NUCLEAR IMPORT OF AGROBACTERIUM VIRD2 AND VIRE2 PROTEINS IN MAIZE ANDTOBACCO

Previously, we have shown that Agrobacterium-plant cell transferred DN A (T-DNA) transport into the host cell nucleus is likely mediated by t wo specific bacterial proteins, VirD2 and VirE2. Here, we used these p roteins to study molecular pathways of nuclear import. First, the role of VirE2 nuclear localization signals (NLSs) in the T-DNA transport p athway was examined by using tobacco plants transgenic for deletion mu tants of VirE2. In these plants, the virulence of wild-type Agrobacter ium was reduced possibly by competition for the cellular nuclear impor t machinery. Second, we analyzed the nuclear localization of VirE2 and VirD2 in the nonhost monocot maize. Part of the known recalcitrance o f monocots to transformation by Agrobacterium could be due to a potent ial selectivity in nuclear import pathways in monocotyledonous and dic otyledonous plants. Nuclear transport of VirD2 and VirE2 in maize leav es and roots was compared to that in tobacco protoplasts and roots. Bo th proteins accumulated in maize leaf and tobacco protoplast nuclei as well as in nuclei of immature root cells. In contrast, VirD2 and VirE 2 expressed in mature roots of maize and tobacco remained cytoplasmic. Point mutations of VirE2 nuclear localization signals, NSE 1 and NSE 2, also revealed that, in maize, the NSE 1 signal was mainly responsib le for nuclear import; in contrast, both signals functioned independen tly in tobacco protoplasts.