BEET WESTERN YELLOWS VIRUS IN PHLOEM TISSUE OF THLASPI-ARVENSE

An EM study was made of pennycress (T. arvense) infected with beet western yellows virus (BWYV) strain B-16. All leaves examined, whether symptomless or chlorotic, contained virus particles. However, the distribution of BWYV in leaves was uneven; many sections did not reveal virus particles. In contrast, large virus aggregations were readily found in stem tissue. In both leaves and stems, virions were seen in sieve elements, companion cells and phloem parenchyma, but not in xylem or mesophyll cells. In sieve elements, virions were located peripherally, often between the stacked cisternae of the endoplasmic reticulum (ER). In companion and other parenchyma cells, the virus was most often visible in the cytoplasm near the plasmalemma. Occasionally, virions were seen in the nucleus, aggregated around the nucleolus. Virus particles were found in plasmodesmata connecting sieve and parenchyma cells and in those between adjacent parenchyma cells. The most common cytopathological change was proliferation of the ER. Some small vesicles were noted, most often within virus aggregates in sieve elements. Healthy pennycress contained no virus-like particles.