Evidence for high activity of xylem parenchyma and ray cells in the interface of host stem and Agrobacterium tumefaciens - induced tumors of Ricinus communis
2001
Pavlovkin, J. (Slovak Academy of Sciences, Ivanka pri Dunaji (Slovak Republic). Institute of Experimental Phytopathology and Entomology) | Okamoto, H. | Wachter, R. | Lauchli, R. | Ullrich, C.I.
Rapidly developing tumors at hypocotyls of Ricinus communis, induced by Agrobacterium tumefaciens strain C58, were characterised by strong differentiation of vascular bundles and functional connection to the host bundles. The stem/tumor interface showed typically increased xylem, with smaller but numerous vessels interrupted by multiseriate unlignified rays. To know the mechanisms by which nutrients efficiently accumulate in the tumor sink tissue, in an electrophysiological approach cell electropotentials (Em) in tumor cross-sections were mapped. Xylem and phloem parenchyma cells and stem/tumor-located rays were hyperpolarised to Em values of around -170 mV, suggesting high plasma-membrane proton pump activities. Fast dividing cells of cambia or small tumor parenchyma cells had relatively low membrane potentials. The tumor aerenchyma and the stem cortex cells displayed values close to the energy-independent diffusion potential. The lowest values were recorded in stem pith cells. Measured cells were identified upon iontophoretically injected fluorescent Lucifer Yellow CH. The pattern of individual cell electropotentials, obtained from tissue sections, was supplemented by whole organ voltage measurements. Voltage difference between the tumor surface and the xylem perfusion solution in stems attached to the tumors, the trans-tumor electropotentials (TTP), originate from some in Ricinus stems not yet known, endodermis-like insulating boundary between the phloem and aerenchyma. The TTP confirm the findings of high membrane pump activity also in intact tumors of mainly xylem parenchyma and ray cells. These data functionally support the structural evidence of nutrient flow priority from the stem to the tumor on expense of supply to the host shoot
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