NEWS FROM
PHYSIOLOGIA PLANTARUM
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Published monthly on behalf of SPPS by Wiley-Blackwell.
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Vine disease causes senescence
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Pierce's disease has become a major problem for wine growers in California and Central America. It is caused by the bacterial pathogen Xylella fastidiosa that uses leafhoppers for transmission. In diseased plants, a gel like substance forms in the xylem tissue and leaves turn yellow and brown. It is generally believed that symptoms arise from occlusion of xylem conduits but this may not be so according to new research conducted by Brendan Choat and colleagues at University of California, Davis. They measured leaf hydraulic conductance (i.e. how easy water is transported) in infected and uninfected Vitis vinifera cv. Chardonnay under different irrigation regimes and found that susceptibility to Pierce's disease was apparently favored by water stress. In addition, hydraulic conductance of infected leaves from field-grown vines was similar to naturally senescing leaves. From these results the researchers concluded that infection of X. fastidiosa leads to a systemic response that accelerates senescence.
Read full article here: Choat et al. (March 2009) Physiologia Plantarum 136: 384-394
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NEWS IN BRIEF
FROM OTHER JOURNALS
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Sex may improve industrial production
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Source: Seidl et al. (18 August 2009) PNAS 106: 13909-13914
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Orchid smells like prey
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Source: Brodmann et al (25 August 2009) Current Biology 19: 1-5
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Scandinavian research institute:
PUMPKIN, Centre for Membrane Pumps in Cells and Disease, Denmark
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The PUMPKIN laboratories are situated both at the University of Aarhus and the University of Copenhagen. From www.pumpkin.au.dk
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Some goals are just too ambitious to meet for a single research group. Deciphering the 3 dimensional structure of complex proteins not only requires the expertise in x-ray crystallography but also a unique knowledge of the proteins biochemistry and the molecular biological processes it participates in. Bearing this in mind, PUMPKIN, Centre for Membrane Pumps in Cells and Disease, was established in 2007 as a highly interdisciplinary research centre funded by the Danish National Research Foundation. The PUMPKIN laboratories are situated both at the University of Aarhus and the University of Copenhagen, and their research is focused on five ion pumps that are fundamental to life and are present in all cells of either plants, animals and bacteria. These are the P-type ATPases pumping either sodium/potassium, proton/potassium, calcium, protons or heavy metals over the the plasma membrane or internal membranes.
Together, the pumps have important physiological functions and they require almost 50% of an organisms energy expenditure. The animal sodium/potassium pump maintains the salt balance and like the plant proton pump it generates electric gradients that are used to energize fundamental cellular processes, e.g. generation of nerve signals and and taking up nutrients through the roots. The calcium pump are critical for muscular contractions, while the proton/potassium pump are required to pump into the stomach and the heavy metal pump is used by organisms to detoxify their cells.
All except the sodium/potassium and proton/potassium pumps are found in plants and they have been studied for over two decades by the group of Professor Mickey Palmgren from TRAP-LABS at the University of Copenhagen. He is a partner and group leader in PUMPKIN and enjoys the expertise in x-ray crystallography of Professor Poul Nissen from Centre for Structural Biology at the University of Aarhus, who is director of the centre.
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Scientists from PUMPKIN made it to the cover of Nature - with three articles inside! From www.nature.com
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In an astonishing move for Danish research, the team brought their research to the cover of Nature in 2007, where not only the 3D crystal structure of the plant plasma membrane H+-ATPase but also the Na+/K+-ATPase and the Ca2+-ATPase from mammals were presented in three separate articles in the prestigious journal (see Nature papers 1, 2 and 3). Among the new information obtained, the scientists were puzzled to find that the structure of Na+/K+-ATPase and the H+-ATPase are fundamentally similar and that they also share the same general pumping mechanism - you can interact for yourself with the 3D structures of the sodium/potassium pump and the proton pump here. This came as a surprise since the former is an exchanger that exchanges three sodium ions for two potassium ions during each cycle of ATP hydrolysis whereas the latter is a regular pump that expels one proton per cycle.
The real difference between the two seems to be the 'pocket' that embeds the ions when they are transported through the protein across the membrane. This might suggest, that the chemical properties of the amino acids adjacent to the pocket determines which ions can be transported. Mickey Palmgren envisions that this can be exploited to design novel crops that can expel sodium and thus can be irrigated with salt water. The classical approach to this task has been to transfer the sodium-potassium pump from animals to transgenic plants, but so far these efforts have failed. Instead, the scientists at PUMPKIN wants to mutagenize the plants own proton pump so the amino acids surrounding the pocket resembles those in the sodium.potassium pump. If succesful, the mutated plants can then use their own modified pump to get rid of excess sodium and thereby filter the salt out of the sea water.
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The plant plasma membrane proton pump is regulated by 14-3-3 protein binding to the C-terminus. From www.traplabs.dk
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In Palmgrens laboratory at the Department of Plant Biology and Biotechnology at University of Copenhagen, Anja Thoe Fuglsang and Morten Buch-Pedersen are group leaders proton pump subgroups dealing with regulation and structure/function, respectively. Among the achievements of the groups, besides from obtaining the crystal structure, is the elucidation of how the pump is regulated by a terminal auto-inhibitory domain. When the second last amino acid of the C-terminal, a threonine residue, gets phosphorylated it binds to 14-3-3 proteins and this causes a conformational changes in the protein. The C-terminal swings down and like a hand break it inhibits the pumping action by literally blocking the door, so protons can not leave the cell. A similar regulatory mechanism has been found for the plant Ca2+-ATPase, however, in this case the regulatory domain is in the N-terminal and the regulatory protein is calcium/calmodulin complex.
The branch of PUMPKIN dealing with the heavy metal pump is headed by Lone B¾kgaard, who previously was working with the plant plasma membrane Ca2+-ATPase in the Palmgren laboratory. The heavy metal pump belongs to the group of type IB ATPases which are able to transport a wide range of ions: Cu+, Ag+, Cu2+, Zn2+, Cd2+, Pb2+ and Co2+. The pump seems to be crucial for controlling zinc transport over the root-shoot barrier and thus for grain filling of this important nutrient in cereals and other crops. A long term goal of the group is to develop plants with increased content of bioavailable nutrients in their edible parts, also referred to as biofortification.
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Plants with mutations in the phospholipid pump can not shed cells from the growing root cap (B, C). From Poulsen et al (2008) Plant Cell 20: 658
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Another, somewhat controversial, pump under investigation in PUMPKIN is the type IV ATPases. Also known as flippases, these enzymes catalyze the movement of phospholipids from the cytoplasmic to the ectoplasmic face of the membrane, thereby maintaining an asymmetric distribution of membrane phospholipids. It is still subject to debate whether this task is really performed by ATPases, but the PUMPKIN scientists have shown that Arabidopsis mutants lacking one member (aminophospholipid ATPase3, ALA3) of this family can not form certain secretory vesicle, leading to impaired growth of roots and shoots.
Due to their crucial role in vital physiological functions, the P-type ATPases are potential targets for drugs and herbicides. A number of pharmaceuticals against peptic ulcer and heart diseases are already directed towards these ATPases, but the new 3D crystal structures might pave the way for many more. E.g. cancer cells need very high activity of their calcium pump to maintain their fast growth, so a drug that specifically inhibits the pump might prove anti-cancerous. And even nanotechnology can benefit from the 3D structures obtained at PUMPKIN. Even though each pump consist of only a single protein, it can generate an electrical gradient of 0.25 V and in the right settings thousands of these can be combined into minuscules batteries that can power nanomachines of the future.
You can find more information about PUMPKIN at the official homepage.
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