PubMed · 14020986
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B J CHOLNOKY. 1963-06-29. Cell structure and environment.. https://doi.org/10.1111/j.1749-6632.1963.tb13390.x
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Ras proteins function as signaling hubs that are activated by convergent signaling pathways initiated by extracellular stimuli. Activated Ras in turn regulates a diversity of downstream cytoplasmic signaling cascades. Ras proteins are founding members of a large superfamily of small GTPases that have significant sequence and biochemical similarities. Recent observations have established a complex signaling interplay between Ras and other members of the family. A key biochemical mechanism facilitating this crosstalk involves guanine nucleotide exchange factors (GEFs), which serve as regulators and effectors, as well as signaling integrators, of Ras signaling.
In this paper, the stages of normal sexual reproduction between pollen tube penetration of the archegonium and early embryo formation in Pinus tabulaeformis are described, emphasizing the transmission of parental cytoplasm, especially the DNA-containing organelles--plastids and mitochondria. The pollen tube growing in the nucellus contained an irregular tube nucleus followed by a pair of sperm cells. The tube cytoplasm contained abundant organelles, including starch-containing plastids and mitochondria. The two sperm cells differed in their volume of cytoplasm. The leading sperm, with more cytoplasm, contained abundant plastids and mitochondria, while the trailing one, with a thin layer of cytoplasm, had very few organelles. The mature egg cell contained a great number of mitochondria, whereas it lacked normal plastids. At fertilization, the pollen tube penetrated into the egg cell at the micropylar end and released all of its contents, including the two sperms. One of the sperm nuclei fused with the egg nucleus, whereas the other one was retained by the receptive vacuole. Very few plastids and mitochondria of male origin were observed around the fusing sperm and egg nuclei, while the retained sperm nucleus was surrounded by a large amount of male cytoplasm. The discharged tube cytoplasm occupied a large micropylar area in the egg cell. In the free nuclear proembryo, organelles of maternal and paternal origins intermingled in the neocytoplasm around the free nuclei. Most of the mitochondria had the same features as those of the egg cell, but some appeared to be from sperm cells and tube cytoplasm. Plastids were obviously of male origin, with an appearance similar to those of the sperm or tube cells. After cellularization of the proembryo, maternal mitochondria became more abundant than the paternal ones and the plastids enlarged and began to accumulate starch. The results reveal the cytological mechanism for paternal inheritance of plastids and biparental inheritance of mitochondria in Chinese pine.
We examined pollen cells of Wisteria sinensis and Robinia pseudoacacia (Leguminosae) to determine a possible mode for cytoplasmic inheritance in these species. Epifluorescence microscopy revealed distinct mature generative cells. Mature generative cells of W. sinensis were associated with large numbers of punctuated fluorescent signals corresponding to cytoplasmic DNA aggregates, but no fluorescent signals were observed in the generative cells of R. pseudoacacia. Closer examination showed that the punctate fluorescent signals corresponded to plastid but not mitochondrial DNA. These results suggest a strong potential for paternal transmission of the plastid genome in W. sinensis. Electron microscopy confirmed the presence of plastids in the generative cells of W. sinensis and the absence of plastids in R. pseudoacacia cells due to an unequal distribution of plastids during the first pollen mitosis. Mitochondria were present and intact in the mature generative cells of both species. The lack of fluoresced mitochondrial DNA suggests a very low level of mitochondrial DNA in the cells. Immunoelectron microscopy demonstrated that the labeling of mitochondrial DNA in these cells was reduced by nearly 90% during pollen development. Such a dramatic reduction suggests an active degradation of paternal mitochondrial DNA, which may contribute greatly to the maternal inheritance of mitochondria. In short, we found that W. sinensis exhibits a strong potential for paternal transmission of plastids and that both W. sinensis and R. pseudoacacia appear to share the same mechanism for maternal mitochondrial inheritance.