Bidirectional anatomical effects in a mistletoe–host relationship: Psittacanthus schiedeanus mistletoe and its hosts Liquidambar styraciflua and Quercus germana

PREMISE OF THE STUDY: During the interactions between a parasitic plant and its host, the parasite affects its host morphologically, anatomically, and physiologically, yet there has been little focus on the effect of hosts on the parasite. Here, the functional interactions between the hemiparasitic mistletoe Psittacanthus schiedeanus and its hosts Liquidambar styraciflua and Quercus germana were interpreted based on the anatomical features of the vascular tissues. METHODS: Using standard techniques for light and transmission electron microscopy, we studied the effects of P. schiedeanus on the phloem anatomy of Liquidambar styraciflua and Quercus germana and vice versa. KEY RESULTS: The phloem of P. schiedeanus has larger sieve elements, companion cells, and sieve plate areas when it is parasitizing L. styraciflua than Q. germana; however, the parasite produces systemic effects on the phloem of its hosts, reducing the size of phloem in L. styraciflua but increasing it in Q. germana. Those seem to be the bidirectional effects. No direct connections between the secondary phloem of the parasite and that of its hosts were observed. Parenchymatic cells of L. styraciflua in contact with connective parenchyma cells of the parasite develop half-plasmodesmata, while those of Q. germana do not. CONCLUSIONS: The bidirectional effects between the parasite and its hosts comprise modifications in secondary phloem that are potentially affected by the phenology of its hosts, a combination of hormonal agents such as auxins, and the symplasmic or apoplasmic pathway for solutes import.