Plant Ceil, Tissue and Organ Culture 21: 17-19, 1990. © 1990 Kluwer Academic Publishers. Printedin the Netherlands. Effects of benzisoxazole and benzisothiazole on tomato plant regeneration in vitro Camillo Branca, Anna Torelli & Maria Bassi Istituto di Botanica, Universit6 di Parma, 43100 Parma, Italy Received 18 May 1989; accepted in revisedform 5 September 1989 Key words: auxins, benzisothiazole, benzisoxazole, tomato, in vitro culture Abstract The effects of two synthetic auxins, BOA and BIA, on plant regeneration in vitro have been studied on explants of tomato cotyledons. The activity of these substances on cell elongation has also been tested on pea stem segments. It has been found that BOA is particularly effective in inducing the formation of shoots but has a weak activity on cell elongation, while BIA, which is more effective in inducing cell elongation, is less active in morphogenesis. It is concluded that (1) the two activities are not related to each other, (2) the receptors involved in the two processes are probably different, (3) the chemical structure of the auxin may be an important factor in organogenetic processes. Abbreviations: BIA-- 1,2-benzisothiazole-3-acetic acid, BOA-- 1,2-benzisoxazole-3-acetic acid, IAA--in- doleacetic acid, MS--Murashige & Skoog medium Introduction In vitro morphogenesis from explants of adult plant tissues is influenced not only by environmen- tal factors such as light, temperature, pH and com- position of the culture medium, but also by the ratio of auxins/cytokinins [8] and by the cytokinin structure [9]. Heyle & Vendrig [5] have demonstra- ted that in thin cell layers of Nicotiana tabaeum organogenesis also depends on the chemical struc- ture of the auxin added to the culture medium. To determine whether this was true in the case of in vitro regeneration from cotyledon explants, we tested the effect of two synthetic auxins on organ formation by explants of tomato cotyledons. The synthetic auxins were 1,2-benzisoxazole-3-acetic acid (BOA) and 1,2-benzisothiazole-3-acetic acid (BIA) Fig. 1). BOA is an isomer of ben- zisoxazolinone, a natural inhibitor of auxin binding to receptor sites and of the auxin-induced growth in oat coleoptile sections [10]. It has a low auxin-like activity on split pea test [4]. BIA's activity is similar to that of the natural auxin IAA. In etiolated pea stem it causes water absorption, cell enlargement, curvature of split internodes and ethylene produc- tion [2]. In explants of dormant tubers of Heliant- hus tuberosus it induces cell proliferation [1]. ••O N cH~ BOA COOH IiC" c°°" N S7 BIA Fig. 1. Chemical structure of 1,2-benzisoxazole-3-aceticacid (BOA) and 1,2-benzisothiazole-3-acetic acid (BIA).