Defense response of resistant host Impatiens balsamina to the parasitic angiosperm Cuscuta japonica

Abstract

The response of the stem of a resistant host (Impatiens baslamina) to infection by the parasitic flowering plant Cuscuta japonica was studied with light and electron microscopy. The intra- and interfascicular cambial cells in the host stem first reacted to the penetrating upper haustorium by dividing, and the differentiation of the host xylem (vascular) tissues proceeded toward interfascicular areas from vascular bundles. When the host vascular tissue was invaded by the endophyte (haustorial portion in the host stem), the host xylem was displaced, and host vessels became occluded with parenchyma cells, resulting in tyloses. As the parasitism progressed, areas of the host stem penetrated by the endophyte became swollen via secondary growth and cell division in the parenchymatous cortex, pith, and interfascicular areas. During this intrusion by the endophyte, darkly stained necrotic reactions were detected at the interface between the host tissue and the invading endophyte. The results suggested that in the host tissues penetrated by the parasite, the formation of secondary tissue and swellings caused by active cell division of ground tissue and host vessel occlusion by tyloses constitute the host structural defense against the parasite.