Histopathology of Aculeastrum americanum on Rubus idaeus and insights into the chloroplast-pathogen interaction

Our findings provide the first microscopic evidence, in rust fungi, of the formation of two haustoria within a single host cell. Chloroplasts were observed in close association with the <i>A. americanum</i> haustorium, and underwent a series of alterations, that help to explain the drastic reduction in leaf gas exchange during late leaf rust infection. Although infected leaves produce defense substances, such as callose and phenolic compounds, raspberry leaves are unable to prevent successful colonization. The occurrence of cell collapses and necrosis, together with the ultrastructural alterations, likely contributes to the early defoliation observed in raspberry plants infected by <i>A. americanum</i>. This study provides novel insights into chloroplast-pathogen interactions, highlighting previously unrecognized aspects of chloroplast alterations during late leaf rust infection. Nevertheless, further investigations are required to deepen our understanding of this relationship in rust fungi as well as in other biotrophic pathogens.