occurring rice mutant that exhibited albino phenotype under continuous rainy days in the field, but gradually recovered to normal green after the rainy season. Molecular and genetic analyses revealed that the phenotypes were caused by a mutation in the OsABCI8 that encoded a member of the ABCI family. Subcellular localization demonstrated that OsABCI8 is a chloroplast ABC transporter. Expression of OsABCI8 is significantly enhanced in rainy days compared to sunny days. Besides defects in chloroplast development and chlorophyll biosynthesis, the mutant phenotype is accompanied by a higher accumulation of iron, suggesting that OsABCI8 is involved in iron transportation and/or homeostasis in rice. Our results demonstrate that OsABCI8 represents a conserved ABCI protein involved in transition metals transportation and/or homeostasis and suggest an important role of the plastid-localized OsABCI8 for chloroplast development.

TE studies in Japan: The third Japanese meeting on host-transposon interactions

The third Japanese meeting entitled “Biological Function and Evolution through Interactions between Hosts and Transposable Elements (TEs)” was held on 5–6 September 2016 at National Institute of Genetics (NIG), Mishima, Japan. Supported by NIG, the goal of the meeting was to bring together researchers who study diverse biological phenomena such as schizophrenia, carcinogenesis, cellular reprograming, skin function, placental formation, plant mutagenesis and epigenetics, and small RNA-mediated heterochromatinization, where TEs are involved in various ways. The meeting included 13 invited speakers. Here we present highlights of these invited talks.

Albino Leaf 2 is involved in the splicing of chloroplast group I and II introns in rice

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.

A residue substitution in the plastid ribosomal protein L12/AL1 produces defective plastid ribosome and causes early seedling lethality in rice

The plastid ribosome is essential for chloroplast biogenesis as well as seedling formation. As the plastid ribosome closely resembles the prokaryotic 70S ribosome, many plastid ribosomal proteins (PRPs) have been identified in higher plants. However, their assembly in the chloroplast ribosome in rice remains unclear. In the present study, we identified a novel rice mutant, albino lethal 1 (al1), from a chromosome segment substitution line population. The al1 mutant displayed an albino phenotype at the seedling stage and did not survive past the three-leaf stage. No other apparent differences in plant morphology were observed in the al1 mutant. The albino phenotype of the al1 mutant was associated with decreased chlorophyll content and abnormal chloroplast morphology. Using fine mapping, AL1 was shown to encode the PRPL12, a protein localized in the chloroplasts of rice, and a spontaneous single-nucleotide mutation (C/T), resulting in a residue substitution from leucine in AL1 to phenylalanine in al1, was found to be responsible for the early seedling lethality. This point mutation is located at the L10 interface feature of the L12/AL1 protein. Yeast two-hybrid analysis showed that there was no physical interaction between al1 and PRPL10. In addition, the mutation had little effect on the transcript abundance of al1, but had a remarkable effect on the protein abundance of al1 and transcript abundance of chloroplast biogenesis-related and photosynthesis-related genes. These results provide a first glimpse into the molecular details of L12's function in rice.

A gain-of-function Bushy dwarf tiller 1 mutation in rice microRNA gene miR156d caused by insertion of the DNA transposon nDart1

A non-autonomous DNA transposon in rice, nDart1, is actively transposed in the presence of an autonomous element, aDart1, under natural conditions. The nDart1-promoted gene tagging line was developed using the endogenous nDart1/aDart1 system to generate various rice mutants effectively. While the dominant mutants were occasionally isolated from the tagging line, it was unclear what causes dominant mutations. A semidominant mutant, Bushy dwarf tiller1 (Bdt1), which has the valuable agronomic traits of multiple tillering and dwarfism, was obtained from the tagging line. Bdt1 mutant carried a newly inserted nDart1 at 38-bp upstream of transcription initiation site of a non-protein-coding gene, miR156d. This insertion caused an upstream shift of the miR156d transcription initiation site and, consequently, increased the functional transcripts producing mature microRNAs. These results indicate that the total amount of miR156d is controlled not only by transcript quantity but also by transcript quality. Furthermore, transgenic lines introduced an miR156d fragment that flanked the nDart1 sequence at the 5′ region, suggesting that insertion of nDart1 in the gene promoter region enhances gene expression as a cis-element. This study demonstrates the ability of nDart1 to produce gain-of-function mutants as well as further insights into the function of transposable elements in genome evolution.

Lipid trafficking in plant cells

Plant cells contain unique organelles such as chloroplasts with an extensive photosynthetic membrane. In addition, specialized epidermal cells produce an extracellular cuticle composed primarily of lipids, and storage cells accumulate large amounts of storage lipids. As lipid assembly is associated only with discrete membranes or organelles, there is a need for extensive lipid trafficking within plant cells, more so in specialized cells and sometimes also in response to changing environmental conditions such as phosphate deprivation. Because of the complexity of plant lipid metabolism and the inherent recalcitrance of membrane lipid transporters, the mechanisms of lipid transport within plant cells are not yet fully understood. Recently, several new proteins have been implicated in different aspects of plant lipid trafficking. While these proteins provide only first insights into limited aspects of lipid transport phenomena in plant cells, they represent exciting opportunities for further studies.