RUPTURED POLLEN GRAIN1, a member of the MtN3/saliva gene family, is crucial for exine pattern formation and cell integrity of microspores in Arabidopsis

During microsporogenesis, the microsporocyte (or microspore) plasma membrane plays multiple roles in pollen wall development, including callose secretion, primexine deposition, and exine pattern determination. However, plasma membrane proteins that participate in these processes are still not well known. Here, we report that a new gene, RUPTURED POLLEN GRAIN1 (RPG1), encodes a plasma membrane protein and is required for exine pattern formation of microspores in Arabidopsis (Arabidopsis thaliana). The rpg1 mutant exhibits severely reduced male fertility with an otherwise normal phenotype, which is largely due to the postmeiotic abortion of microspores. Scanning electron microscopy examination showed that exine pattern formation in the mutant is impaired, as sporopollenin is randomly deposited on the pollen surface. Transmission electron microscopy examination further revealed that the primexine formation of mutant microspores is aberrant at the tetrad stage, which leads to defective sporopollenin deposition on microspores and the locule wall. In addition, microspore rupture and cytoplasmic leakage were evident in the rpg1 mutant, which indicates impaired cell integrity of the mutant microspores. RPG1 encodes an MtN3/saliva family protein that is integral to the plasma membrane. In situ hybridization analysis revealed that RPG1 is strongly expressed in microsporocyte (or microspores) and tapetum during male meiosis. The possible role of RPG1 in microsporogenesis is discussed.     

ABNORMAL POLLEN VACUOLATION1 (APV1) is required for male fertility by contributing to anther cuticle and pollen exine formation in maize

Anther cuticle and pollen exine are the major protective barriers against various stresses. The proper functioning of genes expressed in the tapetum is vital for the development of pollen exine and anther cuticle. In this study, we report a tapetum‐specific gene, Abnormal Pollen Vacuolation1 (APV1), in maize that affects anther cuticle and pollen exine formation. The apv1 mutant was completely male sterile. Its microspores were swollen, less vacuolated, with a flat and empty anther locule. In the mutant, the anther epidermal surface was smooth, shiny, and plate‐shaped compared with the three‐dimensional crowded ridges and randomly formed wax crystals on the epidermal surface of the wild‐type. The wild‐type mature pollen had elaborate exine patterning, whereas the apv1 pollen surface was smooth. Only a few unevenly distributed Ubisch bodies were formed on the apv1 mutant, leading to a more apparent inner surface. A significant reduction in the cutin monomers was observed in the mutant. APV1 encodes a member of the P450 subfamily, CYP703A2‐Zm, which contains 530 amino acids. APV1 appeared to be widely expressed in the tapetum at the vacuolation stage, and its protein signal co‐localized with the endoplasmic reticulum (ER) signal. RNA‐Seq data revealed that most of the genes in the fatty acid metabolism pathway were differentially expressed in the apv1 mutant. Altogether, we suggest that APV1 functions in the fatty acid hydroxylation pathway which is involved in forming sporopollenin precursors and cutin monomers that are essential for the development of pollen exine and anther cuticle in maize.     

Fertile Arabidopsis cyp704b1 mutant,defective in sporopollenin biosynthesis,has a normal pollen coat and lipidic organelles in the tapetum

The exine acts as a protectant of the pollen from environmental stresses, and the pollen coat plays an important role in the attachment and recognition of the pollen to the stigma. The pollen coat is made of lipidic organelles in the tapetum. The pollen coat is necessary for fertility, as pollen coat-less mutants, such as those deficient in sterol biosynthesis, show severe male sterility. In contrast, the exine is made of sporopollenin precursors that are biosynthesized in the tapetum. Some mutants involved in sporopollenin biosynthesis lose the exine but show the fertile phenotype. One of these mutants, cyp704b1, was reported to lose not only the exine but also the pollen coat. To identify the cause of the fertile phenotype of the cyp704b1 mutant, the detailed structures of the tapetum tissue and pollen surface in the mutant were analyzed. As a result, the cyp704b1 mutant completely lost the normal exine but had high-electron-density granules localized where the exine should be present. Furthermore, normal lipidic organelles in the tapetum and pollen coat embedded between high-electron-density granules on the pollen surface were observed, unlike in a previous report, and the pollen coat was attached to the stigma. Therefore, the pollen coat is necessary for fertility, and the structure that functions like the exine, such as high-electron-density granules, on the pollen surface may play important roles in retaining the pollen coat in the cyp704b1 mutant.     

