Triporate pollen in the Arecaceae

Triapertury is rare in monocotyledons. The well-defined, regularly spaced, circular porate apertures that occur in Arecaceae: Areca klingkangensis from Borneo, and species of the West African genus Sclerosperma, appear to be unique in monocotyledons. There is evidence to suggest that tripory in Arecaceae has been derived from trichotomosulcy, although in Areca equatorial zonosulcy may have an important role. The apical triporate, and zonosulcate pollen of Areca are described, as well as examples of mono- and trichotomosulcate pollen within the genus. The sub-apical distal triporate pollen of Sclerosperma gilletii and S. mannii are described. Notably, in Sclerosperma pollen, aperture position at post-meiotic tetrad stage follows the rare ‘Garside's rule’ (four groups of three apertures), previously only demonstrated for Proteaceae and Olacaceae. Possible reasons for the occurrence of these rare triporate pollen phenomena in palms are considered. The bearing this may have on the transition from the distal polar position of the single sulcus, to the radial symmetry of the triaperturate condition in many dicotyledons is discussed in comparison with other examples of triapertury in monocotyledons.     

Constraints and selection: insights from microsporogenesis in Asparagales

Developmental constraints have been proposed to interfere with natural selection in limiting the available set of potential adaptations. Whereas this concept has long been debated on theoretical grounds, it has been investigated empirically only in a few studies. In this article, we evaluate the importance of developmental constraints during microsporogenesis (male meiosis in plants), with an emphasis on phylogenetic patterns in Asparagales. Different developmental constraints were tested by character reshuffling or by simulated distributions. Among the different characteristics of microsporogenesis, only cell wall formation appeared as constrained. We show that constraints may also result from biases in the correlated occurrence of developmental steps (e.g., lack of successive cytokinesis when wall formation is centripetal). We document such biases and their potential outcomes, notably the establishment of intermediate stages, which allow development to bypass such constraints. These insights are discussed with regard to potential selection on pollen morphology.     

绿色荧光蛋白基因结合鼠Talin基因蛋白标记转基因水稻未成熟花粉的微丝骨架

利用绿色荧光蛋白基因结合鼠Talin基因表达技术及水稻转基因技术,在未成熟花粉发育期(即生殖细胞在形成后从靠壁部位移向中央部位的阶段)的水稻(Oryza sativa L.)内发现了一系列前人未曾报道过的微丝骨架的形成和多变过程.在这一发育阶段,未成熟花粉内的生殖细胞呈圆形,中央部位存有一个大液泡,大量微丝在细胞的中央胞质内形成.微丝首先在营养核的核膜表面形成两个集结中心,中心内的微丝呈短粗状.尔后,中心微丝不断延长,最终在细胞中央的胞质内形成一个非常复杂的类似多个纺锤体结合在一起的网络结构.这一网络的中间部位经常包围着营养核和生殖细胞,网络的部分微丝则与存在周缘细胞质(或称周质)的微丝网络形成连接,在连接点部位则形成一些由微丝环状组成的结构.未成熟花粉中央的微丝网络可能与营养核和生殖细胞在未成熟花粉内的运动有密切关系.     

Pollen presenters in the flowering plants–form and function

Pollen presenters are specific floral structures, other than anthers, from which pollen is distributed for cross fertilization between flowers. They occur in only five families of monocotyledons and 20 families of the dicotyledons. Presenters in 15 families are described here. In the largest plant family (Asteraceae) all taxa have pollen presenters, while in others (e.g. Myrtaceae) they occur in only some species in a few genera. Most presenters are associated with the gynoecium and there is a wide range of forms involving hairs or outgrowths of the stylar tissue. Despite the placement of self pollen close to the stigma most taxa are outcrossing and avoid self fertilization by protandry, with the stigma being covered at anthesis, or by precise placement of pollen so that is does not contact the stigma. Likely selective advantages in the development of pollen presenters include greater accuracy in pollen transfer than in the normal anther to stigma movement and avoidance of interference between male and female organs in the flower. In some groups there is enhanced ability for female choice because effective pollen delivery enables a choice from among many pollen grains, while in others pollen presenters enhance male reproductive success. Study of the pollination biology of plants requires a knowledge of which plants have pollen presenters and a full understanding of the structures in the flower which are associated with the pollen presentation action.     

