Adeninic and ureidic cytokinins: primary response events in in vitro tomato caulogenesis

Among the molecules showing cytokinin activity, phenylurea derivatives have been long discussed as regards their direct or indirect action in cytokinin-mediated processes. This paper considers the activity of three cytokinins, an adenine and two phenylurea derivatives, on the caulogenesis onset. Caulogenesis is composed of sequential stages involving peculiar hormonal requirements and the differential activation of genes that can be considered markers of the steps leading to shoot production. Our study shows that the expression of LESK1, marker of caulogenic competence acquisition, is induced by both adeninic and ureidic derivatives, indicating that morphogenesis is induced by these molecules through the activation of common components of response pathway. The detailed analysis on morphogenesis in a system deprived of endogenous cytokinins strengthen the hypothesis of a proper cytokinin activity for the two phenylurea derivatives, 1-phenyl-3-(1,2,3-thiadiazol-5-yl)urea (thidiazuron TDZ) and N-phenyl-N′-benzothiazol-6-ylurea (PBU). A different behaviour of zeatin riboside (ZR), TDZ and PBU, emerges when the growth regulator supply is preceded by a 3-day pre-culture which positively affects the caulogenesis obtained with TDZ, while it strongly inhibits the action of ZR. PBU activity instead resulted more similar to that of the cytokinin/auxin treatment, supporting the idea of an interaction of diphenylurea derivatives with auxin metabolism. The synergistic effect of auxin on shoot production emerges from both the higher rate of caulogenic competence acquisition, marked by a sharper peak of LESK1 expression, which leads to a strong increase in shoot number produced per explant when cytokinin and auxin are supplied together.