KCS1 encodes a fatty acid elongase 3-ketoacyl-CoA synthase affecting wax biosynthesis in Arabidopsis thaliana

An Arabidopsis fatty acid elongase gene, KCS1, with a high degree of sequence identity to FAE1, encodes a 3-ketoacyl-CoA synthase which is involved in very long chain fatty acid synthesis in vegetative tissues, and which also plays a role in wax biosynthesis. Sequence analysis of KCS1 predicted that this synthase was anchored to a membrane by two adjacent N-terminal, membrane-spanning domains. Analysis of a T-DNA tagged kcs1-1 mutant demonstrated the involvement of the KCS1 in wax biosynthesis. Phenotypic changes in the kcs1-1 mutant included thinner stems and less resistance to low humidity stress at a young age. Complete loss of KCS1 expression resulted in decreases of up to 80% in the levels of C26 to C30 wax alcohols and aldehydes, but much smaller effects were observed on the major wax components, i.e. the C29 alkanes and C29 ketones on leaves, stems and siliques. In no case did the loss of KCS1 expression result in complete loss of any individual wax component or significantly decrease the total wax load. This indicated that there was redundancy in the elongase KCS activities involved in wax synthesis. Furthermore, since alcohol, aldehyde, alkane and ketone levels were affected to varying degrees, involvement of the KCS1 synthase in both the decarbonylation and acyl-reduction wax synthesis pathways was demonstrated.     

Rice No Pollen 1 (NP1) is required for anther cuticle formation and pollen exine patterning

Angiosperm male reproductive organs (anthers and pollen grains) have complex and interesting morphological features, but mechanisms that underlie their patterning are poorly understood. Here we report the isolation and characterization of a male sterile mutant of No Pollen 1 (NP1) in rice (Oryza sativa). The np1‐4 mutant exhibited smaller anthers with a smooth cuticle surface, abnormal Ubisch bodies, and aborted pollen grains covered with irregular exine. Wild‐type exine has two continuous layers; but np1‐4 exine showed a discontinuous structure with large granules of varying size. Chemical analysis revealed reduction in most of the cutin monomers in np1‐4 anthers, and less cuticular wax. Map‐based cloning suggested that NP1 encodes a putative glucose‐methanol‐choline oxidoreductase; and expression analyses found NP1 preferentially expressed in the tapetal layer from stage 8 to stage 10 of anther development. Additionally, the expression of several genes involved in biosynthesis and in the transport of lipid monomers of sporopollenin and cutin was decreased in np1‐4 mutant anthers. Taken together, these observations suggest that NP1 is required for anther cuticle formation, and for patterning of Ubisch bodies and the exine. We propose that products of NP1 are likely important metabolites in the development of Ubisch bodies and pollen exine, necessary for polymerization, assembly, or both.     

New views of tapetum ultrastructure and pollen exine development in Arabidopsis thaliana

Background and Aims

The Arabidopsis thaliana pollen cell wall is a complex structure consisting of an outer sporopollenin framework and lipid-rich coat, as well as an inner cellulosic wall. Although mutant analysis has been a useful tool to study pollen cell walls, the ultrastructure of the arabidopsis anther has proved to be challenging to preserve for electron microscopy.

Methods

In this work, high-pressure freezing/freeze substitution and transmission electron microscopy were used to examine the sequence of developmental events in the anther that lead to sporopollenin deposition to form the exine and the dramatic differentiation and death of the tapetum, which produces the pollen coat.

Key Results

Cryo-fixation revealed a new view of the interplay between sporophytic anther tissues and gametophytic microspores over the course of pollen development, especially with respect to the intact microspore/pollen wall and the continuous tapetum epithelium. These data reveal the ultrastructure of tapetosomes and elaioplasts, highly specialized tapetum organelles that accumulate pollen coat components. The tapetum and middle layer of the anther also remain intact into the tricellular pollen and late uninucleate microspore stages, respectively.

Conclusions

This high-quality structural information, interpreted in the context of recent functional studies, provides the groundwork for future mutant studies where tapetum and microspore ultrastructure is assessed.     

