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Ultrastructural changes in the epidermis of petals of the sweet orange infected by Colletotrichum acutatum

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 epidermis 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 epidermis. This pattern consists of vacuolated parietal cells whose cytoplasm contains mitochondria, plastids with an undeveloped 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 reticulum, a large number of hyperactive dictyosomes, numerous 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 vacuoles or the endoplasmic reticulum. Multivesicular bodies and myelin bodies are frequently observed in the vacuole, cytoplasm, 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.

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Acknowledgments

We would like to thank FAPESP for financial support (projects n° 2008/54176-4; 2009/00425-6), FUNDECITRUS for the use of laboratories, and the Electron Microscopy Centre of the UNESP Biosciences Institute (Botucatu) for microscopy facilities.

Author information

Correspondence to Beatriz Appezzato-da-Glória.

Additional information

Handling Editor: Adrienne R. Hardham

Electronic supplementary material

Below is the link to the electronic supplementary material.

Few lipid droplets in epidermal cells of healthy petals of ‘Valencia’ sweet orange stained with Neutral red. Magnification ×63. (WMV 814 kb)

Healthy petals of ‘Valencia’ sweet orange after double staining with Calcofluor White and Nile red. Magnification ×63. (WMV 822 kb)

Numerous lipid droplets, stained by Neutral red, in the epidermal cells of petals of ‘Valencia’ sweet orange three days post-inoculation with the fungus Colletotrichum acutatum. Magnification ×63. (WMV 1447 kb)

Altered cuticular pattern of petals of ‘Valencia’ sweet orange five days post-inoculation with the fungus Colletotrichum acutatum after double staining with Calcofluor White and Nile red. Magnification ×63. (WMV 1462 kb)

Online Resource. 1

Few lipid droplets in epidermal cells of healthy petals of ‘Valencia’ sweet orange stained with Neutral red. Magnification ×63. (WMV 814 kb)

Online Resource. 2

Healthy petals of ‘Valencia’ sweet orange after double staining with Calcofluor White and Nile red. Magnification ×63. (WMV 822 kb)

Online Resource. 3

Numerous lipid droplets, stained by Neutral red, in the epidermal cells of petals of ‘Valencia’ sweet orange three days post-inoculation with the fungus Colletotrichum acutatum. Magnification ×63. (WMV 1447 kb)

Online Resource. 4

Altered cuticular pattern of petals of ‘Valencia’ sweet orange five days post-inoculation with the fungus Colletotrichum acutatum after double staining with Calcofluor White and Nile red. Magnification ×63. (WMV 1462 kb)

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Marques, J.P.R., Amorim, L., Spósito, M.B. et al. Ultrastructural changes in the epidermis of petals of the sweet orange infected by Colletotrichum acutatum . Protoplasma 253, 1233–1242 (2016). https://doi.org/10.1007/s00709-015-0877-3

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Keywords

  • Citrus sinensis
  • Cuticle
  • Dictyosomes
  • Endoplasmic reticulum
  • Multivesicular bodies
  • Plastids