Overexpression of sesame polyketide synthase A leads to abnormal pollen development in Arabidopsis

BACKGROUND

Sesame is a great reservoir of bioactive constituents and unique antioxidant components. It is widely used for its nutritional and medicinal value. The expanding demand for sesame seeds is putting pressure on sesame breeders to develop high-yielding varieties. A hybrid breeding strategy based on male sterility is one of the most effective ways to increase the crop yield. To date, little is known about the genes and mechanism underlying sesame male fertility. Therefore, studies are being conducted to identify and functionally characterize key candidate genes involved in sesame pollen development. Polyketide synthases (PKSs) are critical enzymes involved in the biosynthesis of sporopollenin, the primary component of pollen exine. Their in planta functions are being investigated for applications in crop breeding.

RESULTS

In this study, we cloned the sesame POLYKETIDE SYNTHASE A (SiPKSA) and examined its function in male sterility. SiPKSA was specifically expressed in sesame flower buds, and its expression was significantly higher in sterile sesame anthers than in fertile anthers during the tetrad and microspore development stages. Furthermore, overexpression of SiPKSA in Arabidopsis caused male sterility in transgenic plants. Ultrastructural observation showed that the pollen grains of SiPKSA-overexpressing plants contained few cytoplasmic inclusions and exhibited an abnormal pollen wall structure, with a thicker exine layer compared to the wild type. In agreement with this, the expression of a set of sporopollenin biosynthesis-related genes and the contents of their fatty acids and phenolics were significantly altered in anthers of SiPKSA-overexpressing plants compared with wild type during anther development.

CONCLUSION

These findings highlighted that overexpression of SiPKSA in Arabidopsis might cause male sterility through defective pollen wall formation. Moreover, they suggested that SiPKSA modulates vibrant pollen development via sporopollenin biosynthesis, and a defect in its regulation may induce male sterility. Therefore, genetic manipulation of SiPKSA might promote hybrid breeding in sesame and other crop species.

Development of the permanent tetrad wall in Juncus L. (Juncaceae, Poales)

Juncaceae, a cosmopolitan family, belong to the cyperid clade of Poales together with Cyperaceae and Thurniaceae. Pollen grain of Juncaceae, as in Thurniaceae, is dispersed in a permanent tetrad, and knowledge about the ontogeny of its wall is still incipient, based on data from only one species. This study aims to analyze the formation of the pollen wall of seven Juncus species in order to characterize the timing and the ontogenetic events that lead to the cohesion of the four pollen grains in a permanent tetrad. Anthers at different developmental stages were submitted to techniques of light microscopy and transmission electron microscopy; dehiscent anthers with mature pollens were also analyzed in scanning electron microscopy. In all the species here studied, callose deposits around each microsporocyte, with dissolution prior to meiosis. Microspore wall starts at the end of the second meiotic division with formation of primexine. Exine comprises tectum, columellae, and foot layer. During cytokinesis, cell plates form the internal wall of the pollen tetrad. Mature permanent tetrad is enveloped externally by both the exine and intine and internally by the intine and the foot layer, which forms the continuous internal wall. Callose was detected in the early stages of microsporocytes, although reported to be absent in Juncaceae. Our data confirm the variation in Juncaceae cytokinesis and the occurrence of simple cohesion due to the presence of a continuous tectum along the pollen tetrad.

IMPERFECTIVE EXINE FORMATION (IEF) is required for exine formation and male fertility in Arabidopsis

KEY MESSAGE

IEF, a novel plasma plasma membrane protein, is important for exine formation in Arabidopsis. Exine, an important part of pollen wall, is crucial for male fertility. The major component of exine is sporopollenin which are synthesized and secreted by tapetum. Although sporopollenin synthesis has been well studied, the transportation of it remains elusive. To understand it, we analyzed the gene expression pattern in tapetal microdissection data, and investigated the potential transporter genes that are putatively regulated by ABORTED MICROSPORES (AMS). Among these genes, we identified IMPERFECTIVE EXINE FORMATION (IEF) that is important for exine formation. Compared to the wild type, ief mutants exhibit severe male sterility and pollen abortion, suggesting IEF is crucial for pollen development and male fertility. Using both scanning and transmission electron microscopes, we showed that exine structure was not well defined in ief mutant. The transient expression of IEF-GFP driven by the 35S promoter indicated that IEF-GFP was localized in plasma membrane. Furthermore, AMS can specifically activate the expression of promoterIEF:LUC in vitro, which suggesting AMS regulates IEF for exine formation. The expression of ATP-BINDING CASSETTE TRANSPORTER G26 (AGCB26) was not affected in ief mutants. In addition, SEM and TEM data showed that the sporopollenin deposition is more defective in abcg26/ief-2 than that of in abcg26, which suggesting that IEF is involved in an independent sporopollenin transportation pathway. This work reveal a novel gene, IEF regulated by AMS that is essential for exine formation.

