NEWS FROM
PHYSIOLOGIA PLANTARUM
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Published monthly on behalf of SPPS by Wiley-Blackwell.
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Transcriptome reveals phosphate responses
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Microarrays are increasingly being used for global expression studies and over the last few years this has been used to build up substantial information about the plant transcriptome. Using internet-based data ressources from previous analysis on Arabidopsis thaliana, Danish researchers have dissected the complex regulatory network involved in responses to phosphate deprivation. Tom Hamborg Nielsen and co-workers from University of Copenhagen and Aalborg University evaluated the functional relationship between several transcription factors, microRNAs (miRNAs) and feedback loops that contribute to keep P-homeostasis. The authors propose a model for the complex coordinated responses to phosphate starvation, which affect all parts of the plant and include Pi-signalling miRNAs that are transported via the phloem. However, the model still lacks any sensor of P-status, since the precise role of several recent candidates for this crucial function still needs to be verified.
Read full article free: Nilsson et al (June 2010) Physiologia Plantarum 139: 129-143
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NEWS IN BRIEF
FROM OTHER JOURNALS
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Weed pollute the air
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Source: Hickman et al (1 June 2010) PNAS 107: 10115-10119
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Chromatin Profiling of Individual Cell Types
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Source: Deal & Henikoff (15 June 2010) Developmental Cell 18: 1030-1040
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History of the tree
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The 385 mio years old Wattieza/Eospermatopteris was the first real tree. From www.nature.com
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It was like solving a puzzle and finally finding the last two pieces when archaeologists from New York State Museum in 2005 studied the fossile they had just dug out in the tiny village of Gilboa 200 km north of New York. The area has since the 1870's been known for its numerous fossiles that can be dated to 385 mio years ago. Named Eospermatopteris, they appear to be fossilized trunks, but the canopy, that would be expected to sit at the end of the trunk, has never been found and accordingly most scientists had doubt, that this was indeed a real tree. This time, however, they also found a canopy that resembled the Wattieza species, which was known from Venezuela and Belgium and was believed to be a low fern-like plant.
For the first time ever, the canopy and the trunk - believed for decades to belong to two different species - was unveiled from the same excavation. And when William Stein and Christopher Berry from the USA and UK, respectively, tried to solve the puzzle they realized, that the two pieces made a perfect match: an 8 meter tall and almost intact tree that dated to 385 mio years ago and thereby qualified to be the worlds first, real tree. The discovery was published in Nature in 2007 and the union of Wattieza and Eospermatopteris represented the second major step in the evolution of trees.
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The evolution of plants and trees escalated in Devon 408-374 mio years ago. From illvid.dk
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When plants first emerged from the sea and crawled onto land in Ordovicium around 465 mio years ago they depended completely on direct access to water in abundance. They had no roots and no vascular tissue to take up water and transport it internally, so for 50 mio years terrestrial plants were only a few centimeters tall and virtually had to soak in water. Such 410 mio years old moss-like plants can be found in the Scottish village of Rhynie near Aberdeenshire, but with the acquisition of first vascular tissues and subsequently secondary growth evolution speeded up. Plants could now raise their canopies towards the sun since the trunk would support them and their leaf-like structures received plenty of water and nutrients through the vascular tissue.
Devon - lasting from 408-374 mio years ago - became the next great epoque in the history of the trees. This geological period saw the emergence of Archaeopteris which brought by three important new traits: roots, large leaves and the first seed-like structures. The roots deep penetration into the soil stabilized the trunk by letting it have a firm grip in the soil and also allowed for more efficient uptake of water. Theoretically, roots gave trees the possibility to spread further into the land masses and up in the mountains, but in reality they still depended on staying close to the shore. Water evaporated very easily from their primitive, fern-like leaves so they still needed lots of water.
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The trees' magical formula of succes. Illustration by Gorm Palmgren.
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So the next logical step was to improve on the leaves and by 350 mio years ago the leaves started to get a waterproof, waxy coat on their surface and developed stomata, so water could evaporate in a more controlled fashion. Moreover, leaves grew bigger so they could capture more sunlight and supply more nutrients for further growth of the trees. Despite the big advantages of roots and leaves, the evolution of seeds, however, proved to be the most important step in the history of trees and other plants.
Like contemporary ferns and mosses, trees of early Devon relied on spores for sexual reproduction. Spores do not tolerate drought and must constantly be kept wet from fertilization until germination. This makes timing of reproduction very important since a single day of drought in principle can destroy a generation of trees or other plants in a dry area. Seeds, however, are far more robust due to a hard, thick coat that prevents drying up and a nutritious supply of starch or oil. This allows seeds to rest in dry soil for a long time, while they are waiting for the weather to change so they can succesfully germinate.
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Perm saw the emergence of the gymnosperm Cordaites (left) that later gave rise to the true conifer Gingko (right). From www3.interscience.wiley.com and universe-review.ca
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Archaeopteris and other representatives of the new generation of trees that had taken the first steps to acquire roots, leaves and seeds were extremely succesful and quickly spread to cover most of the globe. Fossiles of Archaeopteris have been found in locations as far apart as Oklahoma, the Atlas mountains of Marocco and the lowlands of Ukraine and they ruled the world for almost 20 mio years. Then, at the end of Devon they had to resign when a new group of trees, Cordaitales, developed more advanced seeds about 305 mio years ago in Carbon. This group af trees originally emerged in the mountains but eventually climbed down and occupied the lowlands as well.
After a series of ice ages the climate started to get warmer in Perm, that began 300 mio years ago, when huge tectonic forces created the supercontinent of Pangaea. The climate now became warmer and drier, and this gave optimal conditions for trees with real seeds like Ginkgo and Cycads. These trees are related to conifers and have relatives that thrives even in our times.
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Trees dramatically changed their surroundings and shaped the Earth as we know it today. From illvid.dk
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During their evolution, trees have dramatically changed their surroundings and have been the driving force to shape the Earth as we know it today. Their presence changed the physical and chemical composition of not only the soil but also of the atmosphere and the landscape itself. Roots penetrating the soil broke down rocks and excreted organic acids that degraded them into even smaller particles. Salts and nutrients were released and taken up by the trees, and when they shed their leaves, they were combusted by microorganism and turned into fertile soil.
Much of the organic material, however, was deposited and turned into the fossile fuels of today, and in this way substantial amounts of CO2 were removed from the air. This lowered the greenhouse effect, so temperatures fell and lead to a severe ice age 360 mio years ago where around 75% of all species became extinct. Whereas animals previously only could live in the water and shallows swamps around them, trees in abundance builded up huge forests that covered the whole globe from the lowlands to the mountains. This created a wealth of new ecological niches, and when the ice later retracted an evolutionary explosion occured.
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