Gametophyte interaction and sexual reproduction: how plants make a zygote

The evolutionary success of higher plants relies on a very short gametophytic phase, which underlies the sexual reproduction cycle. Sexual plant reproduction takes place in special organs of the flower: pollen, the male gametophyte, is released from the anthers and then adheres, grows and interacts along various tissues of the female organs, collectively known as the pistil. Finally, it fertilizes the female gametophyte, the embryo sac. Pollen is released as bi or tricellular, highly de-hydrated and presumably containing all the biochemical components and transcripts to germinate. Upon hydration on the female tissues, it develops a cytoplasmic extension, the pollen tube, which is one of the fastest growing cells in nature. Pollen is completely "ready-to-go", but despite this seemingly simple reaction, very complex interactions take place with the female tissues. In higher animals, genetic mechanisms for sex determination establish striking developmental differences between males and females. In contrast, most higher plant species develop both male and female structures within the same flower, allowing self-fertilization. Outcrossing is ensured by self-incompatibility mechanisms, which evolved under precise genetic control, controlling self-recognition and cell-to-cell interaction. Equally important is pollen selection along the female tissues, where interactions between different cell types with inherent signalling properties correspond to check-points to ensure fertilization. Last but not least, pollen-pistil interaction occurs in a way that enables the correct targeting of the pollen tubes to the receptive ovules. In this review, we cover the basic mechanisms underlying sexual plant reproduction, from the structural and cellular determinants, to the most recent genetic advances

The effects of cutting frequency of guinea grass (Panicum maximum Jacq.) at 30, 45, 60, and 75 days, on the dry matter production, digestibility and mineral contents.

Young Guinea grass plants were transplanted to a soil of the series "Luiz de Queiroz", belonging to Terra Roxa Estruturada (Alfisol) group at Piracicaba, SP, Brasil. On the forthcoming day after planting, each plant received by dressing 2.4 g N (ammonium sulphate), 3.2 g P2O5 (normal superphosphate) and 1.9 g K(2)0 (potassium chloride). The plants were cut at the height of 10 cm from the ground and divided into leaves and stems. The dry matter contents were obtained at 609C. Digestibility was determined in vivo by the nylon bag technique. The mineral contents were determined by conventional laboratory methods for N, P, K, Ca, Mg, S, Cu, Mn, and Zn. The experimental designed was randomised blocs with three replications. CONCLUSIONS Dry matter production The relation steii/Leaf was altered by the aging of the plant. The dry matter contents were higher in the stems than in the leaves. A linear positive relation was observed between dry matter production and age of the plants. Digestibility This decreases with aging of the grass. No differences among leaves and stems were observed. Minerals The concentration of N, P, Cu and Zn decreased with aging of the grass. Concentrations of K, Ca, Mg, S and Mn were not affecred by aging of the grass. Animal nutrition The minimum mineral nutrient requiriments for large animals were not affected by cutting frequency of the grass.Mudas de capim colonião (Panicum maximum Jacq.) foram transplantadas para uma Terra Roxa Estruturada, serie "Luiz de Queiroz", em Piracicaba, SP, e adubadas na razão de 2,5 g de N (sulfato de amônio), 3,2 g de P2O5 (superfosfato simples) e 1,9 g de K2O (cloreto de potássio) por muda. Cortes foram efetuados aos 35, 45, 60 e 75 dias a 15 cm de altura do solo. No material coletado foram separadas hastes e folhas e determinado o peso da matéria seca, o coeficiente de digestibilidade in vitro e as concentrações de N, P, K, Ca, Mg, S, Cu, Fe, Mn e Zn. O delineamento estatístico foi de blocos ao aca so, com três repetições. Concluíram os autores: 1. a relação haste/folha varia com o envelhecimento da planta. A produção de matéria seca e maior nas hastes do que nas folhas. Há uma relação linear positiva entre o aumento de peso da matéria seca e a idade da planta; 2. o coeficiente de digestibilidade da matéria seca diminui com a idade da planta, não havendo diferenças entre haste e folha; 3. a concentração de N, P, Cu, Fe e Zn diminui com o aumento da idade da planta; 4. a idade da planta não afeta os teores mínimos exigidos pelo animal

Localized Intersections of Non-Extremal p-branes and S-branes

A class of solutions to Supergravity in 10 or 11 dimensions is presented which extends the non-standard or semi-local intersections of Dp-branes to the case of non-extremal p-branes. The type of non-extremal solutions involved in the intersection is free and we provide two examples involving black-branes and/or D-\bar{D} systems. After a rotation among the time coordinate and a relatively transverse radial direction the solutions admit the interpretation of an intersection among D-branes and S-branes. We speculate on the relevance of these configurations both to study time dependent phenomena in the AdS/CFT correspondence as well as to construct cosmological brane-world scenarios within String Theory admitting accelerating expansion of the Universe.Comment: 31 pages, latex file; v2: typos corrected and references adde

Prospects for the CERN Axion Solar Telescope Sensitivity to 14.4 keV Axions

The CERN Axion Solar Telescope (CAST) is searching for solar axions using the 9.0 T strong and 9.26 m long transverse magnetic field of a twin aperture LHC test magnet, where axions could be converted into X-rays via reverse Primakoff process. Here we explore the potential of CAST to search for 14.4 keV axions that could be emitted from the Sun in M1 nuclear transition between the first, thermally excited state, and the ground state of 57Fe nuclide. Calculations of the expected signals, with respect to the axion-photon coupling, axion-nucleon coupling and axion mass, are presented in comparison with the experimental sensitivity.Comment: 4 pages, 1 figure. Submitted to Nucl. Instr. and Meth.

Magnetism in Herbig Ae/Be stars and the link to the Ap/Bp stars

Among the A/B stars, about 5% host large-scale organised magnetic fields. These magnetic stars show also abundance anomalies in their spectra, and are therefore called the magnetic Ap/Bp stars. Most of these stars are also slow rotators compared to the normal A and B stars. Today, one of the greatest challenges concerning the Ap/Bp stars is to understand the origin of their slow rotation and their magnetic fields. The favoured hypothesis for the latter is that the fields are fosils, which implies that the magnetic fields subsist throughout the different evolutionary phases, and in particular during the pre-main sequence phase. The existence of magnetic fields at the pre-main sequence phase is also required to explain the slow rotation of Ap/Bp stars. During the last 3 years we performed a spectropolarimetric survey of the Herbig Ae/Be stars in the field and in young clusters, in order to investigate their magnetism and rotation. These investigations have resulted in the detection and/or confirmation of magnetic fields in 8 Herbig Ae/Be stars, ranging in mass from 2 to nearly 15 solar masses. In this paper I will present the results of our survey, as well as their implications for the origin and evolution of the magnetic fields and rotation of the A and B stars.Comment: To appear in the proceedings of "Ecole d'astronomie du CNRS et du PNPS : Les champs magnetiques stellaires", editors: C. Neiner and J.-P. Zahn, EAS Publications Serie