Ultrastructure of Male Gametophyte in wheat I. The Formation of Generative and Vegetative Cells

The present study of the formation of the generative and vegetative cells in wheat has demonstrated some cytological details at the ultrastructural level. The phragmoplast formed in telophase of the first microsporic mitosis extended centrifugally until it connected with the intine of the pollen grain. A new cell wall was then formed to separate the generative and the vegetative cells. By unequal cytokinesis the former is small and the latter large. In early developmental stage of male gametophyte, the organelles in the cytoplasm of the generaVive cell and the vegetative cells are similar, including mitochondria, dictyosomes, rough endoplasmic retieulum, free and clustered ribosomes and plastids, but microtubules were observed only in the early cytokinesis stage. In the further developmental stage of the male gemetophyte, the generative cell gradually detached from the intine of pollen grain and grew inward to the cytoplasm of the vegetation cell. When the generative cell became round and free in the cytoplasm of the vegetative cell, the wall materials between plasma membranes of the cytoplasm of the generative and the vegetative cells disappeared completely, so that it was a naked cell with a double-layer membrane at this time. The heterogeneity between both cells was then very conspiceous. The organelles in the cytoplasm of the generative cell have hardly any changed besides the degeneration of plastids, but in vegetative cytoplasm the mitochondria and plastids increased dramatically both in number and size. The rapid deposition of starch in the plastids of the cytoplasm of the vegetative cell made the most conspicuous feature of the vegetative cell in mature pollen grain. The significance of the presence of a temporary cell wall in generative cell and heterogeneity between generative and vegetative cells are discussed.     

Characterization of genes expressed during the development ofBrassica napus pollen

Summary The study of the formation of pollen in plants has been the focus of extensive morphologic and cytologic observations. This complex developmental process requires the coordinated activity of both gametophytic and sporophytic tissues. The events that occur during microspore development represent a carefully orchestrated program of physiologic, biochemical, and genetic activities. Genes expressed specifically in pollen or in sporophytic tissues that support pollen development have only recently been identified and desribed. In the present paper we describe several genes expressed during pollen development in the important oil seed speciesBrassica napus (oil seed rape/canola). The characterization of three gene families expressed during microspore development is reviewed which provides a basis for comparison with other genes expressed during pollen maturation. The, potential value of these genes for the development of novel plant breeding strategies and hybrid seed production is discussed. Presented in the Session-In-Depth In vitro, Gametophyte Biology at the 1991 World Congress on Cell and Tissue Culture held in Anaheim, CA, June 16–20, 1991.     

油松细胞质遗传的细胞学机理：Ⅰ.雄配子的超微结构和细胞器类核

证明了油松（Ｐｉｎｕｓ ｔａｂｕｌａｅｆｏｒｍｉｓ Ｃａｒｒ．）雄配子存在质体物线粒体及细胞器ＤＮＡ,提供了油松具父系质体和线粒体遗传基础的确切的细胞学证据,结果与松科植物在遗传学上已确定的父系质体遗传的一般规律是一致的。但精细胞中的线粒体是否能传递至胚,还需要追踪其后的发育过程。另一重要的结果是揭示了油松的雄配子是细胞,这与以前将松科植物雄配子归入雄核（精核）的类型不同。精细胞无壁,仅被质膜包围     

Analyses of phylogeny, evolution, conserved sequences and genome-wide expression of the ICK/KRP family of plant CDK inhibitors

Background and Aims

The cell cycle is controlled by cyclin-dependent kinases (CDKs), and CDK inhibitors are major regulators of their activities. The ICK/KRP family of CDK inhibitors has been reported in several plants, with seven members in arabidopsis; however, the phylogenetic relationship among members in different species is unknown. Also, there is a need to understand how these genes and proteins are regulated. Furthermore, little information is available on the functional differences among ICK/KRP family members.

