Embryogenesis in plants is a unique process in the sense that it can be initiated from a wide range of cells other than the zygote. Upon stress, microspores or young pollen grains can be switched... Show moreEmbryogenesis in plants is a unique process in the sense that it can be initiated from a wide range of cells other than the zygote. Upon stress, microspores or young pollen grains can be switched from their normal pollen development towards an embryogenic pathway, a process called androgenesis. Androgenesis represents an important tool for research in plant genetics and breeding, since androgenic embryos can germinate into completely homozygous, double haploid plants. From a developmental point of view, androgenesis is a rewarding system for understanding the process of embryo formation from single, haploid microspores. Androgenic development can be divided into three main characteristic phases: acquisition of embryogenic potential, initiation of cell divisions, and pattern formation. The aim of this review is to provide an overview of the main cellular and molecular events that characterize these three commitment phases. Molecular approaches such as differential screening and cDNA array have been successfully employed in the characterization of the spatiotemporal changes in gene expression during androgenesis. These results suggest that the activation of key regulators of embryogenesis, such as the BABY BOOM transcription factor, is preceded by the stress-induced reprogramming of cellular metabolism. Reprogramming of cellular metabolism includes the repression of gene expression related to starch biosynthesis and the induction of proteolytic genes (e.g. components of the 26S proteasome, metalloprotease, cysteine, and aspartic proteases) and stress-related proteins (e.g. GST, HSP, BI-1, ADH). The combination of cell tracking systems with biochemical markers has allowed the key switches in the developmental pathway of microspores to be determined, as well as programmed cell death to be identified as a feature of successful androgenic embryo development. The mechanisms of androgenesis induction and embryo formation are discussed, in relation to other biological systems, in special zygotic and somatic embryogenesis. Show less
Wang, M.; Hoekstra, S.; Bergen, S. van; Lamers, G.E.M.; Oppedijk, B.J.; Heijden, M.W. van der; ... ; Schilperoort, R.A. 1999
Intra-nucleosomal cleavage of DNA into fragments of about 200 bp was demonstrated to occur in developing anthers, in which microspores had developed into the mid-late to late uni-nucleate stage in... Show moreIntra-nucleosomal cleavage of DNA into fragments of about 200 bp was demonstrated to occur in developing anthers, in which microspores had developed into the mid-late to late uni-nucleate stage in situ, i.e. at the verge of mitosis. The same was observed, but to a much larger extent, if these anthers were pre-treated by a hyper-osmotic shock. Pretreatment of anthers before the actual culture of microspores was required for optimal androgenesis of microspores. The use of the TUNEL reaction, which specifically labels 3' ends of DNA breaks, after intra-nucleosomal cleavage of DNA, revealed that DNA fragmentation mainly occurred in the loculus wall cells, tapetum cells and filament cells. TUNEL staining was absent or infrequently observed in the microspores of developing anthers in situ. Electron microscopy studies showed condensed chromatin in nuclei of loculus wall cells in the developing anthers. These observations at the chromatin and DNA level are known characteristics of programmed cell death, also known as apoptosis. Features of apoptosis were infrequently found in microspores from freshly isolated mature anthers. However, most tapetum cells had disappeared in these anthers and the remaining cell structures showed loss of cellular content. The viability of microspores in pre-treated anthers was comparable to those in freshly isolated anthers and almost four times higher than in anthers from control experiments. This observation was correlated with three to four times less microspores showing TUNEL staining and a two times higher level of ABA in the anther plus medium samples than in controls. Addition of ABA to the controls enhanced the viability and lowered the occurrence of apoptosis linked characteristics in the microspores. These data suggest that pre-treatment is effective in stimulating androgenesis because it leads to an increase in ABA levels which protects microspores from dying by apoptosis. Show less
Chin-A-Woeng, T.F.C.; Priester, W. de; Bij, A.J. van der; Lugtenberg, E.J.J. 1997
Capping of Concanavalin A (Con A) receptors can be inhibited in Dictyostelium by treatment of amoebae with the microtubular drug tubulozole. In cells that were incubated with Con A or with... Show moreCapping of Concanavalin A (Con A) receptors can be inhibited in Dictyostelium by treatment of amoebae with the microtubular drug tubulozole. In cells that were incubated with Con A or with fluorescent Con A conjugate the capping process was completed in 30 min as could be demonstrated by fluorescence microscopy and Con A peroxidase labeling. In the presence of 10(-5) M tubulozole redistribution of the receptors did not proceed beyond a stage that can be characterized as patching. The effect of the drug on microtubule integrity was checked by electron microscopy and immunofluorescence of tubulin. Treatment resulted in shortening of the peripheral parts of the microtubules, in agreement with results described by other authors. Electron microscopy confirmed that the Con A receptor complexes remained on the plasma membrane and were not internalized. The distribution of F-actin in Con A-treated cells showed a pattern closely resembling that of Con A. Cells that were also treated with tubulozole remained spherical and did not resume significant directional movement until tubulozole was removed from the medium. It is concluded that microtubules are involved in the rearrangement of the microfilament network in moving cells. Show less
