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Fine Structure of Puccinia carthami and the Ultrastructural Nature of Exclusionary Seedling-Rust Resistance of Safflower. D. E. Zimmer, Research Plant Pathologist, Crop Research Division, ARS, USDA, Logan, Utah 84321. Phytopathology 60:1157-1163. Accepted for publication 20 February 1970. DOI: 10.1094/Phyto-60-1157.

The fine structure of intercellular hyphae and intracellular haustoria of Puccinia carthami closely parallels that of other Puccinia spp. Lomosomes occurred both in haustoria and hyphal cells, and were believed to be involved in cell wall deposition rather than absorption. No channels through the haustorial wall connecting the haustorial cytoplasm and the sheath matrix or host cytoplasm were observed. Contrary to published reports for other obligate parasites, the sheaths surrounding mature haustoria of P. carthami were not of a single amorphic structure, but were composed of three distinct, perhaps functional, regions. The exclusionary seedling-rust resistance of the safflower, Carthamus tinctorius, Nebraska 1-1-5, resulted from cytoplasmic inhospitality of the cells of the perivascular region. Resistance was manifested by cytoplasmic collapse, degradation of the haustorial sheath, and the apparent deprivation of the haustorium of nutrients essential for glycogen synthesis. Glycogen was abundant in haustoria and hyphae in the perivascular region of Nebraska 8, a susceptible variety, but absent from these structures in the same region of Nebraska 1-1-5. Crystal-containing microbodies were abundant in cells in the region where exclusionary-seedling-rust resistance was operative. The rapid disappearance of these bodies in rust-infected cells suggests that they may play an important role in incompatibility.