The HKM gene, which is identical to the MS1 gene of Arabidopsis thaliana, is essential for primexine formation and exine pattern formation

A male-sterile mutant of Arabidopsis thaliana was isolated by T-DNA tagging screening. Using transmission electron microscopy analysis, we revealed that the microspores of this mutant did not have normal thick primexine on the microspore at the tetrad stage. Instead, a moderately electron-dense layer formed around the microspores. Although microspores without normal primexine failed to form a proper reticulate exine pattern at later stages, sporopollenin was deposited and an exine-like hackly structure was observed on the microspores during the microspore stage. Thus, this mutant was named hackly microspore (hkm). It is speculated that the moderately electron-dense layer was primexine, which partially played its role in sporopollenin deposition onto the microspore. Cytological analysis revealed that the tapetum of the hkm mutant was significantly vacuolated, and that vacuolated tapetal cells crushed the microspores, resulting in the absence of pollen grains within the anther at anthesis. Single nucleotide polymorphism analysis demonstrated that the hkm mutation exists within the MS1 gene, which has been reportedly expressed within the tapetum. Our results suggest that the critical process of primexine formation is under sporophytic control .     

Combined activity of LACS1 and LACS4 is required for proper pollen coat formation in Arabidopsis

Very long chain lipids are important components of the plant cuticle that establishes the boundary surface of aerial organs. In addition, these lipids were detected in the extracellular pollen coat (tryphine), where they play a crucial role in appropriate pollen‐stigma communication. As such they are involved in the early interaction of pollen with the stigma. A substantial reduction in tryphine lipids was shown to compromise pollen germination and, consequently, resulted in male sterility. We investigated the role of two long‐chain acyl‐CoA synthetases (LACSs) in Arabidopsis with respect to their contribution to the production of tryphine lipids. LACS was shown to provide CoA‐activated very long chain fatty acids (VLCFA‐CoAs) to the pathways of wax biosynthesis. The allocation of sufficient quantities of VLCFA‐CoA precursors should therefore be relevant to the generation of tryphine lipids. Here, we report on the identification of lacs1 lacs4 double knock‐out mutant lines that were conditionally sterile and showed significant reductions in pollen coat lipids. Whereas the contributions of both LACS proteins to surface wax levels were roughly additive, their co‐operation in tryphine lipid biosynthesis was clearly more complex. The inactivation of LACS4 resulted in increased levels of tryphine lipids accompanied by morphological anomalies of the pollen grains. The additional inactivation of LACS1 neutralized the morphological defects, decreased the tryphine lipids far below wild‐type levels and resulted in conditionally sterile pollen.     

Ultrastructural characterization of exine development of the transient defective exine 1 mutant suggests the existence of a factor involved in constructing reticulate exine architecture from sporopollenin aggregates

A male-sterile mutant of Arabidopsis thaliana, in which filament elongation was defective although pollen fertility was normal, was isolated by means of T-DNA tagging. Transmission electron microscopy (TEM) analysis revealed that primexine synthesis and probacula formation, which are thought to be the initial steps of exine formation, were defective, and that globular sporopollenin aggregation was randomly deposited onto the microspore at the early uninucleate microspore stage. Sporopollenin aggregation, which failed to anchor to the microspore plasma membrane, was deposited on the locule wall and in the locule at the uninucleate microspore stage. However, visually normal exine with a basic reticulate structure was observed at the middle uninucleate microspore stage, indicating that the exine formation was restored in the mutant. Thus, the mutant was designated transient defective exine 1 (tde1). These results indicated that tde1 mutation affects the initial process of the exine formation, but does not impair any critical processes. Our results also suggest the existence of a certain factor responsible for exine patterning in A. thaliana. The TDE1 gene was found to be identical to the DE-ETIOLATED 2 gene known to be involved in brassinosteroid (BR) biosynthesis, and the tde1 probacula-defective phenotypes were recovered in the presence of BR application. These results suggest that BRs control the rate or efficiency of initial process of exine pattern formation.     