Deficiencies in the formation and regulation of anther cuticle and tryphine contribute to male sterility in cotton PGMS line

BACKGROUND

Male sterility is a simple and efficient pollination control system that is widely exploited in hybrid breeding. In upland cotton, CCRI9106, a photosensitive genetic male sterile (PGMS) mutant isolated from CCRI040029, was reported of great advantages to cotton heterosis. However, little information concerning the male sterility of CCRI9106 is known. Here, comparative transcriptome analysis of CCRI9106 (the mutant, MT) and CCRI040029 (the wild type, WT) anthers in Anyang (long-day, male sterile condition to CCRI9106) was performed to reveal the potential male sterile mechanism of CCRI9106.

RESULTS

Light and electron microscopy revealed that the male sterility phenotype of MT was mainly attributed to irregularly exine, lacking tryphine and immature anther cuticle. Based on the cytological characteristics of MT anthers, anther RNA libraries (18 in total) of tetrad (TTP), late uninucleate (lUNP) and binucleate (BNP) stages in MT and WT were constructed for transcriptomic analysis, therefore revealing a total of 870,4 differentially expressed genes (DEGs). By performing gene expression pattern analysis and protein-protein interaction (PPI) networks construction, we found down-regulation of DEGs, which enriched by the lipid biosynthetic process and the synthesis pathways of several types of secondary metabolites such as terpenoids, flavonoids and steroids, may crucial to the male sterility phenotype of MT, and resulting in the defects of anther cuticle and tryphine, even the irregularly exine. Furthermore, several lipid-related genes together with ABA-related genes and MYB transcription factors were identified as hub genes via weighted gene co-expression network analysis (WGCNA). Additionally, the ABA content of MT anthers was reduced across all stages when compared with WT anthers. At last, genes related to the formation of anther cuticle and tryphine could activated in MT under short-day condition.

CONCLUSIONS

We propose that the down-regulation of genes related to the assembly of anther cuticle and tryphine may lead to the male sterile phenotype of MT, and MYB transcription factors together with ABA played key regulatory roles in these processes. The conversion of fertility in different photoperiods may closely relate to the functional expression of these genes. These findings contribute to elucidate the mechanism of male sterility in upland cotton.

Deficiencies in the formation and regulation of anther cuticle and tryphine contribute to male sterility in cotton PGMS line

Background

Male sterility is a simple and efficient pollination control system that is widely exploited in hybrid breeding. In upland cotton, CCRI9106, a photosensitive genetic male sterile (PGMS) mutant isolated from CCRI040029, was reported of great advantages to cotton heterosis. However, little information concerning the male sterility of CCRI9106 is known. Here, comparative transcriptome analysis of CCRI9106 (the mutant, MT) and CCRI040029 (the wild type, WT) anthers in Anyang (long-day, male sterile condition to CCRI9106) was performed to reveal the potential male sterile mechanism of CCRI9106.

Results

Light and electron microscopy revealed that the male sterility phenotype of MT was mainly attributed to irregularly exine, lacking tryphine and immature anther cuticle. Based on the cytological characteristics of MT anthers, anther RNA libraries (18 in total) of tetrad (TTP), late uninucleate (lUNP) and binucleate (BNP) stages in MT and WT were constructed for transcriptomic analysis, therefore revealing a total of 870,4 differentially expressed genes (DEGs). By performing gene expression pattern analysis and protein-protein interaction (PPI) networks construction, we found down-regulation of DEGs, which enriched by the lipid biosynthetic process and the synthesis pathways of several types of secondary metabolites such as terpenoids, flavonoids and steroids, may crucial to the male sterility phenotype of MT, and resulting in the defects of anther cuticle and tryphine, even the irregularly exine. Furthermore, several lipid-related genes together with ABA-related genes and MYB transcription factors were identified as hub genes via weighted gene co-expression network analysis (WGCNA). Additionally, the ABA content of MT anthers was reduced across all stages when compared with WT anthers. At last, genes related to the formation of anther cuticle and tryphine could activated in MT under short-day condition.

