Premature Tapetum Degeneration: a Major Cause of Abortive Pollen Development in Photoperiod Sensitive Genic Male Sterility in Rice

Photoperiod-sensitive genic male-sterile (PSGMS) rice ( Oryza sativa L.), a natural mutant found in the rice cultivar Nongken 58, is very useful for the development of hybrid rice cultivars. Despite its widespread use in breeding programs, the initial stage of the abortive development of PSGMS rice and the possible cytological mechanisms of pollen abortion have not been determined. In the present study, a systematic cytological comparison of the anther development of PSGMS rice with its normal fertile counterpart is conducted. The results show that pollen abortion in PSGMS rice first occurs before the pollen mother cell (PMC) stage, and continues during the entire process of pollen development until pollen degradation. The abortive process was closely associated with the abnormal behavior of the tapetum. Although tapetum degeneration in PSGMS rice initiates already at the PMC stage, it proceeds slowly and does not complete until the breakdown of the pollen. Such cytological observations were supported by the results of the TUNEL (TdT-mediated dUTP Nick End Labeling) assay, which detects DNA fragmentation resulting from programmed cell death (PCD), indicating that the premature tapetum degeneration is in the process of PCD.     

Expression of CP4 EPSPS in microspores and tapetum cells of cotton (Gossypium hirsutum) is critical for male reproductive development in response to late-stage glyphosate applications

Plants expressing Agrobacterium sp. strain CP4 5-enolpyruvylshikimate-3-phosphate synthase (CP4 EPSPS) are known to be resistant to glyphosate, a potent herbicide that inhibits the activity of the endogenous plant EPSPS. The RR1445 transgenic cotton line (current commercial line for Roundup Ready^® Cotton) was generated using the figwort mosaic virus (FMV) 35S promoter to drive the expression of the CP4 EPSPS gene, and has excellent vegetative tolerance to glyphosate. However, with high glyphosate application rates at developmental stages later than the four-leaf stage (late-stage applications: applications that are inconsistent with the Roundup^® labels), RR1445 shows male sterility. Another transgenic cotton line, RR60, was generated using the FMV 35S promoter and the Arabidopsis elongation factor-1α promoter (AtEF1α) for the expression of CP4 EPSPS. RR60 has excellent vegetative and reproductive tolerance to applications of glyphosate at all developmental stages. Histochemical analyses were conducted to examine the male reproductive development at the cellular level of these cotton lines in response to glyphosate applications, and to investigate the correlation between glyphosate injury and the expression of CP4 EPSPS in male reproductive tissues. The expression of CP4 EPSPS in RR60 was found to be strong in all male reproductive cell types. Conversely, CP4 EPSPS expression in RR1445 was low in pollen mother cells, male gametophytes and tapetum, three crucial male reproductive cell types. Our results indicate that the FMV 35S promoter, although expressing strongly in most vegetative tissues in plants, has extremely low activity in these cell types.     

Contribution of pollen and tapetal characters to the systematics of Triuridaceae

 Pollen and tapetal characters in the mycoheterotrophic monocot family Triuridaceae are here compared with those of their putative relatives, including the lilioid order Pandanales (Pandanaceae, Cyclanthaceae, Velloziaceae and Stemonaceae), with which Triuridaceae have recently been associated following analyses of molecular data. Triuridaceae have small, inaperturate (functionally monoaperturate) pollen grains with the exine reduced to gemmae which have distinctive protruberances or spines. Microsporogenesis is of the successive type. Some genera have a plasmodial tapetum. Orbicules are absent. These characters are compatible with a relationship with Pandanaceae, but a relationship with Alismatales, as suggested by earlier authors, cannot be excluded. Received June 18, 2002; accepted July 22, 2002 Published online: November 22, 2002 Address of the authors: Carol A. Furness (e-mail: c.furness@rbgkew.org.uk), Paula J. Rudall and Alison Eastman, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.     

