Light and electron microscopy of the micromorphology and development of pycniospores and aeciospores of the sunflower rust, Puccinia helianthi

The pathogenic fungus Puccinia helianthi, which causes rust disease and significant economic loss, poses a serious threat to the Helianthus annuus crop. This study is the first to examine the ultrastructure of the spore stages of this rust, aiming to fill the gap in the understanding of Egyptian rusts. The present study aimed to explore the micromorphology and development of pycniospores and aeciospores of Puccinia helianthi on its host, using LM, SEM, and TEM. The immature pycnium grows subepidermally and is bordered by a peripheral layer of pseudoparenchymatous cells. Within the pycnium, uninuclear, compactly packed, and elongated pycniosporophores are formed. Pointed periphyses emerge at the highest point of the pycnium, where numerous pycniospores are produced. The primary pycniospores are then released from the pycniosporophores having an elliptical shape with a wrinkled surface. Aeciospores are produced at the edges of a distinct layer, the peridium with rhomboid-shaped peridial cells. A prosenchymatous stroma develops beneath the hymenium layer (aeciosporophores). Two main types of hyphae can distinguish in this stroma: vacuolated hyphae, at the base of the aecium, and non-vacuolated hyphae with dense, ribosome-rich cytoplasm, oil droplets, and glycogen. Each aeciospore divides to form a young aeciospore and an intercalary cell with elongated or hexagonal shapes and warts on the mature aeciospores. SEM examination reveals cog-like ornaments and refracting bodies on the surfaces of peridial cells and aeciospores. The ultrastructure of these two spore stages is more or less similar to that of other Puccinia species with some differences. The procedures used in this study will assist mycologists in rust identification, taxonomy, and microscopic characterization. In conclusion, this study will provide additional information to help understand the interaction between rusts and their hosts. Future research on ultracytochemical studies may provide insight into controlling these serious pathogens.