Quantitative traits, such as size and weight in animals and seed yield in plants, are distributed normally, even within a population of genetically identical individuals. For example, in plants, various factors, such as local soil quality, microclimate, and sowing depth, affect growth differences among individual plants of isogenic populations. Besides these physical factors, also epigenetic components contribute to differences in growth and yield. The network that regulates crop yield is still not well understood. Although this network is expected to have epigenetic elements, it is completely unclear whether it would be possible to shape the epigenome to increase crop yield. Here we show that energy use efficiency is an important factor in determining seed yield in canola (Brassica napus) and that it can be selected artificially through an epigenetic feature. From an isogenic canola population of which the individual plants and their self-fertilized progenies were recursively selected for respiration intensity, populations with distinct physiological and agronomical characteristics could be generated. These populations were found to be genetically identical, but epigenetically different. Furthermore, both the DNA methylation patterns as well as the agronomical and physiological characteristics of the selected lines were heritable. Hybrids derived from parent lines selected for high energy use efficiencies had a 5% yield increase on top of heterosis. Our results demonstrate that artificial selection allows the increase of the yield potential by selecting populations with particular epigenomic states. Brassica napus ͉ epigenome
Several processes during sexual reproduction in higher plants involve the movement of water between cells or tissues. Before flower anthesis, anther and pollen dehydration takes place before the release of mature pollen at dehiscence. Aquaporins represent a class of proteins that mediates the movement of water over cellular membranes. Aquaporins of the plasmamembrane PIP2 family are expressed in tobacco (Nicotiana tabacum) anthers and may therefore be involved in the movement of water in this organ. To gain more insight into the role these proteins may play in this process, we have analyzed their localization using immunolocalizations and generated plants displaying RNA interference of PIP2 aquaporins. Our results indicate that PIP2 protein expression is modulated during anther development. Furthermore, in tobacco PIP2 RNA interference plants, anther dehydration was slower, and dehiscence occurred later when compared with control plants. Together, our results suggest that aquaporins of the PIP2 class are required for efficient anther dehydration prior to dehiscence.In plant sexual reproduction, control of water movement plays an important role. During development, for example, young anthers must first take up water for growth, but at later stages anthers and pollen dehydrate before dehiscence. In tobacco (Nicotiana tabacum) and many other species, pollen dehydration starts after the degeneration of the tapetum, and pollen grains are partially dehydrated at dehiscence (Goldberg, 1988;Franchi et al., 2002). The significance of correct pollen dehydration for the functions of the pollen grain is illustrated by the Arabidopsis (Arabidopsis thaliana) mutant raring-to-go (Johnson and McCormick, 2001). In these plants, pollen grains do not dehydrate as wild-type pollen and germinate within the anther when it is exposed to high humidity. However, besides this example, not much is known about the detailed mechanism of pollen dehydration.Correct timing of anther dehiscence is important, as the time of pollen release is crucial for successful fertilization. It was shown that plant hormones are implicated in the dehiscence process. In Arabidopsis, mutations in several genes involved in jasmonic acid biosynthesis all result in delayed anther dehiscence, suggesting the involvement of jasmonic acid signaling in its regulation (Sanders et al., 2000;Ishiguro et al., 2001;Park et al., 2002). In tobacco, however, ethylene has been shown to control the timing of anther dehiscence (Rieu et al., 2003). Other processes associated with dehiscence involve the formation of secondary wall thickenings, the consecutive degeneration of various anther tissues, changes in carbohydrate metabolism, and the movement of water out of the anther (Keijzer, 1987;Bonner and Dickinson, 1990;Clement and Audran, 1995;Beals and Goldberg, 1997;Dawson et al., 1999;Steiner-Lange et al., 2003;Scott et al., 2004; see Table I for an overview of tobacco anther development). It has been suggested that the formation of secondary wall thickenings, in combination wi...
Several processes during sexual reproduction in higher plants involve the movement of water between cells or tissues, such as occurs during dehiscence of the anther and hydration of the pollen grain after it is deposited on a stigma. To get more insight in these processes, a set of putative aquaporins was cloned and it was found that at least 15 are expressed in reproductive organs, which indicates that the control of water flow is important for reproduction. Functional studies in Xenopus laevis oocytes using two of the cDNAs showed that NtPIP2;1 is an efficient aquaporin, whereas NtPIP1;1 is not. Expression studies on RNA and protein levels showed that PIP1 and PIP2 genes are differently expressed in reproductive organs: PIP1 RNA accumulates in the stigma, and PIP1 and PIP2 RNA can be detected in most tissues of the anther.
SummaryIn the transgenic tobacco line T17, plants homozygous for the gn1 transgene display developmentally regulated post-transcriptional silencing of basic b-1,3-glucanase genes. Previously, it has been shown that silencing involves a markedly increased turnover of silencingtarget glucanase mRNAs. Using a two-component viral reporter system facilitated a comparison, in a quantitative manner, of the relative silencing ef®ciencies of various sequences derived from the gn1 transgene. The results show that target sites for the silencing mechanism are present throughout the coding region of the gn1 mRNA. Similar-sized coding region sequences along the entire gn1 mRNA display a similar susceptibility to the silencing mechanism. The susceptibility to silencing increases as the coding region elements increase in size. Relative to internal sequences, the 5¢ and 3¢ terminal regions of the gn1 mRNA are inef®cient targets for the silencing machinery. Importantly, sequences of the gn1 transgene that are not part of the mature gn1 mRNA are not recognized by the silencing machinery when expressed in chimeric viral RNAs. These results show that the glucanase silencing mechanism in T17 plants is primarily directed against gn1 mRNA-internal sequences and that terminal sequences of the gn1 mRNA are relatively unaffected by the silencing mechanism.
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