The ATP-binding Cassette Transporter OsABCG15 is Required for Anther Development and Pollen Fertility in Rice

Plant male reproductive development is a complex biological process, but the underlying mechanism is not well understood. Here, we characterized a rice (Oryza sativa L.) male sterile mutant. Based on map‐based cloning and sequence analysis, we identified a 1,459‐bp deletion in an adenosine triphosphate (ATP)‐binding cassette (ABC) transporter gene, OsABCG15, causing abnormal anthers and male sterility. Therefore, we named this mutant osabcg15. Expression analysis showed that OsABCG15 is expressed specifically in developmental anthers from stage 8 (meiosis II stage) to stage 10 (late microspore stage). Two genes CYP704B2 and WDA1, involved in the biosynthesis of very‐long‐chain fatty acids for the establishment of the anther cuticle and pollen exine, were downregulated in osabcg15 mutant, suggesting that OsABCG15 may play a key function in the processes related to sporopollenin biosynthesis or sporopollenin transfer from tapetal cells to anther locules. Consistently, histological analysis showed that osabcg15 mutants developed obvious abnormality in postmeiotic tapetum degeneration, leading to rapid degredation of young microspores. The results suggest that OsABCG15 plays a critical role in exine formation and pollen development, similar to the homologous gene of AtABCG26 in Arabidopsis. This work is helpful to understand the regulatory network in rice anther development.     

Tapetum and middle layer control male fertility in Actinidia deliciosa

Background and Aims

Dioecism characterizes many crop species of economic value, including kiwifruit (Actinidia deliciosa). Kiwifruit male sterility occurs at the microspore stage. The cell walls of the microspores and the pollen of the male-sterile and male-fertile flowers, respectively, differ in glucose and galactose levels. In numerous plants, pollen formation involves normal functioning and degeneration timing of the tapetum, with calcium and carbohydrates provided by the tapetum essential for male fertility. The aim of this study was to determine whether the anther wall controls male fertility in kiwifruit, providing calcium and carbohydrates to the microspores.

Methods

The events occurring in the anther wall and microspores of male-fertile and male-sterile anthers were investigated by analyses of light microscopy, epifluorescence, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL assay) and transmission electron microscopy coupled with electron spectroscopy. The possibility that male sterility was related to anther tissue malfunctioning with regard to calcium/glucose/galactose provision to the microspores was also investigated by in vitro anther culture.

Key Results

Both tapetum and the middle layer showed secretory activity and both degenerated by programmed cell death (PCD), but PCD was later in male-sterile than in male-fertile anthers. Calcium accumulated in cell walls of the middle layer and tapetum and in the exine of microspores and pollen, reaching higher levels in anther wall tissues and dead microspores of male-sterile anthers. A specific supply of glucose and calcium induced normal pollen formation in in vitro-cultured anthers of the male-sterile genotype.

Conclusions

The results show that male sterility in kiwifruit is induced by anther wall tissues through prolonged secretory activity caused by a delay in PCD, in the middle layer in particular. In vitro culture results support the sporophytic control of male fertility in kiwifruit and open the way to applications to overcome dioecism and optimize kiwifruit production.     

An investigation of the role of the anther tapetum during microspore development using genetic cell ablation

The effects on anther development of a fusion of the Arabidopsis anther-specific apg gene promoter to a ribonuclease (barnase) in transgenic tobacco plants were examined. Contrary to expectations, viable pollen grains were produced by these plants despite the demonstration that ribonuclease expression in the microspores and tapetum caused targeted cell ablation. Transformed plants were reduced in male fertility due to ablation of a proportion of pollen dependent on apg-barnase locus number. Plants were otherwise phenotypically normal and fully female fertile, confirming the anther-specific nature of the apg promoter. In microspores inheriting an apg-barnase locus following meiosis, loss of cell viability, as judged by fluorescein diacetate staining, occurred during mid to late microspore development. Microspores not inheriting a transgene went on to mature into viable pollen grains. Premature degeneration of the tapetum was also observed as a result of apg-barnase expression, but this did not appear to disrupt the subsequent microspore and pollen developmental programmes. This was substantiated by observations of microspore development in plants in which the tapetum was rescued from ablation by crossing in a second transgene encoding a tapetum-specific inhibitor of the ribonuclease. It was determined that tapetum cell disruption occurs at the early to mid uninucleate microspore stage in apg-barnase transformants. The data presented show that after this point in microspore development the tapetum is no longer essential for the production of viable pollen in tobacco.     

