Allelopathic Exudates of Cogongrass (Imperata cylindrica): Implications for the Performance of Native Pine Savanna Plant Species in the Southeastern US

We conducted a greenhouse study to assess the effects of cogongrass (Imperata cylindrica) rhizochemicals on a suite of plants native to southeastern US pine savanna ecosystems. Our results indicated a possible allelopathic effect, although it varied by species. A ruderal grass (Andropogon arctatus) and ericaceous shrub (Lyonia ferruginea) were unaffected by irrigation with cogongrass soil “leachate” (relative to leachate from mixed native species), while a mid-successional grass (Aristida stricta Michx. var. beyrichiana) and tree (Pinus elliottii) were negatively affected. For A. stricta, we observed a 35.7 % reduction in aboveground biomass, a 21.9 % reduction in total root length, a 24.6 % reduction in specific root length and a 23.5 % reduction in total mycorrhizal root length, relative to the native leachate treatment. For P. elliottii, there was a 19.5 % reduction in percent mycorrhizal colonization and a 20.1 % reduction in total mycorrhizal root length. Comparisons with a DI water control in year two support the possibility that the treatment effects were due to the negative effects of cogongrass leachate, rather than a facilitative effect from the mixed natives. Chemical analyses identified 12 putative allelopathic compounds (mostly phenolics) in cogongrass leachate. The concentrations of most compounds were significantly lower, if they were present at all, in the native leachate. One compound was an alkaloid with a speculated structure of hexadecahydro-1-azachrysen-8-yl ester (C₂₃H₃₃NO₄). This compound was not found in the native leachate. We hypothesize that the observed treatment effects may be attributable, at least partially, to these qualitative and quantitative differences in leachate chemistry.     

A Pore-forming protein with a protease-activated trigger

Hemolysin (HL) is a 293 amino acid pore-forming toxin, whichis secreted as a water-soluble monomer by Staphylococcus aureus.By forming a hexameric pore, HL damages the plasma membranesof target cells. Previous studies established that HL proteinswith nicks near the midpoint of a central glycine-rich loopare held together by a domain-domain interaction and are hemolyticallyactive. In contrast, HL proteins comprising two HL truncationmutants that overlap in the central loop have no or greatlyreduced pore-forming activity, even though the two chains againform a tight complex. Based on these findings, overlap mutantshave now been designed that are activated when redundant aminoacids in the loop are removed by proteases. Further, the identityof the activating enzyme can be specified by additional mutagenesisof the protease recognition site in the overlap sequence. Mutantsof aHL that are activated by tumor-associated proteases mightbe useful components of immunotoxins