In addition to important differences between plant and animal development, such as plant cells having rigid cell walls that are unable to migrate and the continuous growth of mature plants as opposed to the fixed size of adult animals, there are also several unanticipated similar ities. Although we do not yet have sufficient information to make detailed comparisons between plant and animal embryonic development, some striking parallels are already apparent, particularly given the significant mechanistic differences in how plant and animal embryos undergo morphological changes. One notable similarity is that typical animal and plant embryos are organized into three fundamental germ layers (i.e., ectoderm, mesoderm, and endoderm in animals, and epidermis, cortex, and vasculature in plants). Although this similar basic organization could reflect a need for embryos to be organized into a small number of tissue layers, there does not seem to be anything particularly magical about three. For example, in animals, there are several different mechanisms by which the three primary tissue layers are generated during embryogenesis. In some cases, the mesoderm invaginates before the endoderm (e.g., flies), whereas in other organisms, these tissues invaginate in a concerted fashion (e.g., frogs). In addition, during eye development, distinct series of inductive events and morphological transformations generate the varied and complex arrangements of cell layers characterizing the eyes of diverse species (see Fig. 6.5), indicating once more that it is possible to generate multiple tissue layers by various combinations of invagination and delamination of cells.
Another noteworthy similarity between patterning of early animal and plant embryos is the conspicuous role played by homeobox-type transcription factors. Again, this similarity could be coincidence or could reflect some special property of homeobox proteins that makes them particularly well suited for patterning purposes. This explanation seems unlikely, however, given that there are several other classes of structurally unrelated transcription factors involved in controlling developmental decisions in both plants and animals. Alternatively, the homeobox genes may have played a key developmental role in a common ancestor of plants and animals. In support of this latter possibility, a gene distantly related to plant and animal homeobox genes determines which of two different mating types will be adopted by individual yeast cells.
An intriguing recent finding is the identification of a receptor protein in plants that is clearly related to a receptor in the nervous system of animals for the signal glutamate. Glutamate is among the most widely used signals in our nervous system. The glutamate receptor is a member of a large family of molecules that relay many different kinds of signals in animals cells. Gloria Coruzzi at New York University and Julian Schroeder at UCSD made a very interesting discovery when they identified a cell-surface receptor that is the mustard plant counterpart to the animal glutamate receptor. Although the function of the plant glutamate receptor is unclear, it may be involved in some aspect of responding to light. It is well known, and not surprising, that many types of molecules involved in basic biochemical processes are common to plants and animals. Such molecules typically carry out functions common to all cells, including bacteria and fungi, and therefore have been inherited from an early form of single-cell (unicellular) ancestor of plants and animals that probably evolved more than 3 billion years ago. What is unexpected about a receptor molecule being common to plants and animals is that it begs the question, What was the function of such a protein in a unicellular organism that had no obvious need to communicate with other cells? Perhaps the glutamate receptor evolved independently in plants and animals from a molecule that functioned in a unicellular ancestor to bind the essential amino acid glutamate and transport it into the cell to be used as a building block for synthesizing proteins. This ancestral molecule may not have had any function in receiving signals, but because it could bind glutamate, it was used in two different branches of multicellular life as the starting point from which to evolve a receptor. It also is possible that an ancestral unicellular organism was capable of sending and receiving signals that could have been used for purposes such as building organized colonies of cells such as the ancient stromatolites (>3 billion years old). Stromatolites still exist today in isolated bodies of water (e.g., Shark Bay in Western Australia) that are sheltered from seaweeds and animals (for example, in water that has double the salinity of normal seawater) and can grow to be over a meter tall. Remnants of these ancient stromatolite colonies, which formed giant reefs, are still visible today as cliffs hundreds of feet high.
Finally, another titillating parallel between plant and animal cell communication pointed out by Elliot Meyerowitz is that the receptor encoded by the plant gene CLV1 (described above) is a member of a large family of receptors present in plants, which also includes receptors involved in recognizing plant pathogens. In response to pathogens such as bacteria, plant cells produce a cocktail of toxins to kill infected cells and stop the spread of infection. The family of plant immunity receptors including CLV1 is distantly related to a family of receptors found in animal cells. This family of receptors includes a member in flies that functions during formation of the egg in the mother to concentrate the Dorsal morphogen in ventral cells of early embryos. Intriguingly, this same fly receptor is involved in protecting larvae from bacterial infection, and when activated, leads to the production of bactericidal proteins to limit infection. In addition, the same signaling pathway is present in humans where it mediates an acute-phase immune response. Did the same type of receptor get selected independently in plant and animal lineages to carry out both immune and developmental patterning functions? Alternatively, was this receptor used in an ancient unicellular organism to combat other microorganisms?
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