Viscin tissue of the dwarf misteltoe Arceuthobium americanum (Viscaceae): how does it develop, what is it made of, and how does it function in explosive seed discharge

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Mistletoes, which spread solely by seed, are a group of flowering plants that are parasitic on other plants, absorbing water and minerals from their host.  Dwarf mistletoes, genus Arceuthobium, are parasites of conifers in North America, and, in addition to pilfering water and minerals, also take sugars.  Consequently, these parasites cause disease in their hosts, and are serious forest pests: it is estimated that dwarf mistletoes cause an annual growth loss of 4 million m3 of wood in western Canada, at a cost of several billion dollars.  Furthermore, dwarf mistletoe infestation leads to dead standing forests with excessive branches, notorious as fuel for fire, and predisposes the trees to attack by other pest organisms including mountain pine beetle.        Found within the single-seeded fruit of all mistletoes is a unique gelatinous tissue called viscin, which remains as a coating on the dispersed seed to facilitate its adherence to any surface it strikes, including the next host.  The viscin tissue in dwarf mistletoes plays an additional and critical role in parasite dispersal: whereas the seeds of other mistletoes are spread by birds, those of dwarf mistletoes are dispersed almost exclusively by forcible discharge from mature fruit to a maximum distance of 20 m.  The gelatinous viscin tissue somehow imbibes water during fruit development, which generates a hydrostatic force needed for explosive discharge.      Viscin tissue is crucial for the transmission and proliferation of dwarf mistletoes as insidious forest pathogens.  A project being carried out at Thompson Rivers University in Kamloops, British Columbia is examining the development, structure, and chemistry of the viscin tissue as well as exploring how water channels called aquaporins might direct water toward the viscin tissue in order to build the hydrostatic force.  With a better understanding of the viscin tissue and explosive discharge in hand, work toward reducing disease caused by dwarf mistletoe infection can advance.