All plants studied in natural ecosystems are symbiotic with fungi (1), which obtain nutrients while either positively, negatively, or neutrally affecting host fitness (2). Plant adaptation to selective pressures is considered to be regulated by the plant genome (3). To test whether mutualistic fungi contribute to plant adaptation, we collected 200 Dichanthelium lanuginosum plants from geothermal soils at 10 sites in Lassen Volcanic (LVNP) and Yellowstone ( YNP) National Parks. These soils have annual temperature fluctuations ranging from about 20°to 50°C (4 ).Plants and their roots were removed and assessed for fungal colonization (5). A fungal endophyte was isolated from the roots, crowns, leaves, and seed coats of all plants collected. Cultures established from single
The possibility that plant membrane-bound MgATPases may act as electrogenic proton pumps has been investigated. MgATPases may mediate active proton transport across the plant plasma membrane and tonoplast. Although this idea has gained wide acceptance recently, it is supported primarily by only indirect evidence (19,21,26). Plant cells are known to excrete protons, and potential measurements across higher plant cells indicate that electrogenic proton extrusion pumps may drive the active membrane potential across the plasma membrane (21, 26). In several higher plant systems so far examined, the membrane potential is affected by changes in the intracellular ATP level (14,16). These findings indirectly support the idea that proton extrusion is driven by membrane-bound MgATPases which transduce the free energy of ATP-hydrolysis into a transmembrane electrochemical proton gradient (19,26). The likelihood that MgATPase proton pumps exist in the plant plasma membrane has also been suggested by the results of investigations of hormone-induced plant cell enlargement (22). It is likely that the active transport of protons occurs also at the tonoplast since, in general, the pH of the vacuole is at least one pH unit lower than the cytoplasm (21).
A broad survey of most of the major geyser basins within Yellowstone National Park (Wyoming, USA) was conducted to identify the flowering plants which tolerate high rhizosphere temperatures (> or = 40 degrees C) in geothermally heated environments. Under such conditions, five species of monocots and four species of dicots were repeatedly found. The predominant flowering plants in hot soils (>40 degrees C at 2-5 cm depth) were grasses, primarily Dichanthelium lanuginosum. Long-term (weeks to months) rhizosphere temperatures of individual D. lanuginosum above 40 degrees C were recorded at several different locations, both in the summer and winter. The potential role of heat shock proteins (HSPs) in the apparent adaptation of these plants to chronically high rhizosphere temperatures was examined. Antibodies to cytoplasmic class I small heat shock proteins (sHSPs) and to HSP101 were used in Western immunoblot analyses of protein extracts from plants collected from geothermally heated soils. Relatively high levels of proteins reacting with anti-sHSP antibodies were consistently detected in root extracts from plants experiencing rhizosphere temperatures above 40 degrees C, though these proteins were usually not highly expressed in leaf extracts from the same plants. Proteins reacting with antibodies to HSP101 were also present both in leaf and root extracts from plants collected from geothermal soils, but their levels of expression were not as closely related to the degree of heat exposure as those of sHSPs.
ABSTRACrElectron transport activity at the cell surface of intact oat seedlings (Avena satina L. cv Gary) was examined by measuring the oxidation and/or reduction of agents in the medium bathing the roots. Oxidation of NADH with or without added electron acceptors and reduction of ferncyanide by an endogenous electron donor were detected. The activities appear to be due to electron transfer at, or across, the plasma membrane and not due to reagent uptake or leakage of oxidants or reductants. NADH-fefricyanide oxidoreductase activity was also detected in plasma membrane-enriched preparations from Awaea roots. Based on redox responses to pH, various ions, and to a variety of electron donors and acceptors, the results indicate that more than one electron transport system is present at the plasma membrane.Redox activity, which provides the energy for many important biological processes, is known to be associated with organelles such as mitochondria and chloroplasts. This activity also appears to be present at the plasma membrane of eukaryotic cells. There are reports of electron transport at the surface of intact ascites tumor cells (7), yeast (10), Neurospora (24), carrot phloem cells (8, 23), tobacco callus (2), corn root segments, and protoplasts (12,15,19). Electron transport also has been found in plasma membrane fractions from animal tissues (12), Neurospora (4), corn coleoptiles (17,25), oat roots and shoots, and cauliflower (32). In support of these results, components usually associated with redox activity have been detected on the plasma membrane of animal cells (5, 20, 22), and there is also evidence in plants for flavins and for a b-type Cyt (17,32).Cell surface redox activity might be related to proton excretion in a manner analogous to that which occurs in prokaryotes, and in chloroplasts and mitochondria. Although this process is controlled at least in part by a proton-translocating ATPase (29), it is still possible that some of the proton transport is coupled to energy released by electron flow (8,10,11,19 be converted to its reduced form before entering the cells of certain plants (6). Recent evidence suggests that iron is reduced by a plasma membrane-localized redox system (27).Earlier reports have measured cell surface redox activity in various ways and on a range of plants. We wanted to confirm these reports with one organ of a single species-the roots of intact oat seedlings. Roots were chosen because they need not be peeled or abraded in order for agents to enter the intercellular space; thus, artifacts due to wounding are less likely. Based on data for pH optima, ion requirements, and various electron donors and acceptors, our results indicate that electron transport complexes exist at the cell surface of intact roots; we also show that oxidoreductase activity is present in plasma membraneenriched pieparations from these roots.MATERIALS AND METHODS Plant Culture. Oat 'seeds' (Avena sativa L. cv Garry) were germinated on water-moistened vermiculite for 3.5 to 4 d in total darkness except for a ...
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