Plant life without leaves, roots, or stems: anatomy, development, and 3D structure of the endoparasite Pilostyles blanchetii (Apodanthaceae) in Mimosa hosts.

Background and aims: The vegetative body of Pilostyles (Apodanthaceae) is highly reduced, composed of cell clusters that occupy the bark of the host plant. From these clusters, tracheary elements connect with the host vascular system during the parasite reproductive stages. We analyze the developmental morphology of P. blanchetii to better understand its growth within the host body, as well as xylem and phloem connections between the two plants, providing new interpretations about the parasite's life history.

Methods: Iodine and lead contrasting solutions were perfused through samples of parasitized host material before microtomography scanning to facilitate posterior virtual segmentation of parasite structures within host tissues. Samples were then prepared for light, fluorescence, and confocal microscopy to analyze xylem and phloem connections between parasite and host plants.

Key results: We detected the presence of parasitic tissue within the host body before flower meristem differentiation and reveal the interconnectedness of the parasitic endophyte. Parasite-host xylem connections are mediated by highly modified vessels elements differentiated in the parasite sinkers. Within the host bark, the parasite develops small sieve tube elements lacking companion cells.

Conclusions: Pilostyles blanchetii develops an extensive tissue network within the bark of its hosts, linking pistillate and staminate flowers, thus suggesting monoecy. Vascular tissue is extremely reduced and includes only vessel elements for the xylem and sieve tube elements for the phloem, which connect directly to the corresponding cells of the host.