The structure of anchovy outer retinae (Engraulididae, Clupeiformes) - a comparative light- and electron-microscopic study using museum-stored material

The outer retinal architecture of Engraulididae is uncommon among vertebrates. In some anchovies, e.g., Anchoa, two cone types are arranged alternating in long photoreceptor chains, i.e., polycones. The cones have radially oriented outer segment lamellae in close contact with a complex guanine tapetum, most probably subserving polarization contrast vision. To clarify the distribution of the aberrant polycone architecture within the Engraulididae and to provide indications about polycone evolution, the outer retina morphology of 16 clupeoid species was investigated by light and electron microscopy, predominantly using museum-stored material. The outgroup representatives of four clupeid subfamilies (Clupeonella cultriventris, Dorosoma cepedianum, Ethmalosa fimbriata, Pellonula leonensis) show a row pattern of double cones, partially with single cones at defined positions and a pigment epithelium with lobopodial protrusions containing melanin. The pristigasterid Ilisha africana has double rows of single cones lying between linear curtains of pigment epithelium processes filled with minute crystallites and melanin concentrated near their vitreal tips. Within the Engraulididae, two main architectures are found: Coilia nasus and Thryssa setirostris have linear multiple cones or polycones separated by long pigment epithelium barriers containing tapetal crystallites and melanin in the tips (also found in Setipinna taty), whereas Anchoviella alleni, Encrasicholina heteroloba, Engraulis encrasicolus, Engraulis mordax, Lycengraulis batesii, and Stolephorus indicus exhibit the typical polycone architecture. Cetengraulis mysticetus and Lycothrissa crocodilus show cone patterns and pigment epithelium morphology differing from the other anchovy species. The sets of characters are compared, corroborated with the previous knowledge on clupeoid retinae and discussed in terms of functional morphology and visual ecology. A scenario on polycone evolution is developed that may serve as an aid for the reconstruction of engraulidid phylogeny. Furthermore, this study demonstrates the suitability of museum material for morphological studies, even at the electron microscopic level.     

Morphological and ultrastructural diversity of orbicules in Gentianaceae

Minute granules of sporopollenin, called orbicules, can be observed on the innermost tangential and/or radial walls of secretory tapetum cells. Orbicules were investigated in 53 species of 34 Gentianaceae genera using light microscopy, scanning electron microscopy and transmission electron microscopy. This selection covered all different tribes and subtribes recognized in Gentianaceae (87 genera, +/-1650 species). Orbicules were found in 38 species (23 genera) distributed among the six tribes recognized in Gentianaceae. The orbicule typology is based on those described previously in Rubiaceae. Of the six orbicule types described previously, Type II orbicules are lacking. Type III orbicules are most common (17 species). Hockinia Gardner is the only representative with Type I orbicules. The number of representatives with orbicules belonging to the other orbicule types are equally distributed among the species studied: seven species possess Type IV orbicules, six species Type V and six species Type VI. The systematic usefulness of this typology is discussed in comparison with the latest systematic insights within the family, and palynological trends in Gentianaceae. Orbicule data have proven to be useful for evaluating tribal delimitation within Rubiaceae and Loganiaceae s.l.; however, they seem not to be useful for tribal delimitation in Gentianaceae. In the tribes Potalieae and Gentianeae orbicule data may be useful at subtribal level.     

A novel extinction screen in Arabidopsis thaliana identifies mutant plants defective in early microsporangial development

Few Arabidopsis mutants defective in early male or female germline development have been reported. A novel extinction screen has been devised which permits the identification of mutants deficient in the earliest stages of anther development. Using mutagenized plants carrying GUS reporter constructs driven by tapetal-specific promoters originally derived from Brassica genes, a wide spectrum of mutants have been identified in Arabidopsis, ranging from those defective in archesporial cell differentiation to others expressed later in development. Crosses between these lines and known anther development mutants have enabled the identification of lines carrying mutations in genes expressed during very early anther formation. Initial characterization reveals these early mutants fall into two classes, gne (GUS-negative) 1-like, and gne2-like. Members of the gne1 mutant class initiate all four layers of the anther wall and an appropriate number of sporogenous cells; however, as development proceeds the tapetal and middle-layer cells enlarge, eventually crushing the sporogenous cells. The gne2 class anthers are disrupted at an earlier stage, with the middle and tapetal layers failing to form, and an excess of sporogenous cells developing until the germline aborts late in meiosis II. Analysis of these mutants has already raised questions about the accuracy of current models of angiosperm anther development.