Structural and ultrastructural analysis of root primordia in vitro cultures (RPC) of Solanum lycopersicoides Dun.

The structure and ultrastructure of cell aggregates and derived root primor dia were analysed in established suspension cultures of Solanum lycopersico ides Dun. A total of four modified Murashige and Skoog (1962) (MS) media we re used, two containing a-naphthaleneacetic acid (NAA) and two containing 2 ,4-dichlorophenoxyacetic acid (2,4-D). Considerable differences were observ ed in the size and structure of the aggregates regardless of the medium. Th e largest aggregates had a four-zone structure (cover, starch, dividing and differentiation zones) with distinct ultrastructural organization. The deg ree of histological and ultrastructural differentiation in the aggregates i ndicated rhizogenesis initiation. It begins with the protrusion of mounds o f root primordia, which are the result of radial growth of cells in defined zones of the aggregate. Further growth from the primordium forms a root me ristem with three tiers initial centre and wholly distinguished histogens ( dermatogen, periblem, plerome). Rhizogenesis was more regular on media with NAA than on media with 2,4-D. Primordia and fully organized roots were lib erated due to fragmentation of the aggregate and underwent changes in the p ermanently active developmental cycle of the culture which gave rise to suc cessive generations of aggregates. Growth regulators had different influenc es on the organellar composition and phenolic compounds presence in the dif ferent zones of aggregates. (C) 2000 Elsevier Science Ireland Ltd. All righ ts reserved.