Investigation of spore infection mechanism in sunflowers infected by Puccinia helianthi

Sunflower rust, caused by the fungus Puccinia helianthi, poses a significant threat to global sunflower production. However, the fundamental mechanisms of P. helianthi infection and development remain poorly understood. This study aims to elucidate the infection process of P. helianthi in sunflower leaves through histological and cytological observations. P. helianthi is an autoecious fungus that undergoes five spore stages: teliospores, basidiospores, pycniospores, aeciospores, and urediospores. Our findings revealed that basidiospores germinated by producing a germ tube at 24 hours post-inoculation (hpi). This germ tube entered the leaf through the stomata or directly penetrated the epidermal cells without forming an appressorium, resulting in the formation of monokaryotic intercellular hyphae. The hyphae grew along the cell wall, which thickened when the hyphae came in touch with the mesophyll cells. The monokaryotic haustorium formed within the host cells at 48 hpi. Chlorotic dots began to appear on the upper surface of the leaves five to six days after the basidiospores were inoculated. After 14 days post-inoculation (dpi), flask-shaped orange pycnia developed. By approximately 18 d, honeydew started appearing on the pycnia. Aecia began to develop six or seven days after fertilization; they were cylinder-shaped and adorned with coglike ornaments. Aeciospores and urediospores germinated by producing a germ tube at 4 hpi, which subsequently developed a dome-shaped appressorium that penetrated the sunflower leaves through the stomata at 6 hpi. Intercellular hyphae produced septa apically and further differentiated into haustorial mother cells, which invaded the host cell and formed multinucleate haustoria at 24 hpi, with all the cytoplasm from the haustorial mother cells flowing into the haustorium. Aeciospores with warty surfaces invaded the host and produced oval-shaped urediospores at 14 dpi, which exhibited dense, thorny protrusions on their surfaces. By regulating the temperature, the urediospore-infected leaves were induced to produce teliospores after 20 days, which consisted of two cells with smooth surfaces.