ABCG26-Mediated Polyketide Trafficking and Hydroxycinnamoyl Spermidines Contribute to Pollen Wall Exine Formation in Arabidopsis

Pollen grains are encased by a multilayered, multifunctional wall. The sporopollenin and pollen coat constituents of the outer pollen wall (exine) are contributed by surrounding sporophytic tapetal cells. Because the biosynthesis and development of the exine occurs in the innermost cell layers of the anther, direct observations of this process are difficult. The objective of this study was to investigate the transport and assembly of exine components from tapetal cells to microspores in the intact anthers of Arabidopsis thaliana. Intrinsically fluorescent components of developing tapetum and microspores were imaged in intact, live anthers using two-photon microscopy. Mutants of ABCG26, which encodes an ATP binding cassette transporter required for exine formation, accumulated large fluorescent vacuoles in tapetal cells, with corresponding loss of fluorescence on microspores. These vacuolar inclusions were not observed in tapetal cells of double mutants of abcg26 and genes encoding the proposed sporopollenin polyketide biosynthetic metabolon (ACYL COENZYME A SYNTHETASE5, POLYKETIDE SYNTHASE A [PKSA], PKSB, and TETRAKETIDE α-PYRONE REDUCTASE1), providing a genetic link between transport by ABCG26 and polyketide biosynthesis. Genetic analysis also showed that hydroxycinnamoyl spermidines, known components of the pollen coat, were exported from tapeta prior to programmed cell death in the absence of polyketides, raising the possibility that they are incorporated into the exine prior to pollen coat deposition. We propose a model where ABCG26-exported polyketides traffic from tapetal cells to form the sporopollenin backbone, in coordination with the trafficking of additional constituents, prior to tapetum programmed cell death.     

OsATG7 is required for autophagy-dependent lipid metabolism in rice postmeiotic anther development

In flowering plants, the tapetum, the innermost layer of the anther, provides both nutrient and lipid components to developing microspores, pollen grains, and the pollen coat. Though the programmed cell death of the tapetum is one of the most critical and sensitive steps for fertility and is affected by various environmental stresses, its regulatory mechanisms remain mostly unknown. Here we show that autophagy is required for the metabolic regulation and nutrient supply in anthers and that autophagic degradation within tapetum cells is essential for postmeiotic anther development in rice. Autophagosome-like structures and several vacuole-enclosed lipid bodies were observed in postmeiotic tapetum cells specifically at the uninucleate stage during pollen development, which were completely abolished in a retrotransposon-insertional OsATG7 (autophagy-related 7)-knockout mutant defective in autophagy, suggesting that autophagy is induced in tapetum cells. Surprisingly, the mutant showed complete sporophytic male sterility, failed to accumulate lipidic and starch components in pollen grains at the flowering stage, showed reduced pollen germination activity, and had limited anther dehiscence. Lipidomic analyses suggested impairment of editing of phosphatidylcholines and lipid desaturation in the mutant during pollen maturation. These results indicate a critical involvement of autophagy in a reproductive developmental process of rice, and shed light on the novel autophagy-mediated regulation of lipid metabolism in eukaryotic cells.     

Anther structure and pollen development in Melicoccus lepidopetalus (Sapindaceae): An evolutionary approach to dioecy in the family

Anther and pollen development in staminate and pistillate flowers of dioecious Melicoccus lepidopetalus (Sapindaceae) were examined by light and electron microscopy. Young anthers are similar in both types of flowers; they consist of epidermis, endothecium, two to four middle layers and a secretory tapetum. The microspore tetrads are tetrahedral. The mature anther in staminate flowers presents compressed epidermal cells and endothecium cells with fibrillar thickenings. A single locule is formed in the theca by dissolution of the septum and pollen grains are shed at two-celled stage. The mature anthers of pistillate flowers differ anatomically from those of staminate flowers. The epidermis is not compressed, the endothecium does not develop fibrillar thickenings, middle layers and tapetum are generally persisting, and the stomium is nonfunctional. Microspore degeneration begins after meiosis of microspore mother cells. At anthesis, uninucleate microspores and pollen grains with vegetative and generative nuclei with no cytokinesis are observed. Some pollen walls display an abnormal exine deposition, whereas others show a well formed exine, although both are devoid of intine. These results suggest that in the evolution towards unisexuality, the developmental differences of anther wall tissues and pollen grains between pistillate and staminate flowers might become more pronounced in a derived condition, such as dioecy.     

OsABCG15 encodes a membrane protein that plays an important role in anther cuticle and pollen exine formation in rice

Key message

An ABC transporter gene ( OsABCG15 ) was proven to be involved in pollen development in rice. The corresponding protein was localized on the plasma membrane using subcellular localization.

