Ultrastructure of sugarbeet leaves infected with beet western yellows virus

Abstract

An electron microscope study of sugarbeet (Beta vulgaris L.) leaves infected with beet western yellows virus (BWYV) revealed isometric particles, 24–30 nm in diameter, in phloem and mesophyll cells. Compared to ribosomes the particles were slightly larger, more deeply stained, and sharper in outline. Many particles showed an electron lucent center. The particles are assumed to be the virus. A comparison of leaves of different ages from the same systemically infected plant suggested the following sequence in the spread of the virus. In a given leaf, the particles appear initially in mature sieve elements, then move to adjacent companion and parenchyma cells. From these cells, the virus spreads to phloem and mesophyll cells not in contact with the sieve tubes. The presence of virus particles in plasmodesmata between sieve elements and adjacent nucleate cells, as well as between contiguous parenchyma cells, indicates that complete particles are transported from cell to cell through plasmodesmata. Within the sieve tube, the particles traverse the sieve plates, for they frequently occur in sieve plate pores jointly with the P-protein. In the lumen of the sieve element, the particles occur close to the cell wall, sometimes in association with endoplasmic reticulum. In parenchyma cells (including companion cells), particles are most conspicuous and most numerous in the nuclei. Their close association with the nucleolus suggests that the latter may be involved in viral multiplication. In older infected leaves, particles within the nuclei form crystalline arrays. Within the cytoplasm, particles are commonly located close to the wall and may be aligned along the microtubules.