Viscin Cells in the Dwarf Mistletoe Arceuthobium ... - Davidsonia

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76 Canada (Hawksworth and Wiens 1996). Results of tree disease include the formation of host branch clumps called “witches’ brooms,” dieback, reduced growth, a compromised lifespan, and curtailed reproduction as well as increased susceptibility to other diseases and injury (Geils and Vázquez Collazo 2002). While Dendroctonus ponderosae (the mountain pine beetle) has gained much notoriety as a serious forest problem in BC, infection by Arceuthobium is widespread and has been implicated in the predisposition of trees to beetle attack (Johnson et al. 1976; McGregor 1978; McCambridge et al. 1982). Of the forest pathogens causing mortality in natural conifer systems, Arceuthobium is perhaps one of the most significant (Winder and Shamoun 2006). Ecologically, the implications of Arceuthobium infection are complex: these mistletoes are influential symbionts that affect numerous aspects of population genetics and coevolution (Geils and Vázquez Collazo 2002). It is thought that the presence of Arceuthobium in a stand significantly increases habitat diversity. As such, the role of Arceuthobium in natural conifer systems has often clashed with human interests and harvesting practices (Shamoun et al. 2003). However, with an understanding of the ecology and biology of Arceuthobium and their hosts, wiser management practices may follow. Arceuthobium spreads solely by seed, which is discharged explosively from mature fruit at the end of the growing season (Hawksworth and Wiens 1996). A seed can be dispersed as far as 20 m from its source plant. Viscin tissue is a unique mucilaginous region that forms a layer around the embryo-pole of the single seed within each fruit. The extracellular hygroscopic mucilage imbibes water during fruit development (Hawksworth and Wiens 1996; Ross 2002) and provides the hydrostatic force needed for explosive discharge. Following discharge, the viscin tissue remains around the dispersed seed and facilitates its adherence to any surface the seed may strike, including its next host. Several studies on the general development and chemistry of the tissue include one by Paquet et al. (1986), who described the tissue as being comprised of elongated “viscin cells” that secrete a largely pectinaceous mucilage. These authors also found that the viscin cells of discharged seeds could be wetted and dried several times before the mucilage was washed away. Gedalovich-Shedletzky et al. (1989) noted that viscin cell development begins as slightly elongated parenchyma in the carpellary region and that the non-pectic monosaccharides,
Davidsonia 17:3 77 xylose and glucose, were also present in the mucilage. However, other features of the viscin cells, such as their cell walls, have not been well characterized. My small study on aspects of the viscin cells not directly related to the mucilage has shown that viscin cells seem to function in discharge and remain viable enough to function in adhesion to new host sites. The post discharge fate of the viscin cells has not been described, but chloroplasts were present, and the cells tested positively for the operation of oxidation-reduction biochemistry when stained with tetrazolium chloride. As seed is the only means by which Arceuthobium can spread, an understanding of seed development and dispersal may provide the basis for an effective control program. Methods Collection and Observation of Fruit and Viscin Cells Prior to Explosive Discharge At least 30 pistillate aerial shoots of Arceuthobium americanum (the lodgepole pine dwarf mistletoe) containing mature fruit were marked in the field and each was enclosed in cheesecloth on August 20, 2005. The contents of the cheesecloth bags were sampled daily until September 2, 2005 (the day on which explosive discharge occurred) then 1 day, 3 days and 10 days following discharge. Samples from each shoot were fixed immediately in 2% paraformaldehyde + 2% glutaraldehyde in a 0.1 M phosphate buffer (pH 6.8), kept at 4 °C overnight, rinsed in the same buffer, and postfixed with 2% osmium tetroxide in the same buffer for 4 hours. The material was dehydrated in an ethanol series and embedded in Spurr’s resin (Spurr 1969). Sections (2 µm thick) were obtained using a Sorvall Porter-Blum JB-4 microtome (Sorvall, Newtown, Connecticut) and affixed to gelatin-coated glass slides (Jensen 1962). The slides were stained with 2% crystal violet in a 0.05 M ammonium oxalate buffer (pH 6.7) and were observed and photographed with a Nikon Optiphot compound light microscope (Nikon, Tokyo, Japan). Within-fruit viscin cell lengths were measured using a micrometer slide calibrated to an eyepiece scale. Average viscin cell length was determined by measuring the 10 most apical viscin cells (i.e., those cells proximal to the embryo and stigma, distal to the pedicel) in sections from 10 fruits sampled on September 1, 2005 (the day in 2005 prior to explosive discharge).
Page 1 and 2: Volume 17, Number 3 July 2006 David
Page 3 and 4: Davidsonia 17:3 73 Editorial This i
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Page 9 and 10: Davidsonia 17:3 79 Image: Cynthia M
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Page 13 and 14: Davidsonia 17:3 83 cell mucilage ca
Page 15 and 16: Davidsonia 17:3 85 such activity in
Page 17 and 18: Davidsonia 17:3 87 Comparison of Tw
Page 19 and 20: Davidsonia 17:3 89 Photo: Esson et
Page 21 and 22: Davidsonia 17:3 91 Photo: Esson et
Page 23 and 24: Davidsonia 17:3 93 Based on purely
Page 25 and 26: Davidsonia 17:3 95 Figure 5: Scatte
Page 27 and 28: Davidsonia 17:3 97 REFERENCES Allen
Page 29 and 30: Davidsonia 17:3 99 The next step, t
Page 31 and 32: Davidsonia 17:3 101 plant cuticles
Page 33 and 34: Davidsonia 17:3 103 Gleanings Notes
Page 35 and 36: Table of Contents Editorial Taylor

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