Histochemical and histometrical changes induced by the rust fungi Kuehneola uredinis in distinct blackberry (Rubus spp.) cultivars

Pathogenic fungi that cause rust adversely affect plants, including crops, affecting them differentially depending on the cultivar. This is the case of blackberry (Rubus spp.), a member of Rosaceae, introduced in Brazil by Embrapa during the 1980s and 1990s. Given the advantageous nutritional properties of its fruits and the relatively low management costs associated with its cultivation, blackberry production has emerged as a significant alternative agricultural source for family farmers. We studied nonsymptomatic leaves of Rubus spp. and those infected with Kuehneola uredinis to evaluate structural and histochemical alterations induced by the rust fungi and compared the responses among the three blackberry cultivars. Kuehneola urenidis colonizes the mesophyll region through hyphal growth, leading to the formation of a pseudoparenchyma and to cell hypertrophy. As colonization progresses, the epidermis becomes ruptured, leading to the production of urediniospores and teliospores. While the cultivars exhibit similar anatomical characteristics, their profiles diverge in the presence of K. uredinis. The Guarani cultivar exhibits more intense tissue expansion responses in the presence of the fungus, with an increase in total leaf blade thickness (15%) and adaxial epidermis thickness (25.9%). On the other hand, the BRS Xavante cultivar suffers the greatest structural losses, showing a reduction in mesophyll (9%) and a drastic 40% drop in palisade parenchyma thickness. Finally, the BRS Tupy cultivar proved to be more stable, maintaining better tissue preservation. In regions colonized by K. uredinis, the histochemical profile undergoes significant changes, characterized by the accumulation of phenolic compounds, proteins, lipids, and essential oils, compared with those in nonsymptomatic regions, although essential oils were less detected in the more susceptible BRS Xavante. Ultimately, our results confirm varying degrees of susceptibility among the cultivars, providing insights for future research and breeding programs aimed at developing rust-resistant blackberry varieties.