拟南芥雄性不育突变体ms1142的遗传定位与功能分析

经EMS诱变野生型拟南芥(Arabidopsis thaliana)群体筛选得到一株雄性不育突变体ms1142, 突变体的果荚短小, 不含种子。细胞学观察和扫描电镜结果表明, 突变体花药发育过程中, 花药中小孢子外壁异常、破裂, 最后没有花粉形成。遗传分析表明, 该突变体为隐性单核基因突变所致; 利用图位克隆的方法将MS1142基因定位于第1条染色体的BAC克隆F16P17上44 kb区间内, 目前尚未见该区间内有雄性不育基因的报道。以上结果结合生物信息学分析表明, MS1142是一个新的调控花药发育的关键基因。该工作为花药发育关键基因MS1142的克隆及功能分析奠定了基础。     

拟南芥雄性不育突变体ms1142的遗传定位与功能分析

经EMS诱变野生型拟南芥（Arabidopsis thaliana）群体筛选得到一株雄性不育突变体ms1142,突变体的果荚短小,不含种子。细胞学观察和扫描电镜结果表明,突变体花药发育过程中,花药中小孢子外壁异常、破裂,最后没有花粉形成。遗传分析表明,该突变体为隐性单核基因突变所致;利用图位克隆的方法将MS1142基因定位于第1条染色体的BAC克隆F16P17上44kb区间内,目前尚未见该区间内有雄性不育基因的报道。以上结果结合生物信息学分析表明,MS1142是一个新的调控花药发育的关键基因。该工作为花药发育关键基因MS1142的克隆及功能分析奠定了基础。     

An ABCG/WBC-type ABC transporter is essential for transport of sporopollenin precursors for exine formation in developing pollen

The exine of the pollen wall shows an intricate pattern, primarily comprising sporopollenin, a polymer of fatty acids and phenolic compounds. A series of enzymes synthesize sporopollenin precursors in tapetal cells, and the precursors are transported from the tapetum to the pollen surface. However, the mechanisms underlying the transport of sporopollenin precursors remain elusive. Here, we provide evidence that strongly suggests that the Arabidopsis ABC transporter ABCG26/WBC27 is involved in the transport of sporopollenin precursors. Two independent mutations at ABCG26 coding region caused drastic decrease in seed production. This defect was complemented by expression of ABCG26 driven by its native promoter. The severely reduced fertility of the abcg26 mutants was caused by a failure to produce mature pollen, observed initially as a defect in pollen-wall development. The reticulate pattern of the exine of wild-type microspores was absent in abcg26 microspores at the vacuolate stage, and the vast majority of the mutant pollen degenerated thereafter. ABCG26 was expressed specifically in tapetal cells at the early vacuolate stage of pollen development. It showed high co-expression with genes encoding enzymes required for sporopollenin precursor synthesis, i.e. CYP704B1, ACOS5, MS2 and CYP703A2. Similar to two other mutants with defects in pollen-wall deposition, abcg26 tapetal cells accumulated numerous vesicles and granules. Taken together, these results suggest that ABCG26 plays a crucial role in the transfer of sporopollenin lipid precursors from tapetal cells to anther locules, facilitating exine formation on the pollen surface.     

Re-Exposure of Tapes at High Temperature and Pressure in the Lycopodium Clavatum Spore Exine

Lamellations are visualizable through the staining commonly used in transmission electron microscopy during exine formation on Lycopodium and other spores, and the nexine of pollen grains. The lamellations so exposed consist-of dark tapes at either side of an unstained (white) line. Neither tapes nor white lines are visualizable in the exine of mature spores of Lycopodium. The continued presence of lamellations having tape-white line spacing has been demonstrated with inorganic tracers in the nexine of pollen in which lamellations otherwise appeared to be absent. Through high contrast staining methods for TEM we have observed lamellations in the residual exine following heat treatment (350[ddot]C) of mature spores of Lycopodium clavatum. The surface of these residual exines was etched by treatment with hot 2-aminoethanol and filaments were observed to protrude from the etched surfaces. The residual exine stained darkly. Lycopodium spores heated to 300[ddot]C at 1 kb pressure had long filaments exposed at the surface of the residual exine (sporopollenin). Sections of the pellet remaining after heat and pressure treatment also included bundles of closely parallel filaments and masses of isolated filaments. The filaments were stained while the exine residue, assumed to include sporopollenin, was not. Isolated filaments produce a stable metachromasia with toluidine blue indicating the presence of many closely spaced sites of negative charge. The staining of intact exines with basic dyes may result from anionic sites on filaments embedded within the exine rather than being due to sporopollenin. The results of our experiments indicate that the filaments are more resistant to heat and pressure and 2-aminoethanol than is sporopollenin. We propose that the trilamellar elements commonly called tapes and white lines might be composed of two filaments bridged by polybasic molecules.     

