Control of thiophene accumulation in calli of two Tagetes species

The relation between root differentiation and accumulation of biocidal thiophenes was studied in stem calli of two Tagetes species. Disorganized tissues of T. erecta were low in thiophene production. A sharp rise in thiophene content coincided with the emergence of roots on the calli. Root regeneration and the amount of thiophenes produced were found to be quantitatively related. Callus tissues of T. minuta did not differentiate into organs. Nevertheless, they accumulated thiophenes throughout the incubation period. Light at 12 W m^- stimulated thiophene production in T. erecta without having an apparent effect on root regeneration. In T. minuta even low irradiance levels (2 W m∼²) strongly inhibited thiophene accumulation. Under favorable conditions thiophene concentrations in calli of both species were comparable to or somewhat lower than the levels in roots on the plants and in excised, cultured roots of T. erecta. We conclude that in calli of T. erecta thiophene accumulation is coupled to root regeneration whereas a different control mechanism allowing for accumulation in disorganized tissues is operative in T. minuta.