Plant Cell, Tissue and Organ Culture 33: 39-44, 1993. © 1993 KluwerAcademic Publishers. Printedin the Netherlands. Direct root tip conversion of Catasetum into protocorm-like bodies. Effects of auxin and cytokinin Sandra Colli ~ & Gilberto B. Kerbauy* ~Department of Animal and Plant Biology, Londrina State University, 86.100, Londrina, PR, Brazil; Department of Botany, University of S~o Paulo, P.O. Box 11461 05422-970, S~o Paulo, SP, Brazil (* requests for offprints) Received 8 October 1991; accepted in revised form 13 October 1992 Key words: Catasetum fimbriatum, orchid, regeneration, root bud Abstract Root apex conversion of Catasetum fimbriatum into protocorm-like bodies (PLBs) can occur in the absence of any added plant growth regulator. The presence of exogenous auxins in media drastically reduced the number of PLBs formed; on the other hand the concentrations of these auxins used greatly increased the process of callus formation. No effect on the mean number of root tip conversions into PLBs was observed with chlorogenic acid. However, this process was significantly increased in one of the concentrations used of p-coumaric acid. BA did not have any effect on callus formation, but caused marked acceleration in the process of root tip conversion and on the mean number of PLBs formed. PLB formation observed in the absence of any exogenous growth substance seemed to reflect a disruption in the interactions between the excised roots and the rest of the plants. The presence of light decreased the process of conversion. Abbreviations: BA-6-benzylaminopurine, IAA-indole-3-acetic acid, IBA-indolebutyric acid, 2,4-0-2,4- dichlorophenoxyacetic acid, PLB-protocorm-like body Introduction Root bud formation takes place predominantly in mature parts of the root system. According to Peterson (1975), bud regeneration in the region of root apical meristem could be considered a relatively rare process in higher plants. In the orchid family the direct conversion of root tip cells into protocorm-like bodies (PLBs) under natural conditions was described in Neottia nidus-avis by Champagnat (1971). However, this process has been also observed in vitro with root-tip segments of Catasetum (Kerbauy 1984a) and other allied epiphytic genera studied in this laboratory, and in Cyrtopodium (S~inchez 1988). Orchid PLBs were considered to be true somatic embryos by Morel (1974). Despite the major changes which must occur during the conversion of meristematic root cells into PLB cells, the process as a whole in Catasetum can be considered relatively fast and easily achieved (Kerbauy 1984a) when compared to other patterns of orchid root-tip regeneration in vitro (Kerbauy 1984b, 1988, 1991). Basically, PLB formation in root tips of Catasetum involves a direct conversion of the apices (Kerbauy 1984a), while in most other orchid species studied a callus stage has proved to be a 'sine qua non' condition for in vitro regeneration (Stewart & Button 1978; Kerbauy 1984b, 1988, 1991). These noteworthy features of Catasetum roots seem to represent an interesting and useful system for certain approaches on plant cell dif- ferentiation studies. The present paper investi-