Abstract

Wax, cutin, and sporopollenin are important for normal development of the anther cuticle and pollen exine, respectively. Their lipid soluble precursors, which are produced in the tapetum, are then secreted and transferred to the anther and microspore surface for polymerization. However, little is known about the mechanisms underlying the transport of these precursors. Here, we identified and characterized a member of the G subfamily of ATP-binding cassette (ABC) transporters, OsABCG15, which is required for the secretion of these lipid-soluble precursors in rice. Using map-based cloning, we found a spontaneous A-to-C transition in the fourth exon of OsABCG15 that caused an amino acid substitution of Thr-to-Pro in the predicted ATP-binding domain of the protein sequence. This osabcg15 mutant failed to produce any viable pollen and was completely male sterile. Histological analysis indicated that osabcg15 exhibited an undeveloped anther cuticle, enlarged middle layer, abnormal Ubisch body development, tapetum degeneration with a falling apart style, and collapsed pollen grains without detectable exine. OsABCG15 was expressed preferentially in the tapetum, and the fused GFP-OsABCG15 protein was localized to the plasma membrane. Our results suggested that OsABCG15 played an essential role in the formation of the rice anther cuticle and pollen exine. This role may include the secretion of the lipid precursors from the tapetum to facilitate the transfer of precursors to the surface of the anther epidermis as well as to microspores.     

Identification, subcellular localization, and developmental studies of oleosins in the anther of Brassica napus

mRNAs encoding putative oleosins have been detected in the tapetum of developing anthers in Brassica and Arabidopsis, but the authentic proteins have not been previously documented. Antibodies against a synthetic 15-residue polypeptide that represents a portion of the putative tapetum oleosins encoded by two cloned Brassica napus genes were raised. Using these antibodies for immunoblotting after SDS-PAGE of the sporophytic extracts of B. napus developing anthers, two oleosins of ～ 48 and 45 kDa were detected. These two oleosins were judged to be the putative oleosins encoded by cloned Brassica genes because of their identical N-terminal sequences. The two oleosins were present in the anthers only during the developmental stage when the tapetum cells were packed with organelles. A fraction of low-density organelles was isolated from the developing anthers by flotation centrifugation. The fraction contained plastoglobule-filled plastids and lipid-containing particles. The structures of these two isolated organelles were similar to those in situ in the tapetum cells. Of subcellular fractions of the anther homogenate, the two oleosins were present exclusively in the low-density organelle fraction. They were absent in the surface fractions of the developing microspores and the mature pollen, although fragmented oleosin molecules were earlier reported to be present on the pollen. By immunocytochemistry, immunogold particles were found largely on the periphery of the plastoglobuli inside the plastids in the tapetum cells. The antibodies also detected oleosins on the surface of storage oil bodies inside the maturing microspores. Apparently, the gametophytic microspore oil-body oleosins share common epitopes at the generally non-conserved C-terminal domain with the sporophytic tapetum oleosins.     

芝麻核雄性不育系ms86-1微粉发生的细胞学观察

芝麻(Sesamum indicum)核雄性不育系ms86-1姊妹交后代表现为可育、部分不育(即微粉)及完全不育(简称不育)3种类型。不同育性类型的花药及花粉粒形态差异明显。Alexander染色实验显示微粉植株花粉粒外壁为蓝绿色, 内部为不均一洋红色, 与可育株及不育株花粉粒的染色特征均不相同。为探明芝麻微粉发生机理, 在电子显微镜下比较观察了可育、微粉、不育类型的小孢子发育过程。结果表明, 可育株小孢子母细胞减数分裂时期代谢旺盛, 胞质中出现大量脂质小球; 四分体时期绒毡层细胞开始降解, 单核小孢子时期开始出现乌氏体, 成熟花粉时期花粉囊腔内及花粉粒周围分布着大量乌氏体, 花粉粒外壁有11–13个棱状凸起, 表面存在大量基粒棒, 形成紧密的覆盖层。不育株小孢子发育异常显现于减数分裂时期, 此时胞质中无脂质小球出现, 细胞壁开始积累胼胝质; 四分体时期绒毡层细胞未见降解; 单核小孢子时期无乌氏体出现; 成熟花粉时期花粉囊腔中未发现正常的乌氏体, 存在大量空瘪的败育小孢子, 外壁积累胼胝质, 缺乏基粒棒。微粉株小孢子在减数分裂时期可见胞质内有大量脂质小球, 四分体时期部分绒毡层发生变形, 单核小孢子时期有部分绒毡层开始降解; 绒毡层细胞降解滞后为少量发育进程迟缓的小孢子提供了营养物质, 部分小孢子发育为正常花粉粒; 这些花粉粒比较饱满, 表面有少量颗粒状突起, 但未能形成覆盖层, 花粉囊腔中及小孢子周围存在少量的乌氏体。小孢子形成的育性类型与绒毡层降解是否正常有关。     

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.     

