Glutamine Synthetase Isoenzymes of Striga hermonthica and other Angiosperm Root Parasites

The occurrence of multiple forms of glutamine synthetase inStriga hermonthica and other angiosperm root parasites was investigated.The facultative chlorophyllous parasite Melampyrum arvense exhibitedtwo isoenzymes in leaf tissue, the cytosolic component (GS₁)comprised less than 30% of total glutamine synthetase. In contrastGS₁ was the major component (<70%) in photosynthetic tissueof Striga hermonthica and S. gesnerioides. Only a single isoenzyme(GS₁) was detectable in the achlorophyllous root parasites Orobancheand Lathraea and in non-photosynthetic tissue of S. gesnerioides.The kinetic and physical properties of GS₁ and GS₂ of theseangiosperm parasites were similar to those of the isoenzymesin other non-parasitic angiosperms. Key words: Glutamine synthetase, Angiosperms, Root parasites     

Wall-to-membrane linkers in onion epidermis: some hypotheses

Wall-to-wall linkage may help maintain cell integrity and polarity, and focus mechanical stress from wall to mech-anotransductive ion channels within the plasm a lemma. When cells of onion bulb scale epidermis shrink during plasmolysis with CaCl₂, the plasmalemma remains attached to the cell wall by Hechtian strands which we hypothesize might possibly be drawn out from linkages fulfilling the above functions. We show that at least many of the attachment loci are independent of the plasmodesmata. A priori, wall glycoproteins seem good candidates for the wall-to-membrane linkers; therefore, we investigated the distribution in wall and plasmalemma of antigen recognized by antibody to hydroxyproline-rich glycoprotein (HRGP). Using fluorescent secondary antibodies, we showed that polyclonal antibodies prepared against wall HRGP from soybean bind to the onion walls (following mild depectination), but also bind to the plasmalemma after the wall is enzymatically digested. The distribution of the antibodies is punctate. On the plasmalemma, the points tend to be scattered more or less uniformly, but can cluster at termini of large streaming strands (which rarely form in wall-constrained cells.) These streaming strands can be seen to exert tension on the membrane. We hypothesize that (1) the antigen on the surface of the protoplast may correspond to the antigen in the walls, (2) such antigen may be responsible for adhesion of membrane to wall at the linkage sites visualized by CaCl₂ plasmolysis, and (3) the linkage sites may be transmembrane proteins to which cytoskeleton can attach at the inner surface.