PubMed · 11231163
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D R Smyth. 2001-02-06. Flower development.. https://doi.org/10.1016/s0960-9822(01)00034-3
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Handedness in plant growth may be most familiar to us when we think of tendrils or twining plants, which generally form consistent right- or left-handed helices as they climb. The petals of several species are sometimes arranged like fan blades that twist in the same direction. Another less conspicuous example is 'circumnutation', the oscillating growth of axial organs, which alternates between a clockwise and an anti-clockwise direction. To unravel molecular components and cellular determinants of handedness, we screened Arabidopsis thaliana seedlings for helical growth mutants with fixed handedness. Recessive spiral1 and spiral2 mutants show right-handed helical growth in roots, hypocotyls, petioles and petals; semi-dominant lefty1 and lefty2 mutants show opposite left-handed growth in these organs. lefty mutations are epistatic to spiral mutations. Arabidopsis helical growth mutants with fixed handedness may be impaired in certain aspects of cortical microtubule functions, and characterization of the mutated genes should lead us to a better understanding of how microtubules function in left-right handedness in plants.
There exists extraordinary morphological and genetic diversity among the maize landraces that have been developed by pre-Columbian cultivators. To explain this high level of diversity in maize, several authors have proposed that maize landraces were the products of multiple independent domestications from their wild relative (teosinte). We present phylogenetic analyses based on 264 individual plants, each genotyped at 99 microsatellites, that challenge the multiple-origins hypothesis. Instead, our results indicate that all maize arose from a single domestication in southern Mexico about 9,000 years ago. Our analyses also indicate that the oldest surviving maize types are those of the Mexican highlands with maize spreading from this region over the Americas along two major paths. Our phylogenetic work is consistent with a model based on the archaeological record suggesting that maize diversified in the highlands of Mexico before spreading to the lowlands. We also found only modest evidence for postdomestication gene flow from teosinte into maize.
On the basis of "cell theory," we tend to think that some changes in cellular behavior must be responsible for mutant morphology. According to the cell theory, the unit of morphogenesis of a multicellular organism is the cell. Another interpretation of morphogenesis of plants is the "organismal theory," which postulates that the individual cell is not the basic unit of morphogenesis. Here we examine the validity of the cell and organismal theories, with particular emphasis on the phenotypes of mutant or transgenic Arabidopsis plants with altered leaf morphology. Genetic evidence shows that a compensatory system(s) is involved in leaf morphogenesis, and an increase in cell volume might be triggered by a decrease in cell number. Such evidence provides a new link between cell and organismal theories. In conclusion, the size and number of leaf cells affect the dimensions and sizes of leaves. Moreover, the leaf size is, at least to some extent, uncoupled from the size and number of cells by the compensatory system(s).