Conclusions

We propose that the down-regulation of genes related to the assembly of anther cuticle and tryphine may lead to the male sterile phenotype of MT, and MYB transcription factors together with ABA played key regulatory roles in these processes. The conversion of fertility in different photoperiods may closely relate to the functional expression of these genes. These findings contribute to elucidate the mechanism of male sterility in upland cotton.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07250-1.

Formation of aperture sites on the pollen surface as a model for development of distinct cellular domains

Pollen grains are covered by the complex extracellular structure, called exine, which in most species is deposited on the pollen surface non-uniformly. Certain surface areas receive fewer exine deposits and develop into regions whose structure and morphology differ significantly from the rest of pollen wall. These regions are known as pollen apertures. Across species, pollen apertures can vary in their numbers, positions, and morphology, generating highly diverse patterns. The process of aperture formation involves establishment of cell polarity, formation of distinct plasma membrane domains, and deposition of extracellular materials at precise positions. Thus, pollen apertures present an excellent model for studying the development of cellular domains and formation of patterns at the single-cell level. Until very recently, the molecular mechanisms underlying the specification and formation of aperture sites were completely unknown. Here, we review recent advances in understanding of the molecular processes involved in pollen aperture formation, focusing on the molecular players identified through genetic approaches in the model plant Arabidopsis. We discuss a potential working model that describes the process of aperture formation, including specification of domains, creation of their defining features, and protection of these regions from exine deposition.

Arabidopsis Cys2/His2 zinc-finger protein MAZ1 is essential for intine formation and exine pattern

Cys₂/His₂ zinc-finger protein (C2H2-ZFP) is widely involved in the reproductive development of plants, but its role in pollen development is still elusive. Here, we identified a pollen-related C2H2-ZFP gene named as MALE FERTILITY-ASSOCIATED ZINC FINGER PROTEIN 1 (MAZ1), which was first isolated from Arabidopsis thaliana. MAZ1 showed a preferential expression pattern in early anther development. Its mutation resulted in aberrant primexine deposition at the tetrad stage, followed by a defective multiple-layer pattern of exine with irregular baculum and no tectum. Furthermore, microspore development was arrested, and no intine layer was formed. These developmental defects led to fertility reduction and pollen abortion. This study reveals the essential role of MAZ1 in pollen wall development.

Acyl-CoA synthetases from Physcomitrella, rice and Arabidopsis: different substrate preferences but common regulation by MS188 in sporopollenin synthesis

ACOS5, OsACOS12 and PpACOS6 are all capable of fatty acyl-CoA synthetase activity but exhibit different substrate preferences. The transcriptional regulation of ACOS for sporopollenin synthesis appears to have been conserved in Physcomitrella, rice and Arabidopsis during evolution. Sporopollenin is the major constituent of spore and pollen exines. In Arabidopsis, acyl-CoA synthetase 5 (ACOS5) is an essential enzyme for sporopollenin synthesis, and its orthologues are PpACOS6 from the moss Physcomitrella and OsACOS12 from monocot rice. However, knowledge regarding the evolutionary conservation and divergence of the ACOS gene in sporopollenin synthesis remains limited. In this study, we analysed the function and regulation of PpACOS6 and OsACOS12. A complementation test showed that OsACOS12 driven by the ACOS5 promoter could partially restore the male fertility of the acos5 mutant in Arabidopsis, while PpACOS6 did not rescue the acos5 phenotype. ACOS5, PpACOS6 and OsACOS12 all complemented the acyl-CoA synthetase-deficient yeast strain (YB525) phenotype, although they exhibited different substrate preferences. To understand the conservation of sporopollenin synthesis regulation, we constructed two constructs with ACOS5 driven by the OsACOS12 or PpACOS6 promoter. Both constructs could restore the fertility of acos5 plants. The MYB transcription factor MS188 from Arabidopsis directly regulates ACOS5. We found that MS188 could also bind the promoters of OsACOS12 and PpACOS6 and activate the genes driven by the promoters, suggesting that the transcriptional regulation of these genes was similar to that of ACOS5. These results show that the ACOS gene promoter region from Physcomitrella, rice and Arabidopsis has been functionally conserved during evolution, while the chain lengths of fatty acid-derived monomers of sporopollenin vary in different plant species.