Analysis of TETRAKETIDE α-PYRONE REDUCTASE function in Arabidopsis thaliana reveals a previously unknown, but conserved, biochemical pathway in sporopollenin monomer biosynthesis

The precise structure of the sporopollenin polymer that is the major constituent of exine, the outer pollen wall, remains poorly understood. Recently, characterization of Arabidopsis thaliana genes and corresponding enzymes involved in exine formation has demonstrated the role of fatty acid derivatives as precursors of sporopollenin building units. Fatty acyl-CoA esters synthesized by ACYL-COA SYNTHETASE5 (ACOS5) are condensed with malonyl-CoA by POLYKETIDE SYNTHASE A (PKSA) and PKSB to yield α-pyrone polyketides required for exine formation. Here, we show that two closely related genes encoding oxidoreductases are specifically and transiently expressed in tapetal cells during microspore development in Arabidopsis anthers. Mutants compromised in expression of the reductases displayed a range of pollen exine layer defects, depending on the mutant allele. Phylogenetic studies indicated that the two reductases belong to a large reductase/dehydrogenase gene family and cluster in two distinct clades with putative orthologs from several angiosperm lineages and the moss Physcomitrella patens. Recombinant proteins produced in bacteria reduced the carbonyl function of tetraketide α-pyrone compounds synthesized by PKSA/B, and the proteins were therefore named TETRAKETIDE α-PYRONE REDUCTASE1 (TKPR1) and TKPR2 (previously called DRL1 and CCRL6, respectively). TKPR activities, together with those of ACOS5 and PKSA/B, identify a conserved biosynthetic pathway leading to hydroxylated α-pyrone compounds that were previously unknown to be sporopollenin precursors.     

Cytochemistry of pollen development in Brachypodium distachyon

Brachypodium distachyon is a widely recognized model plant belonging to subfamily Pooideae with a sequenced genome. To gain a better understanding of the male reproductive development in B. distachyon we examined pollen morphology and cytochemical changes of microspore cytoplasm from pollen mother cell stage to mature pollen using light, fluorescent and scanning electron microscopy. Our results show that B. distachyon exhibits a typical monocot-type pollen ontogeny. Meiosis in the pollen mother cells is accomplished by successive cytokinesis generating isobilateral tetrads. Cytochemical examination indicated that microspore cytoplasm contains variable amounts of insoluble carbohydrates and proteins at different developmental stages. Deposition of starch in the cytoplasm of microspores starts at the bicellular stage and continues till the mature pollen stage. The formation of the exine wall progresses by the deposition of sporopollenin from the tapetum layer of the anther. The mature pollen is trinucleate, spheroidal in shape and possesses a single pore with an annulus and operculum. The exine pattern is smooth and of granular type.     

Is the quantity of orbicules released by Dactylis glomerata and Cynosurus echinatus (Poaceae) big enough to play an allergenic role?

Orbicule characteristics of Dactylis glomerata and Cynosurus echinatus (Poaceae) were investigated using light (LM), scanning electron (SEM) and transmission electron microscopy (TEM). Based on SEM micrographs, the number of orbicules per 100?µm² of the locule wall surface was determined in both dehisced and undehisced anthers and was further compared statistically. A total of 100 pollen grains were examined using SEM in search for orbicules attached to the pollen exine. Orbicules were not found distributed freely in the anther locules. They were attached to the locule wall surface through sporopolleninous fibrils, the orbicule wall being firmly embedded in, and often in continuity with the thin layer of sporopollenin lining each locule. The orbicule density on the locule wall surface of both the dehisced and undehisced anthers did not differ significantly. Only a few orbicules were seen attached to the pollen exine in both species. It is concluded that orbicules are not easily removed from the surface of the locule wall and, consequently, that the number of orbicules emitted from the two grass species is too low to play a significant role in triggering allergic diseases.     

