Symplastic continuity in Agrobacterium tumefaciens-induced tumours

The Agrobacterium tumefaciens-induced plant tumour is regarded as a strong sink, containing a well-developed vascular system that guarantees an efficient supply of water and nutrients from the host plant into the tumour. The phloem transport and unloading of the fluorescent dye carboxyfluorescein (CF) was studied to examine the potential pathways for unloading of a low-molecular-mass solute, and was compared with the symplastic movement of potato virus X expressing a green fluorescent protein-coat protein fusion (PVX.GFP-CP). The distribution of both CF and PVX.GFP-CP in the host plant, Nicotiana benthamiana, demonstrated a clear symplastic pathway between the phloem of the host stem and the cells of the tumour, and also a considerable capacity for subsequent cell-to-cell transport between tumour cells. This same pattern of CF transport was also demonstrated independently for the host species Cucurbita maxima and Ricinus communis. In addition to entering the tumour, CF and PVX both moved through the vascular rays of the host stem towards the stele. The results confirm that host and tumour tissues in the Agrobacterium gall are in direct symplastic continuity and emphasize an important symplastic pathway for radial solute transport in stems.Key words: Agrobacterium tumefaciens, carboxyfluorescein, GFP, symplastic phloem unloading, plant tumour, vascular rays