The ATP-binding cassette (ABC) transporter OsABCG3 is essential for pollen development in rice

BACKGROUND

The pollen wall, which protects male gametophyte against various stresses and facilitates pollination, is essential for successful reproduction in flowering plants. The pollen wall consists of gametophyte-derived intine and sporophyte-derived exine. From outside to inside of exine are tectum, bacula, nexine I and nexine II layers. How these structural layers are formed has been under extensive studies, but the molecular mechanisms remain obscure.

RESULTS

Here we identified two osabcg3 allelic mutants and demonstrated that OsABCG3 was required for pollen development in rice. OsABCG3 encodes a half-size ABCG transporter localized on the plasma membrane. It was mainly expressed in anther when exine started to form. Loss-function of OsABCG3 caused abnormal degradation of the tapetum. The mutant pollen lacked the nexine II and intine layers, and shriveled without cytoplasm. The expression of some genes required for pollen wall formation was examined in osabcg3 mutants. The mutation did not alter the expression of the regulatory genes and lipid metabolism genes, but altered the expression of lipid transport genes.

CONCLUSIONS

Base on the genetic and cytological analyses, OsABCG3 was proposed to transport the tapetum-produced materials essential for pollen wall formation. This study provided a new perspective to the genetic regulation of pollen wall development.

An Introduction to Male Germline Development

In this introductory chapter, we describe male germline development in plants taking Arabidopsis thaliana as a reference species. We first describe the transition from sporophytic to germline development, then microsporogenesis including meiosis, followed by male gametophyte development prior to pollination, and finally the progamic phase culminating in double fertilization, which leads to the formation of the embryo and the endosperm. For detailed information on some of these processes or on the molecular underpinning of certain fate transitions, we refer the reader to recent reviews. An important but often neglected aspect of male gametophyte development is the formation of the unique pollen cell wall. In contrast to that of other plant cells, the pollen cell wall is composed of two principal layers, the intine and exine. While the intine, the inner pecto-cellulosic cell wall layer, is biochemically and structurally similar to a "classical" plant cell wall, the exine is a unique composite with sporopollenin as its main component. Biosynthesis of the cell wall is remarkably similar between the spores of mosses and ferns, and pollen of seed plants, although slight differences exist, even between closely related species (reviewed in Wallace et al., AoB Plants 2011:plr027, 2011). In the latter sections of this chapter, we will present a brief overview of cell wall development in Arabidopsis pollen, where this aspect has been intensively studied.

OsPKS2 is required for rice male fertility by participating in pollen wall formation

OsPKS2, the rice orthologous gene of Arabidopsis PKSB/LAP5, encodes a polyketide synthase that is involved in pollen wall formation in rice. In flowering plants, the pollen wall protects male gametes from various environmental stresses and pathogen attacks, as well as promotes pollen germination. The biosynthesis of sporopollenin in tapetal cell is critical for pollen wall formation. Recently, progress has been made in understanding sporopollenin metabolism during pollen wall development in Arabidopsis. However, little is known about the molecular mechanism that underlies the sporopollenin synthesis in pollen wall formation in rice (Oryza sativa). In this study, we identified that a point mutation in OsPKS2, a plant-specific type III polyketide synthase gene, caused male sterility in rice by affecting the normal progress of pollen wall formation. Two other allelic mutants of OsPKS2 were generated using the CRISPR/Cas9 system and are also completely male sterile. This result thus further confirmed that OsPKS2 controls rice male fertility. We also showed that OsPKS2 is an orthologous gene of Arabidopsis PKSB/LAP5 and has a tapetum-specific expression pattern. In addition, its product localizes in the endoplasmic reticulum. Results suggested that OsPKS2 is critical for pollen wall formation, and plays a conserved but differentiated role in sporopollenin biosynthesis from Arabidopsis.