Methods

We searched publicly available databases and identified over 120 unique ICK/KRP protein sequences from more than 60 plant species. Phylogenetic analysis was performed using 101 full-length sequences from 40 species and intron–exon organization of ICK/KRP genes in model species. Conserved sequences and motifs were analysed using ICK/KRP protein sequences from arabidopsis (Arabidopsis thaliana), rice (Orysa sativa) and poplar (Populus trichocarpa). In addition, gene expression was examined using microarray data from arabidopsis, rice and poplar, and further analysed by RT-PCR for arabidopsis.

Key Results and Conclusions

Phylogenetic analysis showed that plant ICK/KRP proteins can be grouped into three major classes. Whereas the C-class contains sequences from dicotyledons, monocotyledons and gymnosperms, the A- and B-classes contain only sequences from dicotyledons or monocotyledons, respectively, suggesting that the A- and B-classes might have evolved from the C-class. This classification is also supported by exon–intron organization. Genes in the A- and B- classes have four exons, whereas genes in the C-class have only three exons. Analysis of sequences from arabidopsis, rice and poplar identified conserved sequence motifs, some of which had not been described previously, and putative functional sites. The presence of conserved motifs in different family members is consistent with the classification. In addition, gene expression analysis showed preferential expression of ICK/KRP genes in certain tissues. A model has been proposed for the evolution of this gene family in plants.     

Cytological Mechanism of Cytoplasmic Inheritance in Pinus tabulaeformis: I. Ultrastructural Aspects and Nucleoids of the Male Gametes

The cytological mechanism of plastid and mitochondrion inheritance in Pinus is an interesting research topic with only a limited number of published articles. The results indicate that the sperms of Pinus tabulaeformis Carr. contain abundant plastids, mitochondria and organelle DNA. These data provide reliable cytological evidence of paternal plastid and mitochondrion inheritance in Pinus. The results are in line with the confirmed general rule of paternal plastid inheritance in Pinaceae. But whether mitochondria in sperm cells can be transmitted into the embryos is an issue needs further developmental studies. Another important finding is that contrary to earlier classification of the male gamete of Pinaceae into the male nuclei type, the results reveal that male gametes in Pinus tabulaeformis are actually cells. However, the sperm cells are only surrounded by plasma membranes without cell walls. The larger leading sperm cell in a pollen tube section is long in shape, with a large amount of cytoplasm; while the second sperm cell is smaller, round in shape and contains less cytoplasm. Whether this feature of the male gamete type could be considered as a representative characteristic of the family is discussed and further conclusions await more experimental evidences from studies on plants from different species.     

Multiple developmental pathways leading to a single morph: monosulcate pollen (examples from the Asparagales)

BACKGROUND AND AIMS: Early developmental events in microsporogenesis are known to play a role in pollen morphology: variation in cytokinesis type, cell wall formation, tetrad shape and aperture polarity are responsible for pollen aperture patterning. Despite the existence of other morphologies, monosulcate pollen is one of the most common aperture types in monocots, and is also considered as the ancestral condition in this group. It is known to occur from either a successive or a simultaneous cytokinesis. In the present study, the developmental sequence of microsporogenesis is investigated in several species of Asparagales that produce such monosulcate pollen, representing most families of this important monocot clade. METHODS: The developmental pathway of microsporogenesis was investigated using light transmission and epifluorescence microscopy for all species studied. Confocal microscopy was used to confirm centripetal cell plate formation. KEY RESULTS: Microsporogenesis is diverse in Asparagales, and most variation is generally found between families. It is confirmed that the whole higher Asparagales clade has a very conserved microsporogenesis, with a successive cytokinesis and centrifugal cell plate formation. Centripetal cell wall formation is described in Tecophilaeaceae and Iridaceae, a feature that had so far only been reported for eudicots. CONCLUSIONS: Monosulcate pollen can be obtained from several developmental pathways, leading thus to homoplasy in the monosulcate character state. Monosulcate pollen should not therefore be considered as the ancestral state unless it is produced through the ancestral developmental pathway. The question about the ancestral developmental pathway leading to monosulcy remains open.     

