Spatial and Temporal Distribution of Bean Dwarf Mosaic Geminivirus inPhaseolus vulgarisandNicotiana benthamiana

Abstract

The distribution of the bipartite geminivirus bean dwarf mosaic (BDMV) within tissue and cell types of systemically infected Phaseolus vulgaris and Nicotiana benthamiana plants was determined at 6, 12, and 21 days post sap-inoculation (dpi) using an immunolocalization technique with a capsid protein antibody. In both hosts, BDMV was detected in most cell types of the inoculated leaves. In systemically infected immature and mature leaves of P vulgaris (6 and 12 dpi) and in immature leaves of N. benthamiana (12 dpi), BDMV was detected in epidermal, mesophyll, bundle sheath, phloem parenchyma, and companion cells. However, by 21 dpi, BDMV appeared to have become restricted to phloem parenchyma, companion, and bundle sheath cells in newly developing leaves of infected P vulgaris plants. Two distinct BDMV infection domains were identified in both hosts: a phloem domain that included protophloem and mature phloem cells, and a nonphloem domain that included bundle sheath and mesophyll cells and, to a lesser extent, epidermal cells. BDMV showed a striking tropism for the protophloem cells of the shoot apex, and the overall number of infected phloem cells decreased sharply from the apex and upper stem to the roots. The results of this study establish that BDMV infection is not phloem-limited in either host, which is consistent with the efficient sap-transmissibility of BDMV, and suggest that the developmental stage of the host can influence the cell/tissue distribution of the virus. A model for the establishment of systemic BDMV infection in sap-inoculated plants is presented.