Developmental and ultrastructural characters of the pollen grains and tapetum in species of Nymphaea subgenus Hydrocallis

Variations in pollen characters and tapetum behavior were recently acknowledged in the early-divergent family Nymphaeaceae and even within the genus Nymphaea, which probably is not monophyletic; some traits such as infratectum and tapetum type are also a matter of different interpretations. In this study, developmental characters of the pollen grains and tapetum in Nymphaea subgenus Hydrocallis are provided for the first time. Observations were made in N. amazonum, N. gardneriana, and N. prolifera using light, scanning, and transmission electron microscopy. Tapetum is of the secretory type and produces orbicules. At microspore and pollen grain stages, the distal and proximal walls differ considerably. This result supports the operculate condition of the aperture in Hydrocallis, and such aperture might be plesiomorphic for Nymphaeoideae. The infratectum is intermediate, composed of inter-columellae granular elements, robust columellae consisting of agglomerated granules, complete columellae, and fused columellae. Narrow microchannels are present and persist until the mature pollen grain stage. The membranous granular layer is often present in the pollen grains of Nymphaeaceae. In N. gardneriana, this layer is most probably a component of the intine because it is lost after acetolysis. Orbicules in the Nymphaeaceae are characterized as spherical or subspherical, with a smooth sporopolleninic wall that surrounds an electron-lucent core and with individual orbicules that usually merge to give irregular aggregations. The aperture, pollen wall ultrastructure, and the tapetum of the studied species are discussed in an evolutionary and systematic context, and these characters are also compared with those of other angiosperm lineages.

Labyrinths, columns and cavities: new internal features of pollen grain walls in the Acanthaceae detected by FIB-SEM

External pollen grain morphology has been widely used in the taxonomy and systematics of flowering plants, especially the Acanthaceae which are noted for pollen diversity. However internal pollen wall features have received far less attention due to the difficulty of examining the wall structure. Advancing technology in the field of microscopy has made it possible, with the use of a focused ion beam-scanning electron microscope (FIB-SEM), to view the structure of pollen grain walls in far greater detail and in three dimensions. In this study the wall structures of 13 species from the Acanthaceae were investigated for features of potential systematic relevance. FIB-SEM was applied to obtain precise cross sections of pollen grains at selected positions for examining the wall ultrastructure. Exploratory studies of the exine have thus far identified five basic structural types. The investigations also show that similar external pollen wall features may have a distinctly different internal structure. FIB-SEM studies have revealed diverse internal pollen wall features which may now be investigated for their systematic and functional significance.

Role of Lipid Metabolism in Plant Pollen Exine Development

Pollen plays important roles in the life cycle of angiosperms plants. It acts as not only a biological protector of male sperms but also a communicator between the male and the female reproductive organs, facilitating pollination and fertilization. Pollen is produced within the anther, and covered by the specialized outer envelope, pollen wall. Although the morphology of pollen varies among different plant species, the pollen wall is mainly comprised of three layers: the pollen coat, the outer exine layer, and the inner intine layer. Except the intine layer, the other two layers are basically of lipidic nature. Particularly, the outer pollen wall layer, the exine, is a highly resistant biopolymer of phenylpropanoid and lipidic monomers covalently coupled by ether and ester linkages. The precise molecular mechanisms underlying pollen coat formation and exine patterning remain largely elusive. Herein, we summarize the current genetic, phenotypic and biochemical studies regarding to the pollen exine development and underlying molecular regulatory mechanisms mainly obtained from monocot rice (Oryza sativa) and dicot Arabidopsis thaliana, aiming to extend our understandings of plant male reproductive biology. Genes, enzymes/proteins and regulatory factors that appear to play conserved and diversified roles in lipid biosynthesis, transportation and modification during pollen exine formation, were highlighted.

Data for proteomic profiling of Anthers from a photosensitive male sterile mutant and wild-type cotton (Gossypium hirsutum L.)

Cotton is an important economic crop, used mainly for the production of textile fiber. Using a space mutation breeding technique, a novel photosensitive genetic male sterile mutant CCRI9106 was isolated from the wild-type upland cotton cultivar CCRI040029. To study the male sterile mechanisms of CCRI9106, histological and iTRAQ-facilitated proteomic analyses of anthers were performed. This data article contains data related to the research article titled iTRAQ-Facilitated Proteomic Profiling of Anthers From a Photosensitive Male Sterile Mutant and Wild-type Cotton (Gossypium hirsutum L.)[1]. This research article describes the iTRAQ-facilitated proteomic analysis of the wild-type and a photosensitive male sterile mutant in cotton. The report indicated that exine formation defect is the key reason for male sterility in mutant plant. The information presented here represents the tables and figures that detail the processing of the raw data obtained from iTRAQ analysis.

iTRAQ-facilitated proteomic profiling of anthers from a photosensitive male sterile mutant and wild-type cotton (Gossypium hirsutum L.)