Dynamics of vegetative cytoplasm during generative cell formation and pollen maturation inArabidopsis thaliana

Summary Ultrastructural changes of pollen cytoplasm during generative cell formation and pollen maturation inArabidopsis thaliana were studied. The pollen cytoplasm develops a complicated ultra-structure and changes dramatically during these stages. Lipid droplets increase after generative cell formation and their organization and distribution change with the developmental stage. Starch grains in amyloplasts increase in number and size during generative and sperm cell formation and decrease at pollen maturity. The shape and membrane system of mitochondria change only slightly. Dictyo-somes become very prominent, and numerous associated vesicles are observed during and after sperm cell formation. Endoplasmic reticulum appears extensively as stacks during sperm cell formation. Free and polyribosomes are abundant in the cytoplasm at all developmental stages although they appear denser at certain stages and in some areas. In mature pollen, all organelles are randomly distributed throughout the vegetative cytoplasm and numerous small particles appear. Organization and distribution of storage substances and appearance of these small particles during generative and sperm cell formation and pollen maturation are discussed.     

The Arabidopsis U–box/ARM repeat E3 ligase AtPUB4 influences growth and degeneration of tapetal cells,and its mutation leads to conditional male sterility

Pollen formation is a complex developmental process that has been extensively investigated to unravel underlying fundamental developmental mechanisms and for genetic manipulation of the male‐sterility trait for hybrid crop production. Here we describe identification of AtPUB4, a U–box/ARM repeat‐containing E3 ubiquitin ligase, as a novel player in male fertility in Arabidopsis. Loss of AtPUB4 function causes hypertrophic growth of the tapetum layer. The Atpub4 mutation also leads to incomplete degeneration of the tapetal cells and strikingly abnormal exine structures of pollen grains. As a result, although the Atpub4 mutant produces viable pollen, the pollen grains adhere to each other and to the remnants of incompletely degenerated tapetal cells, and do not properly disperse from dehisced anthers for successful pollination. We found that the male‐sterility phenotype caused by the Atpub4 mutation is temperature‐dependent: the mutant plants are sterile when grown at 22°C but are partially fertile at 16°C. Our study also indicates that the AtPUB4‐mediated pathway acts in parallel with the brassinosteroid pathway in controlling developmental fates of the tapetal cells to ensure male fertility.     

Pollenkitt – its composition, forms and functions

Two types of sticky pollen coat material exist in angiosperms, both produced by the anther tapetum. Pollenkitt is the most common adhesive material present around pollen grains of almost all angiosperms pollinated by animals, whereas tryphine seems to be restricted only to Brassicaceae. Tapetal cell protoplasts have different patterns of development according to the products formed during their development and degeneration. If tryphine is formed, the tapetal cell protoplasts lose their individuality at the microspore stage. If pollenkitt is formed, their contents degenerate at later stages. Cell content is totally reabsorbed, when ripe pollen is not surrounded by any gluing material. Current knowledge of pollenkitt formation, deposition on pollen grains and chemical composition are reviewed and discussed. Methods for detecting this viscous fluid are also presented. The many functions of pollenkitt, deduced from personal observations and the literature, act in the period between anther opening and pollen hydration on the stigma; they are: (1) to hold pollen in the anther until dispersal; (2) to enable secondary pollen presentation; (3) to facilitate pollen dispersal; (4) to protect pollen from water loss; (5) to protect pollen from ultra-violet radiation; (6) to maintain sporophytic proteins responsible for pollen–stigma recognition inside exine cavities; (7) to protect pollen protoplasts from fungi and bacteria; (8) to keep together pollen grains during transport; (9) to protect pollen from hydrolysis and exocellular enzymes; (10) to render pollen attractive to animals; (11) to render pollen visible to animal eyes; (12) to hide pollen from animal eyes; (13) to avoid predation of pollen through smell; (14) to enable adhesion to insect bodies; (15) to enable pollen packaging by bees and to form corbicules; (16) to provide a digestible reward for pollinators; (17) to enable pollen clumps to reach the stigma; (18) to allow self-pollination; (19) to facilitate adhesion to the stigma; (20) to facilitate pollen rehydration. Depending on the developmental program of the species, these functions may act during pollen presentation, in relation to pollinators, during pollen dispersal and when pollen reaches the stigma.     

