Microstructural and Histochemical Changes in Acer platanoides Rhytidome Caused by Dendrothele acerina (Aphyllophorales) and Mycena meliigena (Agaricales)

Abstract

Rhytidome blocks from Acer platanoides were incubated in vitro with Dendrothele acerina or Mycena meliigena mycelia in order to describe resultant microstructural and histochemical changes using paraffin sections and brightfield microscopy. Rhytidome consists of alternating layers of periderm and nonfunctional secondary phloem. All cells in the rhytidome, except suberized phellem cells, are lignified. Both fungi penetrated blocks, and advanced 1–2 millimeters in 20 weeks. Both species penetrated suberized phellem layers and eroded phellem cell walls from the lumen outward. In nonfunctional phloem, hyphae moved between lumina of the sieve tube elements, companion cells, and parenchyma through enlarged sieve pores, pits, and bore holes. These walls were progressively thinned from the lumen outward. Fiber and sclereid walls were penetrated through pits and bore holes, but the hyphae remained in the middle of the secondary wall, forming crescent-shaped cavities that gradually enlarged to leave a honeycomb pattern of compound middle lamellae. Both fungi displayed selective delignification and nonselective cell wall degradation patterns observed in some white rot fungi. We suggest that D. acerina and M. meliigena should be classified ecologically as saprotrophic inhabitants of the rhytidome of living trees.