Male sterility is a common phenomenon in flowering plants, and it has been successfully developed in several crops by taking advantage of heterosis. Cotton (Gossypium hirsutum L.) is an important economic crop, used mainly for the production of textile fiber. Using a space mutation breeding technique, a novel photosensitive genetic male sterile mutant CCRI9106 was isolated from the wild-type upland cotton cultivar CCRI040029. To use CCRI9106 in cotton hybrid breeding, it is of great importance to study the molecular mechanisms of its male sterility. Here, histological and iTRAQ-facilitated proteomic analyses of anthers were performed to explore male sterility mechanisms of the mutant. Scanning and transmission electron microscopy of the anthers showed that the development of pollen wall in CCRI9106 was severely defective with a lack of exine formation. At the protein level, 6121 high-confidence proteins were identified and 325 of them showed differential expression patterns between mutant and wild-type anthers. The proteins up- or down-regulated in MT anthers were mainly involved in exine formation, protein degradation, calcium ion binding,etc. These findings provide valuable information on the proteins involved in anther and pollen development, and contribute to elucidate the mechanism of male sterility in upland cotton.

The biosynthesis, composition and assembly of the outer pollen wall: A tough case to crack

The formation of the durable outer pollen wall, largely composed of sporopollenin, is essential for the protection of the male gametophyte and plant reproduction. Despite its apparent strict conservation amongst land plants, the composition of sporopollenin and the biosynthetic pathway(s) yielding this recalcitrant biopolymer remain elusive. Recent molecular genetic studies in Arabidopsis thaliana (Arabidopsis) and rice have, however, identified key genes involved in sporopollenin formation, allowing a better understanding of the biochemistry and cell biology underlying sporopollenin biosynthesis and pollen wall development. Herein, current knowledge of the biochemical composition of the outer pollen wall is reviewed, with an emphasis on enzymes with characterized biochemical activities in sporopollenin and pollen coat biosynthesis. The tapetum, which forms the innermost sporophytic cell layer of the anther and envelops developing pollen, plays an essential role in sporopollenin and pollen coat formation. Recent studies show that several tapetum-expressed genes encode enzymes that metabolize fatty acid derived compounds to form putative sporopollenin precursors, including tetraketides derived from fatty acyl-CoA starter molecules, but analysis of mutants defective in pollen wall development indicate that other components are also incorporated into sporopollenin. Also highlighted are the many uncertainties remaining in the development of a sporopollenin-fortified pollen wall, particularly in relation to the mechanisms of sporopollenin precursor transport and assembly into the patterned form of the pollen wall. A working model for sporopollenin biosynthesis is proposed based on the data obtained largely from studies of Arabidopsis, and future challenges to complete our understanding of pollen wall biology are outlined.

The Arabidopsis cytosolic Acyl-CoA-binding proteins play combinatory roles in pollen development

In Arabidopsis, six acyl-CoA-binding proteins (ACBPs) have been identified and they have been demonstrated to function in plant stress responses and development. Three of these AtACBPs (AtACBP4-AtACBP6) are cytosolic proteins and all are expressed in floral organs as well as in other tissues. The roles of cytosolic AtACBPs in floral development were addressed in this study. To this end, a T-DNA insertional knockout mutant of acbp5 was characterized before use in crosses with the already available acbp4 and acbp6 T-DNA knockout mutants to examine their independent and combinatory functions in floral development. The single-gene knockout mutations did not cause any significant phenotypic changes, while phenotypic deficiencies affecting siliques and pollen were observed in the double mutants (acbp4acbp6 and acbp5acbp6) and the acbp4acbp5acbp6 triple mutant. Vacuole accumulation in the acbp4acbp6, acbp5acbp6 and acbp4acbp5acbp6 pollen was the most severe abnormality occurring in the double and triple mutants. Furthermore, scanning electron microscopy and transmission electron microscopy revealed exine and oil body defects in the acbp4acbp5acbp6 mutant, which also displayed reduced ability in in vitro pollen germination. Transgenic Arabidopsis expressing β-glucuronidase (GUS) driven from the various AtACBP promoters indicated that AtACBP6pro::GUS expression overlapped with AtACBP4pro::GUS expression in pollen grains and with AtACBP5pro::GUS expression in the microspores and tapetal cells. Taken together, these results suggest that the three cytosolic AtACBPs play combinatory roles in acyl-lipid metabolism during pollen development.