Effect of the differentiated or dedifferentiated state of tobacco pith tissue on its behaviour after inoculation with Agrobacterium rhizogenes.

The ability of Nicotiana tabacum (cv. Wisconsin 38) pith tissue to give rise to transformed roots after inoculation with Agrobacterium rhizogenes (agropine type strain 1855) has been examined in relation to its state of differentiation. In young plants, all the stem tissues are able to initiate transformed roots while pith of adult plants does not react. Mature pith, composed mostly of polyploid and non-dividing cells, is able to recover susceptibility when placed in vitro on a medium containing auxin and kinetin. The optimal production of transformed roots is obtained after 6-10 days, when proliferating cells redifferentiated "cambial-like" layers and tracheids. The relationships between the inability of fully differentiated pith to give rise to roots and the underlying cell states are discussed.