Identification of kaonashi mutants showing abnormal pollen exine structure in Arabidopsis thaliana

Exine, the outermost architecture of pollen walls, protects male gametes from the environment by virtue of its chemical and physical stability. Although much effort has been devoted to revealing the mechanism of exine construction, still little is known about it. To identify the genes involved in exine formation, we screened for Arabidopsis mutants with pollen grains exhibiting abnormal exine structure using scanning electron microscopy. We isolated 12 mutants, kaonashi1 (kns1) to kns12, and classified them into four types. The type 1 mutants showed a collapsed exine structure resembling a mutant of the callose synthase gene, suggesting that the type 1 genes are involved in callose wall synthesis. The type 2 mutant showed remarkably thin exine structure, presumably due to defective primexine thickening. The type 3 mutants showed defective tectum formation, and thus type 3 genes are required for primordial tectum formation or biosynthesis and deposition of sporopollenin. The type 4 mutants showed densely distributed baculae, suggesting type 4 genes determine the position of probacula formation. All identified kns mutants were recessive, suggesting that these KNS genes are expressed in sporophytic cells. Unlike previously known exine-defective mutants, most of the kns mutants showed normal fertility. Map-based cloning revealed that KNS2, one of the type 4 genes, encodes sucrose phosphate synthase. This enzyme might be required for synthesis of primexine or callose wall, which are both important for probacula positioning. Analysis of kns mutants will provide new knowledge to help understand the mechanism of biosynthesis of exine components and the construction of exine architecture.     

Sporopollenin monomer biosynthesis in arabidopsis

Land plants have evolved aliphatic biopolymers that protect their cell surfaces against dehydration, pathogens, and chemical and physical damage. In flowering plants, a critical event during pollen maturation is the formation of the pollen surface structure. The pollen wall consists essentially of the microspore-derived intine and the sporophyte-derived exine. The major component of the exine is termed sporopollenin, a complex biopolymer. The chemical composition of sporopollenin remains poorlycharacterized because it is extremely resistant to chemical and biological degradation procedures. Recent characterization of Arabidopsis thaliana genes and corresponding enzymes involved in exine formation has demonstrated that the sporopollenin polymer consists of phenolic and fatty acid-derived constituents that are covalently coupled by ether and ester linkages. This review illuminates the outlines of a biosynthetic pathway involved in generating monomer constituents of the sporopollenin biopolymer component of the pollen wall.     

WBC27, an adenosine tri-phosphate-binding cassette protein, controls pollen wall formation and patterning in Arabidopsis

In flowering plants, the exine components are derived from tapetum. Despite its importance to sexual plant reproduction, little is known about the translocation of exine materials from tapetum to developing microspores. Here we report functional characterization of the arabidopsis WBC27 gene. WBC27 encodes an adenosine tri-phosphate binding cassette (ABC) transporter and is expressed preferentially in tapetum. Mutation of WBC27 disrupted the exine formation. The wbc27 mutant microspores began to degenerate once released from tetrads and most of the microspores collapsed at the uninucleate stage. Only a small number of wbc27-1 microspores could develop into tricellular pollen grains. These survival pollen grains lacked exine and germinated in the anther before anthesis. All of these results suggest that the ABC transporter, WBC27 plays important roles in the formation of arabidopsis exine, possibly by translocation of lipidic precursors of sporopollenin from tapetum to developing microspores.     

Disruption of the novel plant protein NEF1 affects lipid accumulation in the plastids of the tapetum and exine formation of pollen, resulting in male sterility in Arabidopsis thaliana

A novel male-sterile mutant of Arabidopsis thaliana was isolated by means of T-DNA tagging. Pollen abortion of the mutant was evident after microspore release, and pollen grains were completely absent at anthesis. Transmission electron microscope analysis revealed that primexine was coarsely developed, and that although sporopollenin was produced, it was not deposited onto the microspore plasma membrane. The sporopollenin that failed to be deposited aggregated and accumulated within the locule and on the locule wall. Finally, as no exine formation was observed, the mutant was named nef1. The plastoglobuli within the plastids of the tapetum were reduced, and lipid accumulation was considerably decreased. The mutant had a significantly altered leaf chloroplast ultrastructure and showed various growth defects. Lipid analysis revealed that the total lipid content in nef1 was lower than that in the wild type, which indicated that Nef1 was involved in lipid metabolism. Cloning of the full-length Nef1 indicated that the gene encodes a novel plant protein of 1123 amino acids with limited sequence similarities to membrane proteins or transporter-like proteins, and the NEF1 is predicted to be a plastid integral membrane protein. Motif analysis revealed that NEF1 contains prokaryotic membrane lipoprotein lipid attachment sites that are involved in maintaining cell envelope integrity. It is predicted that the Nef1 encodes a membrane protein that maintains the envelope integrity in the plastids.