Is the ‘rejection reaction’ inducing ability in sporophytic self-incompatible systems restricted only to pollen and tapetum?

Summary The present investigation describes the stigmatic rejection response, induced by anther or pollen at developmental stages ranging from anther primordia to mature pollen, as well as in other floral and vegetative tissues. This has been studied in a sporophytic self-incompatible system, Brassica campestris (Brassicaceae). The implications of these observations have been discussed.     

PREM-1, a putative maize retroelement has LTR (long terminal repeat) sequences that are preferentially transcribed in pollen

A family of highly dispersed repetitive elements, designated PREM-1, which are transcribed primarily during pollen development, has been identified in maize. Sequence data from six PREM-1-containing genomic clones suggest that the PREM-1 sequences are the LTRs of a family of putative retroelements. PREM-1 LTRs are estimated to be present in about 10 000 to 40 000 copies in the maize genome. Although related sequences have been detected in sorghum and crab grass, highly homologous sequences appear to be specific to the genus Zea (maize and teosinte). A diverse group of RNAs that contain portions of the PREM-1 sequence at their 3 ends are transcribed in pollen; highest levels appear in early uninucleate microspores. The PREM-1-containing cDNAs do not appear to code for protein products since stop codons are present in all three reading frames. The possible significance of expression of retroelements in the male gametophyte, in terms of transposition of DNA, is discussed.     

Triaperturate pollen in the monocotyledons: configurations and conjectures

Triaperturate pollen are known in at least twenty seven genera of monocotyledons. Differences between aperture type and polarity indicate that the development of three apertures has occurred a number of times. Mode of cytokinesis during microsporogenesis is compared with differences in aperture configuration, to assess the extent to which this appears to influence aperture arrangement. Triapertury in monocot pollen tends to fall into one or another of three situations: 1) it is the normal state, 2) it is fairly common, but pollen with more or less apertures also occur in the taxon or sample, 3) it is a rare, or abnormal state for pollen which usually has less than three apertures. The various forms of triaperturate pollen are described, as well as monosulcate pollen of the orchid genera Cypripedium and Paphiopedilum, often misinterpreted as tri-sulcate, and the unusual extended trichotomosulcate pollen of Agrostocrinum (Hemerocallidaceae). Monosulcy, trichotomosulcy, and zonasulcy, with unusual and rare exceptions of zonasulcy in the eudicots, are aperture states shared exclusively with the basal dicots. Furthermore, to some extent all have links with the triaperturate condition in monocots and basal dicotyledons. This is discussed, as well as the association of tripory with polypory in monocots and basal dicots. The fossil pollen record is considered.This paper is dedicated to Klaus Kubitzki in recognition, not only for his extensive contribution to systematic botany, but also for his firm belief that pollen characteristics contribute to a better understanding of plant systematics and evolution.     

绿色荧光蛋白基因结合鼠Talin基因蛋白标记转基因水稻未成熟花粉的微丝骨架

利用绿色荧光蛋白基因结合鼠Talin基因表达技术及水稻转基因技术,在未成熟花粉发育期（即生殖细胞在形成后从靠壁部位移向中央部位的阶段）的水稻（Oryza sativa L.)内发现了一系列前人未曾报道过的微丝骨架的形成和多变过程。在这一发育阶段,未成熟花粉内的生殖细胞呈圆形,中央部位存有一个大液泡,大量微丝在细胞的中央胞质内形成。微丝首先在营养核的核膜表面形成两个集结中心,中心内的微丝呈短粗状。尔后,中心微丝不断瞎长,最终在细胞中央的胞质内形成一个非常 类似多个纺锤体结合在一起的网络结构。这一网络的中间部位经常包围着营养核和生殖细胞,网络的部分微丝则与存在周缘细胞质（或称周质）的微丝网络形成连接,在连接点部位则形成一些由微丝环状组成的结构。未成熟花粉中央的微丝网络可能与营养核和生殖细胞在未成熟花粉内的运动有密切关系。