矮牡丹小孢子发生和雄配子体发育及其与该种濒危的关系

研究了矮牡丹Paeonia jishanensis Hong et W．Z．Zhao的小孢子发生及雄配子体的形成。矮 牡丹花药具4个小孢子囊,药壁结构属双子叶型,腺质绒毡层,小孢子母细胞减数分裂后胞质分裂为 同时型,四分体多为四面体形,少左右对称形,成熟花粉为2-细胞。对芍药属木本类型的雄性发育进行 了全面研究,还对小孢子母细胞减数分裂和单核小孢子发育时期的异常现象进行了观察,对能育花粉 与不育花粉的百分比进行了测定,结果表明,能育花粉为45.03％～84.18％,它们在不同花中,不同花 药中,甚至同一花药的不同花粉囊中表现都不完全一致。联系矮牡丹的致濒原因进行了讨论,认为雄配子体形成过程中的异常现象,并不是导致矮牡丹濒危的主要因素。     

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.     

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 .     

Formation of microspores and development of male gametes in Paeonia jishanensis,with an analysis of factors of endangerment of this entity

Investigated in the present work were development of microspores and formation Of male gametes in Paeonia jishanensis T．Hong et W．Z．Zhao)．Its anthers are 4-sporran giate;structure of anther wall is of the Dicotyledonous type,with glandular tapetum;cy tokinesis at meiosis of microspore mother cells is simultaneous;tetrads are mostly tetrahe dral,less frequently isobilateral,and mature pollen grains are 2-celled． The overall observa tion on formation of microspores and development of male gametes made in this work is the first for woody peonies．In addition,we observed in the present work abnormal phenomena at meiosis of microspore mother cells and uninucleate microspores,and also made measure ments of fertility of pollen grains．The results show that their fertility ranges from 45.0％ to 84.2％ and varies among flowers,among anthers in a flower and among microsporangia in an anther． The abnormal processes found in development of male gametes are not considered as an important factor responsible for the endangerment of the species．     

OsATG7 is required for autophagy-dependent lipid metabolism in rice postmeiotic anther development

In flowering plants, the tapetum, the innermost layer of the anther, provides both nutrient and lipid components to developing microspores, pollen grains, and the pollen coat. Though the programmed cell death of the tapetum is one of the most critical and sensitive steps for fertility and is affected by various environmental stresses, its regulatory mechanisms remain mostly unknown. Here we show that autophagy is required for the metabolic regulation and nutrient supply in anthers and that autophagic degradation within tapetum cells is essential for postmeiotic anther development in rice. Autophagosome-like structures and several vacuole-enclosed lipid bodies were observed in postmeiotic tapetum cells specifically at the uninucleate stage during pollen development, which were completely abolished in a retrotransposon-insertional OsATG7 (autophagy-related 7)-knockout mutant defective in autophagy, suggesting that autophagy is induced in tapetum cells. Surprisingly, the mutant showed complete sporophytic male sterility, failed to accumulate lipidic and starch components in pollen grains at the flowering stage, showed reduced pollen germination activity, and had limited anther dehiscence. Lipidomic analyses suggested impairment of editing of phosphatidylcholines and lipid desaturation in the mutant during pollen maturation. These results indicate a critical involvement of autophagy in a reproductive developmental process of rice, and shed light on the novel autophagy-mediated regulation of lipid metabolism in eukaryotic cells.     

Observation of Androgenesis in Cultured Wheat Anthers at Meiosis,Tetrad, Early Uninucleate and Trinucleate Stage

