Background: Cucumber mosaic virus (CMV) is known to occur in sugar beet plants in Egypt and may produce severe damage to infected plants. However, studies on the effect of the CMV on the cellular and internal structures of sugar beet leaves were rare.
Methods: The CMV was isolated from sugar beet samples collected during November 2018 from the Fayoum governorate, exhibited symptoms of mosaic and leaf malformation. Detection was performed depending on the presence of specific antibodies against CMV, and isolation from local lesions on Chenopodium amaranticolor as a local lesion host. Eleven plant species belonging to four families were used to confirm the presence of CMV in the inoculum. Detection of the coat protein gene of CMV in infected leaves has been done by reverse transcriptase-polymerase chain reaction (RT-PCR), and the appearance of 678 bp bands confirmed the expected size of such gene. Light and transmission electron microscopy were used to study the cytological and histological changes that occurred in the leaves of the sugar beet plant by the pathogen, as well as, to determine some morphometric parameters such as upper and lower epidermis thickness, midrib thickness, blade thickness, palisade tissue thickness, spongy tissue thickness, height and width of the vascular bundle.
Results: Phylogenetic analysis results indicated that SEA-CMV-isolate under study (acc. no. MT491996) closely related (99.9%) to Beet-EG-CMV-isolate (JX826591), isolated from sugar beet plants from Kafr El-Sheikh governorate, Egypt during 2012. Results of sequence analysis confirmed the classification of isolate SEA as a member of group IA. Infection of sugar beet leaves with CMV resulted in the formation of amorphous inclusion bodies in the cytoplasm. Semi thin-sections of the diseased sugar beet leaf blade showed changes in cellular organization and vascular bundles that reflect the progression of the external symptoms. Electron micrograph showed isometric spherical virus particles, measuring approximately 28 nm in diameter. Ultrathin sections showed chloroplast malformations. The malformation appeared as a chloroplast broken envelope with the presence of numerous long starch globules. Ultrathin sections revealed the virus-like particles filled the nucleus while those in chloroplast formed a distinctive crystalline shape. The shape of the cell walls and the structures of mitochondria changed under the effect of CMV in most cells.
Conclusion: The results demonstrate that the virus possesses dangerous effects that reduce the functions of the chloroplasts in sugar beet plants that may perturb the photosynthesis in chloroplasts and the synthesis of ATP in mitochondria. The results also show that the virus exerted influence on all investigated parameters. The present study provides useful information for the cytological effects and structural changes in sugar beet cells resulting from CMV infection.
Key words: Sugar beet, Cucumovirus, DAS-ELISA, RT-PCR, Coat protein (CP), Sequence alignment, Inclusion bodies, Histopathology.