To Be or Not to Be — A Question of Plasma Membrane Redox? In: Behnke H, Esser K, Kadereit JW, Lüttge U, Runge M, editors. Progress in Botany. Berlin: Springer Berlin Heidelberg; 1998. pp. 328-54. [DOI: 10.1007/978-3-642-80446-5_12]

Plasma membrane phylloquinone biosynthesis in nonphotosynthetic parasitic plants

Nonphotosynthetic holoparasites exploit flexible targeting of phylloquinone biosynthesis to facilitate plasma membrane redox signaling. Phylloquinone is a lipophilic naphthoquinone found predominantly in chloroplasts and best known for its function in photosystem I electron transport and disulfide bridge formation of photosystem II subunits. Phylloquinone has also been detected in plasma membrane (PM) preparations of heterotrophic tissues with potential transmembrane redox function, but the molecular basis for this noncanonical pathway is unknown. Here, we provide evidence of PM phylloquinone biosynthesis in a nonphotosynthetic holoparasite Phelipanche aegyptiaca. A nonphotosynthetic and nonplastidial role for phylloquinone is supported by transcription of phylloquinone biosynthetic genes during seed germination and haustorium development, by PM-localization of alternative terminal enzymes, and by detection of phylloquinone in germinated seeds. Comparative gene network analysis with photosynthetically competent parasites revealed a bias of P. aegyptiaca phylloquinone genes toward coexpression with oxidoreductases involved in PM electron transport. Genes encoding the PM phylloquinone pathway are also present in several photoautotrophic taxa of Asterids, suggesting an ancient origin of multifunctionality. Our findings suggest that nonphotosynthetic holoparasites exploit alternative targeting of phylloquinone for transmembrane redox signaling associated with parasitism.

Phylloquinone, what can we learn from plants?

The plant plasma membrane contains redox proteins able to mediate a trans-membrane electron flow. This electron flow might be responsible for the generation of the active oxygen species observed as a reaction to pathogen attack or stress. Vitamin K1 could be identified as a possible lipid soluble electron carrier in plant plasma membrane preparations. Such a function would be analogous to coenzyme Q in animal plasma membranes. What we are going to outline in this contribution is a concept of how the electron transport system of the plant plasma membrane could interact with quinones, thus contributing to the metabolism of free radicals in plants.

Lipid Composition and Fluidity of Plasma Membranes Isolated from Corn (Zea mays L.) Roots

Although the results of lipid analyses from several plant species have been available for many years a complete characterization of the corn root plasma membrane is still lacking. The present study provides a detailed analysis of individual lipids and a characterization of the membrane fluidity of corn (Zea mays L.) root plasma membranes isolated by phase-partitioning. Phospholipids (43.9 mol%), sterols (40.8 mol%), and sphingolipids in the form of glucocerebroside (6.8 mol%) constitute the major lipid classes. Stigmasterol (19.8 mol%), campesterol (13.0 mol%), phosphatidylcholine 15.8 mol%), and phosphatidylethanolamine (14.2 mol%) represent the most ubiquitous individual lipids. Hydroxy fatty acids make up 80.9 mol% and very long chain fatty acids are almost 78% of fatty acids in glucocerebroside. Hydroxy arachidic acid (20:0 h) and hydroxy lignoceric acid (24:0 h) are most prominent and glucocerebroside from corn root plasma membranes contains virtually no unsaturated fatty acids. Among the phospholipids only phosphatidylserine displayed a high proportion of very long chain fatty acids (e.g., behenic and lignoceric acid). Membrane fluidity was estimated by fluorescence anisotropy. Due to the high sterol content the plasma membrane of corn roots is relatively rigid.