The obtaining of calluses and plantlets from cultured wheat anthersat the stages from pollen mother cell to trinucleate microspore has been reported previously. Haploids as well as diploids existed among the regenerated plantlets derivedfrom anthers at these stages. Present paper reports the study on androgenesis patter-ns of cultured anthers at meiosis, tetrad, early mid- and late uninucleate and trinucleate stage. Cytological evidence of pollen-origin of calluses produced by anthers atthese stages was given. Observation showed that meiosis of wheat anthers was able tocomplete under culture conditions, resulting in releasing microspores, from which multinucleate and multicellular pollen grains formed. In meiosis anthers, abnormal cells,including syncytium and two kinds of binueleate calls were sometimes observed. Theymight be products of abnormal meiosis and abnormal development of tapetum cells. Itwas noted that failure and/or uncomplction of forming callus wall and/or pollen wallin in vitro anthers at meiosis, tetrad and early uninucleate stage occured often. Itmight lead to the low frequency of callus induction. Mature wheat anthers (trinucleate stage) contained both normal and abnormal pollen grains (pollen dimorphism); onlythe abnormal pollen grains developed into embryoids while all the normai trinucleatepollen grains degenerated rapidly. However, the date of the frequency of equal divisionof microspores suggested that abnormal pollen (N pollen, small pollen) could not be theonly source of androgenic pollens in cultured anthers at late uninucleate and other earlier stages.     

Localization of the Ca(2+)-binding protein, Bra r 1, in anthers and pollen tubes

Calcium plays an essential role during pollen development and pollen tube growth, and several Ca(2+)-binding proteins are expressed in anthers. We have previously reported that Brassica pollen allergens encoded by Bra r 1 and Bra r 2 show sequence similarities to Ca(2+)-binding proteins [Toriyama et al. (1995) Plant Mol. Biol. 29: 1157]. Herein, we report that both genes are expressed in the diploid tapetum and haploid microspores, as detected by in situ RNA hybridization. Immunoblot analysis revealed that Bra r 1 and Bra r 2 were accumulated in anthers during pollen development. When pollen grains were suspended in an aqueous solution, both proteins were mainly detected in the pollen extracellular fraction, indicating that Bra r 1 and Bra r 2 are released from the pollen upon hydration. Localization of Bra r 1 was further investigated in sections of anthers and pollen tubes. Bra r 1 was detected in the tapetum, microspores and pollen grains. In longitudinal sections of cross-pollinated pistils. Bra r 1 was detected throughout pollen tubes elongating in transmitting-tissue. These findings suggest that Bra r 1 may be involved in pollen-pistil interaction and pollen tube growth.     

Ultrastructure of microsporogenesis and microgametogenesis in Campsis radicans (L.) Seem. (Bignoniaceae)

In the present study, microsporogenesis, microgametogenesis and pollen wall ontogeny in Campsis radicans (L.) Seem. were studied from sporogenous cell stage to mature pollen using transmission electron microscopy. To observe the ultrastructural changes that occur in sporogenous cells, microspores and pollen through progressive developmental stages, anthers at different stages of development were fixed and embedded in Araldite. Microspore and pollen development in C. radicans follows the basic scheme in angiosperms. Microsporocytes secrete callose wall before meiotic division. Meiocytes undergo meiosis and simultaneous cytokinesis which result in the formation of tetrads mostly with a tetrahedral arrangement. After the development of free and vacuolated microspores, respectively, first mitotic division occurs and two-celled pollen grain is produced. Pollen grains are shed from the anther at two-celled stage. Pollen wall formation in C. radicans starts at tetrad stage by the formation of exine template called primexine. By the accumulation of electron dense material, produced by microspore, in the special places of the primexine, first of all protectum then columellae of exine elements are formed on the reticulate-patterned plasma membrane. After free microspore stage, exine development is completed by the addition of sporopollenin from tapetum. Formation of intine layer of pollen wall starts at the late vacuolated stage of pollen development and continue through the bicellular pollen stage.     

Post-meiotic deficient anther1 (PDA1) encodes an ABC transporter required for the development of anther cuticle and pollen exine in rice

The tapetum of the anther locule encloses the male reproductive cells and plays a supportive role for normal pollen development. However, the underlying mechanism remains less understood. Previously, we identified a complete recessive male sterile mutant, post-meiotic deficient anther1 (pda1), with abnormal postmeiotic tapetal development. In this study we comprehensively characterized pda1. Chemical analysis uncovered that pda1 anther had significant lower levels of cutin monomers and cuticular waxes. PDA1 gene encodes an ATP-binding cassette (ABC) half-transporter, namely OsABCG15, which is conserved from algae to higher plants. In situ RNA hybridization assay showed that PDA1 is strongly expressed in tapetal cells, and weakly in microspores during the anther development. Additionally, the expression of two pollen exine biosynthetic genes CYP704B2 and CYP703A3 was dramatically reduced in pda1 mutant anthers. Altogether, these observations suggest that the tapetum-expressed ABC transporter PDA1 plays a crucial role in secreting lipidic precursors from the tapetum to developing microspores and the anther epidermis.