ORIGINAL ARTICLE Ultrastructural changes in the epidermis of petals of the sweet orange infected by Colletotrichum acutatum João Paulo R. Marques 1 & Lilian Amorim 1 & Marcel B. Spósito 1 & Beatriz Appezzato-da-Glória 1 Received: 22 June 2015 /Accepted: 17 August 2015 # Springer-Verlag Wien 2015 Abstract Postbloom fruit drop (PFD) is an important disease caused by the fungus Colletotrichum acutatum. PFD is characterised by the formation of necrotic lesions on the petals and stigmas of flowers as well as premature abscission of the fruit in Citrus spp. We compare the ultrastructure of the epi- dermis of uninoculated Citrus sinensis petals with that of petals inoculated with the fungus to understand the changes that occur upon C. acutatum infection. Healthy petals have a cuticle with parallel striations covering the uniseriate epider- mis. This pattern consists of vacuolated parietal cells whose cytoplasm contains mitochondria, plastids with an undevel- oped endomembrane system and a slightly dense stroma, a poorly developed rough endoplasmic reticulum, polysomes, few lipid droplets, and a nucleus positioned near the inner periclinal wall. In damaged regions, the cytoplasm of some cells is densely packed with well-developed endoplasmic re- ticulum, a large number of hyperactive dictyosomes, numer- ous mitochondria, and many lipid droplets. The plastids have an electron-dense stroma, starch grains, and a large amount of electron-dense lipid droplets, which can be released into vac- uoles or the endoplasmic reticulum. Multivesicular bodies and myelin bodies are frequently observed in the vacuole, cyto- plasm, and periplasmic space. Vesicles migrate through the cell wall and are involved in the deposition of cuticular material. In the later stages of infection, there is deposition of new cuticle layers in plaques. The outer periclinal walls can be thick. These observations indicate that epidermal cells respond to the pathogen, resulting in cuticular and parietal changes, which may limit further infection. Keywords Citrus sinensis . Cuticle . Dictyosomes . Endoplasmic reticulum . Multivesicular bodies . Plastids Introduction Citrus postbloom fruit drop (PFD), caused by Colletotrichum acutatum (Peres et al. 2005 ) and Colletotrichum gloeosporioides (Lima et al. 2011; McGovern et al. 2012), is an important disease in São Paulo, Brazil, due to high yield losses when blooming occurs during a rainy period. The typ- ical symptoms of the disease are necrotic lesions on petals bearing salmon pink acervuli, necrotic lesions on the stigma, and premature fruit drop (Denham and Waller 1981; Lin et al. 2001). It was demonstrated that C. acutatum can penetrate the host through stomata as well as directly through the cuticle via appressorium formation (Marques et al. 2013). The induction of appressoria is related to the recognition of topographic sig- nals and the hardness and hydrophobicity of the cuticle (Read et al. 1992). These cuticular characteristics can influence the adhesion process of the spore over the plant surface (Read et al. 1992). The composition of the cuticle, particularly that of the epicuticular waxes, is specific to each species and di- rectly influences spore germination and appressorium differ- entiation (Podila et al. 1993; Smith et al. 2006). Citrus flower extracts can stimulate C. acutatum growth and sporulation (MacKenzie et al. 2010) that can survive in citrus leaves sur- face as a latent infection (Zulfiqar et al. 1996). Additionally, it was stated that citrus floral buds smaller than 8 mm present Handling Editor: Adrienne R. Hardham Electronic supplementary material The online version of this article (doi:10.1007/s00709-015-0877-3) contains supplementary material, which is available to authorized users. * Beatriz Appezzato-da-Glória bagloria@usp.br 1 Luiz de Queiroz College of Agriculture, University of São Paulo, Cx. Postal 9, CEP 13418-900 Piracicaba, São Paulo, Brazil Protoplasma DOI 10.1007/s00709-015-0877-3