Higher-plant plasma membrane cytochrome b561: a protein in search of a function

During the past twenty years evidence has accumulated on the presence of a specific high-potential, ascorbate-reducible b-type cytochrome in the plasma membrane (PM) of higher plants. This cytochrome is named cytochrome b561 (cyt b561) according to the wavelength maximum of its alpha-band in the reduced form. More recent evidence suggests that this protein is homologous to a b-type cytochrome present in chromaffin granules of animal cells. The plant and animal cytochromes share a number of strikingly similar features, including the high redox potential, the ascorbate reducibility, and most importantly the capacity to transport electrons across the membrane they are located in. The PM cyt b561 is found in all plant species and in a variety of tissues tested so far. It thus appears to be a ubiquitous electron transport component of the PM. The cytochromes b561 probably constitute a novel class of transmembrane electron transport proteins present in a large variety of eukaryotic cells. Of particular interest is the recent discovery of a number of plant genes that show striking homologies to the genes coding for the mammalian cytochromes b561. A number of highly relevant structural features, including hydrophobic domains, heme ligation sites, and possible ascorbate and monodehydroascorbate binding sites are almost perfectly conserved in all these proteins. At the same time the plant gene products show interesting differences related to their specific location at the PM, such as potentially N-linked glycosylation sites. It is also clear that at least in several plants cyt b561 is represented by a multigene family. The current paper presents the first overview focusing exclusively on the plant PM cyt b561, compares it to the animal cyt b561, and discusses the possible physiological function of these proteins in plants.

Interaction between electron transport at the plasma membrane and nitrate uptake by maize (Zea mays L.) roots

In the present study nitrate uptake by maize (Zea mays L.) roots was investigated in the presence or absence of ferricyanide (hexacyanoferrate III) or dicumarol. Nitrate uptake caused an alkalization of the medium. Nitrate uptake of intact maize seedlings was inhibited by ferricyanide while the effect of dicumarol was not very pronounced. Nitrite was not detected in the incubation medium, neither with dicumarol-treated nor with control plants after application of 100 microM nitrate to the incubation solution. In a second set of experiments interactions between nitrate and ferricyanide were investigated in vivo and in vitro. Nitrate (1 or 3 mM) did neither influence ferricyanide reductase activity of intact maize roots nor NADH-ferricyanide oxidoreductase activity of isolated plasma membranes. Nitrate reductase activity of plasma-membrane-enriched fractions was slightly stimulated by 25 microM dicumarol but was not altered by 100 microM dicumarol, while NADH-ferricyanide oxidoreductase activity was inhibited in the presence of dicumarol. These data suggest that plasma-membrane-bound standard-ferricyanide reductase and nitrate reductase activities of maize roots may be different. A possible regulation of nitrate uptake by plasmalemma redox activity, as proposed by other groups, is discussed.

Inhibition of trans-membrane hexacyanoferrate III reductase activity and proton secretion of maize (Zea mays L.) roots by thenoyltrifluoroacetone

Intact plants can reduce external oxidants by an appearingly trans-membrane electron transport. In vivo an increase in net medium acidification accompanies the reduction of the apoplastic substrate. Up to now, several NAD(P)H dehydrogenases, b-type cytochromes, and a phylloquinone have been identified and partially purified from plant plasma membranes. The occurrence of a quinone in the plasma membrane of maize roots supports the hypothetical model of a proton-transferring redox system, i.e., an electron transport chain with a quinone as mobile electron and proton carrier. In the present study the trans-membrane electron transport system of intact maize (Zea mays L.) roots was investigated. Flow-through and ionostat systems have been used to estimate the electron and proton transport activity of this material. Application of 4,4,4-trifluoro-1-(2-thienyl)-butane-1,3-dione (thenoyltrifluoroacetone) inhibited the reduction of ferricyanide in the incubation solution of intact maize roots up to 70%. This inhibition could not be washed off by rinsing the roots with fresh incubation medium. The acidification of the medium induced after ferricyanide application was inhibited to about 62%. The effects of thenoyltrifluoroacetone on proton fluxes in the absence of ferricyanide have been characterized in a pH-stat system. The net medium acidification by maize roots was inhibited up to 75% by thenoyltrifluoroacetone in the absence of ferricyanide, while dicumarol inhibited net acidification completely. The inhibition of H(+)-ATPase activity was estimated with plasma membrane vesicles isolated by phase partitioning and treated with 0.05% (w/v) Brij 58. ATP-dependent proton gradients and Pi release were measured after preincubation with the effectors. The proton pumping activity by those plasma membrane vesicles was inhibited by dicumarol (53.6%) and thenoyltrifluoroacctone (77.8%), while the release of Pi was unaffected by both inhibitors.