Summary: The ability of wheat root tips to serve as efficient vertical vectors for passive transfer of Cd was evaluated in sterilized growth chambers containing agar, sand or soil. Most root tips, whether from main or lateral roots, were colonized by Cd and were capable of transferring this bacterium to a depth of 290 mm from the inoculation site. The location of Cd was directly dependent on root tip location; whenever root tips passed through a bacterial layer, regardless of depth, they became inoculated. However, when root tips failed to reach the inoculation site they were not colonized by Cd. Seed inoculation resulted in an even distribution of Cd along the entire root system. Inoculation at various depths in the growth medium resulted in an uneven bacterial distribution and bacteria were concentrated mainly in the elongation and small root-hair zones as well as in the inoculation site. Cd multiplied on the root tip during its vertical movement. However, bacterial movement in the root vicinity was minimal. Cd did not spread from the root surface to the rhizosphere when exogenous nutrients were supplied in this area, but did so in the presence of chemoattractants. It is suggested that the prevalence of Cd deeper than the initial inoculation site is a result of their passive transfer by the growing root tip.


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